Hard Carbon as Anodes for Potassium-Ion Batteries: Developments and Prospects

Abstract

Potassium-ion batteries (PIBs) are regarded as a potential substitute for LIBs owing to the benefits of potassium’s abundance, low cost, and high safety. Nonetheless, the practical implementation of potassium-ion batteries still encounters numerous challenges, with the selection and design of anode materials standing out as a key factor impeding their progress. Hard carbon, characterized by its amorphous structure, high specific surface area, and well-developed pore structure, facilitates the insertion/extraction of potassium ions, demonstrating excellent rate performance and cycling stability. This review synthesizes the recent advancements in hard carbon materials utilized in PIB anodes, with a particular focus on the potassium storage mechanism, electrochemical properties, and modification strategies of hard carbon. Ultimately, we present a summary of the current challenges and future development directions of hard carbon materials, with the objective of providing a reference for the design and optimization of hard carbon materials for PIBs.

1. Introduction

In the twenty-first century, the development of innovative renewable energy sources is crucial given the two urgent problems it faces: the energy crisis, and environmental issues [1,2,3]. Relying for an extended period on traditional energy sources has led to the depletion of resources, pollution of the environment, and the release of greenhouse gases, spurring a shift towards cleaner and renewable energy sources such as solar, wind, and hydrogen power [4,5,6,7,8]. Nevertheless, these clean energy sources are constrained by geographical limitations and intermittent energy outputs, necessitating their integration with energy storage devices for the storage and redistribution of energy [9,10,11,12,13]. Among all energy storage technologies, rechargeable batteries are increasingly garnering attention as efficient energy storage solutions [14,15,16]. Within the realm of battery systems, lithium-ion batteries (LIBs), with high energy density, have advanced swiftly, becoming essential energy storage devices in our daily lives [17,18,19]. They are utilized in a variety of applications, ranging from handheld devices to electric cars and intelligent grids for storing energy [20,21,22]. However, the escalating cost of LIBs, attributed to limited lithium resources, poses a challenge, restricting their widespread deployment in energy storage systems [23,24,25,26]. Therefore, it is imperative to develop economically efficient energy storage batteries.

PIBs operate using a ‘rocking chair’ mechanism similar to LIBs, with K⁺ moving between the cathode and anode as the battery charges and discharges [27,28,29]. PIBs offer various benefits over LIBs, including the fact that potassium is evenly spread throughout the Earth’s crust at a rate of 2.09 wt%, whereas lithium is only present at 0.0017 wt% [30,31]. Furthermore, the typical electrode potential of K/K⁺ (−2.71 V) is similar to that of Li/Li⁺ (−3.04 V), resulting in a greater energy density for PIBs [32,33]. Additionally, the size of the K⁺ in organic solvents is smaller than that of Li⁺, leading to faster transport kinetics of K⁺ in the electrolyte [34,35,36]. Because potassium and aluminum do not form an alloy at low pressures, using aluminum foil instead of copper foil as the anode material collector in PIBs can significantly reduce costs. These advantages have prompted a considerable interest in PIBs among researchers [37]. Nevertheless, the relatively large radius of K⁺ presents several challenges, including electrode volume expansion, diminished cycling stability, and constrained energy and power densities. This presents a significant challenge in the manufacturing of high-performance anode materials.

To date, researchers have investigated a range of anode substances for PIBs, such as alloy materials, metal oxides, metal–organic frameworks, and carbon-based materials [38]. Carbon-based materials, including hard carbon [39,40,41], soft carbon [42], graphite, and graphene [43,44], are highly regarded as potential anode options for potassium-ion batteries because of their affordability, eco-friendliness, and superior performance. Graphite is commonly used as a negative electrode in PIBs. In 2015, the research team led by Ji illustrated the potential utility of a layered graphite structure for K⁺ intercalation, resulting in the development of potassium–graphite intercalation compounds (KC₈) with a theoretical specific capacity of 279 mAh g⁻¹ [45,46,47]. When potassium ions are inserted between graphite layers, the large size of the potassium ion causes a volumetric expansion of over 60%, leading to permanent structural harm and seriously affecting the performance of the electrode [48,49,50,51]. Consequently, the investigation of innovative anode materials exhibiting exceptional electrochemical characteristics remains a pivotal area of focus within the domain of PIB research.

Hard carbon is not graphitizable at high temperatures and is sourced from diverse materials such as resin carbon, pyrolysis carbon from organic polymers, carbon black, and biomass-derived carbon [29,52,53,54]. HC comprises micrographene sheets, amorphous regions with defects, and abundant micropores, enabling the accommodation of K⁺ with a large radius [55,56,57,58]. When used as an anode material in PIBs, its disordered arrangement and wider interlayer spacing than that of graphite help to reduce volume fluctuations during insertion and extraction of K⁺ [59]. The OMC synthesized by Wang et al. [60] exhibited a mere 7% alteration in volume during the processes of potassication and depotassication. This results in a higher specific capacity and long-lasting cycling stability. While the electrochemical behavior of HC, commonly employed in LIBs, offers insights into K⁺ storage in HC, differences exist [61,62,63]. Although a ramp region akin to lithium-ion batteries appears in PIB charge/discharge curves, a distinct plateau is absent. The prevailing theory suggests two K⁺ storage mechanisms: diffusion-controlled embedding, and surface-driven adsorption [23,36,64,65]. Notably, the adsorption-driven pseudocapacitive behavior contributes to HC materials exceeding theoretical capacities. Nonetheless, challenges like low initial Coulombic efficiency and poor conductivity persist in HC’s application for PIBs [66,67,68]. Researchers have investigated various alterations, such as elemental doping, biomass derivation, and composites, with the aim of enhancing the electrochemical properties of rigid carbon.

