Challenges and Issues Facing Ultrafast-Charging Lithium-Ion Batteries

Abstract

Ultrafast-charging (UFC) technology for electric vehicles (EVs) and energy storage devices has brought with it an increase in demand for lithium-ion batteries (LIBs). However, although they pose advantages in driving range and charging time, LIBs face several challenges such as mechanical degradation, lithium dendrite formation, electrolyte decomposition, and concerns about thermal runaway safety. This review evaluates the key challenges and advances in LIB components (anodes, cathodes, electrolytes, separators, and binders), alongside innovations in charging protocols and safety concerns. Material-level solutions such as nanostructuring, doping, and composite architectures are investigated to improve ion diffusion, conductivity, and electrode stability. Electrolyte modifications, separator enhancements, and binder optimizations are discussed in terms of their roles in reducing high-rate degradation. Furthermore, charging protocols are addressed; adjustments can reduce mechanical and electrochemical stress on LIBs, decreasing capacity fade while providing rapid charging. This review highlights the key technological advancements that are enabling ultrafast charging and that are assisting us in overcoming severe limitations, paving the way for the development of next-generation high-performance LIBs.

1. Introduction

The transition towards electrification in transportation and portable electronics has significantly raised the demand for lithium-ion batteries (LIBs). LIBs are currently considered to be the most important form of energy storage technology, and they have undergone rapid improvements to keep up with the needs of different applications, especially in EVs and renewable energy systems [1,2]. The increasing popularity of BEVs in global car stocks in recent years, as shown in Figure 1A, is the main reason for this, since they need efficient and high-capacity batteries with longer driving ranges and shorter charging periods [3].

The market for LIBs has grown exponentially in the last decade, owing to the growing uptake of EVs and the demand for portable electronic devices. By 2030, demand for lithium in pursuit of battery-related applications is expected to increase significantly, with almost 60% of presently available lithium already being used for such purposes; this proportion could reach 95% by the end of the decade. The LIBs market reached USD 64.84 billion by 2023, and it is predicted to grow at a compound annual growth rate (CAGR) of 23.33% from 2024 to 2032 [4,5]. In 2023, the demand for lithium for EV batteries rose to approximately 140 kilotons, which is a 30% increase from the previous year; this increment highlights the importance of LIBs in the future of transportation and consumer electronics—the number of EVs in the market is expected to reach 230 million by 2030 [3]. Such growing demand emphasizes the fact that LIBs are increasingly becoming the linchpin of the transition towards a sustainable and electrified future. Pursuing faster and more effective charging technologies has become a critical area of research that aims to overcome the limitations of conventional LIBs and promote their use in different sectors [6,7].

The rapid charging of LIBs is the primary factor in ensuring the effectiveness and comfort of their use; this is especially the case in the EV market. The typical charging time of an EV is six to eight hours; this is not comparable to traditional cars, which can be refueled in less than 10 min [8,9,10]. The demand for faster charging has driven significant developments that have revolutionized the EV industry in the computable market, e.g., Tesla provides superfast chargers that provide a 320 km drive with only a 15 min charging time [11]; the Avion V model from Guangzhou Automobile Group can drive 370 km on the same charging duration [12]. In this regard, Figure 1B is used to demonstrate the growth in the EV market and the dramatic increase in the availability of charging stations, especially fast-charging stations, in recent years [3]. In addition, LIBs are the power source for cell phones and laptop computers, further increasing the demand for fast-charging LIBs. The driving forces of fast-charging LIBs—such as lithium iron phosphate (LFP), which is used in ultrafast-charging batteries—have led to the development of batteries that are able to reach 20% to 80% charge in 6 min; this example of development provides an excellent indicator for the technological developments that have been undertaken in this sector [13].

Generally, the developers of fast- and ultrafast-charging technologies are promising that charging time will be reduced to below one hour, or even minutes; thus, if these promises are fulfilled, then the primary problem facing the widespread use of EVs and mobility electronics would be solved. The charging process for LIBs is categorized into various levels based on the rate of charge, commonly expressed in terms of “C-rate”, which measures how quickly a battery can be charged relative to its capacity [14,15]. A standard charge, defined as 1C, takes approximately one hour to fully charge a battery. Fast charging (FC) occurs at higher C-rates in less than an hour, typically between 4C and 6C, enabling the battery to charge within 10 to 20 min. Ultrafast charging (UFC) involves even higher rates, greater than 6C, reducing the charging time to under 10 min. The fastest category, termed “extremely fast charging”, (XFC), is achieved at rates exceeding 10C, allowing batteries to charge in less than 6 min [16,17,18,19]. As discussed above, current demands are driving certain advancements, improving charging rates to UFC levels; accordingly, adoption of EVs is increasing, signifying a move toward a replication of the convenience of refueling traditional internal combustion engine vehicles.

While many advances have been proposed to achieve UFC, high charging rates can cause some problems, ranging from LIB components and charging methods to safety issues. Mechanical cracking, lithium dendrite, electrolyte decomposition, increased heat generation, and thermal runaway are some of the difficulties that impact battery safety and longevity with high charging rates [20,21,22]. Addressing these challenges necessitates innovations in battery materials, thermal management strategies, and charging techniques, along with a concerted effort to improve the safety of LIBs for widespread UFC adoption. However, achieving these high charging rates comes with some requirements, such as high reaction kinetics, increased Li-ion diffusivity and electron conductivity, minimized internal resistance, and robust electrode structure able to tolerate high currents [17,22]. Therefore, continuous research will produce novel materials and methods to effectively obtain UFC conditions.

This review aims to investigate the challenges facing ultrafast-charging LIBs; a detailed examination of LIB components (anodes, cathodes, electrolytes, separators, and binders), charging protocols, and safety issues is presented. The objectives of this study are as follows: to explain the principles of the charging process and detail effective parameters, to introduce the requirements of and possible strategies for achieving ultrafast charging, to highlight recent advancements in materials, to identify the primary challenges faced in the development, and to discuss potential solutions and strategies for overcoming these challenges. Furthermore, we discuss charging protocols and their impacts; charging protocols are one of the main impactful factors for LIB performance, ensuring that degradation is mitigated while charging speed is maximized. The last section addresses the safety concerns associated with high-rate LIBs, such as lithium dendrite, thermal runaway risks, and remarks on recent safety developments in thermal management systems.

2. Overview of Ultrafast LIBs

Research into the ultrafast charging of LIBs must be grounded in an understanding of the fundamental mechanism of the process—the transport of Li-ions and electrons between electrodes and electrolytes through a series of electrochemical reactions. Various key factors control the kinetics of the charging process, such as Li-ion diffusion within the electrode material, resistance due to charge transfer at the electrode–electrolyte interface, and the conductivity of both ionic and electronic pathways [22,23,24]. Thermodynamically, migration energy barriers on Li-ions and the electrochemical stability of the materials involved during high-rate cycling hinder the process without substantial degradation. There is a requirement of material-level innovations, such as developing electrode materials with high ionic and electronic conductivity with shortened diffusion path lengths and stable interfacial properties [25,26,27]. Approaches for achieving higher fast-charging capabilities include nanostructuring electrode materials that provide 3D ion diffusion pathways and shorten lithium diffusion distances, doping, and defect engineering. Examples of such approaches include the following: graphite modifications like hard-carbon-riveted graphite, phosphorus–sulfur-doped graphite to improve lithium insertion kinetics and surface reactivity and reduce lithium plating, oxygen deficiency to increase electronic conductivity and lithium storage capacity, and carbon coating, which creates efficient ion transport networks while maintaining structural stability. Novel electrolytes focus on rapid ion transport through enhanced formulations. High-concentration electrolytes can be used to modify solvation structures for accelerated lithium desolvation and interfacial kinetics; meanwhile, coatings on solid electrolytes can be used to enable lithium diffusion in the bulk phase [22,28]. This chapter introduces the key requirements of ultrafast charging and material design strategies that can aid in achieving high-rate Li-ion transport without compromising battery cycle life or safety.

Achieving ultrafast charging requires a deep understanding of fundamental charging processes of electrochemical reactions; additionally, it is important to focus on optimizing the transport pathways for Li-ions and electrons—as shown in Figure 2—throughout the entire battery system, which will enable us to efficiently increase the energy storage and conversion rates. The process of charging an LIB involves the following steps, considering Li-ions and electrons: (1) the Li-ions move from the cathode to the electrolyte, while the electrons release from the cathode and move in the opposite direction to the anode; (2) the Li-ions diffuse through the electrolyte to approach the electrolyte–electrolyte interface, and then they migrate through the separator, heading toward the anode side; (3) the electrons migrate via the external circuit driven by the higher potential of the anode, moving through the current collector at the cathode to then pass through the circuit and approach the anode’s current collector; (4) at the anode–electrolyte interface, Li-ions dissolve and move into the anode material, and the Li-ions and electrons reconnect at the anode surface [23,24,29].

The storage and release of energy in an LIB during charge–discharge cycles are controlled by electrochemical reactions on the electrodes. Basically, the driving force of the reactions is the Gibbs free energy change, which determines the voltage and energy efficiency of the system [30,31,32]. As indicated in Equation (1), Gibbs free energy can be figured out by multiplying cell voltage (V), the number of electron transfer (n), and the Faraday constant (F).

$$\Delta G=-nFV$$

(1)

The internal resistance and voltage affect the power capability of the battery, which reflects how fast energy can be delivered or released [17,33]. This is described in Equation (2), where Q is the charge capacity, V_op is the operating voltage, t is the time, and R represents the internal resistance, including the battery’s electronic, ionic, and interfacial resistances. The main impedance for high power density in fast-charging LIBs would be the increasing polarization losses, which effectively decrease the operating voltage. Polarization effects result in voltage deviations from their equilibrium values; this is generally called overpotential, η, and is considered to be a factor that limits kinetics [27,34,35,36].

$$P={\displaystyle \frac{Q{V}_{\mathrm{op}}}{t}}={\displaystyle \frac{V_{\mathrm{op}}^2}{R}}$$

(2)

$$\eta =V_{\mathrm{oc}}-V_{\mathrm{op}}$$

(3)

The overpotential can be determined using Equation (3), where $V_{\mathrm{oc}}$ is the open-circuit voltage and $V_{\mathrm{op}}$ is the operation voltage of the battery. Polarization is induced by three key resistances: charge transfer resistance at the electrode–electrolyte interface, ohmic resistance from the conductive components, and concentration resistance due to Li-ion diffusion limitations in the electrolyte [37,38,39]. These resistances limit the electrochemical reactions and generate heat, thereby reducing charging speed. To model the relationship between current flow ($I$) and overpotential—which is known as a charge-transfer-controlled battery reaction—we refer to the Butler–Volmer equation [40]:

$$I=I_0\left[\mathit{exp}\left({\displaystyle \frac{nF\beta }{RT}}\eta \right)-\mathit{exp}\left({\displaystyle \frac{nF(1-\beta )}{RT}}\eta \right)\right]$$

(4)

In Equation (4), R is the gas constant, T is the temperature, β is the charge transfer coefficient, i₀ is the exchange current density (that is, i₀ = K₀FA), K₀ is the electrode constant reaction rate, and A is the reactant activity. At high overpotential, this equation can be simplified to the Tafel equation (Equation (5)); this shows that reaction rates are exponentially dependent on the voltage drop-off. Therefore, to enable high charging speeds, it is important to minimize polarization and resistance effects and enhance charge transfer efficiency through the use of active materials [41,42].

$$\eta =a-b\mathit{log}I/I_0$$

(5)

Li-ion transport through the electrode structures is a critical factor in determining how quickly a battery can be charged. A material’s intrinsic property to show the ability of Li-ion transport through the electrode is ionic diffusivity (D_i). The Arrhenius equation (Equation (6)) can be used to calculate ionic diffusivity; here, D₀ is a pre-exponential factor, ΔG is the energy barrier, and k_B is the Boltzmann constant. Since diffusion rates are exponentially related to temperature and activation energy—though increasing temperature can lead to safety concerns—reducing the energy barrier for Li-ion diffusion within the material is essential for improving fast-charging capability [20,43,44].

$$D_i=D_0\mathit{exp}\left(-{\displaystyle \frac{\Delta G}{k_{B}T}}\right)$$

(6)

The diffusion time (τ) is dependent on the material’s diffusion coefficient (D_i) and diffusion length (λ); here, a shorter diffusion path or a higher diffusion coefficient results in faster lithium transport [45].

$$\tau ={\displaystyle \frac{{\lambda }^2}{D_i}}$$

(7)

The transport of Li-ions in the electrolyte proceeds through a diffusion-based process. The electrolyte should possess high Li-ion diffusivity (D_L) and Li-ion conductivity (σ_Li) to enable effective fast charging. Both depend upon the lithium salt concentration and solvent properties of the electrolyte [46,47]. Equations (8) and (9) describe lithium-ion diffusivity and its direct correlation with lithium-ion conductivity.

$$D_L=u{k}_{B}T={\displaystyle \frac{k_{B}T}{6\pi \mu R_0}}$$

(8)

$${\sigma }_{\mathrm{Li}}={\displaystyle \frac{q^{2}c}{k_{B}T}}{D}_L$$

(9)

where $\mu$ is the viscosity of the electrolyte solution, R₀ is the solvated ion radius, $u$ is the mobility of the ion, q is the charge of a Li-ion, and c is the Li-ion concentration. From these equations, it is obvious that, to achieve fast charging, the electrolyte must have high Li-ion conductivity, which depends on the lithium salt concentration, the electrolyte viscosity, and the solvent properties. However, during fast charging, concentration gradients can form due to limited ionic mobility, leading to lithium-ion depletion near the electrode interface [48,49]. This issue becomes more severe at high charging currents and increases the internal resistance, with the possibility of lithium plating on the anode potentially leading to reduced battery performance. Low-viscosity solvents can be used to ensure better lithium-ion mobility, improving conductivity. Hence, electrolyte solvents and salt concentration play important roles in fast-charging performance optimization [50,51,52].

In parallel with Li-ion transport, electrons must travel through the electrode to complete the circuit during charging and discharging [53,54]. The electronic conductivity (σ_e) in electrode materials is described by Equation (10), in which n is the charge carrier, $p_i$ is the hole concentration, and μ represents the mobility of the electron and the hole.

$${\sigma }_e=n_{i}e{\mu }_e+p_{i}e{\mu }_h$$

(10)

For semiconducting electrode materials, conductivity is also affected by the material’s bandgap (E_g), which influences electron and hole availability, as follows:

$${\sigma }_e=4.84\times 10^{15}\left({\displaystyle \frac{m^{*}}{m_0}}\right)T^{3/2}3\left({\mu }_e+{\mu }_h\right)\mathit{exp}\left[-\left({\displaystyle \frac{E_g}{2k_{B}T}}\right)\right]$$

(11)

where m*/m₀ is the effective mass ratio of charge carriers (electron and hole). A smaller bandgap or intrinsic metallic conductivity can enhance electronic transport, reducing internal resistance and providing high charge transfer. Strategies such as doping with conductive elements and surface modifications with carbon coatings are commonly employed to enhance electronic conductivity [55,56].

