Hard Carbon Anodes for Sodium‐Ion Batteries: Industrial Precursors, Structural Regulation, and Future Prospects

ABSTRACT

Hard carbon is widely regarded as the most promising anode material for commercializing sodium‐ion batteries. However, the successful industrial application of hard carbon requires not only the proper selection of cost‐effective precursors but also precise regulation of the structures. This review aims to provide a comprehensive summary of advanced hard carbons by using commercial raw materials as precursors and the corresponding structural engineering strategies for the industrialization process. Specifically, we systematically summarize three representative and industrially feasible precursors—(resins, biomass, and needle coke)—with particular emphasis on their origins, synthesis pathways, structural optimization, and corresponding sodium‐ion storage performance. In addition, we discuss the remaining challenges associated with the large‐scale deployment of hard carbon and provide perspectives on future directions, including economic efficiency through low‐cost precursors, environmentally friendly synthetic routes, and the implementation of recyclable manufacturing technologies. This review is expected to offer clear insights and practical guidance for the rational design and preparation of commercial hard carbon anode materials.

This review systematically summarizes three representative and industrially feasible precursors (resin, biomass, and needle coke), including their origins, synthesis routes, structural optimization, and sodium‐ion storage performance. Furthermore, we address the lingering challenges in large‐scale applications and outline prospects for future development. This review is expected to offer clear insights and practical guidance for the rational design and preparation of commercial hard carbon anode materials.

1. Introduction

With the increasing demand for renewable energy and large‐scale energy storage, sodium‐ion batteries (SIBs) have attracted considerable attention as a promising complementary technology to current lithium‐ion systems. SIBs are particularly suitable for grid‐scale electrical storage and low‐speed electric vehicles due to their resource abundance, cost‐effectiveness, and long‐term sustainability [1, 2]. Sodium exhibits a significantly higher crustal abundance (2.3%) compared to lithium (0.006%), while its homogeneous distribution effectively mitigates the supply chain risks associated with lithium's resource scarcity [3, 4]. This inherent resource advantage, coupled with the similar physicochemical properties between SIBs and lithium‐ion batteries (LIBs), makes SIBs a practical complement to existing energy storage technologies and alleviates the pressure on lithium resources in large‐scale applications [5, 6].

The commercial application of SIBs largely depends on the development of high–performance and low‐cost electrode materials, which directly govern key performance metrics, such as energy density, cycling life, rate capability, and overall cost [7]. Over the past decade, cathode materials have received considerable attention in SIBs research, which play a key role in not only determining the overall electrochemical performance of SIBs but also take a large percentage of the total cell cost [8]. Consequently, extensive research has focused on developing cathode materials with optimized compositions and tailored microstructures. These efforts have led to a range of materials with mature synthesis methods and stable electrochemical performance, including layered oxides [9, 10], polyanionic compounds [11, 12], and Prussian blue analogues [13, 14], et al. These extensive research achievements have been comprehensively reviewed in recent review articles, further advancing the development of cathode materials.

With the gradual maturation of cathode materials, research focus has increasingly shifted toward anode materials, with the hope of further reducing costs and improving performance. Specifically, improving the reversible capacity, initial Coulombic efficiency (ICE), and rate capability of anode materials are critical for boosting the energy density and cycling life of SIBs, thereby addressing remaining bottlenecks in commercialization [15, 16]. To address these challenges, researchers have explored diverse anode materials for SIBs, including alloys [17], carbonaceous [18], and organic materials [19, 20], et al. Among these candidates, hard carbon (HC) has attracted particular attention as the most promising and commercially viable anode material. This is mainly attributed to its abundant and low–cost precursors as well as its high specific capacity, which has spurred increasing interest in its design, synthesis, and performance optimization [21, 22]. These features make HC particularly attractive for large‐scale applications where cost–effectiveness and performance scalability are paramount. The commercialization of HC anode materials hinges on the selection of suitable precursors, which must balance electrochemical performance, resource availability, cost–effectiveness, facile synthesis processes, and scalability [4]. These factors ensure that HC can be produced at scale while maintaining consistent electrochemical performance, which is essential for commercial SIBs applications. A variety of production routes for manufacturing HC anodes are currently being explored, with unique advantages and challenges. Among the diverse precursors explored, three primary precursors have emerged as frontrunners due to their inherent properties and suitability for industrial‐scale manufacturing processes (Figure 1).