This article offers insight into the current research on the storage of potassium ions in hard carbon materials and presents some of the most recent applications of hard carbon in PIBs. Furthermore, it delves into the impact of various modification techniques on the performance of hard carbon anodes, including heteroatom doping, biomass derivatization, composite materials, and structural engineering (Figure 1). In conclusion, this article presents a summary of the obstacles and potential future developments for the ongoing progress of hard carbon anodes.

2. Potassium Storage Mechanism

An investigation into the lithium and sodium storage mechanisms of hard carbon anodes may facilitate a more comprehensive analysis of the storage mechanisms of potassium ions in hard carbon. In general, alkali metal ions are stored in hard carbon through three primary mechanisms: intercalation, capacitive adsorption, and nanopore filling [69,70]. There are similarities in the storage of alkali metal ions at high potentials, where lithium ions are mainly adsorbed at defects, edges, or functional groups, and the capacity is mainly provided by the high-potential tilted region, but the pore filling contributes less to the lithium ion storage capacity. Unlike the storage mechanism of lithium ions, the capacity of sodium-ion batteries is mainly provided by pore filling, except for adsorption in the high-potential tilted region, which forms a potential plateau region below 0.1 V [71]. Unlike lithium and sodium storage, the low-voltage platform in potassium storage is as high as 0.25 V; based on the varying storage sites of K⁺, the storage mechanisms can be classified into the intercalation mechanism and the adsorption mechanism [72,73]. Specifically, the intercalation mechanism entails the deposition of potassium ions within the interstitial gaps of the carbon layers, whereas the adsorption mechanism operates by adsorbing K⁺ in the flawed areas and on the surface. A comprehensive investigation into the electrochemical behavior and storage mechanism of hard carbon materials in PIBs is crucial for advancing the practical utilization of PIBs.

2.1. Interpolation Mechanism

Graphite typically exhibits a layered structure, comprising multiple layers of graphite arranged in an ABAB order. In contrast, hard carbon has a non-graphitizable structure consisting mainly of graphite-like microcrystals randomly distributed during high-temperature pyrolysis. Owing to the unique microscopic morphology of hard carbon, its K⁺ intercalation mechanism differs from that of traditional graphite. The localized intercalation reaction predominantly occurs within the graphite-like microcrystals of hard carbon during the process of potassication.

The microstructure of hard carbons exerts a pronounced influence on the storage mechanism of potassium ions and, as a consequence, on their electrochemical properties. Xu et al. [74] synthesized hard carbons with different structural variations at temperatures ranging from 800 to 2900 °C. In Figure 2a, the precursor RF undergoes aromatization and condensation at 800 °C, resulting in a highly disordered structure rich in defects and pores, which enhances K⁺ adsorption. Upon pyrolysis in the range of 1000 to 1500 °C, the formation and growth of graphene layers lead to the carbonization of amorphous domains into partially ordered pseudo-graphitic domains. At this stage, both adsorption and intercalation mechanisms coexist, resulting in the optimal electrochemical performance. With further pyrolysis in the range of 1800 to 2900 °C, graphene layers continue to grow and stack in an orderly fashion, resulting in highly ordered graphite-like microcrystals. At this juncture, potassium ions are predominantly incorporated into the graphite-like microcrystalline layers, which exhibit suboptimal rate performance and stability properties.

When large potassium ions are inserted into graphitized microcrystals, the material’s microstructure expands. Consequently, the K⁺ embedding process can be indirectly inferred by monitoring the change in layer spacing. Zhu’s team [75] investigated the K⁺ embedding process utilizing in situ XRD (Figure 2b). In the high-potential ramp region (stage I), the diffraction peaks exhibited no significant movement or intensity variation. During the discharge process, slight splitting of diffraction peaks was observed in the low-potential plateau region (stage II), along with some peaks shifting towards smaller angles and the appearance of new diffraction peaks. The presence of potassium ions in the graphite microcrystals was believed to cause a decrease in Bragg diffraction and spacing. Moreover, the introduction of potassium ions led to a rise in the electron density on the graphite microcrystalline layer, keeping I_G consistent before the diffusion period but causing a significant shift after reaching 0.2 V. This change was also associated with a marked reduction in the I_D/I_G ratio of the CMGL. It could be inferred that the insertion of potassium ions between graphite microcrystalline layers in the low-potential plateau region of CMGL did not result in the formation of a segmented intercalation compound structure. Instead, the observed phenomenon conformed to the single-phase intercalation mechanism.

The intercalation behavior of potassium ions in hard carbon materials is significantly different from that previously documented for graphite-based materials. The repeated insertion and detachment of K⁺ does not result in the formation or disappearance of new phases. In graphite anodes, the intercalation of K⁺ typically leads to the creation of GICs. However, research indicates that hard carbon materials may not establish stable GICs, and the development of GICs is linked to the level of graphitization in hard carbon. Kim’s team [76] investigated the potassium ion storage mechanism by systematically adjusting the degree of local graphitization of hard carbon. In the process of adding potassium to the less crystallized DGC-1600, the wide peak of disordered graphite (002) expands as K ions are incorporated, and then it contracts during the removal of potassium, suggesting a one-step intercalation reaction (Figure 2c). Conversely, the DGC-2800 specimen displayed characteristic two-phase intercalation behavior, creating two-phase intercalation compounds KC₄₈ (phase IV), KC₃₆ (phase III), and KC₂₄ (phase II) due to its well-organized local graphite structures (Figure 2d). Due to the presence of these complex local microstructures, K⁺ preferentially inserts into the graphite lattice, leading to the absence of the high-specific-capacity first-stage compound KC₈.