Fast-charging strategies are summarized in Table 1. Nanostructuring electrode materials is one of the most effective methods, as it reduces the diffusion distance of lithium-ions and increases the surface area. By forming nanoscale structures, such as nanoparticles, nanowires, nanosheets, and hollow structures, the lithium-ion transport distance is reduced and diffusion kinetics are significantly enhanced [57,58]. Another significant technique is doping and defect engineering, in which the intrinsic material characteristics are modified in order to improve their stability and conductivity. The addition of dopants such as titanium or magnesium can modify the electronic structures of materials so that they are electrically more conductive and reduce ion migration barriers [57,58,59]. Creating controlled defects, for example, oxygen vacancy in oxides or tunnels in lithium-layer-structured compounds, promotes easier ionic transport. The hybrid design provides conductive shells and high-capacity cores, achieving balance between energies and power densities. Examples of such architectures include the high-capacity core material within the conductive, stable shell, which inhibits structural degradation during cycling [60,61]. Phase-mixed materials, such as blends of spinel and layered oxides, create synergies that enhance lithium diffusion and stabilize the electrode during high-rate operation. Incorporating conductive additives like graphene and carbon nanotubes further improves electronic conductivity by forming interconnected networks that facilitate rapid electron transport [62,63,64]. Electrode engineering can be conducted through the optimization of structural parameters such as porosity and particle connectivity. Electrode porosity increases, enhances electrolyte penetration, and minimizes concentration polarization; meanwhile, good particle connectivity minimizes electronic resistance and enhances charge transfer. Techniques such as the creation of gradient porosity, where porosity varies across the electrode thickness, mitigate issues such as lithium plating and non-uniform ion distribution.

Additionally, electrode thickness control is crucial—although thicker electrodes provide higher energy density, they increase resistance and suppress ion transport [65,66,67]. System-level improvements, such as dynamic charging strategies and thermal management systems, minimize heat production and polarization under rapid charging conditions [68,69]. In addition, improved stable materials and high-voltage operation enable increased power density.

Table 1. Summary of fast-charging strategies for lithium-ion batteries [23,24,70,71].

Strategies	Approaches	Mechanism	Schematics
Nanostructuring	Nanoparticles, nanotubes, nanosheets, etc.	Shorten diffusion length, increase surface area, and enhance ion and electron transportation rates
Surface modification	Carbon coating and metallic coating	Enhance the electronic and ionic conductivity to increase rate capability
Dopant manipulation	Homogenous/heterogeneous metal ions	Improve ionic conductivity due to deficiency or excess in electrons
Electrode engineering	Minimizing the strain effect and electrostatic interaction	Increase Li-ion diffusivity by reducing the activation energy
Hybrid composite design	Synthesizing composite materials by conducting additives	Enhancing the electronic and ionic transport rates; conducting materials act as electron pathways and reduce lithium-ion diffusion length

Table 1. Summary of fast-charging strategies for lithium-ion batteries [23,24,70,71].

Strategies	Approaches	Mechanism	Schematics
Nanostructuring	Nanoparticles, nanotubes, nanosheets, etc.	Shorten diffusion length, increase surface area, and enhance ion and electron transportation rates
Surface modification	Carbon coating and metallic coating	Enhance the electronic and ionic conductivity to increase rate capability
Dopant manipulation	Homogenous/heterogeneous metal ions	Improve ionic conductivity due to deficiency or excess in electrons
Electrode engineering	Minimizing the strain effect and electrostatic interaction	Increase Li-ion diffusivity by reducing the activation energy
Hybrid composite design	Synthesizing composite materials by conducting additives	Enhancing the electronic and ionic transport rates; conducting materials act as electron pathways and reduce lithium-ion diffusion length

3. Material Designs for Ultrafast-Charging Lithium-Ion Batteries

The design of materials for ultrafast-charging LIBs requires considerations and optimization of all key components—including the anodes, cathodes, electrolytes, separators, and binders—to enhance ionic and electronic transport in addition to ensuring long-term structural integrity. The main material design challenges that are faced in designing ultrafast-charging LIBs are summarized in Figure 3. The anode is generally made up of graphite; however, various disadvantages regarding lithium plating when high charge rates are applied need further development, with other material alternatives such as lithium titanate (LTO), silicon composites, and transition metal oxides showing high rates of capability and stability [29,72,73]. The cathodes, while acting as hosts to store and release Li-ions, should exhibit excellent electronic and ionic conductivity. Inventions featuring nanostructures, doped composition of layered oxides, spinel structures, and polyanionic compounds have greatly improved the charge kinetics of LIBs [74,75]. Electrolyte optimization becomes crucial for ultrafast charging; it has to offer high Li-ion conductivity with electrochemical and thermal stability based on different strategies, including using electrolyte additives, high-concentration electrolytes (HCEs) or localized high-concentration electrolytes (LHCEs), low-viscosity cosolvents, and advanced electrolyte systems. It involves great improvements made on liquid electrolytes, gel–polymer systems, and solid-state electrolytes that can be used to decrease resistance and consequently enable fast ion transport [66,71]. Moreover, the separator is optimized to avoid short circuits and has allowed Li-ion passage through the use of high-porosity ultrathin membranes, providing safety in operation at high charge–discharge rates [76,77]. The binder also plays an important role in maintaining electrode integrity; next-generation binders with enhanced elasticity and conductivity, such as polymeric and self-healing binders, are under development to accommodate electrode expansion and improve long-term cycling stability [78,79]. These developments collectively contribute to the realization of ultrafast-charging LIBs, where every component needs to be optimized if we are to attain high power density, rapid-charge acceptance, and extended lifespan.

3.1. Anode Materials

As the source of electrons in Li-ion batteries, enabling the oxidation reaction during battery discharge, anodes are a crucial component in Li-ion batteries. Candidate anode materials can be categorized based on their interaction mechanisms for the storage of Li-ions in anodes: intercalation materials (carbon nanotubes, graphite, Ti oxide-based materials, Nb oxide-based materials, etc.), conversion materials (transition metal oxides: Co, Mn, Ni, Fe, Mo, V, Cu, etc.), and alloy materials (metallic and semi-metallic Li-ions (Li_x M): Si, Sn, Sb, Ge, P, etc.) [29,73,80]. The transfer of Li-ions is based on the local electric field; bulk-phase diffusion is mainly affected by concentration gradients in anode materials during lithium storage, both of which are crucial for ensuring good charging rate performance [71,80,81]. Table 2 shows different types of anode materials and their specifications for LIBs.

Graphite-based materials have typically been the most-used anode material, especially in commercial applications, because of their high theoretical capacity of 372 mAh g⁻¹, high electrical conductivity, low costs, and excellent electrochemical and cycling stability [82,83,84]. During the charging process, Li-ions intercalate into the layered structure of graphite upon charging and undergo deintercalation during discharging, enabling a reversible mode of energy storage. However, in using graphite, as a traditional anode material, one encounters some problems; these are caused by poor lithium intercalation kinetics and lithium plating at high current rates, which induce capacity degradation and safety risks [83,85,86]. In this regard, several strategies have been developed so far, such as structural modification, which includes the introduction of nanopores to reduce lithium diffusion paths and expanding graphite interlayer spacing to lower resistance. These modifications provide a way of enabling faster Li-ion transport at elevated currents to further stabilize the material. Also, surface modification through coating or doping has been tested to modify the SEI, providing better control of the issues related to lithium plating and polarization in fast charging.

Table 2. Different types of anode materials and their specifications for fast charging [23,29,73,80,81,87].

Anode Types	Materials	Specifications	Mechanism	Structure
Intercalation	Carbon materials, TiOx, NbOx	Capacity: <400 mAh/g C-rate: 2C–10C Cycle life: >500 Volume expansion: <10%	Layered structure with rapid Li-ion intercalation
Conversion	Metal oxides (Fe2O3, Co3O4)	Capacity: 500–1000 mAh/g C-rate: 2–5C Cycle life: ~200–500 cycles Volume expansion: 10–200%	Conversion reaction; forms new phases upon fast charging
Alloy	Silicon-based materials	Capacity: 1000–4200 mAh/g C-rate: 2–10C Cycle life: ~200–500 cycles Volume expansion: 100–300%	Alloy formation; significant expansion on lithiation

Table 2. Different types of anode materials and their specifications for fast charging [23,29,73,80,81,87].

Anode Types	Materials	Specifications	Mechanism	Structure
Intercalation	Carbon materials, TiOx, NbOx	Capacity: <400 mAh/g C-rate: 2C–10C Cycle life: >500 Volume expansion: <10%	Layered structure with rapid Li-ion intercalation
Conversion	Metal oxides (Fe2O3, Co3O4)	Capacity: 500–1000 mAh/g C-rate: 2–5C Cycle life: ~200–500 cycles Volume expansion: 10–200%	Conversion reaction; forms new phases upon fast charging
Alloy	Silicon-based materials	Capacity: 1000–4200 mAh/g C-rate: 2–10C Cycle life: ~200–500 cycles Volume expansion: 100–300%	Alloy formation; significant expansion on lithiation

The Li₃P-based crystalline SEI in the graphite anode improves Li-ion transport kinetics, significantly reducing Li-ion desolvation barriers. The low desolvation energy and high ionic conductivity of Li₃P SEI enable ultrafast charging with 91.2% in 6C, a long cycle life of 82.9% after 2000 cycles, and enhanced safety via suppression of lithium plating [88]. Chen et al. (2021) have developed hybrid graphite–hard carbon designs and SEI engineering techniques by blending graphite with hard carbon in a 50/50 ratio, improving current homogeneity and mitigating lithium plating, achieving a capacity retention of 87% after 500 cycles at 6C [89]. Figure 4A demonstrates the comparison of capacity performance of different anodes at 6C; some 3D representations of graphite, Gr-50, and hard-carbon electrodes are shown in Figure 4B [89]. In another work, hard-carbon-riveted graphite was used in a demonstration of enhanced Li-ion intercalation and suppressed formation of dead lithium (Figure 4C) in high-current cycling at 10C with a capacity retention of 98.2% after 4000 cycles; these findings represent a pathway for composite designs in achieving higher charge acceptance with a reduction in degradation [90]. Du et al. (2022) enhanced the interfacial performance of graphite with activated edge structures that address polarization and enhance lithium diffusion [91]. The modification achieved a reversible capacity of 150.3 mAh/g at 10C and steady cycling for 700 cycles at 5C [91].

A suitable anode material candidate for fast charging should have fast Li-ion diffusion, superior capacity, a long cycling life, and robust safety characteristics. Furthermore, research into alternative anode materials, such as transition metal oxides and silicon composites to tackle the limitations of graphite, should not be overlooked [70,71]. Titanium-based oxides, for example, allow for a “zero strain” structure with minimal volume changes during lithiation and delithiation; hence, they are ideal for attaining high-rate performances [80,87,92]. Also, silicon anodes can deliver a higher theoretical capacity limit; yet, the engineering that is required to sustain the achieved volume expansion during cycling is more complex [93,94]. Techniques such as nanocomposite materials, hybrid composites, and conductive coating insertion are being employed to increase their quick-charging capabilities. These innovations are introducing shorter charging times in order to respond to the increased need for quickly recharging batteries; such batteries will provide the foundation for increasingly widespread electric mobility.

Li₄Ti₅O₁₂ (LTO) is also a potential candidate for high-power, ultrafast-charging batteries due to structural stability and the extremely low possibility of lithium plating. Wang et al. (2019) synthesized nanosheet-assembled hierarchical LTO microspheres with 99.5% capacity retention at 50C after 2000 cycles [95]. The microsphere design can be used to maximize lithium-ion diffusion and minimize volumetric changes during cycling [95]. Wu et al. (2021) employed a UV-assisted sol-gel synthesis to create nanostructured LTO with refined crystal grains, which enhances Li-ion diffusion and electron transport; the researchers demonstrated a specific capacity of 120 mAh/g at 10C, nearly three times higher than that of non-UV-treated LTO [96]. This study remarks on the potential of photo-assisted methods for the synthesis of high-performance anode materials [96]. Recent advances in research into Li₃V₂O₅ (LVO) anodes has highlighted their potential in ultrafast charging applications. Wang et al. (2023) utilized dual modifications, i.e., polyaniline intercalation and sodium doping, to achieve 230 mAh/g with 94% retention at 1000 cycles at 10C [97]. The comparison of rate capability of LVO, P-LVO, and SP-LVO anodes at various current densities can be seen in Figure 4D [97]. Xu et al. (2021) also designed neural network architectures embedding LVO nanodots, achieving capacities of 375 mAh/g at 10C over 1000 cycles [98].

Silicon is considered to be a potential candidate for use in high-charging-rate applications because of its high theoretical capacity (~4200 mAh/g); however, issues like high volume expansion and unstable SEI formation remain major challenges [70,99]. Li et al. (2024) [100] developed a modification of the silicon anode with an electroactive covalent organic framework (COF). As shown in Figure 4E, the COF layer effectively enhances lithium-ion transport, reduces electrode polarization, and stabilizes the SEI through the formation of a protective LiF-/LiN-rich film. The specific capacity obtained from the modified Si anode reached as high as 2089.2 mAh/g at 10C and remained within 1188.7 mAh/g after 500 cycles at 0.5C. The Si–COF anode exhibited better cycling stability and capacity retention in full cell configurations with NCM811 cathodes (Figure 4F), proving the viability of this anode material for next-generation ultrafast-charging lithium-ion batteries. In another approach, a scalable and cost-effective strategy for synthesizing silicon–carbon (Si–C) hybrid composite anodes was proposed. The Si–C hybrid anode achieved a specific capacity of 1800 mAh/g, maintained 80% capacity retention over 500 cycles, and delivered 1150 mAh/g at 10C [101].

Figure 4. (A) Normalized capacity vs. cycle number plots during 6C fast-charge cycling after 100 cycles [89]; (B) 3D representations of graphite, Gr-50, and hard-carbon electrodes [89]; (C) principle of cooperative promoter design of hard-carbon-riveted graphite (HCRG) anode and the difference of lithium plating between pristine graphite and HCRG anode [90]; (D) rates for LVO, P-LVO, and SP-LVO anodes at various current densities [97]; (E) schematic of the structural evolution and interphase composition of Si and Si@2 %COF anodes during cycling [100]; (F) cycling performance of NCM811//Si and NCM811//Si@COF full cells [100]; (G) SEM images of cycled α-MoO₃ electrode and image of α-MoO₃/SWCNH composite after cycling [102].

