Protective Layer and Current Collector Design for Interface Stabilization in Lithium-Metal Batteries

Abstract

Recent advancements in lithium-metal-based battery technology have garnered significant attention, driven by the increasing demand for high-energy storage devices such as electric vehicles (EVs). Lithium (Li) metal has long been considered an ideal negative electrode due to its high theoretical specific capacity (3860 mAh g⁻¹) and low redox potential. However, the commercialization of Li-metal batteries (LMBs) faces significant challenges, primarily related to the safety and cyclability of the negative electrodes. The formation of lithium dendrites and uneven solid electrolyte interphases, along with volumetric expansion during cycling, severely hinder the commercial viability of LMBs. Among the various strategies developed to overcome these challenges, the introduction of artificial protective layers and the structural engineering of current collectors have emerged as highly promising approaches. These techniques are critical for regulating Li deposition behavior, mitigating dendrite growth, and enhancing interfacial and mechanical stability. This review summarizes the current state of Li-negative electrodes and introduces methods of enhancing their performance using a protective layer and current collector design.

1. Introduction

Rechargeable lithium-ion batteries (LIBs) are fundamental components of modern society. The global energy crisis and climate change have accelerated the development of eco-friendly energy technologies, leading to increased interest in LIBs. The growing demand for electric vehicles has heightened the need to develop high-energy-density batteries [1,2,3,4,5,6,7,8,9,10,11,12,13]. Currently, the energy density of LIBs can reach levels ranging from 120 to 270 Wh kg⁻¹. However, this range remains considerably below the values required for high-energy applications, thus posing significant challenges for overcoming the driving-range limitations of electric vehicles [14,15,16,17,18,19]. In response to this demand, research efforts have focused on increasing the energy density of LIBs [20,21,22,23,24,25,26,27,28]. However, using graphite-negative electrodes with a relatively low capacity significantly constrains the achievable energy density [17,29,30]. To overcome this limitation, lithium-metal batteries (LMBs) have been proposed as a next-generation energy storage solution. Li metal, with a theoretical maximum capacity of 3860 mAh g⁻¹ and low density (0.534 g cm⁻³), is considered an ideal negative electrode material for energy storage systems [2,31,32,33]. However, significant challenges related to safety and cyclability must be addressed before attempting to commercialize LMBs [34,35]. The formation of Li dendrites and an uneven solid electrolyte interphase (SEI), along with volumetric expansion during cycling, severely hinder the development of LMBs [36]. These issues lead to the continuous loss of Li and electrolytes, which results in low Coulombic efficiency (CE) and eventual battery failure [37,38]. The LMBs, which operate without a conventional host structure at the negative electrode, enable Li storage and release through repeated Li-ion plating and stripping reactions [39]. Similar to many other metals, Li tends to be deposited in dendritic forms, recognized as the primary cause of thermal runaway and explosion risks owing to internal short-circuits in the cell [32,34]. Thus, achieving dendrite-free Li deposition is crucial. The SEI formed on the surface of Li metal is known to be unstable in corrosive electrolytes [40]. Although an appropriately formed SEI can enhance the battery performance, in practice, the unstable SEI film is often cited as the major cause of LMB failure. During charging or Li plating, the locally varying chemical composition and morphology of the SEI can lead to a non-uniform current density and irregular Li deposition, resulting in the formation of Li dendrites [41,42]. The volumetric expansion of LMBs is theoretically indefinite owing to the hostless nature of the negative electrode [34]. Thus, the interfacial stability must be enhanced to ensure reliable performance. Recent research efforts have increasingly focused on better understanding Li-metal chemistry and developing improved Li-negative electrodes. Methods of improving the performance of LMBs can be categorized into three key approaches: electrolyte modification, SEI stabilization, and modification of the negative electrode surface. This review focuses on the most effective techniques for modifying the negative electrode surface [43]. To provide a structural overview of these strategies, Scheme 1 illustrates two main approaches—protective layer integration and current collector design—and their respective subcategories discussed in this review. The first area of focus is the design of a protective layer for stabilizing the Li-metal interface. These protective layers are categorized as polymer-based, inorganic, or composite materials. The second area of focus concerns the rational design of the current collector to prevent dendrite growth commonly associated with conventional, planar current collectors. These current collectors involve the design of 3D porous structures to overcome the low metal affinity of traditional Cu/C current collectors, the incorporation of lithiophilic materials, and the use of carbon-based frameworks to provide uniform nucleation sites and reduce nucleation overpotential, thereby preventing the growth of Li dendrites. This review provides a comprehensive summary of recent methodologies, innovative composites, and advanced technologies for improving the safety and cyclability of LMBs for next-generation, high-energy LMBs.

2. Li-Metal Battery

2.1. Mechanisms of Nucleation and Growth of Li Dendrites

When metallic Li undergoes repeated plating and stripping on a specific substrate, its initial deposition morphology plays a crucial role in determining the final structure, which in turn profoundly affects the cycling lifespan of LMBs. Dissolved Li ions migrate from the bulk electrolyte to the vicinity of the electrode surface, shedding solvent molecules while being reduced to adatoms, which diffuse across the surface and integrate into the metal lattice [44,45]. During the Li deposition process, the density and spatial distribution of initial nucleation sites play crucial roles in determining the characteristics and behavior of Li during subsequent deposition [42]. Non-uniform Li nucleation results in concentrated ion gradients in specific regions of the electrode surface, leading to inhomogeneous deposition [46,47]. This non-uniform nucleation is influenced by several factors, including current density, surface characteristics, and electrolyte composition. A high current density accelerates Li-ion transport, leading to localized lithium deposition, whereas the lithiophilicity of the surface facilitates more homogeneous nucleation [48]. Electrolyte additives also affect the formation of the SEI layer, thereby influencing the spatial distribution of Li nuclei and potentially enhancing the uniformity of the nucleation process [49].

Uneven Li nucleation can lead to the growth of Li dendrites. Initially deposited in small protuberances, Li grows into needle-like, moss-like, or tree-like structures through repeated charge and discharge cycles, accelerating the detachment of Li nuclei and particles from the LMBs [50]. Sustained growth of lithium dendrites may penetrate the separator, leading to internal short circuits and uncontrolled current flow, thereby generating excessive heat and elevating the risk of thermal runaway and explosion [51]. With increasing applied voltage, the vertical growth of many dendrites ceases, causing the dendrites to abruptly bend and collapse due to the substantial axial compressive forces exerted on the fragile dendrites [50]. The rapid and uneven dissolution of Li dendrites near the active sites leads to detachment from the matrix, resulting in the formation of “dead lithium”. The formation of electrochemically inactive “dead lithium” contributes to a progressive reduction in the reversible capacity and intensifies cell polarization by obstructing the Li-ion transport pathways. This accumulation of dead lithium on the negative electrode surface increases the resistance, limits ion mobility, and degrades the overall cell performance [50,52]. Figure 1 illustrates the destruction of the SEI layer due to uneven Li deposition and the formation of Li dendrites [53].