The precise mechanism of K⁺ intercalation in hard carbon remains a topic of contention within the scientific community. Nevertheless, it is evident that the storage of potassium ions entails solid-phase diffusion, resulting in substantial alterations in the volume of hard carbon materials during the charging and discharging processes. Therefore, strategies such as modulating the microstructure of hard carbon, shortening the diffusion path of K⁺, and increasing the carbon layer spacing are crucial for enhancing the electrochemical performance of hard carbon.

2.2. Adsorption Mechanism

Hard carbon materials exhibit diverse microstructures and morphologies, which significantly influence the potassium storage mechanism. The intercalation mechanism belongs to the diffusion-controlled Faraday process, and the electrode exhibits inadequate rate and cycling performance due to the excessive radius of potassium ions and the limitation of the lamellar structure. Conversely, surface adsorption involves the adsorption of potassium ions on defects, surfaces, and nanopores of hard carbon materials, representing a surface-driven capacitive process. This mechanism preserves the structural integrity of the electrode material and facilitates rapid storage and release of potassium ions compared to the intercalation mechanism, enhancing kinetic processes. Hence, analyzing the capacitive contribution is crucial for understanding differences in electrochemical performance arising from distinct storage behaviors. Cyclic voltammetry is commonly used to assess the storage of potassium ions; the most accurate results can only be obtained in a three-electrode cell [77,78]. Measurement of the chemical diffusion coefficient using cyclic voltammetry satisfies equation $I_P=2.69\times 10^5{n}^{1/2}A(D_K{)}^{1/2}{\nu }^{1/2}\mathsf{\Delta }{C}_0$, where I_P is the magnitude of the peak current, n is the number of electrons involved in the reaction, A is the area of the electrode immersed in the solution, F is Faraday’s constant, D_K is the diffusion coefficient of K in the electrode, v is the scanning rate, and ΔC₀ is the change in the concentration to be measured before and after the reaction. The power-law relationship between the peak current (i) and the scan rate (v) is usually utilized to evaluate the pseudocapacitance contribution: $i=a{v}^b$, where a and b are adjustable constants. The value of the power exponent b can be found by graphing the logarithm of i against the logarithm of v, where b = 1 signifies a pseudocapacitive process primarily driven by surface phenomena, while b = 0.5 indicates an intercalation process primarily controlled by diffusion processes. Moreover, the contributions of diffusion and pseudocapacitance to specific capacity can be assessed using the equation $i=k_{1}v+k_2{v}_{1/2}$, where $k_{1}v$ and $k_2{v}_{1/2}$ represent capacitance-controlled and diffusion-controlled contributions to specific capacity, respectively.

Past research has indicated that the disparities in the insertion and adsorption processes can be identified through the charge/discharge graphs. The embedding process usually displays a plateau at low potential, whereas the capacitive behavior demonstrates a sloping curve at higher potential, with the majority of its specific capacity coming from this region. Cheng et al. [79] synthesized bio-derived carbon (SP-HC) from peppercorns through high-temperature pyrolysis, resulting in a charge/discharge curve that was separated into two different sections: (I) the high-potential ramp area and (II) the low-potential plateau area. The hard carbon produced at 1000 °C (SP-HC-1000) exhibited an amorphous structure. As the pyrolysis temperature increased, the proportion of graphite microcrystals in the hard carbon gradually rose, with SP-HC-1200 presenting a partially ordered structure. In zone I (the slope region), a portion of K⁺ was adsorbed onto active surface sites. Upon potential reduction to 0.4 V, K⁺ readily infiltrated the gaps created by the stacking structure of microcrystalline graphite (Figure 3a,b). The synergistic effect of the intercalation and adsorption mechanisms resulted in the high reversible specific capacity and stable cycling performance of SP-HC-1200 even at high current densities.

More defects can be introduced by heteroatom doping to increase the adsorption sites for K⁺, resulting in higher capacitance contributions. Lu et al. [80] used sodium citrate and thiourea as raw materials. After a simple mixed carbonation process, Na⁺ was converted to Na₂CO₃ and acted as a template for the precursor. Following washing with deionized water to remove the template, NOSHC-101-500-1 exhibited a well-developed porous structure (Figure 3c). The S and N co-doping resulted in a high quantity of imperfections and empty spaces, significantly enhancing the absorption of potassium ions. Capacitive control driven by the surface was its main mode of potassium ion storage, with a capacitive contribution of 71.6% at a scan rate of 0.5 mV S⁻¹. However, an overabundance of heteroatom doping resulted in a rise in sp³ defects, which hinder rapid electron transmission. At a rate of 0.5 mV S⁻¹, the capacitance impact of NOSHC-11-500-1, which had an increased thiourea proportion, was merely 55.5% (Figure 3d,e). The discrepancy in capacitance contribution had a discernible impact on the electrochemical performance, as illustrated in Figure 3f,g. NOSHC-101-500-1 displayed exemplary rate capability and unwavering cycling performance, providing 125 mAh g⁻¹ at 5 A g⁻¹ and retaining a specific capacity of 210 mAh g⁻¹ after 1000 cycles at a current density of 1 A g⁻¹. Hence, accurate management of heteroatom doping levels and distribution allows for efficient control of the structural characteristics and electrochemical attributes of the substances.