Figure 4. (A) Normalized capacity vs. cycle number plots during 6C fast-charge cycling after 100 cycles [89]; (B) 3D representations of graphite, Gr-50, and hard-carbon electrodes [89]; (C) principle of cooperative promoter design of hard-carbon-riveted graphite (HCRG) anode and the difference of lithium plating between pristine graphite and HCRG anode [90]; (D) rates for LVO, P-LVO, and SP-LVO anodes at various current densities [97]; (E) schematic of the structural evolution and interphase composition of Si and Si@2 %COF anodes during cycling [100]; (F) cycling performance of NCM811//Si and NCM811//Si@COF full cells [100]; (G) SEM images of cycled α-MoO₃ electrode and image of α-MoO₃/SWCNH composite after cycling [102].

As an example of metal oxide anodes, Sahu et al. (2020) presented the synthesis of an α-MoO₃/single-walled carbon nano-horn (SWCNH) composite, achieving an extraordinary capacity of 654 mAh/g at 1C and stable operation at ultrahigh rates of up to 100C [102]. The carbon matrix improved conductivity and buffered the mechanical stress caused by volume changes during charge–discharge cycles. The SEM images of the cycled α-MoO₃ electrode and α-MoO₃/SWCNH composite after cycling presented in Figure 4G obviously show the stability enabled by the nanocomposite after cycling [102]. Xia et al. (2021) designed a hollow, gradient-structured Fe₃O₄@C anode; it was used to minimize radial expansion to ~7%, and the researchers achieved stable operation, with a capacity of ~750 mAh/g after 10,000 cycles at 10 A/g, and retained ~500 mAh/g at an ultrafast rate of 20 A/g [103].

3.2. Cathode Materials

Cathode-active materials (CAMs) are considered to be one of the most important components of LIBs in terms of electrochemical characteristics such as energy density, power output, and safety. Cathode composites consist of active material, conductive carbon additives, binders, and current collectors that serve as the bases for Li-ions and are the sources of ions during the battery’s charging and discharging cycles; thus, they affect the battery’s total capacity and voltage [22,103,104]. The Li-ions mainly enter and exit seating sites (planes and tunnels) to solvate and desolvate from the cathode; this process takes place on the composite of the cathode called the insertion mechanism. Thus, the cathode materials’ chemical compositions, crystal structures, and morphologies mostly contribute to battery efficiency [22,70].

Different types of CAMs and their specifications, as shown in Table 3, are layered oxides, spinel oxides, polyanionic compounds, disordered rock salt oxides, and conversion cathode materials. Each material has specific benefits: layered oxides give high energy density and specific capacity; polyanionic compounds such as olivine cathodes enable good thermal stability, safety, and long cycle life; spinel oxides can provide 3D network channels for Li-ion diffusion and robust structure; disordered rock salt oxides (DRXs) have good stability and high capacity; and conversion cathode materials exhibit high theoretical potential and high specific capacity [24,105,106]. Apart from each material’s advantages, their disadvantages cannot be considered in terms of their high rates for achieving fast charging. For instance, the main challenge for layered oxides is structural instability, whereas low electronic conductivity and Li-ion diffusivity are the primary challenges facing the use of olivine polyanions [107,108]. Doping, surface coating, and nanostructuring are common methods that recent studies have employed to improve cathode material performance. These methods effectively improve electrochemical properties and overcome some problems in structure degradation, capacity fading, and the intrinsically low Li-ion and electronic transport rates that arise from the raw materials. The cathode material’s synthesis modification is key to providing high-rate, long-lasting, sustainable energy storage [109,110,111].

To achieve ultrafast-charging cathode materials, it is necessary to ensure the rapid diffusion of Li-ions at the cathode–electrolyte interface (CEI), fast transport of Li-ions, and the conductivity of electrons in the structure of the composite cathode. The high diffusivity of Li-ions induces internal particle stresses that often develop during operating batteries with increased heterogeneity in the material. Such heterogeneity amplifies the distribution of stress across the cathode, leading to structural degradation and a loss of capacity [112,113]. The current design for fast-charging cathode materials creates highly conductive pathways, i.e., designing materials in such a way that Li ions have shorter diffusion distances—this, among other actions, is a critical approach for evading stress and protecting material integrity. Developing cathode materials to enable ultrafast charging is critical for LIBs since the materials must show high ionic conductivity, structural stability, and excellent cycling performances under high charging rates for full-configuration battery cells [81,114,115]. Recent research has focused on employing various modification strategies for advancing layered oxides, spinel oxides, phosphates, and composite cathode materials to improve performance metrics such as rate capability, capacity retention, and energy density.

Table 3. Different types of cathode materials and their specifications for fast charging [22,23,24,70,71,116].

Cathode Types	Materials	Specifications	Mechanism	Structure
Layered oxides	LiCoO2 (LCO), LiNiCoMnO2 (NMC)	Capacity: 150–250 mAh/g C-rate: Up to 10C Cycle life: 500–2000 cycles Thermal stability: <200 °C	Layered structure; fast reversible Li-ion extraction
Spinel oxides	LiMn2O4, LiNiMnCoO2	Capacity: <150 mAh/g C-rate: Up to 10C Cycle life: 500–2000 cycles Thermal stability: <250 °C	Three-dimensional spinel structure; fast diffusion of Li-ions
Polyanion compounds	LiFePO4, Li3V2(PO4)3	Capacity: 150–170 mAh/g C-rate: Up to 5C Cycle life: 2500 cycles Thermal stability: <300 °C	Olivine structure; stable under fast charging
Disordered rock salt oxides (DRXs)	α-LiFeO2 structure, LiFeSO4F transition	Capacity: >300 mAh/g C-rate: Up to 5C Cycle life: 500–2000 cycles Thermal stability: ~200–300 °C	Disordered rock salt structure with random Li and transition metal positioning; enables high-rate lithium diffusion
Conversion cathodes	FeF3, FeF2	Capacity: 200–300 mAh/g C-rate: Up to 5C Cycle life: 500–1000 cycles Thermal stability: <150 °C	Reversible electrochemical conversion reaction providing high specific capacity and energy density

Table 3. Different types of cathode materials and their specifications for fast charging [22,23,24,70,71,116].

Cathode Types	Materials	Specifications	Mechanism	Structure
Layered oxides	LiCoO2 (LCO), LiNiCoMnO2 (NMC)	Capacity: 150–250 mAh/g C-rate: Up to 10C Cycle life: 500–2000 cycles Thermal stability: <200 °C	Layered structure; fast reversible Li-ion extraction
Spinel oxides	LiMn2O4, LiNiMnCoO2	Capacity: <150 mAh/g C-rate: Up to 10C Cycle life: 500–2000 cycles Thermal stability: <250 °C	Three-dimensional spinel structure; fast diffusion of Li-ions
Polyanion compounds	LiFePO4, Li3V2(PO4)3	Capacity: 150–170 mAh/g C-rate: Up to 5C Cycle life: 2500 cycles Thermal stability: <300 °C	Olivine structure; stable under fast charging
Disordered rock salt oxides (DRXs)	α-LiFeO2 structure, LiFeSO4F transition	Capacity: >300 mAh/g C-rate: Up to 5C Cycle life: 500–2000 cycles Thermal stability: ~200–300 °C	Disordered rock salt structure with random Li and transition metal positioning; enables high-rate lithium diffusion
Conversion cathodes	FeF3, FeF2	Capacity: 200–300 mAh/g C-rate: Up to 5C Cycle life: 500–1000 cycles Thermal stability: <150 °C	Reversible electrochemical conversion reaction providing high specific capacity and energy density

Layered oxides, like lithium cobalt oxide (LCO) and Ni-rich layered oxides (NCMs), are extensively researched for their electrochemical stability and high energy density [117,118]. A fluorinated LiCoO₂ cathode was developed by Bi et al. (2024) with an ultrathin LiF surface coating and a near-surface gradient fluorination lattice, which largely improved stability during high-voltage (4.6 V) and fast-charging (3C–5C) applications [119]. The F-LCO cathode had 92% capacity retention following 1000 cycles at 3C and 1100 cycles at 5C in a graphite/F-LCO full cell with reduced oxygen loss and improved Li-ion diffusion [119]. Sn and S co-doping was used by Zhu et al. (2023) to achieve the high-voltage stabilization of LCO, with 114.1 mAh/g being delivered at 20C and 63.32% capacity, and maintained after 300 cycles [120]. This study showed that co-doping enhances the formation of a stable, uniform cathode–electrolyte interface (CEI) with high LiF content, reducing interface resistance and preventing cobalt dissolution, which improved Li-ion transport and structural stability, resulting in improved capacity (as shown in Figure 5A).

Ni-rich cathodes like NCMs suffer from severe cracking and capacity loss at high charge/discharge rates; these also benefit from doping and surface engineering strategies [121,122]. Lu et al. (2022) optimized NCM811 via controlled surface facets, exhibiting 95% capacity retention at 6C after 100 cycles with a reduction in structural degradation [123]. The capacity retention and discharge capacity performance of different NMC cathodes are demonstrated in Figure 5B [123]. Zhang et al. (2022) further improved the stability and energy density of NCM811 by coating it with a poly(3,4-ethylene dioxythiophene) (PEDOT) layer, which enhanced interfacial stability and provided a specific capacity of 170.4 mAh/g at 7C, which can be seen in Figure 5C [124].

Lithium-rich manganese-based layered oxides (LLOs) exhibit large specific capacities (>250 mAh/g) due to redox processes that include both anionic and cationic reactions [125,126]. Li et al. (2022) built up a dual surface modification framework for LLOs by creating lithium phosphate and spinel layers (Figure 5D), which boosted cycling stability and rate performance [127]. They also achieve good specific capacity performance (132.2 mAh/g) at a high rate of 10C. However, challenges like loss of oxygen and decay of voltage still exist, requiring optimization [127]. Zhao et al. (2021) [128] synthesized a lithium/manganese-rich layered cathode with dual surface coatings of graphene/carbon nanotubes and an ion-conductive coating, which had a volumetric energy density of 2234 Wh/L and a specific capacity of 180 mAh/g at 10C. The material also retained 94.5% capacity after 1000 cycles at 0.1C. Hybrid cathode designs leveraging composite materials have gained attention for their ability to combine high capacity and rate performance.

Spinel oxides, such as LiMn₂O₄, are known for fast Li-ion diffusion through 3D transport pathways; meanwhile, in their application, researchers encounter problems like capacity fading and new phase generating with electrolytes. Wang et al. (2021) introduced twin boundary defects into LiMn₂O₄ to enhance diffusion kinetics, achieving 78 mAh/g at 10C and 94% capacity retention over 500 cycles [129]. The rate performance and schematic diagram of Li-ion diffusion in the asymmetrical twin boundary are shown in Figure 5E [129]. Expanding on these advancements, Gao et al. (2024) used a co-doping strategy with antimony (Sb) and fluorine (F) to address the limitations of high-voltage spinel LNMO cathodes [130]. The co-doped LNMO-SbF exhibited significant improvements in structural stability, charge transfer resistance, and lithium-ion transport (Figure 5F). The material achieved a discharge capacity of 113.1 mAh/g after 450 cycles at 1C, a capacity retention of 69.4% after 1000 cycles, and capacities of 111.4 mAh/g and 70.2 mAh/g at 5C and 10C, respectively. These enhancements were attributed to the Sb-ions stabilizing the lattice structure and the strong TM–F bonds suppressing Mn dissolution and phase transitions [130].

Olivine cathodes, particularly lithium iron phosphate (LiFePO₄, LFP), despite low electronic conductivity and moderate ionic diffusion, are well known for their safety, thermal stability, and long cycle life, making them a reliable choice for ultrafast charging applications [131,132]. Nguyen et al. (2023) fabricated a hybrid conductive coating combining carbon (C) and Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), a solid electrolyte, for LFP cathodes. This dual-layer coating enhanced both electronic and ionic conductivities [133].

Figure 5. (A) Rate performance of LCO and Sn, S co-doped LCO [120]; (B) comparison of discharge capacity and capacity retention at 6C cycling of NMC811 samples [123]; (C) rate capability evaluation of pristine and PEDOT-enfolded NCM811 and schematic of remote effect on LMA [124]; (D) schematic of enhanced cycling stability and fast charging of pretreated LLOs [127]; (E) rate capacities compared with LMO cathode materials and schematic diagram of Li-ion diffusion in the asymmetrical twin boundary [129]; (F) cycling performance of pure LNMO and LNMO-SbF at 10C and the structural models for Li⁺ migration in LNMO-SbF and pure LNMO [130]; (G) cycle performances at different C-rates of LFP@2.5C_7.5LATP and LFP@10.0C_0.0LATP [133].

Figure 5. (A) Rate performance of LCO and Sn, S co-doped LCO [120]; (B) comparison of discharge capacity and capacity retention at 6C cycling of NMC811 samples [123]; (C) rate capability evaluation of pristine and PEDOT-enfolded NCM811 and schematic of remote effect on LMA [124]; (D) schematic of enhanced cycling stability and fast charging of pretreated LLOs [127]; (E) rate capacities compared with LMO cathode materials and schematic diagram of Li-ion diffusion in the asymmetrical twin boundary [129]; (F) cycling performance of pure LNMO and LNMO-SbF at 10C and the structural models for Li⁺ migration in LNMO-SbF and pure LNMO [130]; (G) cycle performances at different C-rates of LFP@2.5C_7.5LATP and LFP@10.0C_0.0LATP [133].

The hybrid-coated LFP cathodes achieved impressive discharge capacities of 164.5, 137.1, and 121.3 mAh/g at 0.1C, 5C, and 10C; they also achieved good performance at 60C, as can be seen in Figure 5G [133]. Okada et al. (2018) used sulfur doping to improve LFP’s rate capability, achieving discharge capacities of 121.6 mAh/g at 5C and 112.7 mAh/g at 10C [134]. This work shows the potential of anion doping in improving the rate capabilities and cycling stability of LiFePO₄-based cathodes, as well as a scalable approach for high-performance energy storage systems [134]. Shi et al. (2021) used a novel copolymer carbon source to create thin, porous carbon layers, resulting in a 96.5% capacity retention after 500 cycles at 30C [135].

3.3. Electrolytes and Additives

Since electrolytes fill the whole battery cell and act as a battery’s blood, this medium is crucial for Li-ion transport between the anode and cathode, enabling electrochemical reactions to occur during charging and discharging. During the operation of a battery, the electrolyte acts like a bridge between the electrodes, fulfilling the following roles in an LIB: desolvation of Li-ions from the surface of one electrode, diffusion to the electrolyte, movement to the opposite side, and solvation to another electrode [22,70,71]. In this regard, the conductivity of the electrolyte for the bulk transportation of Li-ions to reach an electrode surface can greatly impact the charging speed. Additionally, electrolytes can affect different battery specifications, such as charging/discharging rate, capacity, temperature, internal resistance, and battery safety [136,137]. Investigating the factors that influence electrodes’ performances at high rates, previous studies have also shown that poor ionic conductivity—which comes from Li-ion transportation by electrolytes—can cause salt depletion and lithium plating on the anode surface; this is a main limiting parameter for charging speeds in electrodes [138,139]. As a result, ionic conductivity in electrolytes is rarely the rate-limiting factor for fast charging; exceptions to this arise in cases of very thick electrodes that extend the lithium-ion diffusion path.