2.2. SEI Formation and Associated Challenges in LMBs

The SEI forms spontaneously on Li metal in organic electrolytes and serves as a passivation layer [54,55]. The SEI layer exhibits ionic conductivity, allowing Li ions to pass through, whereas its electronic insulating nature blocks electron transport across the interface, effectively preventing parasitic reactions and internal short circuits [56]. In 1983, Peled proposed that the SEI has a dual-layer structure comprising a thin but dense layer near the electrode and a thick but porous layer near the electrolyte [57]. This dual-layer configuration contributes to the effectiveness of the SEI in enhancing the performance and stability of LMBs. Subsequently, a “mosaic structure” of the SEI, comprising multiple organic and inorganic layers, was proposed [58]. Both structural models were later directly confirmed via cryo-electron microscopy by Li et al. in 2017, who visualized the layered SEI structure on lithium metal at high resolution [59]. The thin and compact layer near the electrode surface consists of inorganic Li compounds, such as Li₂O, Li₂CO₃, and LiF—all thermodynamically compatible with Li metal. The layers further from the electrode and closer to the electrolyte mainly consist of oligomeric organic compounds such as semicarbonates and polyolefins [56]. This intricate layering enhances the protective and functional properties of the SEI, contributing to the improved stability and performance of LMBs. The SEI is considered a critical component in determining the safety and cycling performance of electrodes [60]. The stability and composition of the SEI directly influence the longevity and efficiency of LMBs, highlighting the importance of optimizing the formation and properties of the SEI in enhancing the battery performance. An ideal SEI layer should possess high Li-ion conductivity, low electronic conductivity, an appropriate thickness, excellent chemical stability during long-term cycling, and robust mechanical performance [61,62].

A suboptimal SEI layer can lead to various issues in LMBs, including reduced cycling stability, lower CE, and the formation of Li dendrites, which pose safety risks and decrease the overall battery performance. Electrolyte solvents such as ethylene carbonate (EC) and dimethyl carbonate (DMC) undergo spontaneous reduction and decomposition upon contact with the Li-metal electrode due to its intrinsically low electrochemical potential, leading to the formation of an SEI [63]. The SEI formed on Li metal tends to be fragile and heterogeneous, leading to an uneven Li-ion flux and random Li deposition beneath the SEI owing to the varying spatial resistance [38,64]. This non-uniform SEI facilitates the growth of Li dendrites and the formation of dead lithium. As SEI formation and dead lithium accumulation continue, the electrolyte is consistently consumed, leading to a decrease in the reversible capacity and CE of the battery. Moreover, repeated SEI formation results in the volumetric expansion of lithium metal, inducing mechanical stress within the cell. An excessively thick or unstable SEI can reduce the ionic conductivity, increase the internal resistance of the electrode, and lower the battery-charging speed, thus underscoring the importance of stabilizing the SEI to improve battery performance and prolong the cycle life of LMBs.

3. Suppression of Li Dendrites: Structured Electrode Strategy

3.1. Protective Layer

The formation of Li dendrites, which undermine the safety and cyclability of LMBs, originates from interfacial issues related to the transport and reduction of Li ions at various interfaces, such as non-uniform Li⁺ flux, uneven electron distribution, and instability of the SEI at the Li/electrolyte interface [65,66]. These interfacial inhomogeneities promote localized Li nucleation and uncontrolled dendritic growth during repeated plating and stripping cycles [16,67]. Achieving uniform Li-metal deposition necessitates stabilization of the LMB interface. Research on stabilizing the interface has primarily focused on introducing protective layers through two approaches: (1) applying artificial layers and (2) designing a stable SEI through electrolyte engineering. These techniques aim to enhance the ability of the SEI to uniformly distribute the Li-ion flux and prevent the localized deposition of Li, which leads to dendrite formation. An ideal protective layer must be chemically and electrochemically stable and possess a high ionic conductivity, high Young’s modulus, and high yield strength to prevent direct contact between the Li metal and electrolyte, suppress dendrite growth, and withstand significant volumetric changes in the LMBs. Therefore, key factors such as the chemical stability, mechanical properties, ionic conductivity, and corrosion resistance of the protective layers are discussed as they are critical for the design of high-performance LMBs [68].

3.1.1. Polymer-Based Protective Layers

Polymers with excellent electrically insulating properties and the ability to accommodate volume changes are particularly suitable for use as protective layers [69]. Polymeric materials such as poly(dimethylsiloxane) (PDMS) [70,71], polyvinyl alcohol (PVA) [72], Nafion [73], and thermoplastic polyurethane (TPU) [74] have been employed as protective layers in LMBs. These polymers vary widely in their electrochemical stability, ionic conductivity, and compatibility with Li metal.

Table 1. Ionic conductivity at 25 °C and Li metal compatibility of polymer electrolytes under GPE conditions.

Materials	Ionic Conductivity (S·cm−1 at 25 °C)	Compatibility with Li Metal	Characteristic	Ref.
PEO (Polyethylene oxide)	~10−6 to ~10−1	Moderate—unstable SEI over the long term	Widely used; limited at room temperature	[75]
PVDF-HFP	~10−5 to 10−4	Good	High dielectric constant and flexibility	[76]
PAN (Polyacrylonitrile)	~10−5	Moderate to Good	Good electrochemical stability	[77]
PVA (Polyvinyl alcohol)	~10−6 to 10−5	Good	Chemically stable, often modified	[78]
TPU (Thermoplastic polyurethane)	~10−5 to 10−4	Good	Elasticity supports volume change buffering	[79]
Nafion (lithiated)	~10−4 to 10−3	Excellent	Cation-selective; used in single-ion conductors	[80]
PDMS (Polydimethylsiloxane)	~10−7 to 10−6	Good	Inert, flexible; often used in composites	[81]

summarizes key polymer electrolytes commonly employed in LMBs, highlighting their room-temperature ionic conductivities and interfacial performance.

Zhu et al. developed a protective layer consisting of a modified, nanoporous PDMS film to enhance the cycling stability of LMBs [70]. PDMS is widely utilized in microfluidics and other fields because of its processing convenience and chemical inertness. However, standard PDMS films are not ion-conductive, which necessitates introducing pores to create pathways for Li-ion transport. To address this issue, researchers utilized hydrofluoric acid etching (5% HF, 5 min) to introduce nanopores approximately 40~100 nm in size into a spin-coated 500 nm thick PDMS film. This modification enabled efficient Li-ion transport and provided mechanical and chemical stability during electrochemical cycling, effectively suppressing the formation of lithium dendrites (Figure 2a,b). The cells were assembled in a Li‖Cu configuration using 1 M LiPF₆ in EC/DEC (1:1 v/v) with 2 wt% VC additive as the electrolyte. Li foil was used as the counter electrode, and 1 mAh cm⁻² of Li was plated at current densities of 0.25, 0.5, and 1 mA cm⁻². Additionally, owing to its chemical inertness, PDMS-based protection is compatible with other strategies, such as electrolyte development. Compared to bare Cu electrodes, which showed rapid CE degradation and voltage hysteresis exceeding 200 mV after 100 cycles, the PDMS-coated Cu electrode maintained an average CE of ~94.5% over 200 cycles at 0.5 mA cm⁻² with a stable voltage hysteresis of ~80 mV. These results confirm that the modified PDMS layer effectively stabilizes the Li plating/stripping interface and improves the electrochemical longevity of LMBs.