3. Rational Design

In light of the insights into potassium storage mechanisms previously discussed, it is of paramount importance to strategically design carbon materials into a multitude of configurations, with the objective of enhancing their electrochemical performance. Research design strategies can be broadly categorized into four types: (1) heteroatom doping, (2) biomass-derived inheritance of their original microstructures with in situ heteroatom introduction, (3) carbon composites to achieve synergistic effects and (4) structural engineering to enhance electrochemical performance and ICE.

3.1. Heteroatom Doping

Introducing heteroatom doping is a potent approach to enhance the electrochemical characteristics of hard carbon. Embedding heteroatoms can substantially augment the active sites for potassium ion storage [81,82]. Furthermore, adding larger atoms like sulfur and phosphorus can widen the gaps between carbon layers, making it easier for potassium ions to be stored and released, ultimately boosting the storage capacity of hard carbon anodes for potassium. As a result, numerous studies have focused on improving electrochemical efficiency by introducing heteroatoms, which can be mainly divided into the following methods: single-atom doping and double-atom doping.

3.1.1. Single-Atom Doping

N, P, and S are commonly used as doping elements to enhance electrochemical properties by increasing active sites, improving conductivity, and enlarging interlayer distances [83]. N doping was initially investigated for K⁺ storage due to the abundant presence of nitrogen on Earth. Nitrogen can be incorporated into the honeycomb lattice through sp² hybridization, providing more electrically active sites and improving the local electronic structure as an electron donor. Additionally, the increased Fermi energy level can enhance K⁺ adsorption to some extent [84].

Li and colleagues [85] created nitrogen-doped hard carbon (NPC) with a nitrogen content of 6.88 atomic percent through a straightforward self-templating technique. The nitrogen-rich doping and highly developed pore structure endowed the NPC with excellent electrochemical properties. As shown in Figure 4a, the carbon layer spacing expanded from 0.335 nm to 0.383 nm following the N-doping process. The increase in the mean layer distance facilitated the rapid insertion and removal of K⁺ and mitigated the expansion of volume during the electrode’s cycling. To elucidate the enhancement of NPC’s electrochemical performance following N doping, they calculated the binding energies of different N-doped active sites with K⁺ using DFT. As shown in Figure 4b, it was demonstrated that the pyridine nitrogen and pyrrole nitrogen sites had lower adsorption energies compared to the pristine sites, which increased the adsorption ability for potassium ions and, consequently, enhanced the rate performance of the NPC electrode. Additionally, the DOS of different carbon structures was investigated (Figure 4c). Carbon with defects and nitrogen doping exhibited enhanced DOS near the Fermi energy level, indicating superior electronic conductivity compared to pristine graphene layers. Additionally, they computed the adsorption energy of nitrogen sites in the graphitic structure of the graphene layer, which are rich in electrons. Therefore, increasing the contents of pyridine nitrogen and pyrrole nitrogen can help achieve high-performance PIBs. Chu et al. [86] created carbon nanosheets (ENCN-600) with a high concentration of edge nitrogen (pyridine/pyrrole nitrogen), at 88.36%. The presence of edge nitrogen increased the lattice spacing of carbon nanosheets, thereby accelerating K⁺ diffusion and improving K⁺ intercalation capacity. Moreover, the introduction of nitrogen at the edge created additional sites for K⁺ adsorption, synergistically leading to the excellent rate performance and remarkable cycling stability of the ENCN-600. Yuan and colleagues [87] created ENC-850, a nitrogen-doped porous carbon material, through the direct pyrolysis of self-assembled precursors consisting of terephthalic acid (BDC) and melamine (MA). The stability of the triazine structure in MA compared to BDC results in a greater N-doping content in the carbon material. Additionally, the decomposition of the S-triazine structure releases NCNH₂ gas, which positions the majority of doped N atoms in an edge-N configuration. Furthermore, the released gas creates a substantial number of micropores, which not only provide active sites but also increase the spacing between carbon layers.

Nevertheless, nitrogen doping is inadequate for increasing the layer spacing. The presence of certain highly electronegative heteroatoms, such as sulfur and phosphorus, causes distortions in the graphite lattice, resulting in an increase in the distance between its layers. Cheng et al. [88] used polystyrene and sulfur as precursors to create synthesized hard carbon with high sulfur doping, reaching a sulfur content of 27.05 at.%. The sulfur doping increased the electrical conductivity and introduced numerous active sites, which facilitated potassium ion adsorption. Moreover, the increased distance between layers (0.382 nm) in SHC-3 allowed for easier insertion of potassium ions and successfully prevented volume expansion, ultimately preserving the structural stability throughout the potassication/depotassication process. Kim’s group [89] reported 1.1 at.% P-doped hard carbon for PIBs. The main chemical bond in phosphorus-doped hard carbon is the -P-O bond, which leads to an increase in the distance between graphitic carbon layers from 0.375 nm to 0.387 nm. Additionally, P doping maintains a high ICE of the electrodes while enhancing the specific capacity of the low-potential platform.