Depending on an LIB’s application, electrolytes can be divided into different categories: organic liquid, aqueous liquid, ionic liquid, inorganic solid, polymer solid, and composite electrolyte [104,138,139]. The diffusion coefficient of Li-ions in liquid electrolytes is significantly higher than in solid electrodes, but they also offer lower cost, safety, and greener electrolytes [140,141]. The electrolyte typically contains lithium salt dissolved in a solvent mixture (mostly linear carbonates) [139,140], providing Li-ion conduction while maintaining electronic insulation between the electrodes with minor adjustments made for the additive, solvent ratio, and salt concentration in the electrolyte [138,142]. Salts commonly used in EVs, like lithium hexafluorophosphate (LiPF₆), are chosen for their high ionic conductivity, thermal stability, and compatibility with graphite anodes and lithium transition metal oxide cathodes [138,143,144]. Mixtures of organic solvents, usually ethylene carbonate (EC), with linear carbonates such as dimethyl carbonate (DMC) and diethyl carbonate (DEC) provide thermal stability; the flash point is increased, while viscosity and compatibility are decreased. However, most of the conventional organic solvents are flammable and volatile; hence, they are unsafe, especially at high charging rates or elevated temperatures [104,145,146].

Recently, electrolyte formulation design has focused on advancing safety, stability, and performance to meet requirements for rapid charging [17,139]. The lithium salt-solvent mixture is the key in determining the electrode–electrolyte interface and electrolyte properties. Since ionic conductivity is not the main limiting factor for Li-ion diffusion, the SEI layer, which forms mainly from electrolyte components, is crucial for fast-charging performance [147,148]. Moreover, side reactions during fast charging could further destabilize the electrolyte. Heat generation and lithium dendrite result in further conductivity degradation and may trigger exothermic reactions [138,149]. Additives such as fluoroethylene carbonate (FEC) or vinylene carbonate (VC) are typically added to stabilize the solid electrolyte interface (SEI) on the anode and the cathode–electrolyte interface (CEI) by forming a passivation layer on the composite of electrodes. This reduces side reactions, prevents the decomposition of the electrodes, and improves cycle life. In addition, low-viscosity cosolvents, for example, methyl acetate (MA) and acetonitrile (AN), enhance ionic conductivity and reduce desolvation barriers [139,150].

Additives play a critical role in stabilizing the electrolyte interface and enhancing the overall battery performance [151]. Bifunctional sulfone additives such as diphenyl sulfone (DPS) have been shown to increase capacity retention to 89 mAh/g at 5C after 500 cycles through the formation of a uniform, LiF-rich SEI [152]. Dong et al. (2021) demonstrated the use of fluoroalkyl ethers in the design of solvation-free Li⁺ transfer for ultrafast 10C charging with minimal capacity loss of 0.0012% per cycle for up to 5000 cycles [153]. Figure 6A exhibits the SEM and cross-sectional SEM images of Li anodes in 1,3-dioxolane (DOL) and dimethoxyether (DME) solvent and 60% fluoroalkyl ether 2,2,2-trifluoroethyl-1,1,2,3,3,3-hexafluoropropyl ether (THE) electrolytes after 100 cycles. These advances demonstrate the critical role of a design-specific additive strategy in enabling high-rate, long-term-stable battery operation [153].

HCEs and LHCEs can be used in approaches for improving ion transport through modifying the solvation structure and reducing free solvent content. Novel electrolytes, including ionic liquids, solid-state electrolytes, and gel-based systems, are being developed in a move toward better thermal stability and the suppression of dendrite formation at high current densities. These developments are targeted at achieving higher energy density, thermal stability, and safety, with a view to supporting the fast-charging capabilities of next-generation LIBs [154,155,156].

HCEs and LHCEs are innovations in improving interfacial stability and ionic transport [157,158]. For example, Lin et al. (2021) presented an advanced LHCE to enhance electrochemical stability, ionic conductivity, and electrode compatibility [159]. The study demonstrates that Li||Ni_0.5Co_0.2Mn_0.3O₂ (NCM523) cells using this LHCE achieve 89% capacity retention after 200 cycles at 1C (200 mA g⁻¹), with an excellent high-rate performance retaining 65% capacity (~130 mAh g⁻¹) at 10C (see Figure 6B). Additionally, this electrolyte remains liquid at −80 °C and enables stable cycling at −40 °C, retaining more than 50% of room-temperature capacity after 200 cycles at 1C [159]. Compared to conventional HCEs, the LHCE significantly reduces viscosity while maintaining high ionic conductivity, leading to enhanced Li-ion transport and passivation film stability. LHCEs mitigate solvent co-intercalation and promote reversible Li-ion transport. The solvation structures of LCEs, HCEs, and LHCEs are shown in Figure 6C. Additives like fluoroethylene carbonate and trimethylsilyl isothiocyanate further improve SEI and CEI stability, enabling high-rate cycling with minimal degradation [138,158].

Weakly solvated electrolytes have been seen to be promising in reducing desolvation barriers and facilitating Li-ion transport [160]. Yang et al. (2022) prepared an electrolyte with a film-forming cosolvent (fluoroethylene carbonate) and a weakly solvated solvent (ethyl trifluoroacetate) with higher ion transport, lower overpotential, and stable SEI formation with reversible capacities of 183 mAh/g at −30 °C and a 6C rate [149]. Tu et al. (2023) investigated Li₃P-based crystalline SEIs, for which they observed a 91.2% capacity recovery after 10 min (6C), with 82.9% capacity retained after 2000 cycles [88].

Molten salt and hybrid electrolytes are studied on the basis of their optimal combination of cost, safety, and ionic conductivity. In addition, molten salts avoid limitations such as volatility and flammability, serving as a better option over conventional liquid electrolytes [151,161]. A room-temperature molten salt (RTMS) electrolyte, comprising LiNO₃·3H₂O and NaNO₃, was used by Wang et al. (2021) [162]. As demonstrated in Figure 6D, this system delivered 90 mAh/g capacities at 20C and retained 94.7% capacity over 500 cycles at 3C. Controlled solvation electrolytes (CSEs) are engineered to optimize the balance between solvation and ionic mobility. Kautz et al. (2023) [163] introduced a CSE with minimal free solvents and achieved 165 and 160 mAh/g at 4 and 5C and retained 89.2 and 86.5% capacity for 300 cycles, respectively. The robust SEI and cathode–electrolyte interface (CEI) formed by the CSE suppressed structural degradation and enhanced ion transport. Huang et al. (2022) created a mixed-solvent electrolyte that blended fluoroethylene carbonate (FEC) and acetonitrile (AN), which exhibited over 300 mAh/g at 20C and retained 80% capacity for 2500 cycles [164]. The cycling performance at 8C is shown in Figure 6E. It shows that the solvent-assisted hopping mechanism shortened the lithium-ion diffusion times, enabling ultrafast charging [164].

Solid-state electrolytes (SSEs) are increasingly recognized as a transformative solution for ultrafast-charging applications due to their inherent safety and mechanical robustness. Li et al. (2022) addressed the poor solid–solid interfacial contact in conventional bipolar SSBs, which limits power performance. The research introduced an in situ-formed nonflammable ionogel that enhances ionic conductance and interfacial contact [165].

Figure 6. (A) SEM and cross-sectional SEM images of Li anodes in 1, 3-dioxolane (DOL) and dimethoxyether (DME) solvent and 60% THE electrolytes after 100 cycles [153]; (B) rate performance of Li||NCM523 batteries with commercial 301 electrolyte and TEH-2m-LiTD+10%FEC [159]; (C) solvation structures of LCEs, HCEs, and LHCEs [158]; (D) the capacity performance of a NaCoHCF/PTCDA full cell at different rates in 20 m LiNO₃ and RTMS [162]; (E) cycling performance of Li||NMC811 cells with different electrolytes consisting of 1 M LiPF6 in the indicated solvent mixture at 8C [164]; (F) cold-cranking capabilities of ionogel-introduced LMO/LLZO–LTO and ionogel-introduced LMO–LATSP/LTO cell units with a rate of 10C at −18 °C [165].

Figure 6. (A) SEM and cross-sectional SEM images of Li anodes in 1, 3-dioxolane (DOL) and dimethoxyether (DME) solvent and 60% THE electrolytes after 100 cycles [153]; (B) rate performance of Li||NCM523 batteries with commercial 301 electrolyte and TEH-2m-LiTD+10%FEC [159]; (C) solvation structures of LCEs, HCEs, and LHCEs [158]; (D) the capacity performance of a NaCoHCF/PTCDA full cell at different rates in 20 m LiNO₃ and RTMS [162]; (E) cycling performance of Li||NMC811 cells with different electrolytes consisting of 1 M LiPF6 in the indicated solvent mixture at 8C [164]; (F) cold-cranking capabilities of ionogel-introduced LMO/LLZO–LTO and ionogel-introduced LMO–LATSP/LTO cell units with a rate of 10C at −18 °C [165].

The full cell battery design included a garnet-type Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte, a spinel LiMn₂O₄ (LMO) cathode, and a Li₄Ti₅O₁₂ (LTO) anode. The ionogel significantly reduced interfacial resistance, enhanced rate performance (74% capacity retained at 5C), and enabled cold-cranking to −18 °C (Figure 6F), meeting automotive industry specifications [165]. By focusing on improving fast-charging capability and interface stability, Niitani et al. 2021 used a hard-carbon anode paired with a sodium carborane solid electrolyte, (Na(CB₉H₁₀)_0.7(CB₁₁H₁₂)_0.3), which exhibits superionic conductivity of 2 × 10⁻² S cm⁻¹ [166] The electrolyte’s high deformability facilitates intimate and stable contact with the hard-carbon anode, forming a thin oxide interphase and drastically reducing interfacial resistance. The battery delivers a capacity of over 1 mAh cm⁻² within 5 min (approximately 12C), excellent coulombic efficiency (99.9% after 100 cycles), and stable cycling life. Despite all their advantages, SSEs suffer from interfacial resistance, poor compatibility with high-voltage cathodes, and mechanical stress from cycling [166].

3.4. Separator

A Li-ion battery separator is usually a thin microporous membrane, significantly affecting battery performance and safety. Even though the separator does not directly participate in electrochemical reactions, the physical separation of the anode and cathode provides the necessary medium for the transportation of Li-ions across the electrolyte [167,168,169]. The capacity, internal resistance, cycling stability, and safety of the batteries are closely related to the microstructural properties and chemical composition of the separators [170,171]. Desirable separator characteristics include high ionic conductivity, low internal resistance, high porosity, superior mechanical strength, and thermal stability [137,170]. Recent advancements in materials engineering have emphasized thin separators, which reduce internal resistance and enhance ionic conductivity. However, thinner separators can compromise mechanical strength, which increases the risk of short circuits [167,172]. A proper trade-off is made between mechanical robustness and electrochemical performance; the separators should be thick enough to bear the pressure caused by dendrite growth, yet they must still maintain excellent ionic conductivity [77,173].

Polyolefin-based separators, such as polyethylene (PE) and polypropylene (PP)—unless their low melting points (PE = 135 °C and PP = 165 °C) (which result in poor thermal stability) make them vulnerable to shrinkage and rupture under thermal abuse, leading to internal short circuits—are the most widely used material due to their high mechanical strength and chemical stability [77,104,170]. To address the limitations of these, nonwoven nanofiber membranes made from materials such as polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), and polyethylene terephthalate (PET) have been explored because of their highly porous structure and excellent thermal stability [174,175]. The materials have weak physical interactions, with lower mechanical strength; hence, they are liable to lithium dendrite penetration. Advanced separator designs with ceramic coatings and hybrid materials also contribute to thermal stability and mechanical integrity [170,172]. Additionally, solid electrolytes that might replace conventional separators can significantly reduce some of the risks associated with the use of liquid electrolytes; however, insufficient ionic conductivity and electrode interface compatibility at room temperature are the main problems that limit the development of high-performance solid-state batteries [167,171,176,177].

The separator needs to efficiently support the passage of Li-ions and endure heat and pressure, which can weaken the separator [170,176,178]. The key factors that serve as turning points for ions in the fast-charging–discharge process are high ionic conductivity and low internal resistance. High mechanical strength and proper thermal stability are necessary for enabling separators to last in the raised conditions in which they are used, and these features assist in mitigating the risks caused by lithium dendrite growth [179,180]. Recent improvements in the development of separators with the usage of ceramic coatings, nonwoven nanofibers, and 2D materials like graphene oxide (GO) and boron nitride (BN) have helped in overcoming the challenges these materials brought [178,181]. Because of graphene and boron nitride’s durability and high surface-to-volume ratio, 2D materials ensure that the robustness of the separator structure is maintained while enabling efficient ions to move. The implementation of additive manufacturing (AM) and 3D-printing technologies like projection micro-stereolithography (PμSL) to make 3D porous separators is another option for the production of well-tailored microstructures. These separators, through their architectural design—based on polymerization-induced phase separation (PIPS)—can precisely control the porosity and morphology; thus, they can enhance Li-ion transport by reducing internal resistance [177].

Recent advances have focused on developing composite separators with functional additives for enhanced ionic conductivity, mechanical stability, and thermal stability [179,182]. Polydopamine-coated carbon nanotube (CNT@PDA) separators can be used in polyvinylidene fluoride–hexafluoropropylene (PVDF-HFP) matrices. This composite separator has been demonstrated to enhance thermal conductivity (10.46 W/m·K) and ionic conductivity (0.49 × 10⁻³ S/cm), with 87.35% capacity retention after 800 cycles at 5C (Figure 7A,B) [183]. Zhang (2025) made a barium sulfate (BS) and bacterial cellulose (BC) dielectric separator to accelerate Li-ion desolvation and promote uniform flux at the anode interface [184]. This separator achieved a coulombic efficiency of 99.0%, significantly outperforming conventional polypropylene (PP) separators (retaining 27% after 500 cycles) by extending cycling stability to 100 cycles at 2 mA/cm² and keeping 70% capacity after 500 cycles at 2C [184]. Frankenberger (2019) investigated the impact of electrode–separator lamination on electrochemical performance and reported that laminated separators reduce charge-transfer resistance and enhance rate stability, as can be seen in Figure 7C [185]. LIBs utilizing laminated separators exhibited 84% nominal capacity retention at 5C, compared to 72% in non-laminated cells [185]. Polyethylene (PE)-based separators were modified by Dang in 2024 with Al₂O₃ nanoparticles using a green binder approach. This modification significantly increased electrolyte uptake (95.16%) and ionic conductivity, resulting in a 40% increase in capacity at 10C, relative to conventional separators. The rate performances of LFP/Li cells with various separators and comparisons of the pristine PE separator and different binder-modified separators are shown in Figure 7D [186].