FIGURE 1.

Commercial HC precursors of resin, biomass, and needle coke with industrial advantages.

Resin–derived precursors, such as phenolic resins and epoxy resins, have also garnered attention for their well‐defined molecular structures and industrial processability [23, 24, 25]. The crosslinked networks in these resins undergo predictable aromatization and heteroatom removal during pyrolysis, enabling the synthesis of HC with uniform grain sizes and pore distributions [23]. Such control is particularly beneficial for ensuring reproducible electrochemical performance. Recent studies on phenolic resin–derived HC have demonstrated this advantage by optimizing pyrolysis temperatures [26]. Furthermore, the functionalization of resin precursors through the addition of specific chemical groups provides additional pathways for tuning surface chemistry properties and Na⁺ storage kinetics [27].

Biomass‐derived precursors, including agricultural wastes [28, 29], lignocellulosic [30, 31], and bamboo [32, 33], are particularly favorable due to their abundant raw materials, affordability, and environmental friendliness. The natural hierarchical structure and intrinsic heteroatom doping, such as oxygen and nitrogen, can be transformed into HC with enhanced ion diffusion pathways and Na⁺ storage locations, leading to performance enhancement [34, 35]. Recent progress in pre‐treatment methods, like acid washing and hydrothermal carbonization, has significantly improved the reproducibility of biomass–derived HC by minimizing composition variations [36, 37]. For instance, acid washing can effectively remove inorganic impurities, while hydrothermal carbonization can precisely control the pore size distribution, which is critical for ensuring consistent electrochemical performance in industrial production.

Needle coke, a traditional precursor in graphite production, has been repurposed for HC synthesis due to its highly oriented graphite structure and distinct layered structure. The graphitizable properties of needle coke endow the resulting HC with excellent electrical conductivity and mechanical strength, which are critical for enhancing the overall performance of SIBs [38]. By carefully tailoring the chemical composition of needle coke and optimizing the pre‐treatment and carbonization conditions, it is possible to obtain high‐performance needle coke‐derived HC. Recent efforts have focused on heteroatom doping and the introduction of oxygen–containing functional groups to enhance the Na⁺ storage capacity of needle coke‐derived HC [39, 40]. Notably, its established industrial production chain, such as in metallurgy, provides a significant advantage for rapid scalability, making it an economically viable candidate for large‐scale SIBs production [38]. Other precursors, such as polymers [41], have demonstrated promise but are frequently constrained by high raw material costs, complex processing requirements, or inconsistent performance, making them less competitive for industrial–scale implementation. In contrast, the aforementioned precursors (biomass, resin, and needle coke) collectively highlight the significance of balancing performance, cost, and scalability to achieve HC toward commercial applications in SIBs.

Currently, most existing reviews have focused primarily on fundamental sodium storage mechanisms or general modification strategies, with limited attention to the industrial feasibility of precursors and scalable structural regulation. Despite the enormous progress in developing HC materials, a critical gap remains between laboratory‐scale synthesis and practical industrialization. Given the scarcity of reviews on HC materials for SIBs suitable for industrialization, this review aims to provide a comprehensive overview of commercial HC anodes for SIBs and answer a central question: how can we rationally design and produce high‐performance HC anodes for SIBs by integrating scalable, cost‐effective precursors with targeted structural tuning strategies? First, we systematically analyzed the sodium storage mechanisms of HC and integrated the existing models to clarify the key structural features and nanopores that control the sodiation–desodiation process. Second, we compared the synthetic routes for HC derived from front‐running precursors (biomass, resin, and needle coke). We also highlight strategies to optimize HC properties by tuning key parameters such as surface functional groups, interlayer spacing, and pore structure. Particular attention is given to how these modifications influence critical electrochemical metrics, including specific capacity, ICE, and cycling stability. Finally, based on the comparative analysis, we outline the remaining challenges hindering their commercialization. We also provide our perspective on future directions, including cost reduction, sustainable synthesis, and recyclable manufacturing, aiming to provide a comprehensive guideline for both academic research and the industrial scale‐up of SIBs.