3.1.2. Dual-Atom Doping

On occasion, the incorporation of a single heteroatom may prove inadequate in meeting the comprehensive performance criteria associated with anode materials. The introduction of two heteroatoms within the carbon matrix, which allows for a synergistic interplay between the two, has been demonstrated to significantly enhance the electrochemical performance of PIBs. The main types of dual-atom doping methods currently available include N/O co-doping [90,91,92], N/S co-doping [32,93], and N/P co-doping [94] as the primary forms. These approaches have been extensively studied for their ability to enhance active sites, conductivity, and stability.

Chong’s group [95] employed a combination of soft templates and hydrothermal methods to synthesize NO-YS-CSs. The NO-YS-CSs had a developed pore structure and high specific area, which is conducive to alleviating the volume expansion of the electrode during the charging and discharging process. The synergistic doping of nitrogen and oxygen heteroatoms not only helps to enhance the adsorption and embedding ability of the material for potassium ions, it also effectively promotes electron conduction. Cui’s group [92] successfully prepared a new type of nitrogen–oxygen-co-doped hard carbon materials (NOHCs) by using the pith tissue of sorghum straw as the raw material and carbonization treatment. NOHCs-800 has a high reversible specific capacity and excellent cycling stability. The excellent electrochemical performance of NOHCs can be attributed to the following aspects: The introduction of elemental N and O through doping creates heteroatomic and edge defects, which, in turn, add more active sites and effectively expand the layer spacing, ultimately decreasing the volume change following K⁺ embedding. Additionally, nitrogen and oxygen can create various functional groups like pyridine nitrogen, pyrrole nitrogen, graphite nitrogen, and C-OH hydroxyl groups, which improve the surface wetting properties of the electrode material and boost conductivity by speeding up electron diffusion.

Sulfur–nitrogen double doping is another widely used synergistic heteroatom doping method. Liu’s team [93] produced S/N-co-doped hard carbon SNHCs with nitrogen and sulfur contents of 4.16 wt% and 1.81 wt%, respectively, utilizing cost-effective sulfur and polyacrylonitrile as precursors through a pyrolysis process. Due to the S/N co-doping, the SNHC has a graded porous structure with plenty of defects and active sites, which is beneficial for storing potassium ions. Thus, SNHC electrodes exhibit excellent rate and cycling performance. Tao’s team [96] synthesized a S-doped nitrogen-rich hard carbon (S-NC); S was introduced into the polymer by the reaction between sodium thiosulfate and dilute hydrochloric acid, and the S and N contents of the resulting hard carbon could reach 12.9 at.% and 9.9 at.%, respectively. The nitrogen-containing groups have the ability to add more imperfections and reactive sites, ultimately improving the electrode’s reversible specific capacity. Additionally, the introduction of sulfur increases the amount of graphitic nitrogen (N-Q), leading to improved electronic conductivity in the hard carbon and facilitating charge transfer [97]. Zhang’s team [98] designed a S/N-co-doped three-dimensional (3D) flower-like hard carbon for PIBs. The 3D flower-shaped configuration offers numerous surface active sites and serves as a highly conductive pathway for electron transport. Introducing S can widen the layer gaps, facilitating the incorporation of K⁺ into the carbon layer and reducing the electrode’s volume expansion during cycling (Figure 5a). S groups have lower ΔEa compared to N groups, which improves the adsorption capacity of potassium ions (Figure 5b).

Expanding the spacing of the hard carbon layer is the most efficient way to speed up K⁺ diffusion and ensure structural stability. This can be achieved through the introduction of P doping, which significantly expands the interlayer spacing. As illustrated in Figure 5c, Chen et al. [99] developed an innovative N/P-co-doped hollow bowl-shaped hard carbon. The introduction of P enlarged the layer spacing of the bowl-shaped carbon to 0.375 nm, and the large layer spacing was favorable for rapid K⁺ migration, while N doping could effectively improve the K⁺ adsorption capacity and conductivity of the electrode. As shown in Figure 5d,e, the synergistic effect of N/P co-doping resulted in the N/P-HPCB having a high specific capacity (providing 458.3 mAh g⁻¹ after 100 cycles at 0.1 A g⁻¹) and an excellent cycling performance (exhibiting 205.2 mAh g⁻¹ after 1000 cycles at 2 A g⁻¹).

Additionally, the incorporation of P doping has been demonstrated to influence the nitrogen conformation. Jin et al. [100] engineered hollow porous carbon spheres with a high proportion of edge nitrogen through structural manipulation and P/N co-doping. Following P doping, the nitrogen content in the N/P-HPCS increased from 2.94% to 4.24%, and the edge nitrogen fraction rose from 80.6% to 85.6%. These edge-nitrogen-dominated groups endowed the electrode with exceptional K⁺ adsorption capacity. In addition, DFT showed that P atom doping is able to reduce the adsorption activation energy, and the ΔEa values of N5/P and N6/P are lower than those of N5 and N6, which are −3.03 eV and −3.07 eV, respectively (Figure 6a). Figure 6b,e show the densities of states for different nitrogen configurations, and the N/P-HPCS with and without K⁺ adsorption shows higher DOS near the Fermi energy level, indicating that additional P doping improves the electronic conductivity of the material. These results confirm that the ratio of active edge nitrogen can be tuned by P doping to improve the K⁺ adsorption capacity and electronic conductivity of N/P-HPCS.