Reactive polymers contribute to SEI formation by undergoing chemical and electrochemical reactions with Li metal at the interface [82,83,84]. When effectively utilized, this strategy allows precise control of the composition and mechanical properties of the SEI layer. Zhao et al. developed an effective protective layer for LMBs using a low-cost and environmentally friendly PVA polymer [72]. In their study, Li‖Cu configurations were assembled using 1 M LiTFSI in DOL/DME (1:1 v/v) with 4 wt% LiNO₃ as the electrolyte. The current density and capacity were systematically varied (1~3 mAh cm⁻² and 2 mA cm⁻²), and the cycling tests were conducted at room temperature in CR2016-type coin cells. This protective layer was compatible with both ether- and carbonate-based electrolytes. By participating in the formation of a high-quality polymer composite SEI layer, the protective layer mitigates Li dendrite formation and electrolyte consumption. The performance of the bare Cu electrode as a reference was also examined: at a capacity of 1 mAh cm⁻² and current density of 2 mA cm⁻², its CE declined rapidly after 150 cycles. In contrast, the cell with the PVA-protected Cu foil demonstrated remarkable cycling stability, maintaining a consistent CE of 98.3% over 630 cycles. At higher deposition capacities of 2 and 3 mAh cm⁻², the PVA-protected cells achieved average CEs of 98.4% (320 cycles) and 98.5% (220 cycles), respectively. Additionally, voltage polarization was lower and stabilized within 40 mV after 20 cycles, with reduced interfacial resistance confirmed by EIS, indicating more stable Li plating/stripping behavior. Li et al. developed a robust, all-organic, interfacial protection layer for achieving a highly efficient dendrite-free LMB [73]. The layer-integrated porous hypercrosslinked poly(4-chloromethylstyrene) nanospheres were grafted with poly(oligo(ethylene glycol) methyl ether methacrylate) (xPCMS-g-PEGMA) using single-ion-conductive lithiated Nafion. xPCMS-g-PEGMA provides a porous structure that facilitates uniform lithium-ion transport and prevents dendrite growth by evenly distributing the lithium flux. Lithiated Nafion enhances stability by enabling selective lithium-ion migration, reducing dendrite formation, and improving safety. The experimental setup involved Li symmetric cells assembled with 1 M LiTFSI in DOL/DME (1:1 v/v) as the electrolyte. Electrochemical cycling was conducted at high current densities of up to 10 mA cm⁻² using CR2032 coin-type cells. Control experiments using unprotected Li electrodes showed unstable voltage profiles and cell failure within ~300 h, whereas the protected electrodes demonstrated stable cycling for over 9100 h at 10 mA cm⁻² and for over 2800 h at 10 mAh cm⁻². Zhao et al. developed an artificial SEI layer for stabilizing Li-negative electrodes using TPU elastomers via a straightforward and mild fabrication method [74]. Owing to its high elasticity, TPU effectively suppresses the growth of Li dendrites and limits the volumetric expansion of LMBs. The ion-conductive channels of the coating layer were provided by the soft-segment polyethylene oxide, while the hard-segment isophorone diisocyanate enhanced the overall elasticity and flexibility. The TPU-modified Li‖Cu cells exhibited impressive cycling stability, maintaining a CE of >97% even after 400 cycles at a capacity of 0.5 mAh cm⁻² and a current density of 0.5 mA cm⁻². Moreover, Li‖LFP cells demonstrated exceptional long-term stability, with 90% capacity retention after 1500 cycles at a rate of 5 °C. These results highlight the excellent durability and performance of the modified cells under extended cycling conditions. Ordaz et al. recently developed a composite protective layer comprising a cellulose-based matrix (TDMSC), a single-ion conducting polymer [P(LiMTFSI)], and LiNO₃ to enhance the stability of Li metal in liquid electrolytes [85]. The protective layer was fabricated by drop-casting onto mechanically polished Li foils (200 μm thick), forming a uniform film approximately 1.4~1.6 μm in thickness. An electrochemical evaluation was conducted in Li‖Cu and Li‖Li cells using 1 M LiFSI in FEC/DEC or 1 M LiFSI in DME solvents. The coated Li-negative electrode achieved a CE of 99.3% in Li‖Cu cells, compared to 95.9% for bare Li, and exhibited stable plating/stripping in symmetric cells with significantly reduced overpotential. Moreover, Li‖LFP cells employing the PTL-coated negative electrode delivered improved cycle stability and interfacial morphology under both liquid and solid-state electrolyte conditions. In particular, in all-solid-state configurations using a PEO-based electrolyte at 60 °C, the coated cell retained 99.4% of its capacity over 200 cycles at C/3 with an average CE of 99.8%.

3.1.2. Inorganic-Based Protective Layers

Various inorganic materials have been investigated as protective layers for Li-ion-negative electrodes [38,86,87,88,89]. The formation of stable Li-metal/electrolyte interfaces using Li-based alloys has been explored, with MXenes utilized as platforms for Li deposition [87,88]. Among the inorganic components of the SEI, LiF is notable owing to its high electron-tunneling barrier and excellent mechanical strength [90,91]. An increase in the LiF content of the SEI can reduce the likelihood of electron tunneling at the SEI/Li interface, effectively preventing dendrites from penetrating the SEI [92]. Considering these advantages, protective layers containing LiF have been developed [88,93,94]. Li-based alloys have been investigated as protective phases owing to their ability to reduce the Li-ion diffusion barrier and improve the stability of the Li interphase [95,96]. Approaches utilizing alloys include the in-situ formation of phases such as Sn-Li, Li₁₃-In₃, Li-Zn, Li₃-Bi, Li₃-As, Au-Li, and Si-Li [95,97].

Pathak et al. developed an efficient methodology for creating an artificial Li-metal/electrolyte interphase by treating a Li-negative electrode with a tin-containing electrolyte [87]. The resulting artificial SEI, composed of Li fluoride, Sn, and a Sn-Li alloy, demonstrated a synergistic effect. This effectively traps Li dendrites and electrically isolated (dead) Li within its structure and facilitates Li deposition beneath the alloy layer. Compared to bare lithium, a Li symmetrical cell employing this artificial SEI exhibited a reduced overpotential by approximately 100 mV and excellent plating/stripping cycling performance (~2325 h). MXene is recognized as an ideal platform for Li deposition due to its excellent electrical conductivity, low Li-ion-diffusion barrier, and the abundance of lithiophilic functional groups on its surface [98,99]. The general formula for MXenes is M_n+1X_nT_x, where M represents an early transition metal, X denotes carbon and/or nitrogen, and T indicates surface terminal groups such as -OH, -O, and -F [100]. Ti₃C₂T_x MXenes are the most commonly used platforms. Among the terminal groups, the F-termination groups can form a robust LiF-containing SEI layer, achieving uniform Li deposition [99]. However, the heterogeneous lithiophilic sites and dense structure of MXenes lead to anisotropic Li nucleation. To address the drawbacks of MXenes, Zhao et al. developed a Ti₃C₂T_x/g-C₃N₄ composite electrode, which ensured uniform Li plating/stripping, prevented continuous Li corrosion by the electrolyte, and inhibited the growth of Li dendrites (Figure 3j) [88]. Owing to the insulating properties of g-C₃N₄, Li was not directly deposited on the g-C₃N₄ surface; instead, it was uniformly plated within the Ti₃C₂T_x/g-C₃N₄ composite structure, and its unique atomic structure provided Li-ion conduction pathways [101]. To investigate the effect of the Ti₃C₂T_x/g-C₃N₄ composite on Li deposition, varying amounts of lithium were plated onto the Ti₃C₂T_x/g-C₃N₄ and Ti₃C₂T_x electrodes. Figure 3a–f show the SEM images of the Ti₃C₂T_x/g-C₃N₄ electrode after plating with Li at capacities of 1, 2, and 3 mAh cm⁻² with a current density of 0.5 mA cm⁻². The electrode surface appeared smooth and uniform without the formation of Li dendrites or large Li agglomerates. Cross-sectional analysis revealed that the overall morphology of the electrode remained intact, confirming that Li was evenly distributed within the composite structure and across the electrode surface without dendrite formation [88]. To explore ion diffusion within the SEI, Hu et al. fabricated an artificial hybrid SEI layer composed of a Li-antimony (Li₃Sb) alloy and Li fluoride (LiF) [94]. The protective layer was prepared via spontaneous chemical reaction by immersing Li-metal foil into a DME solution containing 5 mM SbF₃ at room temperature, forming a ~5 μm thick Li₃Sb/LiF composite interphase. Symmetric Li‖Li cells were assembled using 1 M LiTFSI in DOL/DME (1:1 v/v) with 2 wt% LiNO₃ and Celgard 2400 as the separator. These protective layers enabled dendrite-free Li deposition at the SEI/Li interface along with stable Li plating/stripping at a current density of 20 mA cm⁻². The hybrid-modified Li cell exhibited long-term cycling stability over 1360 h with a low voltage hysteresis of ~100 mV, while the bare Li cell failed within ~84 h under identical conditions, highlighting the critical importance of the modified interface. The internal mechanisms of Li-ion transport within the SEI components were investigated using experimental measurements, density functional theory (DFT), and finite element simulations, which revealed that Li₃Sb acts as a fast Li⁺ conductor and structural stabilizer, whereas LiF serves as an electron-blocking component. This synergistic combination ensured fast and uniform Li⁺ flux across the interface and mitigated dendritic growth, even under ultrahigh-rate cycling. You et al. developed a composite protective layer containing TiO₂ and ROLi by reacting a metal alkoxide (titanium butoxide) with the hydroxyl groups on Li metal [89]. This coating formed a dendrite-free LMB, which afforded significantly improved cycling stability. Figure 3k presents a schematic of the suppression of dendrite formation owing to the protective layer formed on the surface of Li metal. Symmetric cells utilizing the modified electrode demonstrated stable cycling for over 1500 h at a current density of 2 mA cm⁻² in ether-based electrolytes. In carbonate-based electrolytes, the modified electrode maintained stable cycling for over 80 h at a current density of 5 mA cm⁻². This performance contrasts sharply with that of the bare Li electrodes, which exhibited significant voltage fluctuations after only 10 h of cycling under the same conditions. Additionally, Li‖LCO cells incorporating the coated Li-negative electrode and a high-loading LCO positive electrode (approximately 15 mg cm⁻²) exhibited outstanding cycling stability, with 98.1% capacity retention and an enhanced average CE exceeding 99.6% over 150 cycles at 0.2 C.