Figure 7. (A) Cycling behavior of the various separators at 5C with an LFP cathode [183]; (B) FLIR images of PE, PVDF-HFP, and CNT@PDA/PVDF-HFP separators at different temperatures [183]; (C) cross-section SEM images of laminated single-cell stack—NMC cathode in upper part, self-standing inorganic filled separator film in central part, graphite anode in lower part [185]; (D) rate performances of LFP/Li cells with various separators and electrochemical performance for the pristine PE separator and different binder-modified separators [186]; (E) cycling test of the anodic half-cells with different binders at 10C [187].

Figure 7. (A) Cycling behavior of the various separators at 5C with an LFP cathode [183]; (B) FLIR images of PE, PVDF-HFP, and CNT@PDA/PVDF-HFP separators at different temperatures [183]; (C) cross-section SEM images of laminated single-cell stack—NMC cathode in upper part, self-standing inorganic filled separator film in central part, graphite anode in lower part [185]; (D) rate performances of LFP/Li cells with various separators and electrochemical performance for the pristine PE separator and different binder-modified separators [186]; (E) cycling test of the anodic half-cells with different binders at 10C [187].

Shkrob (2023) investigated the performance of separator membranes during polarization effects upon rapid charging, comparing Celgard 2320 and Celgard 2500 [188]. The study found that higher porosity (55%) and better electrolyte wettability reduced ionic resistance by 2.23 Ω/cm², leading to enhancements in charge capacity of 57% at 2C and 47% at 3C. A bifunctional composite separator (PP-PHY) consisting of a PVDF-HFP matrix coating and YSZ (Y_0.08Zr_0.92O₂-δ) nanoparticles was found to redistribute Li-ion flux and prevent dendrite growth. This separator demonstrated stable coulombic efficiency (98.4% over 500 cycles in Li/Cu cells) and maintained a high specific capacity of 155 mAh/g at 10C in NCM712/Li full cells [189]. Makki (2023) studied the stress distribution in separators at fast-charging conditions and confirmed that tensile creep and pore closure cause mechanical degradation under 4C cycling conditions [190]. There are experiments showing the values of von Mises stress for PE separators of 74 MPa, which generates microstructural degradation and more inefficient ion transport [190].

3.5. Binders

Even though binders make up only 3–5% of the total weight of a Li-ion battery, they play an important role in maintaining the structural integrity and performance of electrodes [191,192]. These polymeric materials serve to stabilize the battery by ensuring the homogeneous adhesion of active materials onto the current collector, supporting mechanical strength during lithiation and delithiation, and maintaining the integrity of the electrode in spite of the volume changes [193,194]. Moreover, they also enable the formation of a homogeneous coating during the fabrication process of the electrode—this involves the mixing of the electrode material with the binder in a paste form, which is coated onto the collector and dried [191,193,195]. Therefore, the binders should have enough elasticity, flexibility, and hardness to bear the mechanical stresses and provide an electrochemical window so that unwanted reactions with electrode materials can be prevented for the transport of Li-ions and electrons [196,197,198].

Polyvinylidene fluoride (PVDF) is the most widely used binder in the cathode of commercial Li-ion batteries, owing to its electrochemical stability and ability to tolerate mechanical stresses upon charge/discharge [79,199]. Nevertheless, PVDF and other conventional binders possess drawbacks like poor mechanical properties and low thermal stability, which can negatively impact battery safety and performance, particularly under fast-charging conditions [191,194]. For anodes, styrene–butadiene rubber (SBR) and carboxymethyl cellulose sodium (CMC) have been studied for their strong bonding, flexibility, and stability abilities in the formation of SEIs, which are important factors in ensuring the long life and safety of fast-charging batteries [78,194]. Compared to SBR, these materials provide higher flexibility, stronger bonding, and higher heat resistance; meanwhile, CMC, with carboxylate anion and hydroxyl functional groups, acts as an effective dispersing and thickening agent in aqueous suspensions [191,198]. A composite binder comprising SBR/CMC achieves excellent thermal stability with minimal usage, increasing electrolyte absorption and reducing heat shrinkage, especially when a ceramic coating is applied [198,200].

The potential of a bio-derived lithium borate polymer binder being utilized for facilitating the fast-charging performance of graphite anodes was investigated by Pradhan et al. (2023) [187]. The study focused on addressing issues of lithium plating and inefficient Li-ion diffusion, which are common issues in high-rate charging. By offering inherent Li-ions and a low-lying lowest unoccupied molecular orbital (LUMO) energy level, the binder facilitated the creation of a boron-rich SEI with excellent ionic conductivity and low impedance. As demonstrated in Figure 7E, electrochemical studies demonstrated that the binder significantly improved lithium diffusion with a high diffusion coefficient that is significantly greater than the value in conventional polyvinylidene fluoride (PVDF) and carboxymethyl cellulose (CMC)–styrene butadiene rubber (SBR) systems. Furthermore, under a high rate of charging of 10C, the lithium borate binder allowed for a capacity discharge of 73 mAh/g, with a 93% capacity retention of 1200 cycles [187].

To further enhance charge transfer and electrode stability, Rao et al. (2024) [201] investigated the use of conductive polymer-based binders for lithium iron phosphate (LFP) cathodes. The study combined lithium polyacrylate (LiPAA) with conductive polymers such as polyaniline (PANI), polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT:PSS). The polymers formed a conductive, web-like network around active LFP particles, significantly enhancing in-plane conductivity by up to 71%, with PEDOT:PSS achieving the highest, at 0.219 S/cm. Compared to conventional PVDF-based systems, the modified LFP cathodes exhibited superior fast-charging capabilities, retaining 85–88% of their initial capacity at a 5C charging rate; meanwhile, conventional PVDF-based cathodes retained only 63%. The incorporation of conductive polymers also led to lower charge transfer resistance and improved voltage stability at high discharge rates. However, the issue of uniform polymer distribution—as well as the need for the optimization of initial coulombic efficiency (ICE) and a reduction in production costs—should be considered to be a problematic area in need of further development. Another development is the weight loading of the carbon-binder domain (CBD); this has emerged as a strategy for improving Li-ion transport in fast-charging the electrodes. Meanwhile, Usseglio Viretta et al. (2022) have researched the effect of CBD content interactions with LIB electrodes on rate capability [202]. The results show that decreasing the CBD weight loading from a conventional 10% to an optimized 4% improved ionic diffusion through a reduction in electrode tortuosity and an enhancement in electrolyte penetration. This optimization resulted in a 48% improvement in capacity retention during 6C charging and a 20% reduction in cathode film resistance [202].

For graphite anodes, which are widely used in commercial LIBs, Scheck et al. (2023) developed an alternative water-based binder system composed of sodium alginate, polyethylene oxide (PEO), and polyvinylpyrrolidone (PVP) in a 2:1:1 ratio [203]. This binder system was specifically designed to improve Li-ion mobility, ionic conductivity, and electrode wettability. Electrochemical tests revealed that, at a 4C charge rate, the alternative binder increased the specific charge capacity by over 150% during the constant current (CC) phase compared to conventional CMC-SBR systems. Furthermore, it demonstrated excellent thermal stability, retaining 96% of its charge capacity at 45 °C and enabling a state of charge (SOC) of 80% to be reached in just 12 min and 23 s [203].

Since silicon suffers from severe volume expansion during lithiation, which leads to particle pulverization and electrode failure, Zhang et al. (2019) [204] investigated the use of MXene-based binders (Ti₃C₂T_x) to address these challenges. The study demonstrated that the MXene-bonded silicon–carbon (Si@C) electrode achieved a specific capacity of 1040.7 mAh/g after 150 cycles at 420 mA/g, significantly outperforming conventional CMC- and PVDF-based binders. MXene’s high conductivity (208.7 S/cm) and porous structure facilitated superior Li-ion diffusion and mechanical flexibility, effectively buffering the large volume changes in silicon anodes. Despite these advantages, MXene binders are still expensive and face scalability challenges in commercial battery production.

At separators, binders are equally important for offering high Li-ion conductivity, mechanical robustness, and attaining thermal stability [79,137,198]. These are materials that prevent the shrinkage or rupture of separators, mostly ceramic-coated or hybrid separators, under high temperature or quick charge conditions by improving their structure. Binders also play a critical role in the fabrication process by forming a durable porous structure that can enable good Li-ion transportation while maintaining separation between anodes and cathodes [79,198]. Preparation of separators with polyolefin-based, PVDF, and other conventional binders is usually conducted with the aim of enhancing their mechanical strength and/or their compatibility with different electrolyte systems [77,205]. Deficiencies regarding thermal stability and flexibility at high mechanical stresses have driven the search for advanced binders showing flame-retardant, conductive, or self-healing properties [193,206].

Next-generation binders will definitely see an extension of their role with separators for LIBs under fast-charge and high-energy-density conditions. Among aqueous-based binders, cellulose-derived materials are becoming prominent, including carboxymethyl cellulose (CMC), and these come with assurances of eco-friendliness and cost-efficiency, with good adhesion. Such binders enhance the structural integrity of ceramic-coated separators and their hybrid counterparts against shrinkage or rupture under high-temperature and mechanical stresses. Ceramic-coating strategies further amplify such benefits through an additional layer, which enhances thermal stability and wettability by the electrolyte, and reduces the risk of thermal shrinkage; hence, they render the separators more robust under demanding conditions [167,170,205].

4. Charging Protocols

Charging protocols are used to control the charging of LIBs and to keep parameters such as voltage, current, and temperature under control; this ensures efficient energy transfer. However, in the case of ultrafast charging (UFC) applications, with power up to 350 kW or even more, the exceptionally high charge rates introduce gigantic challenges, such as rapid ion transport, electrolyte decomposition, and accelerated thermal aging [17,207,208]. The high current densities in fast charging can also result in mechanical stress on electrode materials, especially in nickel-rich cathodes like NCAs [209]. Additionally, ultrafast charging can also result in SOC imbalance between cells in a battery pack due to uneven charge distribution, which can reduce the performance and safety of battery cells. Figure 8 illustrates the key charging protocol risks in ultrafast charging and shows the critical challenges. The protocol challenges are mainly related to high current density, electrochemical stress, and overvoltage conditions [21,210,211]. Beyond these electrochemical challenges, another major obstacle is the compatibility of the existing charging infrastructure. Ultrafast charging requires high-power charging stations and a strong electrical grid. However, several current charging networks cannot handle such high energy demands, so upgrading charging stations and improving power management systems will be essential. Thus, the development and optimization of UFC protocols is necessary in balancing fast charging with battery safety and long-term performance. Charging protocols can be passive and optimized. Passive charging, such as CC/CV, is a fixed pattern but does not optimize charging efficiency and can accelerate degradation due to its static nature. Optimized protocols adapt dynamically based on the battery’s internal state, which makes it an efficient and safer charging process [212,213]. Thus, optimized protocols dynamically adjust the charging profile based on the internal states of the battery (e.g., SOC, temperature, and aging state). Constant risk (CR) protocols are a specific type of optimized strategy that regulates charging parameters to control the degradation. For instance, CR protocols have been demonstrated to achieve 80% charge in 10 min for NMC532/graphite batteries, which can minimize the degradation risk [214].

Different protocols have been developed to enhance fast and ultrafast charging performance. Several charging methods, such as constant current–constant voltage (CC-CV), multistage constant current (MSCC), pulse charging, and adaptive charging algorithms, have been implemented [215]. The following section overviews these protocols and their advancements in fast- and ultrafast-charging conditions.

4.1. CC-CV

CC-CV protocols keep a fixed current during the initial charging phase (e.g., 2C–6C) to rapidly increase SOC to up to ~80%. Then, the battery approaches full capacity, and the constant voltage (CV) phase maintains a fixed voltage while the current gradually decreases to prevent overcharging, as shown in Figure 9. Prolonged exposure to high currents during the CC stage can result in non-uniform Li-ion intercalation that promotes lithium plating. For instance, graphite anodes are very prone to dendrite formation under such conditions [216]. The CC-CV technique also suffers from charging rate limits, and the temperature rise during the CV stage can lead to material degradation [217]. CC-CV charging is the most common protocol, but its extended CV phase results in longer charging times at high currents or low temperatures [218].

Figure 8. Charging protocol challenges in ultrafast charging of LIBs.

Figure 8. Charging protocol challenges in ultrafast charging of LIBs.

In addition to CC-CV, constant power–constant voltage (CP-CV) (Figure 9) follows a similar charging structure to CC-CV. Still, it applies a constant power instead of maintaining a fixed current in the initial phase [219,220]. Studies have shown that CP-CV provides better capacity retention, especially at higher charge rates like 1C, compared to CC-CV. It reduces the risk of lithium plating and mechanical stress, which are critical factors in battery degradation. However, at lower charge rates such as 0.5C, CC-CV tends to be less damaging [6]. The second modification protocol in CC-CV is the varying current decay (VCD) method (Figure 9), which was proposed to achieve greater charging speed and minimize the degradation that takes place in pure CV charging. Contrary to constant current phase with a constant voltage phase, VCD gradually decreases the current during charging. This configuration was proposed to achieve maximum capacity utilization in the early cycles [221,222]. Experimental results indicated that VCD caused a substantially higher capacity fade rate than CC-CV under the same average current [223]. While VCD enables faster charging, the observed trade-off between speed and longevity indicates that additional optimization is required. Methods such as the addition of temperature management or current decay profile adjustment may maximize the efficiency of the protocol [221,222,224].

Figure 9. Schematic representation of common types of charging protocols proposed for fast charging. Constant current–constant voltage (CC-CV); constant power–constant voltage (CP-CV); multistage constant current–constant voltage (MCC-CV); pulse charging; boost charging with a CC-CV-CC-CV scheme; varying current decay (VCD).

Figure 9. Schematic representation of common types of charging protocols proposed for fast charging. Constant current–constant voltage (CC-CV); constant power–constant voltage (CP-CV); multistage constant current–constant voltage (MCC-CV); pulse charging; boost charging with a CC-CV-CC-CV scheme; varying current decay (VCD).

One of the main concerns in CC-CV fast charging is the trade-off between reducing charging time and limiting heat generation. Experimental data [225] showed that increasing the C-rate from 1C to 2C reduces charging time by approximately 48% while increasing the maximum temperature rise from 28.2 °C to 31.6 °C. An optimized charging protocol was developed that reduced total charge time by 7.6% compared to conventional CC-CV (3938 s vs. 4263 s) and by over 60% compared to 1/3C CC-CV, while temperature rise was kept at less than 3.8 °C [226]. Their findings reveal that controlling existing profiles based on SOC and polarization behavior allows for fast charging without exceeding safe thermal limits. Another challenge in CC-CV charging is its inability to dynamically adjust current based on real-time battery conditions, which results in heat generation and lithium plating. Real-time adaptive charging strategies can improve the charging protocols, especially for ultrafast-charging situations. A study addressed this by applying Pontryagin’s minimum principle (PMP) approach for optimizing fast charging in real time. Compared to a 2C CC-CV protocol, this method achieved similar charging times while reducing temperature increase [227].