In addition, other diatom-doped hard carbon materials have also been developed as anodes for PIBs. Chen’s research group [101] developed an F/O double-doped porous carbon for PIBs. Through density functional theory (DFT) calculations, they found that the F/O double-doping strategy not only improves the adsorption capacity of potassium ions but also induces only slight structural distortion while adsorbing multiple potassium ions. This F/O double-doped porous carbon exhibits high specific capacity, excellent rate performance, and remarkable cycling stability. Guo’s group [40] prepared S/O-co-doped porous hard carbon microspheres (PCMs). S/O co-doping facilitates the enhancement of K⁺ adsorption capacity and also mitigates structural distortion during the potassium embedding/de-embedding process. As a result, PCMs exhibit high specific capacity and excellent rate performance. Zhang et al. [102] synthesized an I/N-co-doped hard carbon material. They investigated both I₃- and I₅- configurations adsorbed on the carbon layer surface. Iodine doping increased the layer spacing, while I/N co-doping lowered the K⁺ adsorption energy (−4.00 eV), thus promoting K⁺ adsorption.

Table 1. Recently reported hard carbon anode materials for PIBs.

Materials	ICE (%)	Rate Capability (mAh g −1)	Cycling Stability [Specific Capacity (mAh g−1) (Cycle Number) @Current Density (A g−1)]	Reference
Single-atom doping for carbon materials
High N-doped carbon	35.7	111.8 at 10 A g −1	203.9 (8500) @1	[103]
Edge-N doped carbon	46.8	217.9 at 2 A g −1	189.9 (2000) @2	[87]
N-doped hard carbon microspheres	53	93 at 2 A g −1	201.6 (600) @0.2	[104]
N-doped porous hard carbon	20.04	146 at 1.4 A g −1	193.1 (1000) @0.28	[105]
Highly graphitic nitrogen-doped hard carbon	65	118 at 2 A g −1	176 (260) @1	[106]
P-doped hard carbon	44.4	81.6 at 2 A g −1	72.6 (1000) @1	[107]
S-doped hard carbon	35.12	229 at 2 A g −1	220 (5000) @2	[108]
S-doped hollow porous carbon spheres	36.25	210 at 2 A g −1	211.4 (1000) @1	[109]
S-doped hard carbon	48.2	86.1 at 10 A g −1	128.2 (1500) @2	[110]
High S-doped hard carbon	42	162 at 10 A g −1	159 (1500) @2	[111]
S-doped carbon nanorods	—	43 at 5 A g −1	81 (1000) @1	[112]
High S-doped hard carbon	47.3	216 at 1 A g −1	188.9 (1000) @0.5	[113]
Dual-atom doping for carbon materials
N/O-co-doped yolk–shell carbon spheres	31	183.3 at 1 A g −1	189.2 (2500) @0.5	[95]
N/O-co-doped biomass carbon	77.74	220.5 at 5 A g −1	124.19 (5000) @10	[114]
N/O dual-doped hard carbon	50.7	178.9 at 5 A g −1	189.5 (5000) @10	[92]
N/O-co-doped porous hard carbon nanobelts	49	235 at 1.6 A g −1	277 (1600) @1	[115]
N/O-co-doped volcanic rock-like carbon	—	103 at 4 A g −1	81 (4000) @2	[116]
Hollow porous N/P-co-doped carbon spheres	20	193 at 4 A g −1	137.6 (1500) @2	[100]
N/P-co-doped hard carbon	44	179 at 5 A g −1	138 (2000) @2	[117]
N/P-co-doped hard carbon	—	98 at 2 A g −1	154.2 (200) @0.1	[118]
N/P dual-doped hollow porous bowl-like hard carbon	58.3	213.6 at 4 A g −1	205.2 (1000) @2	[99]
N/P-cofunctionalized porous carbon monoliths	63.6	168 at 5 A g −1	218 (3000) @1	[119]
N/S-co-doped hard carbon	60	90 at 2 A g −1	268 (2400) @0.1	[120]
Hollow stalagmite-like N/S-doped carbon	50	256 at 1 A g −1	148 (1000) @1	[121]

summarizes the important parameters recently reported for hard-carbon-based PIBs, including first-time Coulombic efficiency, rate performance, and long-cycle performance.

3.2. Biomass-Derived Carbon

Environmentally friendly and renewable biomass-derived materials are an important source of hard carbon. Through simple pyrolysis treatment, the carbonized biomass precursors inherit their original special biological structures, such as spherical, fibrous, caged, and reticulated, which provide natural transport channels for K⁺. Meanwhile, their high specific surface area and hierarchical porous structure can effectively promote the migration and diffusion of K⁺. In addition, their internal heteroatoms, such as N, P, S, etc., can be directly introduced in situ in the carbon matrix through carbonization, providing additional active sites to enhance potassium ion adsorption.

The majority of plants contain fiber and sugar, making them suitable carbon sources for application as anodes in potassium-ion batteries. Qian et al. [122] prepared carbon microspheres (OCMSs) with the same orientation using a high-temperature hydrothermal method, with cedar as the raw material. Figure 7a shows a diagram of the formation mechanism of OCMSs. In the process of hydrothermal treatment at high temperatures, hemicellulose, cellulose, and lignin break down in order and create OCMSs oriented towards (002) under elevated pressure (Figure 7b). The graphite layers are positioned towards the middle of the sphere, thus shortening the transport path of potassium ions. As a result, OCMS anodes have excellent rate performance and superior cycling stability. The presence of microscopic pore structures helps mitigate volume expansion and maintain the stability of the carbon structure. Scientists are now focusing on unique biomass substances that have intricate pore structures within them. Gao’s team [58] synthesized nitrogen-rich biomass carbon (N-CHC) from hemp culm cores. The dense honeycomb porous structure of N-CHC (Figure 7c) is a natural channel for the transport of K⁺, and its large specific surface area of 1185.3 m² g⁻¹ facilitates the penetration of electrolytes. In addition, the nitrogen content of N-CHC is as high as 8.56%. Therefore, N-CHC has excellent potassium storage properties.