3.1.3. Composite Protective Layers

Owing to the difficulty in fulfilling all the requirements of an ideal protective layer with a single material, inorganic/organic composite protective layers are commonly employed. Using such composite layers enhances both the mechanical properties and ionic conductivity of the material. Although elastic polymers contribute to the flexibility of the material, their limited rigidity can restrict the suppression of dendrite growth. To address this issue, rigid inorganic particles have been incorporated into polymer matrices, resulting in increased mechanical strength and more effective dendrite inhibition [68]. This synergistic approach balances flexibility and mechanical stiffness, enabling the composite layer to more effectively accommodate volumetric changes in the LMB. Consequently, a more stable interface is maintained, thus improving the overall performance and cycling stability of the LMBs.

To enhance the performance of LMBs, Lee et al. introduced a stabilized Li electrode incorporating a composite protective layer composed of inorganic and organic components with Li-ion conductivity [102]. The composite protective layer consisted of 1.7 μm Al₂O₃ particles dispersed in a PVDF-HFP matrix with a plasticizer electrolyte (1 M LiClO₄ in EC/PC, 1:1 v/v) and was fabricated via doctor blade casting directly onto Li foil to form a ~25 μm thick layer. Incorporating the composite protective layer effectively suppressed the growth of Li dendrites and electrolyte decomposition, enabling stable Li plating/stripping even at high current densities of up to 10 mA cm⁻². Li‖Li symmetric cells and Li‖LCO cells were assembled using 1 M LiClO₄ in EC/PC (1:1 v/v) as the electrolyte and a glass fiber membrane as the separator. The composite protective layer cell retained 91.8% of its initial discharge capacity after 400 cycles, with a CE exceeding 99.8% after 400 cycles. In contrast, the bare Li cell exhibited rapid capacity fading after only 100 cycles and showed scattered CE due to unstable SEI formation and electrolyte consumption. Nanoindentation tests revealed that the shear modulus of the composite protective layer is 1.8 times higher than that of Li metal in narrow indentation regions. The CPL is composed of Al₂O₃ particles and a poly(vinylidene-co-hexafluoropropylene) (PVDF-HFP) copolymer, where Al₂O₃ provides mechanical strength and the copolymer facilitates rapid Li-ion transport [103]. Using the composite protective layer as a protective layer for LMBs enhanced the cycle life by more than four-fold compared to that of bare Li, owing to the effective inhibition of Li dendrite growth and reduced electrolyte decomposition. To provide excellent interfacial stability during the long-term cycling of LMBs, an artificial protective layer with synergistic flexible-rigid characteristics was developed [104]. This protective layer suppressed localized Li deposition and isolated Li formation, enabling a dendrite-free LMB and preventing the depletion of Li metal and the electrolyte. The hybrid composite film composed of PVDF-HFP and LiF exhibited high mechanical strength, ion conductivity, and excellent conformability, facilitating uniform Li deposition and long-term cycling stability (Figure 4a). The Li‖Cu cells with the modified electrode maintained a relatively stable CE (~96.3%) over 60 cycles at a relatively high current density of 1.0 mA cm⁻² and sustained the capacity over 100 cycles. In contrast, the CE of the control cell with the bare Cu electrode dropped sharply to 40% after only 40 cycles. The Li‖LFP cells protected by APL demonstrated a lifespan of 2.5 times longer and a higher CE compared to the control cell, along with the optimal morphology of deposited Li. The results highlight the synergistic effect of organic/inorganic PLs, providing new insights into LMB stabilization and practical applications of high-energy-density batteries. The spontaneous reaction between Li metal and FEC solvent formed a bilayer film on the LMB. FEC is employed as an electrolyte additive because it effectively enhances the capacity retention of LMBs [105,106]. This protective film is dense and robust, suppressing side reactions between the electrolyte and electrode. Additionally, the interface induces a low overpotential, which contributes to cell reversibility [107]. Yan et al. developed a typical bilayer structure on an LMB featuring small organic components (ROCO₂Li and ROLi) in the upper layer and rigid inorganic components (Li₂CO₃ and LiF) in the lower layer [108]. The bilayer interfacial film was formed by immersing Li metal in pure FEC for 5 h, which induced spontaneous surface reactions to form a ~75 nm thick dual-layered SEI (≈25 nm organic-rich top and ≈50 nm inorganic-rich bottom). This bilayer interfacial film protected the LMB from corrosion by organic electrolytes and regulated the uniform deposition of Li ions, thereby affording a dendrite-free LMB (Figure 4b). The bilayer interface on the Li surface enabled uniform arrangement of the nucleation seeds of Li ions during Li plating, thus inducing a dendrite-free morphology. Electrochemical tests were performed in both Li‖Cu cells and Li‖NCM cells using 1 M LiPF₆ in EC with FEC. At a current density of 1.0 mA cm⁻², the cell employing the FEC solvent achieved an average CE of 98.3%, significantly higher than that observed for the cell with EC solvent (75.3%). In Li‖NCM cell tests at 0.5 C and 3.0~4.3 V, the protected Li exhibited 68.2% capacity retention after 120 cycles, compared to only 19.1% in the EC system.