Several methods have been proposed for optimizing CC-CV, especially for fast-charging situations. A study adjusted C-rates (1.2C–1.4C) and voltage limits (4.25 V–4.4 V) to balance charging speed, cycle life, and thermal stability. It was found that the best result, 1.4C with 4.35 V, reduced charging time to 60 min, maintained 97% capacity after 500 cycles, and kept the cell temperature below 40 °C [228]. One of the fast-charging CC-CV-optimized techniques is the two-step CV protocol. A study employed pre-charge at 4V for 5 min before switching to CV at 4.3 V. This approach achieved 15 min full charging of NCM811 cathodes with reduced capacity loss (>90% after 200 cycles), impedance growth (~15% in 200 cycles), and peak C-rates (>6C at high SOC) compared to conventional CV-only charging (Figure 10A); this showed ∼20% capacity loss after 100 cycles and ∼30% impedance increase [229]. Moreover, charging at a constant 6C current up to 4.1 V, then transitioning to constant voltage (CV) charging, was investigated; this was found to allow stable charging within 10 min. Increasing the charging current to 7C and using a gradual voltage ramp of 3.5 mV/10 s up to 4.15 V improved charge acceptance by 5%, reducing lithium plating. Figure 10B shows the voltage vs. C-rate profiles of the three-stage constant-current charging protocol, in addition to the temperature variation vs. anode interface potentials and cell SOCs after 10 min of charging using CCCV [230]. A study compared 4C CC-CV and 6C-4C-1C CC charging protocols for LFP-based batteries at 20 °C and 10 °C. In 4C CC-CV, charging starts with 4C constant current before switching to constant voltage, while 6C-4C-1C CC uses staged current steps without a voltage phase. At 20 °C, 4C CC-CV reduced temperature rise but increased internal resistance, whereas 6C-4C-1C CC allowed for faster charging but accelerated LFP degradation. At 10 °C, lithium plating was minimal, but electrolyte degradation dominated [228]. The self-adaptive multistage constant current–constant voltage (SMCCCV) is a type of adaptive CC-CV charging strategy that dynamically adjusts the charging current in multiple stages to optimize fast charging. As an example, this strategy decreased charging time by 37.7% over 1C CC-CV [231]. Still, the minimum aging strategy increased aging loss by 14.3% while decreasing charging time by 47.9%. The balanced strategy decreases aging loss by 11.5% over 1.18C CC-CV while keeping anode potential safe; the optimized SMCCCV approach is a promising charging protocol for EV battery management systems [231]. Different algorithms have been proposed to optimize CC-CV fast charging by dynamically adjusting charge parameters to improve efficiency and battery lifespan. The voltage ramping protocol improved no-plating capacity by 10.7% at 30 °C and enabled 80% charge in 10 min at a lower temperature than with CC-CV. The multiple CC-CV protocol optimized the charge rate by 18.8% for 4 mAh cm⁻² cells while reducing degradation. These adaptive methods enhance extreme fast charging beyond traditional CC-CV approaches [230,232]. These methods can optimize charging by dynamically adjusting charge parameters to prevent lithium plating. Additionally, several experimental studies have investigated optimized charging protocols. An experimental study analyzed an optimized CC-CV fast-charging approach using 18650 high-power Li-ion cells. It was found that current at high SOC and adjusting cut-off voltage significantly improved cycle life. A two-step CV protocol (pre-charging at 4V for 5 min before CV at 4.3V) reduced extreme C-rates and enhanced stability. Voltage ramping and multiple CC-CV protocols increased charging efficiency by up to 10.7% (2.5 mAh cm⁻²) and 18.8% (4 mAh cm⁻²) while preventing degradation [213].

It should be noted that CC-CV is the most utilized charging protocol in EVs due to its reliability. However, ongoing research has been continuously conducted to optimize it for faster charging. Adaptive current control, voltage ramping, and multi-step CV methods have been developed. Commercial development has been undertaken, for instance, to optimize CC-CV charging efficiency and battery life. XTAR, for instance, developed the TC-CC-CV charging method. Here, a three-step process begins with a trickle charge (TC), then constant current (CC), and finally constant voltage (CV) [233].

4.2. MSCC

MSCC protocols initiate the charging process with a high current (e.g., 2C up to 50% SOC) to achieve rapid charging and transitions to lower current levels (e.g., 1C to 80% SOC, then 0.5C until full charge), protecting the battery from excessive stress and reducing lithium plating [234]. The process is shown in Figure 9. However, the process can induce thermal hotspots and mechanical stress in the electrodes. Nickel-rich cathodes such as NMCs are especially prone to structural degradation under these circumstances, whereas graphite anodes commonly experience problems of lithium plating, which diminishes capacity and enhances the safety hazards [235]. One study enhanced the LFP cathode by modifying it with carbon and LATP for the purpose of increasing Li-ion transport, voltage stabilization, and polarization reduction. The performance was examined under an MSCC-CV protocol to obtain 161.7 mAh g⁻¹ within 10 min at 10C and 90.7 mAh g⁻¹ at 60C, as shown in Figure 10C [236].

There have been studies on MSCC charging, where the number of stages can range from three to ten depending on charging efficiency and battery cycle life. The three-stage MSCC method typically applies 1.8C → 1.5C → 0.9C for faster charging; meanwhile, it was shown that the five-stage optimized approach can obtain a 79.6% cycle life improvement and 12.54% higher efficiency. The ten-stage MSCC strategy could also enhance charging control and lifetime. It was found that the five-stage MSCC strategy appears to be the most effective overall, balancing fast charging, efficiency, and long-term battery lifespan [235,237].

One method for mitigating challenges in MSCC is the MSCC-CV protocol, which uses staged current reductions where the charging current gradually decreases in steps to balance speed and efficiency. For instance, an LFP cell maintained 83% capacity after 4200 cycles using this approach [238]. An improvement in MSCC is the optimization of charge rates, where the initial stage applies a higher current (e.g., 2C) to accelerate charging, followed by gradual reductions (e.g., 1.5C → 0.9C) to minimize lithium plating and polarization. This approach reduces charging time while maintaining high-capacity retention (99.6–99.9% after 510 cycles), particularly at 25 °C and 50 °C, making it more effective than conventional 1.5C CC charging [239]. Another optimization method is the integration of active control pulse (ACP), which dynamically introduces discharge pulses during fast charging to mitigate lithium plating. It has been observed that this approach optimizes charge rates (e.g., 2.2C → 1.5C) and reduces charging time from 16 min to 10 min while maintaining stable cycling over 2000 cycles in 60 Ah pouch cells [240].

Numerical models have been introduced to optimize MSCC charging by predicting lithium plating. A reduced-order electrochemical thermal life model, refined with a genetic algorithm (GA), has been posited to reduce computational time by 36.4%. The nonlinear model predictive control (NMPC) algorithm optimized charging currents and could reduce charging time by 11.7% and capacity loss by 59.4% [238]. In another study, an electro-thermal model was used to dynamically adjust currents to reduce charging time and degradation [241]. At 25 °C, MSCC-B (optimized MSCC with balanced charge time and SOC) achieved 93.5% SOC in 52 min, whereas CC-CV required 61 min for 96.2% SOC. MSCC-C (an aggressive, fast-charging variant) further reduced the charging time to 37 min, but this came at the cost of a lower SOC of 78.4%. It was observed that, when high currents of >2C were applied at low SOC, no degradation was found; meanwhile, stepwise current reductions at higher SOCs prevented lithium plating and overheating [241].

Optimization techniques have been used to balance conflicting objectives such as charging time, energy loss, and battery degradation. A study optimized CC-CV and MCC-CV charging patterns for LIBs using an ensemble multi-objective biogeography-based optimization (EM-BBO) approach. It was found that the optimized MCC-CV patterns reduced battery degradation by 16.5% compared to CC-CV while slightly increasing the charging time by 1.7%. It was also shown that the temperature plays a crucial role; for instance, higher ambient temperatures reduce charging time by 24.9% while accelerating battery aging by 225.3% [242].

Additionally, a machine-learning-based method using deep reinforcement learning (DRL) was implemented in order to optimize MSCC during fast charging [243]. It was found that it can reduce charging time to 6 min for thin cathodes and 14 min for thick cathodes (Figure 10D), and could perform better than 6CC-CV. The method adapts to electrode thickness and porosity and can be integrated into battery management systems (BMSs). Another study proposed a closed-loop optimization (CLO) algorithm using Bayesian optimization and an early cycle life estimation model to formulate MSCC fast-charging protocols. Their method evaluates 224 charging protocols in 16 days instead of 500+ days. The optimized six-step, ten-minute charging method extended the cycle life by 30%, and thus improved battery stability [217]. Some studies have shown 13.3% faster charging while maintaining battery performance. However, most EVs still use CC-CV due to its simplicity and reliability [225], and MSCC adoption is expected to grow for improved efficiency and longevity.

4.3. PC

Pulse charging (PC) alternates between high-current charging pulses and short rest or discharge pulses (Figure 9) to mitigate lithium deposition, reducing polarization effects and controlling temperature rise, which improves battery life and charging speed. However, its effectiveness depends on electrode and electrolyte properties [232]. Pulse charging reduces polarization and mechanical stress, improving active material utilization and limiting degradation mechanisms like lithium dendrite growth. It has been demonstrated that PC charging significantly improves the cycling stability and lifespan of NMC532/graphite lithium-ion batteries. Moreover, it was shown that PC charging lowers polarization by 19.5% [244]. Although it has advantages, pulse charging poses challenges such as enhanced heat generation and inhomogeneous thermal distribution within a battery pack, with up to 15 °C cell temperature differences at high current loads. The highest temperature recorded on a 12P7S battery pack at a 100 A pulse load was 70 °C, which was above the recommended 60 °C safe operation limit [245]. Studies have shown that PC can slow down lithium plating by briefly reversing electrochemical reactions, but its application comes at the expense of increased complexity in the charging system [246].

Several studies compared different charging protocols. For instance, different charging protocol methods, including CC-CV, MSCC, and PC, have been evaluated to minimize battery degradation in fast-charging conditions [247]. Their results showed that the PC protocol provides the best balance between fast charging and battery degradation. MSCC achieves the fastest charging time (38.7 min) but causes significant degradation (>20% capacity loss). Standard CC-CV charging requires 57.7 min and leads to gradual capacity. PC maintains lower degradation (<10%), with a charging time of 50.76 min. Thus, it was stated that PC would be more suitable for practical battery charging applications [247]. It was shown that, compared to CC charging, PC charging at 2000 Hz extended the cycle life beyond 1000 cycles, with 81.7% capacity retention; meanwhile, CC charging resulted in rapid degradation after 500 cycles [244].

It has been stated that PC combined with MCC protocols could reduce diffusion time constants and slowed capacity fade, and could thus enhance battery longevity under fast-charging conditions [248,249]. An optimized PC method has been proposed to mitigate lithium dendrite growth and plating by adjusting pulse duration and rest periods. Their method applies 1 ms charging pulses followed by 3 ms rest periods, which significantly reduced dendrites by ~2.5 times compared to continuous charging [250]. A method has been proposed that combines MSCC-CV with PC in order to minimize degradation and improve ultrafast-charging efficiency. Using a high-fidelity lithium-ion battery model based on porous electrode theory, the optimized protocol (which is refined with the Bayesian method) reduced charge time to ≈42 min for 80% SOC, compared to ≈47 min with the standard CC-CV [251].

Empirical and model-based studies have also been used to optimize the charging protocols, especially for high rates of charging. Pulse charging in the 60–80% SOC range controls temperature rise; meanwhile, lowering the C-rate in the 40–60% SOC stage prevents overheating. A method is introduced that adjusts the current dynamically to avoid critical temperature and voltage levels, achieving faster charging while preserving battery health [252]. A pulse-charging model (non-dc waveform pulses) has been proposed to prevent lithium saturation; this could reduce charging time from ~3–4 h to 0.72–0.85 h. By maintaining a lithium concentration below 0.077 mole/cm³, the method enhanced charging to a rate that was 2.4–2.9× faster than that of CC-CV [253]. An experimental study optimized PC for Si-Gr 18650 cells that could reduce charging time from 4 h to 1 h while maintaining the temperature below 45 °C. It was found that, although pulse charging within 15–80% SOC improves long-term performance, higher voltage charging (>4.2 V) led to faster capacity fading (>10% after 200 cycles) and increased degradation. It was observed that temperature profiles showed a ~5 °C reduction compared to standard CC-CV [254]. However, further analysis is needed to evaluate long-term stability and degradation mechanisms. While several companies have been exploring advanced fast-charging technologies, specific implementations of pulse charging in commercial EVs have not been widely reported. TAE Power Solutions is developing pulse-charging technology that is applicable to both EV batteries and stationary storage systems. Their goal is to optimize energy transfer and improve charging efficiency [255].

4.4. BC

Boost charging (BC) introduces an initial high-current phase at the start of charging. The cell is charged with a constant boost current that is higher than the following constant current charge. The cell then discharges while maintaining a constant voltage (as shown in Figure 9). In fact, BC is derived from the CC-CV protocol that introduces an extra CC interval at the beginning of the CC phase of a CC-CV profile, with a higher value of charging current. This supplementary boost interval reduces the total charging [256,257].

Boost charging was first proposed by Notten et al., who proposed an ultrafast-charging algorithm for Li-ion batteries that could recharge a fully discharged battery to one third of its nominal capacity (30% SOC) in 5 min. Notten et al. revealed that BC is able to inhibit side reactions such as lithium plating, limiting degradation and enhancing battery performance. Boost charging involves a high initial current, expediting charging and reducing degradation. Research has demonstrated that the process can shorten charging time by 40% without major capacity loss in cylindrical cells, although prismatic cells have higher rates of degradation [257].

Compared to CC-CV, BC increases charging without additional degradation when properly managed [210]. However, studies have illustrated that BC could induce high and low SOC degradation, making SOC selection critical [213]. It has been stated that applying BC at a low SOC (e.g., <30%), where the electrolyte–anode concentration gradient is high, can result in safer high-current charging while minimizing lithium plating and thermal stress [210]. Despite these advancements, trade-offs between charging speed, thermal management, and long-term cycle stability remain a focus for ongoing research, in the hope of ensuring that BC can be widely adopted in commercial applications [258,259].