Animal biomass contains a more complex composition than plant biomass, with derived materials such as chitosan, chitin, and animal protein. Many scholars have attempted to use various protein-rich biomasses to synthesize derived carbon materials. The team of Li [123] used a mixed biomass of glucose and ovalbumin as the precursor and ultimately formed multi-heteroatom-doped flower-like hierarchical porous carbon microspheres (NPS-FCMs) through the polymerization and thermal decomposition of biomolecules. The highly disordered structure and N, P, and S doping enabled NPS-FCM to have a larger lattice spacing (0.41 nm) and more active sites, which are favorable for electrolyte penetration and potassium ion transport (Figure 7d). In situ Raman spectroscopy showed (Figure 7e) that the I_D/I_G ratio gradually increased at the beginning of the discharge process, attributed to adsorption at the surface heteroatom/defect sites. With the deepening of the discharge process, the I_D/I_G ratio increased sharply at potential below 0.5 V, which was attributed to the expansion of the carbon lattice due to the embedding/porosity filling of K⁺ and the increase in the disorder of the hard carbon. The NPS-FCM electrode features a hybrid potassium storage mechanism involving both adsorption and intercalation, resulting in a high ICE of 56.1%, excellent rate performance, and stable cycling performance. Chitin is a linear polysaccharide polymer and the second-largest biomass material on Earth, after cellulose. It is cheap, renewable, and contains natural nitrogen-containing functional groups. Zhang’s team [115] synthesized N/O-co-doped hard carbon nanoribbons (NOCNB) by pyrolysis, using discarded lobster shells as raw materials. The natural CaCO₃ nanoparticles within the lobster shells act as self-templates to obtain a robust and porous hard carbon skeleton. After HCl etching, the NOCNB exhibits a three-dimensionally interconnected nanoribbon morphology, mainly consisting of aligned hollow carbon nanotubes (Figure 7f,g). The in situ doping of N and O atoms in the carbon skeleton introduces additional active sites, extending the spacing of the carbon layers to 0.4 nm, which facilitates K⁺ adsorption and alleviates electrode volume expansion. As a result, NOCNB has excellent long-cycle stability (retaining a specific capacity of 277 mAh g⁻¹ after 1600 cycles at 1 A g⁻¹). Deacetylation of chitin can extract chitosan, which possesses free amino groups and is the only natural alkaline polysaccharide. Wang’s team [124] developed a flexible biomass carbon film from chitosan (CS). CS-1000 has a porous honeycomb structure with a N-doping content of 6.3%. This flexible carbon film can be directly used as a negative electrode for PIBs without the need for a collector, organic binder, or additional conductive agents (Figure 7h). Due to its abundant N doping and high conductivity, the CS-1000 has stable cycling performance and high reversible specific capacity.

3.3. Carbon Composite Materials

A single hard carbon material is often unable to achieve the comprehensive performance of PIBs. Soft carbon has high conductivity (5.7 S m⁻¹) and low discharge potential, but its cycling stability is poor. The disordered structure of hard carbon produces abundant active sites with long-term cycling stability. However, its low conductivity (1.7 S m⁻¹) and high operating potential reduce the battery’s energy density [125]. The advantages of hard and soft carbon are complementary, so combining the two is an effective strategy for improving battery performance. Jian et al. [125] mixed spherical hard carbon (HCS) obtained by hydrothermalization of sucrose and soft carbon (SC) obtained by pyrolysis of PTCDA through mechanical ball-milling to obtain the soft and hard carbon composite HCS-SC (Figure 8a–c). As shown in Figure 8d–f, HCS-SC exhibited the highest charge specific capacity of 261 mAh g⁻¹ compared to HCS and SC. Thanks to the combination of soft and hard carbon, HCS-SC offers the best multiplier performance and stable cycle life. Nevertheless, the physical method exhibits limited mixing homogeneity and partial separation of components, leading to an incomplete utilization of the synergistic effect between hard carbon and soft carbon. Xu et al. [126] used PTCDA and glucose as SC and HC precursors, respectively. After dissolution in water, acid-catalyzed esterification was used to achieve the binding of precursors at the molecular level. The prepared CHC anodes exhibited a strong synergistic effect as the precursor molecules were immobilized by chemical bonds and HC was uniformly distributed in the SC matrix. This method effectively enhances the diffusion kinetics of K⁺, which improves the cycling stability of the electrode (only 0.078% specific capacity decay per cycle after 500 cycles at 1 A g⁻¹) while increasing the rate performance (it exhibits a high specific capacity of 121 mAh g⁻¹ at 3.2 A g⁻¹).

In addition, coating hard carbon materials on metal-based particles is also a new way to enhance the performance of PIBs. Liu’s team [127] synthesized Bi@N-doped carbon composites (SPB@NC). As shown in Figure 8g, the potassium ion storage in SPB@NC is dominated by the alloying/de-alloying reaction of Bi-K₂Bi-K₃Bi₂-K₃Bi, and the N-doped carbon layer can keep the electrode stable and accelerate the electron transfer. The special interconnection structure, sufficient internal voids, and fast electron transfer channels endow the SPB@NC electrode with excellent potassium storage performance.