Li et al. fabricated a 2.5 μm thick lithiated Nafion/lithium salt interface (NLI) via a straightforward dip-casting method [109]. The soft, lithiated Nafion polymer provided rapid ion-transport pathways and conformal interfacial contact, whereas the rigid Li salt enhanced the interfacial stability and mechanical modulus to prevent dendrite growth during cell cycling. At a low current density of 1.0 mAh cm⁻² (Figure 5a), the coin cell with the bare Cu electrode exhibited a short lifespan of only 120 cycles and an average CE of 97.0%. Subsequently, owing to the irreversible detachment of the fragile dendrites, the CE decreased sharply to 65.5% over the next 20 cycles. In contrast, the NLI-modified Li‖Cu cells demonstrated an extended lifespan of 250 cycles with a stable CE of 98.1%, indicating the effective suppression of dendritic Li growth through surface protection. Even at a high current density of 3.0 mA cm⁻² (Figure 5b), the cell with NLI maintained a stable CE of approximately 96% for over 110 cycles. Owing to its high ionic conductivity and stability, the NLI layer enabled the symmetric Li cell to operate at a high current density of 8 mA cm⁻². The NLI-coated electrodes exhibited extended cycling stability in cells configured with various positive electrode materials, including Li₄Ti₅O₁₂, LFP, and sulfur-based cathodes, demonstrating enhanced performance and durability. Zhu et al. developed a bilayer SEI consisting of alternating layers of graphene-based materials (graphene and h-BN) and inorganic components (LiF, Li₂O, Li₃N, Li₂CO₃) [93]. The graphene layer imparted mechanical robustness, allowing it to accommodate and redistribute stress induced by irregular Li deposition. The inorganic layer prevented corrosion of the graphene layer by the electrolyte and covered the surface of Li metal, ensuring enhanced stability [110,111]. Li₃Sb functions as a fast Li-ion conducting phase and contributes to interfacial stabilization, while LiF serves as an electronic barrier, suppressing electron leakage from the Li metal to the SEI. Liu et al. [112] developed an organic/inorganic composite protective layer consisting of poly(propylene carbonate) (PPC), LiTFSI, and Li₃N, which was fabricated via a controllable spin-coating process to improve the stability and performance of LMBs. The selection of Li₃N as the inorganic filler was motivated by its high Li⁺ ionic conductivity (~10⁻³ S cm⁻¹), high mechanical modulus, and thermodynamic stability, whereas PPC was chosen for its polar carbonate groups that facilitate salt dissociation and Li-ion transport [113]. The resulting composite film exhibited a thickness of ~5 μm and achieved a maximum ionic conductivity of 3.57 × 10⁻⁴ S cm⁻¹. They demonstrated that the PPC-Li₃N-LiTFSI (PLNL)-coated Li-negative electrode significantly reduced side reactions and enabled a Li–composite film cell to retain a high discharge capacity (940 mAh g⁻¹ at 0.1 C) and remarkable storage performance over 60 days at both room temperature and 55 °C. The PLNL@Li‖LFP cell (positive electrode loading ~1.6 mg cm⁻²) was assembled using 1 M LiTFSI with 1 wt% LiNO₃ in DOL/DME (1:1 v/v) as the electrolyte. These results confirm that the composite film not only stabilizes the SEI through the formation of a LiF/Li₃N-rich layer but also improves interfacial compatibility and long-term retention under harsh storage conditions. Yang et al. [114] proposed a novel inorganic/organic hybrid protective layer synthesized by ionic layer epitaxy (ILE) to achieve uniform Li deposition and extended cycling performance. The protective coating, comprising a LiF-rich inorganic matrix interlaced with SiO₂ domains, was directly grown on the Li surface under mild conditions. This hybrid interlayer served as an ion-conductive, electron-blocking scaffold, enabling uniform Li⁺ flux and suppressing dendritic growth. An electrochemical evaluation of Li‖Cu and Li‖LFP cells with 1 M LiTFSI in DOL/DME (1:1 v/v) + LiNO₃ demonstrated stable plating/stripping for over 500 cycles at a current density of 1 mA cm⁻², with CE consistently exceeding 99%. This approach highlights the effectiveness of ILE-grown inorganic interfaces in achieving large-area, dendrite-free Li-metal-negative electrodes for next-generation batteries.

3.2. Structural Design of Current Collectors for Lithium Metal

The current collector is a key component that significantly affects the Li-negative electrode. Its surface energy, roughness, and lithiophilicity influence the nucleation barrier and the spatial distribution of Li nuclei. These factors determine the initial deposition pattern, which strongly impacts subsequent Li growth. Most current collectors used in Li batteries are planar, such as conventional Li foil [115]. When Li is initially plated onto these planar current collectors, the Li particles are unevenly deposited, facilitating the subsequent growth of dendrites on the deposited Li particles [115,116]. Additionally, the volumetric fluctuations resulting from the “hostless” nature of the LMB present a significant challenge for the practical application of LMBs. The hostless LMB undergoes substantial volume changes during the deposition and stripping processes, leading to a lower CE and making it difficult to form a stable SEI. Therefore, the rational development of a current collector host for Li metal is required. The primary objectives in designing the current collector are to enhance the electrochemical stability through uniform Li deposition and stripping, suppress dendrite growth, and mitigate the volume changes of Li metal during charging and discharging. Conventional current collectors for negative electrodes typically use planar Cu foils. However, the performance can be optimized by developing current collectors with porous structures or modification with hybrid materials, carbon-based materials, or metal alloy collectors. Although the aforementioned protective layers can effectively suppress dendrite formation, they face limitations in accommodating the volume expansion during cycling, thus highlighting the necessity of designing current collectors for LMBs, specifically 3D host structures, to mitigate volume expansion and ensure stable performance. Several approaches can be employed in the design of current collectors. First, using porous structures, such as a 3D Cu mesh, can significantly increase the surface area while reducing the localized current density, thus facilitating uniform Li plating [117]. Second, introducing lithiophilic materials such as silver, zinc oxide, and gold nanoparticles (NPs) may be beneficial. Standard current collectors, such as copper or carbon, exhibit a low affinity for Li, leading to high nucleation overpotentials and non-uniform Li deposition. Incorporating lithiophilic materials can mitigate these issues and enable uniform Li plating. Finally, a 3D framework can be constructed using carbon-based materials such as graphene, carbon nanotubes, and carbon fibers. These structures provide lightweight characteristics and high conductivity, significantly enhancing the performance of Li-metal electrodes.

3.2.1. Porous Copper Current Collectors

Commercial copper foil is commonly used in the industry as a current collector owing to its electrical conductivity, chemical stability, mechanical strength, and cost-effectiveness. As mentioned, most current collectors used in Li batteries, such as conventional copper and Li foil, are flat. When Li is initially plated onto a flat current collector, it tends to be deposited as uneven particles, which then act as nucleation sites for dendrite growth. This irregular deposition promotes the formation of Li dendrites, leading to instability and potential safety issues during cycling [115]. Furthermore, the functional lithiophobicity of planar Cu surfaces originates not from the intrinsic chemistry of copper but from its smooth morphology, which lacks energetically favorable sites for Li nucleation. This results in high nucleation overpotentials and promotes inhomogeneous Li-ion flux distributions [117,118]. In contrast, 3D Cu architectures—by increasing surface area, edge density, and defect concentration—effectively reduce the nucleation barrier and exhibit behavior that is characteristic of lithiophilic surfaces [119,120].

Li et al. conducted galvanostatic analyses on both symmetric and Li‖Li₄Ti₅O₁₂ cells with a porous copper mesh, revealing that the electrode enhanced the cell lifespan and stabilized the cycling behavior [117]. In conventional, flat current collectors, irregular features such as rough surfaces and fine structures along the Cu surface can lead to an uneven charge distribution, causing a concentration of Li ions near the edges of the Cu foil that can accelerate local deposition (Figure 6a). The newly formed Li deposits increase the surface roughness, promoting further formation of dendritic structures via the same mechanism. In addition, such uneven surfaces often lead to unstable SEI formation, which accelerates dendrite growth. In contrast, as shown in Figure 6b, using a 3D copper mesh as a current collector effectively suppresses the formation of Li dendrites, enabling stable Li deposition. Moreover, Figure 7 presents SEM-based evidence for how a 3D Cu (M400)/Li composite structure buffers volumetric expansion during prolonged cycling. Cross-sectional images show that while a bare Li electrode swells irregularly over 100 cycles, the 3D host structure maintains a uniform thickness and stable morphology. The enhanced performance of the Cu-mesh current collector compared with that of the planar current collector can be attributed to four main reasons: (1) The porous structure facilitates faster charge transfer and minimizes resistance at the electrode/electrolyte interface. (2) The large specific surface area lowers the local current density compared to that of bare Li metal. (3) The 3D host structure accommodates Li growth and mitigates volume changes during plating/stripping. (4) This process minimizes thickness variations in the negative electrode during cycling, thereby alleviating internal stress and stabilizing the electrode/electrolyte interface.