In addition to the above charging protocols, universal voltage protocol (UVP) is an advanced charging method that optimizes current profiles to enhance efficiency and extend battery lifespan. It offers a more adaptive approach than fixed-current methods like BC and PC. It has been shown that UVP maintains high charging efficiency, even for aged cells, with LCO/NMC cells achieving 80% capacity retention after 370 cycles compared to only 100 cycles with 2C CC-CV charging [224]. Another novel charging protocol, especially for fast charging, is the fast charging with negative pulse (FCNP) method, which provides negative pulse charging to reduce lithium plating [260]. Unlike CC-CV or MSCC, FCNP actively adjusts the current profile by introducing negative pulse discharges during the charging process. It has been stated that this method is particularly effective in reducing charging time by 50% compared to 2C CC-CV in the 0–40% SOC range while also extending cycle life by 23% compared to 3C CC-CV after 60 cycles. Although charging time to 100% SOC is initially longer than 3C CC-CV, FCNP becomes faster after 40 cycles [260].

Figure 10. (A) voltage profile with normalized capacity after 50 cycles of CV charging and CCCV charging [229]; (B) voltage and C-rate profiles of the three-stage constant-current charging protocol, and anode interface potential vs. cell SOC after 10 min charging using CCCV [230]; (C) long cycle performance of ultrafast charging for the LFP@10.0C_7.5LATP [236]; (D) cathode thickness and anode thickness variation vs. charge time [243].

Figure 10. (A) voltage profile with normalized capacity after 50 cycles of CV charging and CCCV charging [229]; (B) voltage and C-rate profiles of the three-stage constant-current charging protocol, and anode interface potential vs. cell SOC after 10 min charging using CCCV [230]; (C) long cycle performance of ultrafast charging for the LFP@10.0C_7.5LATP [236]; (D) cathode thickness and anode thickness variation vs. charge time [243].

5. Safety

Several challenges are faced in efforts to enable 10 min extremely fast charging of LIBs, requiring a 6C charging rate. As shown in Figure 11, these challenges are mainly related to material limitations and electrochemical instabilities under high current densities. One of the most critical issues is lithium plating, particularly in graphite anodes, where high overpotentials and sluggish Li-ion transport at low temperatures lead to dendrite formation. This dendrite growth in LIBs penetrates the separator and has direct contact between the anode and cathode, which results in severe internal short circuits and catastrophic failure [261].

Additionally, cathode degradation, including particle cracking and interphase deterioration, along with electrolyte depletion, is more significant in high-loading cells (>4 mAh/cm²) and could lead to capacity fade, increased resistance, and reduced cycle life under ultrafast-charging conditions [262]. These can ultimately cause mechanical stress and expansion within the cell. Charge distribution at high C-rates leads to uneven charging profiles, increasing the risk of localized overheating [263]. At 1C, Li-ion batteries maintain 80% of capacity after 1000 cycles at 25 °C, but only 60% at 3C after 500 cycles. High rate charging at 7C increases internal temperature by 22.5 °C in 5 min, accelerating degradation. Moreover, environmental stressors are important factors in the aging of LIBs. It has been demonstrated that vibration increases impedance by 5–10% after 200 h, while low-pressure conditions accelerate lithium plating, reducing capacity by 15% in 500 cycles at 0.2 kPa [264].

Different Li-ion battery components, including the anode, cathode, and electrolyte, play substantial roles in safety and aging behavior. The aging mechanisms of Li-ion batteries significantly impact capacity, resistance, and safety. One of the critical factors influencing battery durability and aging is temperature [263]. A research study demonstrated that optimal aging conditions varied based on cell chemistry: 45 °C for NMC811/graphite-SiO, 5 °C for NCA/graphite, and 25 °C for LFP/graphite. Higher charge currents and voltages accelerated degradation in NMC811 and LFP cells; meanwhile, NCA was found to be more resistant due to minimum lithium plating. Thus, temperature is crucial in mitigating aging effects and optimizing cycle life in LIBs [265]. Anode aging is strongly influenced by temperature. Lithium plating at low temperatures becomes a major issue and reduces cycle life by 50% when operating below −10 °C. At high temperatures, SEI growth accelerates, which leads to a 40% increase in impedance after 200 cycles at 60 °C [263,266,267]. Another challenge is mechanical instability caused by dendrite-induced volume expansion. Unlike graphite anodes, lithium metal can experience significant volumetric changes during cycling, which creates internal mechanical stress and electrode cracking. In high-voltage Li-ion batteries, dendrite growth becomes more aggressive, triggering cell aging [268].

Electrolyte degradation at high temperatures significantly impacts battery performance and safety. Dendrite growth also leads to increased side reactions between lithium and the electrolyte. This could increase gas generation and internal pressure buildup, finally deforming the separator and electrode structure and increasing the risk of electrolyte leakage [268]. Studies have shown that this increases electrolyte consumption by 30%, resulting in gas formation and a 20–30% rise in internal pressure, which can accelerate aging and thermal instability [263]. Cathodes such as NCM, LFP, and LCO showed degradation mechanisms, especially in fast-charging situations. NCM suffers from oxygen release, which results in mechanical cracking. LFP performs poorly at low temperatures and can maintain only 67.4% capacity at −20 °C, whereas NCM maintains 84.5%. Ni-rich NMC cathodes subjected to high-rate charging show significant lithium plating and surface degradation, especially in high-loading electrodes. Thus, NMC811 has the most particle cracking. This shows the need to adjust the composition for better stability at high charge rates (≥6C) to 80% SOC [269]. It has been observed that NCM811 cathodes lose over 30% of capacity after 500 cycles at 55 °C due to phase transitions and metal dissolution; meanwhile, LFP cathodes, though they are more thermally stable, experience a 10% fade after 800 cycles at 55 °C due to high resistance.

Fast charging causes significant safety challenges across the battery cell, particularly due to temperature regulation issues. Excessive heat generation at high charging rates can lower the trigger temperature for thermal runaway (TR) [270]. This increases risks such as gas production and internal pressure buildup, particularly in Li-plated cells [271]. The self-heating rate of the battery exceeds 10 °C/min at 6C charging, which can be a significant safety risk [272]. The temperature for self-heating in fast-charged cells begins at 60 °C, which includes lithium plating reactions with the electrolyte, separator collapse, and redox reactions between electrodes that lead to maximum runaway temperatures [273].

It has been shown that the self-heating temperature of TR sharply declines with increasing overcharge [274]. When the charge rate increased from 1C to 5C, the thermal runaway activation time dropped by 62.3 s, and the triggering temperature decreased by 30.3 °C [275]. There have been several fire accident reports, such as the Tesla Model S incidents in California, Zurich, and Amsterdam, which indicate the severe risks that are caused by internal chemical reactions in LIBs [276]. It has been demonstrated that the battery’s overall thermal stability is significantly reduced to 60 °C or lower once a large amount of lithium is plated on the anode surface (this occurs at high charge rates, particularly around 6C charging) [272].

Several tests have illustrated the importance of TR in the safety of batteries. Differential scanning calorimetry (DSC) tests showed that the onset temperature for thermal runaway decreased when lithium plating was present, with heat release exceeding 800 J/g [272]. Accelerated rate calorimetry (ARC) tests indicate that LIBs subjected to fast charging exhibit reduced thermal stability due to lithium plating on the anode surface. The results show that, when SOC is at 0%, there is no ignition or explosion, even after heating the cell for 2 h. However, thermal runaway propagation (TRP) only occurs if the SOC exceeds 50%. Specifically, at 80% SOC, the critical distance for TRP is 6 mm, and at 100% SOC, it increases to 8 mm [277].

Different battery cells show varying heat accumulation during high-rate charge. Cylindrical cells trap heat in the core [278]. For instance, in 18,650 cylindrical cells, cycling at −20 °C and 3C reduces thermal stability. After 150 cycles, SOH drops to ~70%, and heat generation increases by 67%. Thermal runaway peaks at 614 °C after 25 cycles but decreases to 585.8 °C at 150 cycles due to lithium loss. Mass loss reaches 19.56%, with severe electrode deformation that accelerates TR. Thus, better thermal management and SOH monitoring is required in cold fast-charging conditions [279]. A study analyzed the thermal behavior of large prismatic LiFePO₄ cells under various charging rates (0.3C–1C), thicknesses (11.3 mm–90 mm), and cooling strategies [280]. Higher charging rates and thicker cells increased temperature and led to uneven heat distribution. Bottom cooling was found to be the most effective approach for thick cells (90 mm), while single-side cooling worked better for thinner ones (≤22.5 mm). Double-side cooling was found to reduce temperature deviations by up to 73.5% [280]. Pouch cells developed large temperature gradients at 5C that increased lithium dendrites at low temperatures and thus led to faster aging at high temperatures [281].

Figure 12 illustrates the thermal degradation stages of lithium-ion batteries under elevated temperatures. Elevated temperatures beyond 40 °C degrade the SEI layer, which leads to instability at the anode–electrolyte interface. At 66.5 °C, SEI decomposition triggers exothermic reactions and produces flammable gases like ethane and methane. Beyond 100 °C, thermal runaway begins, causing electrolyte breakdown and separator failure. At 145 °C, the cathode releases oxygen, and between 200 and 300 °C, the PVDF binder decomposes, emitting hazardous gases (HF, PF₅, POF₃) that can rapidly increase pressure and lead to explosion risks [282,283]. As can be seen in Figure 12, at approximately 130 °C, the separator melts, which results in direct contact between the anode and cathode, which leads to short circuits occurring internally. At 145 °C, the cathode starts releasing oxygen, which leads to exothermic reactions and increasing internal temperature. Between 200 and 300 °C, the PVDF binder in the electrode decomposes and emits hazardous gases such as HF, PF₅, and POF₃. These results reflect the critical need for effective thermal management strategies, especially in fast- and ultrafast-charging applications.

Furthermore, mechanical stresses by repeated lithium intercalation and deintercalation during fast charging can cause volume changes of up to 10% in graphite anodes, resulting in mechanical fatigue. This can lead to pulverization and cracking of electrode materials due to uneven lithium concentration and stress distribution, reducing active material and structural integrity [281]. Fast charging induces mechanical stresses in LIBs due to electrode expansion and separator deformation, which can lead to internal short circuits (ISCs). During rapid lithium intercalation and deintercalation, electrode materials expand and contract [284]. Ni-rich NCM cathodes experience severe mechanical stress due to oxygen release, resulting in micro-cracks. Studies have shown that, after 500 cycles at 55 °C, NCM811 loses over 30% of its capacity [284]. Additionally, mechanical stress can originate from high stress on the separator, leading to long-term degradation. A study found that, during 4C fast charging, the separator experienced a maximum von Mises stress of 74 MPa, significantly accelerating mechanical degradation [190]. Creep tests confirmed this effect, showing that, under 70 MPa stress, the separator deformed by 60 mm in just 37 h; meanwhile, at 50 MPa, deformation of 48.45 mm occurred over six days. Moreover, it was observed that rapid charging caused the temperature to rise from 20 °C to 24.6 °C, contributing to polymer degradation in the separator [190].

One of the most important areas of battery safety is the choice of electrode materials; this is because different chemistries react differently to fast charging. Lithium dendrite growth is a critical issue, which triggers internal short circuits and safety threats. A study provided a comprehensive review of the lithium plating mechanism, detection, and mitigation in LIBs [285]. Some techniques, such as electrochemical processes such as voltage plateau analysis, differential voltage (DV) analysis, and incremental capacity (IC) measurements, have been researched [285]. Mitigation methods, including material-based approaches like particle size decrease and lithium solid diffusion enhancement, have also been proposed [281]. It was found that safety issues in fast charging, particularly for materials like LCO, can develop phase heterogeneity, where regions within the cathode experience uneven lithium-ion distribution, resulting in safety risks [286]. The thick electrodes, which are essential for high-energy-density batteries, can bring charge-transport constraints and mechanical instability during ultrafast charging [287]. Accordingly, studies have shown that densities above 4 mA/cm² in graphite-based cells should be avoided to avoid these effects [288]. In addition to electrode materials, electrolyte vapor pressure is a significant factor in LIBs safety through the influence of evaporation, gas evolution, and thermal runaway risk. Conventional carbonate-based electrolytes, such as LiPF₆ in EC/DMC, have vapor pressures of 300–800 Pa at 25 °C that contribute to solvent evaporation and combustibility. In contrast to this, ionic liquid (IL) electrolytes possess quasi-zero vapor pressure (<1 Pa at 100 °C), greatly reducing fire hazards [289]. Thus, electrolyte formulations with optimized interfacial compatibility and conductivity must balance safety and performance.

Several advancements, such as enhanced porosity, reduced tortuosity, and robust electrode designs, have been made in improving battery safety. There have been developments regarding the mechanical stability of LIBs, such as anode composition and separator durability. Research shows that optimizing the composition of anodes, such as incorporating 8–10 wt% SiO in graphite and placing SiO particles near separators, minimizes mechanical damage [290]. In order to attain a more accurate assessment of mechanical degradation, a hybrid multi-physics model, integrating electrochemical, mechanical, and thermal sub-models, was developed to predict stress distribution in batteries under different charging conditions. It has been found that separator materials with higher creep resistances are required if batteries are to withstand stress levels beyond 70 MPa. Also, the importance of temperature management should be illustrated; the fast-charging process has been found to increase cell temperature from 20 °C to 24.6 °C, resulting in polymer degradation in the separator [190].

As mentioned earlier, one of the issues in high-rate charging is managing heat generation, which affects battery performance and safety. One effective solution is preheating the battery to 60 °C, which reduces cooling requirements by a factor of 15 and doubles the charging speed [291]. Combining preheating and adiabatic fast charging improves temperature uniformity and minimizes cooling demands. High-precision thermo-electrochemical coupling models have been developed to mitigate these safety risks. For instance, optimizing battery pressure relief design increases the thermal runaway threshold to over 300 °C, which can enhance overall safety [292].

Different thermal management systems are used in LIB packs, including liquid cooling, phase-change materials (PCMs), and hybrid cooling methods. Further details of these methods and their advancements and processes have been provided by other researchers [293,294,295,296]. Liquid cooling systems effectively dissipate heat and maintain uniform temperatures during high C-rate charging. PCM-based systems absorb excess heat and could reduce thermal runaway by at least 40 min, enhancing safety. A hybrid system combining liquid cooling and PCMs has been proposed that can maintain temperature control for an 8C fast-charging battery pack. During thermal runaway, LIBs can release over 10 kW of heat; thus, advanced cooling strategies are essential in the pursuit of preventing catastrophic failures [297]. Advanced liquid cooling is required to counter the extreme heat generated during ultrafast charging (≥5C). It has been observed that a three-sided cooling plate layout reduces maximum temperatures from 58 °C to 49 °C with 4C charging. A 5 °C coolant was found to lower the temperature by 1 °C–3 °C, and it expanded the safe 5C charging range by 50.1% at 20% SOC. An optimized coolant temperature at 5 °C was found to improve the charging range by 29.5% at 15 °C, 49.3% at 25 °C, and 43.3% at 35 °C, increasing EV mileage by 117 km under 5C charging [298]. Several companies have been developing advanced methods for thermal management and safety, especially for the ultrafast charging of EVs. For instance, pulsating heat pipe (PHP) [299], two-phase immersion cooling (which submerges battery cells in a dielectric liquid) [300], and immersion cooling (fully submerging battery modules in a thermally conductive fluid) have been proposed to support ultrafast charging [301]. It has been found that these methods minimize hotspots and could be promising approaches for high-power charging applications.