3.4. Structural Engineering

The large specific surface area and abundant defects of hard carbon tend to lead to irreversible depletion of the electrolyte and, thus, exhibit low ICE. Structural engineering optimization of hard carbon materials, such as constructing hierarchical pore structures and increasing the carbon layer spacing, can not only enhance the initial Coulombic efficiency of PIBs but also improve the long-cycle and multiplicity performance. Li et al. [128] synthesized porous hard carbon (KC) with adjustable layer spacing by an in situ templating method. The rich pore structure of KC accelerated the transport of potassium ions and improved the electrical conductivity of hard carbon (Figure 9a). In particular, with the expansion of the layer spacing, the carbon layer can accommodate more potassium ions embedded in it, and the energy consumed by potassium ions embedded in the carbon layer for the formation of KC₈ is reduced, so the KC electrodes have faster diffusion kinetics and, thus, exhibit excellent multiplicity performance (Figure 9b). Yuan et al. [129] successfully synthesized carbon spheres with a hierarchical pore structure (AHCS) using a chemical activation method (Figure 9c). As shown in Figure 9d, the micropores not only facilitate the penetration of electrolytes but also provide more active sites for K⁺ adsorption, bringing a greater capacitive contribution. The abundant mesopores of AHC were able to accommodate the insertion of more potassium ions and achieve a high ICE (49.6%). Hwang’s [130] team prepared hard carbon materials with open nano–mesopores as anodes for PIBs. The oe-MCS open mesoporous channels enhanced the K⁺ adsorption capacity and reduced the K⁺ diffusion length. As a result, the oe-MCS anode exhibited a higher reversible specific capacity and had a greater capacitance control contribution and lower charge-transfer resistance. Meso-C synthesized by Guo et al. [131] using a self-templating method had short-range ordered mesopores, and the specific surface area of Meso-C was reduced by a factor of 31.5, to 22.4 m²g⁻¹. It thus had an extremely high ICE (76.7%), much higher than that of Micro-C (31.1%) and other hard carbon anodes reported in the literature. Tan et al. [132] synthesized CS-HC from coconut shells with enlarged layer spacing and small specific surface area, which effectively reduced the side reactions of the electrolyte; thus, it exhibited an ultra-high ICE of 87.32%. The pre-potassication method can significantly enhance the reversible cycling and ICE of the boosted electrodes. Nam et al. synthesized a-CB using a pre-potassication method [133] to provide additional potassium ions to replenish the potassium ions lost from the initial cycling; a-CB exhibited 120.7% ICE, high multiplicity capacity, and significant cycling stability.

4. Conclusions and Prospects

Researchers have shown great interest in hard carbon as an anode material for PIBs, due to its exceptional electrochemical performance and adjustable structural characteristics. Although the utilization of hard carbon in PIBs is advancing rapidly, a range of challenges persist, and the optimization of its performance remains imperative for commercialization. Potassium ions are mainly stored in hard carbon through intercalation within graphite-like microcrystals and adsorption on the material’s surface, defects, and nanopores. These mechanisms are intertwined and influence each other. The intercalation mechanism establishes stabilized potassium storage sites within hard carbon, whereas the adsorption mechanism introduces additional pseudocapacitance contributions, allowing hard carbon to surpass its theoretical specific capacity. Nevertheless, the objective of ongoing research is to enhance the stability and cycle life of the material while preserving a high specific capacity.

As shown in Figure 10, to optimize the electrochemical properties of hard carbon, fine-tuning is required in several aspects. Firstly, moderate heteroatom doping can increase the defects and active sites of the material, promoting the adsorption and transport of potassium ions. However, excessive doping may block the conductive network and reduce the material’s conductivity. Therefore, it is necessary to balance the positive and negative effects of heteroatom doping to find the optimal doping species and content. Secondly, a large specific surface area is favorable for the rapid adsorption and desorption of potassium ions, shortening the ion diffusion path and improving the rate performance of the material. However, an excessively large surface area may also lead to a decrease in first-time Coulombic efficiency and an increase in electrolyte consumption. Meanwhile, an extensive pore structure may weaken the mechanical strength of the material and trigger structural collapse. Hence, precise regulation of the pore structure of hard carbon is essential to establish a well-organized hierarchical pore space. This ensures the structural integrity of the electrode material while preserving the pathways for ion transport. The low first-time Coulombic efficiency is another key issue limiting the development of potassium-ion batteries. This mainly stems from the irreversible depletion of potassium ions during the first embedding process and the decomposition of the electrolyte on the material surface. In order to improve the first-time Coulombic efficiency, pre-potassication treatment can be adopted to compensate the irreversible loss of the first cycle by introducing potassium ions in advance. In addition, ether-based electrolytes have a more stable solid electrolyte interfacial film than ester-based electrolyte systems, resulting in higher charge/discharge capacity and higher ICE. Optimization of the electrolyte formulation and rational design of the interfacial film can not only inhibit the continuous decomposition of the electrolyte but also promote the reversible embedding/de-embedding of potassium ions and improve the cycling stability of the material.

With its excellent performance and tunability, hard carbon is expected to stand out among many anode materials. In the future, in-depth investigation of the potassium storage mechanism of hard carbon materials through theoretical calculations and advanced characterization techniques, optimization of the heteroatom doping strategies, precise tuning of its microstructure, and enhancements in the stability of the electrode–electrolyte interface are anticipated to bolster the practical implementation of PIBs based on hard carbon.