Yang et al. demonstrated a 3D current collector with a submicron skeleton and porous structure, which could significantly alter the plating behavior of Li [115]. When a porous copper foil was used as the 3D current collector, Li grew within the submicron Cu framework, filling the pores of the structure [45]. Figure 8a shows the formation of dendrites when Li was plated on a conventional flat Cu current collector, whereas Figure 8b illustrates Li plating on a porous current collector. The 3D current collector was fabricated as follows: Firstly, a flat Cu foil was immersed in an ammonia solution to facilitate the self-assembled deposition of Cu(OH)₂. After dehydration, Cu(OH)₂ converted to CuO, which was then thermally reduced to metallic Cu by annealing in a hydrogen-containing (Ar/H₂) atmosphere at elevated temperatures (300~400 °C), resulting in a porous Cu structure [121]. The plating/stripping of Li on the 3D Cu foil led to excellent cycling stability with minimal voltage fluctuations, with no short-circuiting observed even after 600 h of cycling, indicating significantly delayed growth of dendritic Li. This phenomenon is attributed to the increased surface area of porous copper relative to flat Cu foil. The enlarged electroactive surface facilitated broader contact between lithium and the electrolyte, resulting in a lower local current density and improved charge transfer kinetics during cycling. Unlike the fluctuating Coulombic efficiency (70–100%) observed on flat Cu foil—caused by unstable dendritic growth and poor interfacial contact—the porous Cu with its submicron architecture maintained a more consistent CE of around 97% over 50 cycles at 0.5 mA cm⁻². This improved stability is advantageous for reducing hysteresis and promoting low-voltage polarization during the discharge/charge cycles in LMBs. Numerous studies have demonstrated that employing micro- or nanostructured current collectors increases the electroactive surface area, thereby reducing the effective current density and promoting uniform Li plating and stripping during cell cycling [115,122]. Lu et al. [123] reported the rational design of a free-standing Cu nanowire (CuNW) membrane that accommodated Li metal within a 3D nanostructure, effectively suppressing the growth of dendritic Li. In traditional, flat Cu current collectors, the Li-ion flux tends to concentrate at the crack sites of the SEI. During Li plating, this concentrated ion flux accelerates Li growth at these cracks, leading to rapid dendrite formation (Figure 8c). In contrast, the interconnected CuNW membrane with its high surface area significantly reduced the ion flux density and improved the homogeneity of the Li-ion flux distribution (Figure 8d), promoting uniform Li plating on the porous collector. Additionally, even if Li dendrites were occasionally formed on the Li-CuNW composite, these structures tended to merge within the network, forming bulk Li before puncturing the separator. The CuNW-based LMB exhibited a high CE (average of 98.6% over 200 cycles) and demonstrated excellent rate performance owing to the suppressed growth of Li dendrites and the high conductivity of the CuNW membrane [123].

Wang et al. developed a 3D copper framework with open, micrometer-sized pores fabricated via a NaCl-assisted powder sintering process [124]. One of the challenges of traditional 3D porous copper structures is that the low porosity of the current collector can limit the diffusion paths for Li-ions, causing some Li to be preferentially deposited in the upper sections near the separator [123]. When the top surface of the 3D current collector is covered, the Li⁺ diffusion pathways may be obstructed by previously deposited Li. However, the 3D Cu framework developed in the aforementioned study featured a high surface area and an open, porous structure, which effectively dispersed the Li-ion flux, provided ample space for Li deposition, and helped mitigate volumetric changes. The Li deposited within the 3D Cu framework had a smooth, spherical shape rather than the dendritic, needle-like structures, which played a crucial role in suppressing dendrite growth and extending the lifespan of the LMB. As a result, the 3D Cu framework demonstrated significant advantages in enhancing the cycling efficiency of LMBs, where the CE was maintained above 95% for 700 cycles at a cycling current density of 0.5 mA cm⁻² and for 400 cycles at 1 mA cm⁻² (Figure 8e).

Additionally, stabilized LMBs were obtained by controlling the Li plating/stripping in vertically aligned microchannels [125]. As the strength of the electric field increases, ions are more readily adsorbed at the tip regions, a phenomenon commonly referred to as the “tip effect”. The tip effect plays a crucial role in modulating the electric field and facilitating ion orientation—it is essential for achieving uniform metal deposition. Concentrating the electric field at the tip of the electrode enhances the localization of the ionic species, promoting their directed movement toward the electrode surface. This process aids in the coherent arrangement of ions, leading to more consistent and uniform metal electroplating [110,126,127,128]. The tip effect guided the preferential deposition of Li ions along the microchannel walls, effectively suppressing the growth of Li dendrites. Consequently, the Li‖LFP cells exhibited excellent rate performance and cycling stability. The assembly of porous Cu current collectors with an LFP positive electrode resulted in a high capacity retention of 90% after 100 cycles. In contrast, the flat Cu-negative electrodes exhibited 80% capacity retention. This significant difference underscores the advantages of the porous structure in suppressing the formation of Li dendrites during cycling. The increased surface area and improved ion transport pathways in porous Cu contributed to more stable Li deposition, enhancing the overall battery performance. Liu et al. recently proposed a scalable and time-efficient method to fabricate porous Cu–Cu_xO heterostructured current collectors using a flashlight irradiation technique, wherein intense pulsed light rapidly sinters Cu(OH)₂ nanowires grown on Cu foil to form a porous, oxide-rich surface layer [129]. This irradiation-induced thermal reduction process, which occurs within seconds, enables uniform formation of nanostructured features without damaging the bulk Cu foil, offering scalability for industrial applications. The resulting 3D Cu–Cu_xO framework effectively promotes uniform Li nucleation by reducing local current density and increasing surface area. Symmetric Li‖Li cells were assembled using ether-based electrolytes (1 M LiTFSI in DOL/DME with 2 wt% LiNO₃) and demonstrated stable Li plating/stripping over 700 h at a current density of 1 mA cm⁻² with a fixed capacity of 1 mAh cm⁻², indicating robust suppression of dendritic growth and improved interfacial stability compared to planar Cu electrodes.

3.2.2. Effects of Incorporating Lithiophilic Materials

Incorporating lithiophilic sites into Cu current collectors has emerged as a promising strategy for stabilizing LMBs by promoting uniform Li deposition and mitigating volume fluctuations [130]. As a lithiophobic substrate, Cu presents a significant nucleation overpotential for Li deposition owing to the pronounced thermodynamic mismatch between Li and the Cu surface. A promising strategy for overcoming this heterogeneous nucleation barrier involves introducing lithiophilic materials into the Cu current collector. Lithiophilic materials, which lack an overpotential, act as nucleation sites that selectively attract Li metal, thereby facilitating uniform nucleation at the designated seed points. These materials help facilitate Li nucleation primarily due to the partial solubility of lithium in them, which allows Li atoms to diffuse into the substrate before forming a distinct Li phase, thereby lowering the nucleation overpotential [46]. This mechanism enables uniform Li deposition and stripping during cycling, effectively suppressing dendrite growth and promoting stable interfacial behavior [49]. Studies on the Li nucleation overpotentials of materials such as Au, Ag, Zn, Mg, Cu, C, and Sn have shown distinct differences [97,118,122]. Metals such as Au, Ag, Zn, and Mg, which exhibit partial solubility in Li, uniquely enable Li to dissolve into their surfaces, effectively mitigating the nucleation barrier before forming a pure-Li phase. In contrast, materials insoluble in lithium, such as Cu, C, and Sn, exhibit finite overpotentials that persist throughout the deposition process and contribute to heterogeneous lithium nucleation. For Cu, the overpotential is approximately 30 mV, whereas Sn and C, both of which form Li alloy phases, have lower overpotentials of 16 mV and 14 mV, respectively [97]. Incorporating lithiophilic materials significantly improves the stability of LMBs by promoting uniform Li nucleation and reducing dendrite growth. This approach effectively lowers the nucleation overpotential, leading to enhanced interfacial stability and prolonged cycle life in LMBs.