Significant thermal management strategies have been developed to optimize heat regulation, especially for ultrafast charging. A thermally modulated charging protocol (TMCP) with active thermal switching (ATS) has been proposed. TMCP enables 80% SOC in under 15 min while extending the cycle life by 54.6% (773 cycles vs. 500 DOE target). Unlike conventional cooling, TMCP allows stable 6C charging, suppresses lithium plating, and reduces SEI layer growth [302]. A study proposed active thermal switching, where self-generated heat is maintained during charging (switch OFF) to enhance ion transport and dissipation after charging (switch ON) to reduce degradation. This method enabled <15 min charging to 80% SOC without modifying the cell materials, and could help in maintaining a cycle life similar to that of 1 h charge–discharge cycles [303]. Another study proposed asymmetric temperature modulation (ATM) in order to maintain cycle life. The method involved charging at 60 °C for 10 min per cycle to enhance ion transport and limit SEI growth. Using this approach, a 9.5 Ah, 170 Wh/kg cell achieved 1700 cycles (compared to just 60 cycles under conventional fast charging), and a 209 Wh/kg BEV cell kept 91.7% capacity after 2500 cycles. Additionally, charging at 60 °C significantly reduced cooling demands by over 123%, improving overall charging efficiency [304].

In order to inhibit lithium dendrite growth, several enhancements have been proposed in electrode design and material optimization. Electrode microstructure engineering has achieved 25.9% reduction in internal resistance and 12.3 times superior capacity retention at 59.7C compared to traditional electrodes [305]. Graphite anodes release 658 J/g, while polyimide-based anodes only released 242 J/g. High-power anode materials like LTO might be a safer alternative, as seen in Toshiba SCiB batteries, which achieve 90% charge in 10 min. Modified anodes (such as polyimide-coated anodes) exhibit better thermal stability, which is crucial for preventing overheating during rapid charging [306]. One study presented a solution by increasing the anode lithiation potential, which reduces dendrite formation; additionally, the authors proposed doping and structural adjustments to stabilize the SEI and improve safety. A simplified material synthesis method utilizing advanced anode materials like black phosphorus has been proposed (~2600 mAh/g) to be a safer and more reliable solution for fast-charging applications [81]. Lithium plating on graphite anodes during high charging rates reduces capacity and leads to safety risks. Another approach is modifying the graphite structure—for instance, expanding graphite’s lamellar spacing (interlayer distance), as is the case for EG60, for example [307,308]. It has been illustrated that reduced graphite interlayer spacing (~3.37 Å vs. 3.42 Å) improves structural stability and improves lithium-ion diffusion (Figure 13) [307].

Several strategies have been developed for suppressing dendrite growth in LIBs [268]. Modifying the anode structure is a critical strategy for overcoming dendrite formation. For instance, nanostructured scaffolds like carbon nanotubes (CNTs), graphene frameworks, and 3D lithium can provide a high surface area for lithium plating that would minimize dendrite growth and could enhance cycle life beyond 1000 cycles at 5 mA cm⁻² (as shown in Figure 13) [268]. Moreover, Samsung Electronics developed micron-scale protective layers with a Young’s modulus above 10⁶ Pa, providing mechanical stability to suppress dendrites and extend cycle life. Oerlikon Surface Solutions designed 3D lithium anodes with columnar structures, increasing lithium loading density and improving cycling stability at high current densities (>5 mA cm⁻²) [309].

SEI engineering has evolved with improved safety, particularly for high charging rates. Inorganic-rich, high-modulus SEIs can withstand temperatures of 220 °C, whereas conventional Si-based anodes will degrade at 175 °C. At a charging rate greater than 4C, lithium dendrites may be larger than 30 µm and may penetrate the separator and cause internal short circuits. In addition, SEI decomposition at 69 °C leads to gas generation, thereby increasing internal pressure and battery swelling. Artificial SEI layers, such as Al₂O₃ coatings fabricated using atomic layer deposition (ALD), have been observed to provide a thermally stable interphase and are capable of reducing SEI decomposition by 40% at high temperatures [310]. Another advancement includes the development of artificial SEI layers, such as aluminum fluoride (AlF₃)-coated graphite or lithium titanate oxide (LTO) coatings, which stabilize the electrode surface [311]. Thus, AlF₃ and LTO coatings directly contribute to dendrite suppression by ensuring stable Li-ion transport (Figure 13). In artificial SEI formation, it has been found that Sn-protected SEI films reduce dendritic growth and could reduce overpotential by 20% [312].

Conventional polyethylene (PE) and polypropylene (PP) separators have melting points of 130 °C and 170 °C, respectively, resulting in structural failure and internal short circuits [310]. An advancement for thermal management in LIBs is the development of high-stability separator designs [313]. A sponge-like separator (nanoporous SiO₂ separator) can be highly porous, allowing for high electrolyte absorption (362%). Thus, more stable ion transport is developed at high charge rates. This development also showed high ionic conductivity (7.8 × 10⁻³ S/cm at 25 °C) and a wide electrochemical stability window (5.1 V), preventing degradation. Furthermore, thermal shrinkage (1.2% MD, 0.5% TD) is minimized, lowering the self-heating temperature to 117 °C; thus, the risk of thermal runaway is significantly reduced [314]. One factor of improvement in the separator is high electrolyte retention (200%); this allows for stable ion transport to occur during fast charging, preventing electrolyte depletion. In one study, the authors developed a semi-interpenetrating polymer network (SPN) ceramic separator to improve the safety and performance of fast-charging LIBs. Here, the NMC811/SPN@CCS/graphite cell achieved 93.0% capacity retention and 99.5% coulombic efficiency over 300 cycles at 1.5C. The self-heating temperature remained at 110 °C, compared to 70 °C in commercial cells, reducing thermal runaway risks. Thus, this advanced separator is a strong candidate for safe, high-performance, fast-charging LIBs [315].

Separator coatings can be designed to make batteries safer, enhance ion conductivity, and suppress the growth of lithium dendrites. Different developments have been proposed when it comes to separator coatings. Ceramic-coated separators, thermally stable polymers like PI and PAN, and inorganic structure membranes have been proposed. It has been observed that multi-layer separators (PP/PE/PP) can delay thermal collapse; meanwhile, ceramic and polymer separators have flame retardancy, improving thermal stability [316]. Another study focused on improving the safety of LIBs to regulate lithium dendrite growth during fast charging. A functional separator coated with an ultrathin (20 nm) layer of gold (Au) nanoparticles has been developed. The Li/graphite cells with this functional separator showed an areal capacity retention of 90.5% over 95 cycles at a current density of 0.72 mA cm⁻². Under a high current density of 1C, the battery maintained satisfactory capacity retention [317]. In another study, a plate-shaped zeolite separator was coated to prevent dendrite propagation. The separator, with a 40 µm thick coating, contained intraparticle crystalline micropores that promoted uniform Li-ion flux at the separator–anode interface. At a 3C rate, the LMB cell with the zeolite separator kept its capacity for 100 cycles, whereas the γ-alumina separator cell started to degrade by the 75th cycle. Moreover, the thermal stability tests showed minimal mass loss of 5% at 400 °C, which confirms its nonflammable nature [318].

A functional separator, formed by depositing an ultrathin 20 nm gold (Au) nanoparticle layer via a thermal evaporation coating (Figure 13), was developed in order to regulate lithium dendrite growth and prevent short circuits. It was found that a Li/graphite battery with a Au-coated separator achieved 90.5% areal capacity retention over 95 cycles at 0.72 mA cm⁻² with high current density (1C) compared to conventional separators [317]. Moreover, the Tata Institute of Fundamental Research Hyderabad (TIFRH) developed a separator modification using graphitic fluorocarbon to suppress dendrite growth in lithium metal batteries. This coating improved Li-ion transport in order to attain uniform deposition and reduce dendrite formation. The modified separator increased energy density by 10×, improving safety and cycle life [319].

One of the improvements that has been made in cathodes in terms of safety is achieved through element substitution (Al, Ni, Mn) and protective coatings (AlF₃, SiO₂). AlF₃ delays thermal runaway by 20 °C, while SiO₂ lowers heat generation by 35%, enhancing stability [320,321]. A study explored a systematic approach to achieving 10 min charging in LIBs. NMC811 cathodes, dual-layer anodes (DLAs), high-pore-volume separators (Celgard tri-layer (PP/PE/PP) 2320), advanced electrolyte mixtures, and voltage ramp protocols were implemented. These strategies enabled 6C charging for hundreds of cycles with >90% charge acceptance and minimal degradation. Moreover, higher-Ni cathodes like NMC811 are shown to outperform NMC532 due to superior electrochemical properties that reduce overpotential [322]. Additionally, optimized electrode architecture, such as reduced tortuosity and secondary pore networks, can mitigate degradation, especially in fast and ultrafast charging [323].

Electrolytes play a significant role in Li-ion transport and the safety of batteries. Inhomogeneous transport increases polarization and overpotential, while SEI films may crack under high rates. Traditional ester-based electrolytes decompose above 60 °C, raising thermal risks [324,325,326]. One approach involves increasing the electrolyte oxidation reaction potential to 4.7 V using functional additives that can delay thermal runaway until the battery reaches 80% SOC. One improvement in electrolyte design for fast charging is the use of LiFSI instead of LiPF₆, which significantly reduces internal heating and polarization [327]. An electrochemical–thermal P2D model was implemented; the researchers found that, at 5C charging, batteries with LiFSI experienced a 40 K temperature rise, compared to 57 K for LiPF₆ due to higher ionic conductivity (10.5 vs. 8.1 mS/cm) and lower polarization (~18%) [327]. Studies show that replacing carbonate solvents with ILs lowers the risk of thermal runaway by over 60%, delaying self-heating onset and reducing pressure buildup inside cells. Solid and gel electrolytes further suppress vaporization, ensuring safer battery operation even under >150 °C abuse conditions [289]. Organic liquid electrolytes might be at risk of overheating and short circuiting; these risks can be mitigated by solid-state electrolytes (SSEs). Thus, in addition to liquid electrolytes, solid-state electrolytes provide safety advantages; however, they face challenges with ionic conductivity, poor interface stability, and mechanical detachment during volume changes. For instance, polyrotaxane (PRX)-based solid polymer electrolytes (SPEs) have high ionic conductivity (5.93 × 10⁻³ S/cm) and oxidative stability above 4.7 V, enabling safe fast charging with a coulombic efficiency of 98.5% [328]. To address these issues, flame-retardant additives like trifluoroethoxy pentafluorocyclotriphosphazene (TFPN) have been employed, which can restrict dendrite growth and improve thermal stability. Studies show that, at 5 wt% TFPN, the lithium metal batteries of Ni-rich cathodes retained their capacity at a high level, with over 90 cycles compared to only 48 cycles achieved with regular electrolytes [329]. Additionally, one of the most impactful solutions is high-modulus solid electrolytes, which can be used in preventing dendrite penetration. Asymmetric solid electrolytes (ASEs) have shown 90.2% capacity retention after 200 cycles at 0.2C [330]. Moreover, electrolyte engineering using fluorinated solvents and lithium nitrate additives has reduced dendrite growth by 50%, impacting LIB lifespan under high-rate cycling conditions. Furthermore, three-dimensional metallic electrode architectures, e.g., nickel foam precoated with lithium, minimize local overpotentials by 30%, leading to more stable lithium deposition.

6. Conclusions

There is a strong need for the rapid development of ultrafast-charging lithium-ion batteries (LIBs) in order to meet the increasing demand for electric vehicles (EVs), consumer devices, and grid energy storage systems. Though significant advancements have been made, some major challenges remain, such as electrode degradation, lithium dendrite growth, electrolyte instability, and safety issues under high charging rates. Overcoming these challenges requires a multi-disciplinary strategy combining materials engineering, electrochemical optimization, and thermal management techniques.

Improvements in cathode and anode materials (for example, nanostructured electrodes, doped and hybrid materials, and high-rate conductive coatings) have shown enhanced Li-ion diffusion and charge transfer efficiency. Next-generation electrolytes, including high-concentration electrolytes (HCEs), localized high-concentration electrolytes (LHCEs), and solid-state electrolytes (SSEs), have shown potential in enhancing ionic conductivity, electrochemical stability, and safety. Separator advancements, like self-healing, conductive, and flame-retardant binders, are also contributing to the development of more robust and durable ultrafast-charging LIBs. Beyond material-level improvements, optimized charging protocols are crucial in balancing charging speed, efficiency, and cycle life. Strategies such as pulse charging, adaptive current control, and multistage voltage profiles offer potential solutions to mitigate lithium plating, heat generation, and irreversible capacity loss. Additionally, enhancement in thermal management systems will be essential to maintaining battery stability under high-power operations.

The successful commercialization of ultrafast-charging LIBs will depend on continued innovations in material science, electrochemical engineering, and battery management systems (BMSs). There are various areas of focus: material optimization and novel chemistries must focus on developing high-conductivity and high-power anodes like LTO and LVO, high-capacity anodes (e.g., silicon composites, LTO, etc.) and hybrid anodes with enhanced structural stability must be developed, and, in addition, there is a need for advanced cathode materials, including high-voltage layered oxides, disordered rock salt oxides, and polyanion-based compounds, to improve energy density and rapid charge acceptance. Moreover, the investigation of hybrid electrolytes that integrate liquid- and solid-state properties will be essential for improving charge transport and safety.

Electrode and interface engineering techniques can be of great importance in the development of UFC. For fast ion and electron transport, the architecture of the electrode is improved. Such improvements can be achieved through the use of 3D nanostructures, hybrid coatings, and conductive networks. The development of self-healing, dendrite-resistant interfaces can also prevent lithium plating and structural degradation during UFC. Battery safety and thermal management are issues that require the integration of advanced thermal management solutions, such as phase-change materials (PCMs), active cooling systems, and heat-controlled battery designs. Real-time safety diagnostics and AI-driven predictive models should be implemented to monitor degradation and prevent catastrophic failures.

Optimized charging protocols and advanced battery management systems (BMSs) are crucial to the longevity and safety of ultrafast-charging LIBs. Adaptive, dynamic charging algorithms that maximize charging speed while maintaining low stress on battery components should be developed; real-time electrochemical impedance spectroscopy (EIS), machine learning models, and predictive analytics should be developed to extend the lifespan of batteries, along with those of battery electric vehicles (BEVs). The sustainability and scalability of ultrafast-charging LIBs are concerns that can be met by using environmentally friendly and cost-effective methods of material sourcing and developing recycling strategies and second-life applications for LIBs, which can help promote sustainable energy storage solutions.