Chen et al. fabricated a Cu current collector with 3D pillar morphology, deposited an ultrathin ZnO layer on the surface of the current collector via atomic layer deposition, and analyzed the formation of dendrites as a function of the pillar thickness and spacing [122]. Figure 9a illustrates the synthesis of the vertically aligned Cu pillar arrays. Copper was electrochemically deposited into uniform cylindrical pores defined by a porous polycarbonate track-etched membrane, enabling precise control over pore dimensions. Figure 9b–d show the SEM images of the Cu pillar arrays with diameters of 10, 2, and 0.2 µm, respectively. Cross-sectional views revealed that the Cu pillars were vertically aligned and uniformly distributed on the planar Cu substrate. A ZnO coating layer enhanced the uniformity of lithium nucleation, thereby impacting both the morphology and reversibility of subsequent Li plating and stripping processes [131,132]. Adjusting the geometric parameters of the 3D Cu current collector—including pillar diameter, spacing, and height—enabled control over lithium morphology during cycling, contributing to improved electrochemical performance. At a current density of 1 mA cm⁻², the CE reached 95.7% for the planar Cu current collector, while 10 µm, 2 µm, and 0.2 µm Cu pillar structures yielded efficiencies of 96.7%, 97.7%, and 97.1%, respectively (Figure 9e). Additionally, the uniform deposition of spherical Li with a consistent size and coverage across the entire electrode surface was observed in the 2 µm Cu pillar array at a lithium plating density of 0.5 mAh cm⁻².

Au NPs were introduced into hollow carbon spheres to find an appropriate host for LMBs [97]. Considering Au has a lithium nucleation overpotential of 0 V, lithium metal preferentially nucleates at the Au seed sites rather than at the amorphous carbon (with a higher nucleation overpotential of 14 mV). As a result, Li nucleation occurred more readily on the Au NPs within the carbon nanoshells than on the shell exterior. This favorable nucleation within the nanoshell lowered the thermodynamic barrier for Li growth and effectively suppressed the formation of dendrites during cycling. Nanoencapsulation of Li metal within the hollow carbon shells significantly reduced direct contact between Li and the electrolyte, enhancing the stability of Li during electrochemical cycling and minimizing side reactions. The AuNP-loaded nanocapsules demonstrated improved cycling performance, maintaining a CE of 98% for over 300 cycles in alkyl carbonate electrolytes. Cui et al. developed a straightforward and commercially viable strategy for the mass production of Ag-NP-modified copper foil through a simple and eco-friendly galvanic replacement reaction between pure Cu and Ag⁺ [118]. The modification was achieved by uniformly distributing Ag NPs on the surface of Cu foil by directly spraying an AgNO₃ solution. The Ag NPs increased the uniformity of the electric field and ion distribution on the surface and served as seeds for depositing Li metal because of the strong affinity between Ag and Li, facilitating uniform Li deposition on the planar Cu substrate (denoted as Cu-Ag). At a current density of 1.0 mA cm⁻², the Ag-Cu collector enabled a significant increase in the CE from 93.36% in the first plating cycle to 98.48% by the fifth cycle, maintaining a high CE of 98.54% even after 600 cycles. This low capacity loss was attributed to the restoration of the SEI layer over the surface of the Ag-Cu electrode. Additionally, the CE remained at approximately 99.72% at a current density of 2.0 mA cm⁻² and approximately 98.23% at 3.0 mA cm⁻². The Cu-Ag current collector exhibited a consistent deposition morphology even at a high Li capacity of 5.0 mAh cm⁻². In the Li‖CuAg@Li cell, the Li-plated Cu-Ag collector demonstrated stable performance for over 800, 900, and 500 h at low-voltage polarizations of 0.5, 1.0, and 3.0 mA cm⁻², respectively. Additionally, the Cu-Ag@Li‖LiNi_0.8Co_0.1Mn_0.1O₂ configuration exhibited higher capacity retention compared to the Li‖LiNi_0.8Co_0.1Mn_0.1O₂ counterpart. Liu et al. proposed a method of stabilizing LMBs by creating a uniform Sn layer on the surface of carbon foam (CF) through pre-electroplating to form Sn-coated CF (SCF) [133]. This Sn layer enhanced the lithiophilicity of the structure, effectively preventing non-uniform deposition caused by the “tip effect”. The synergistic effect of the porous structure and the lithiophilic Sn layer resulted in strong compatibility with the LMB. Figure 9f–k present SEM images illustrating Li deposition on CF and SCF at a current density of 1 mA cm⁻². Figure 9f shows the CF surface after depositing 0.2 mAh cm⁻² of Li, where granular Li deposition was observed. When the capacity was increased to 0.5 mAh cm⁻² (Figure 9g) and 1.0 mAh cm⁻² (Figure 9h), Li growth and aggregation within the CF were observed, driven by the low initial lithiophilicity of Cu and the concentration of the electric field. This aggregation reflects the accumulation of Li⁺ ions at the original nucleation sites, leading to dense clusters with increased deposition. In contrast, after Li deposition at 0.2, 0.5, and 1.0 mAh cm⁻², the SCF surface (Figure 9i–k) remained smooth and uniform. This stability results from the high lithiophilicity of the Sn layer, which mitigates the effect of the accumulation of Li⁺ ions at the initial nucleation points. The SCF symmetric cell demonstrated a longer lifespan compared to that of pristine CF. At a current density of 1 mA cm⁻² and a capacity of 1 mAh cm⁻², the CE of the SCF was stably maintained above 98% after 550 cycles. The Li@SCF ‖LFP cell delivered a satisfactory capacity of 117.1 mAh g⁻¹ at 1 C after 130 cycles. In addition, the electroplating solution used for Sn deposition could be recycled with minimal contamination during the preparation process, thereby reducing energy costs and preventing environmental pollution. Zhao et al. engineered a lithiophilic copper current collector by thermally inducing a Zn-Cu alloy layer on the Cu surface, thereby creating nucleation sites that facilitate uniform Li plating and reduce dendrite formation [134]. The Zn was electrochemically deposited onto commercial Cu foil and annealed at 300 °C, forming an alloy layer with a strong affinity for Li metal. This modification significantly reduced the nucleation overpotential and improved interfacial uniformity. Li‖Cu cells assembled with 1 M LiTFSI in DOL/DME and 1 wt% LiNO₃ as the electrolyte demonstrated stable plating/stripping for over 400 h at 1 mA cm⁻² with 1 mAh cm⁻², while maintaining a high CE and smooth surface morphology. The results confirm that the incorporation of Zn-Cu alloy sites can serve as an effective strategy to enhance the lithiophilicity and interfacial stability of Cu current collectors in LMBs.

3.2.3. 3D Carbon-Based Frameworks

The “hostless” nature of LMBs leads to Li plating and accumulation on the negative electrode surface during cycling, resulting in significant volume changes that cause SEI breakdown and degradation of the electrochemical performance. A stable plating framework is essential for achieving a high capacity and long cycle life for LMBs. Numerous studies have demonstrated that 3D-structured current collectors with porous characteristics and high surface areas present a promising host structure for accommodating Li deposition [115,117,122,123,124,125]. However, the high mass of traditional host structures often limits improvements in the energy density [135]. Lightweight carbon-based frameworks can mitigate these issues, offering a more affordable and lightweight 3D current collector that promotes uniform Li nucleation and growth while enhancing energy density and cycling stability.

Conductive CF is considered a suitable matrix for LMBs owing to its high electrical conductivity and porosity, where the latter facilitates the storage of metallic Li. However, the low lithiophilicity of CF frameworks makes preloading Li via molten infusion methods challenging. To address this issue, Jhang et al. electroplated Ag onto a CF framework and synthesized lithiophilic Ag particles with adjustable sizes, which enhanced the Li affinity and facilitated uniform Li deposition [136]. Introducing Ag also resulted in stable Li plating and stripping during charge and discharge cycles [136,137,138]. The ultrafine Ag NPs uniformly fixed on the carbon nanofibers reduced the nucleation overpotential of Li and induced uniform Li deposition in the 3D carbon matrix. This 3D host demonstrated low overpotentials and facilitated the reversible operation of the LMBs. These heterogeneous seeds effectively induced Li deposition and achieved spatial control over Li nucleation. Figure 10a illustrates the synthesis of CF/Ag-Li, which involved two main steps: Ag coating and Li infusion. As shown in the SEM image of Figure 10b, the CF structure, with a uniform pore diameter of 10 µm, provides ample space for high-capacity lithium storage and low resistance for Li⁺ transport. Galvanostatic cycling was performed in a Li‖Li symmetric cell to assess the long-term cycling stability of the CF/Ag-Li electrode. At a current density of 1.0 mA cm⁻² with a plating/stripping capacity of 1.0 mAh cm⁻², the Li‖Li cells exhibited increasing hysteresis up to 80 mV over 70 cycles, whereas the Li‖CF/Ag-Li cells maintained a lower, stable hysteresis of 60 mV for over 200 cycles. Theoretical calculations indicate that matrices with N-containing functional groups, such as pyridinic and pyrrolic nitrogen, undergo stronger interactions with Li atoms, thereby promoting uniform Li nucleation [139]. Lin et al. designed N-doped graphitic carbon foams (NGCFs) to enhance the long-term stability of LMBs [135]. The well-dispersed nitrogen-containing functional groups on the NGCF created a low nucleation overpotential, promoting uniform Li nucleation and NP growth during deposition. Considering these initial Li seeds guide subsequent Li growth, nitrogen functional groups effectively inhibited the formation of dendritic Li by ensuring controlled and homogeneous deposition. Furthermore, the extensive surface area of the 3D skeleton resulted in a reduced local current density, thereby promoting uniform Li growth. Figure 10c illustrates Li deposition on the NGCF, which differs significantly from that on the conventional Cu current collector. Leveraging the advantages of Li growth, electrochemical evaluations revealed a high CE, large capacity, extended lifespan, and low overpotential. At a high current density of 2 mA cm⁻² and capacity of 2 mAh cm⁻², NGCF achieved a remarkably high and stable CE of approximately 99.6% over 300 cycles, indicating excellent cycling stability and efficient Li plating/stripping, even under demanding conditions.

Gan et al. fabricated a fluorine-enriched, nitrogen-doped, hollow carbon sphere-decorated carbon fiber (FNCS@CF) skeleton [140]. LiF, one of the components of the SEI, was analyzed using density functional theory (DFT), with the results demonstrating that the LiF-rich SEI layer can effectively block Li dendrites [141]. Various fluorinated electrolytes and substances form LiF in SEI layers [142]. Fabricating an LMB with a LiF-rich SEI layer improved the CE and extended the lifespan of the LMB [143]. FNCS@CF effectively integrated uniformly distributed lithiophilic sites and a mechanically robust Li-ion-rich SEI, imparting durability and dendrite-free characteristics to the LMB composite [140]. The FNCS@CF skeleton demonstrated excellent electrochemical performance, with an exceptionally high average CE of 99.6% over 240 cycles at 3 mAh cm⁻² and remarkable cycling stability over 1300 h with a low deposition overpotential of 10 mV.

To clearly delineate the functional differences and relative merits of each approach,

Table 2. Comparative summary of the protective layer and current collector strategies in LMBs. Performance is evaluated based on dendrite suppression, volume accommodation, cycle stability, and fabrication complexity. CE values refer to representative tests using symmetric cells or Li-metal cells paired with various positive electrode materials, as reported in the literature.

Strategy	Dendrite Suppression	Volume Accommodation	Cycling Stability	Fabrication Complexity	Key Limitations
Polymer-based protective layer	Moderate	Moderate	CE ~ 98% for 200–500 cycles @ 1–2 mA cm−2	Low	Limited ionic conductivity; thermal instability
Inorganic-based protective layer	High	Low	CE > 98% for 300–800 cycles @ 1–3 mA cm−2	Moderate	Brittle; prone to cracking and interfacial mismatch
Composite protective layer	High	Moderately High	CE > 98% for >600 cycles @ 1–5 mA cm−2	High	Complex synthesis; integration challenges
Porous copper current collector	Moderate	Moderate	CE ~ 97–98% for 200–500 cycles @ 1–2 mA cm−2	Moderate	Requires precise 3D architecture control
Effect of incorporating lithiophilic materials	High	Low	CE > 98% for 300–700 cycles @ 1–3 mA cm−2	High	Costly materials; surface treatment complexity
Carbon-based frameworks	High	High	CE > 98% for >1000 cycles @ 1–5 mA cm−2	Moderately high	Requires tailored interface design and conductivity balance

provides a comparative overview of protective layer strategies and current collector modifications discussed in this review. This summary highlights their respective capabilities in dendrite suppression, volume accommodation, and long-term cycling stability, along with fabrication considerations and common limitations.

4. Conclusions

The expanding electric vehicle market necessitates the development of high energy density and high power batteries, positioning LMBs as a promising next-generation energy storage solution. However, the unstable interface of LMBs restricts their safety and longevity, highlighting the critical need for interface stabilization to enable commercial viability. This review explored strategies to stabilize the LMB interface, focusing on protective layers and current collector designs. Protective layers, categorized as polymer-based, inorganic-based, and composite, each offer unique benefits, such as enhanced flexibility, structural robustness, and synergistic properties that improve interfacial stability. Current collector designs, including porous copper structures, lithiophilic material incorporation, and 3D carbon frameworks, mitigate nucleation barriers, accommodate volume changes, and support uniform lithium deposition over extended cycling. Both approaches address distinct failure modes and contribute to overall interface stabilization through different mechanisms. By examining these strategies individually, this review highlights functional design principles such as ionic conductivity, mechanical modulus, chemical compatibility, and lithiophilicity—factors that are essential for achieving uniform Li plating, stable SEI formation, and long-term cycling performance. Compared to existing reviews that often address either the interfacial layer or the current collector in isolation, this work presents a broader design perspective emphasizing the independent contributions of both strategies.

Despite these advances, challenges such as material compatibility, mechanical durability, and energy density optimization persist. Among the most critical remaining barriers are interfacial instability under high current densities and poor compatibility with practical positive electrodes, both of which hinder the commercial viability of LMBs. Future research should prioritize the development of adaptive artificial interphases that can respond dynamically during cycling, as well as multifunctional current collectors capable of strain absorption and guided ion transport. Advancements in either the protective layer materials or current collector engineering can individually enhance interfacial stability, and their continued refinement remains essential for realizing high-performance lithium-metal batteries. By bridging interfacial and structural design strategies, this review provides a design-oriented roadmap to guide future innovations in practical LMB systems.