2. Sodium Storage Mechanisms and Performance Evaluation Metrics of Hard Carbon

2.1. Sodium Storage Mechanisms of Hard Carbon

The Na⁺ storage behavior of HC is primarily governed by its unique turbostratic structure. This structure is characterized by curved graphene nanosheets, intrinsic defects, and a hierarchical network of closed and open nanopores, which collectively give rise to complex electrochemical processes [42, 43, 44, 45]. The typical charge‐discharge curves of HC anodes in SIBs exhibit two distinct voltage regions, a slope region above 0.1 V and a plateau region below 0.1 V, which correspond to Na⁺ storage sites with different structures in HC [46]. Over the past two decades, extensive researchers had been dedicated to unravel the underlying mechanisms governing these regions, leading to the proposal of four basic mechanisms (Figure 2). Although these mechanisms differ in their interpretation of specific processes, they collectively provide a fundamental framework for understanding how precursor selection and structural engineering govern the electrochemical behavior of HC, which is crucial for guiding its industrial‐scale applications.

FIGURE 2.

The schematic illustration of the sodium storage mechanism in hard carbon for SIBs, microstructures, storage behaviors, and voltage regions.

2.1.1. “Intercalation–Filling” Mechanism

The “Intercalation–Filling” mechanism, first proposed by Dahn et al. in 2000 through the study of glucose‐derived HC [47], remained one of the most influential models for elucidating Na⁺ storage in HC. They described a structure similar to “House of Cards”, which is composed of many tiny fragments. Meanwhile, they noted that the high‐voltage sloping region was attributed to reversible Na⁺ intercalation into the expanded graphitic interlayers of HC, and the low‐voltage plateau region was explained by Na⁺ filling into micropores formed by chaotic stacking of pseudographitic domains, as shown in Figure 2a. Komaba et al. [48] employed ex situ XRD to investigate the correlation between graphite interlayer spacing and reaction potential in HC materials. When the Na⁺ insertion potential was reduced to 0.2 V, the (002) diffraction peak of HC shifted to a lower angle, indicating that the carbon layer spacing increased, which might be caused by Na⁺ insertion into the graphite layer. When charged to 2.0 V, the peak position returned to its initial state, confirming the reversible intercalation/deintercalation of Na⁺ within the graphitic layers. They utilized Raman spectroscopy and found that the G band of HC exhibited a redshift when the discharge voltage was in the slope region, whereas no shift was observed when the discharge voltage was in the low‐voltage plateau region. Thus, it was proposed that the slope capacity originates from Na⁺ insertion into the graphite layer. When the discharge voltage was below 0.2 V, small‐angle X‐ray scattering (SAXS) detected a significant decrease in micropore electron density within the range of 0.03–0.07 Å⁻¹, indicating that the low‐voltage plateau region corresponds to Na⁺ filling in micropores. Furthermore, Myung et al. [49] conducted comprehensive experiments on commercial HC and reached the same conclusion, thereby further supporting the “Insertion–Filling” mechanism.

2.1.2. “Adsorption–Intercalation” Mechanism

Although some research findings align with the “Intercalation–Filling” mechanism, certain experimental observations show clear contradictions. In 2012, Cao et al. [50] reported significant differences in the electrochemical behavior of sodium and lithium storage for HC anodes derived from polyaniline pyrolysis. They inferred that the sodium storage mechanism of HC in the low‐voltage plateau region (0–0.1 V) resembles the lithium storage mechanism of graphite, and accordingly proposed the “Adsorption–Intercalation” mechanism. In this model, Na⁺ ions are first adsorbed at defect sites on the HC surface, followed by intercalation into graphitic layers in the low‐voltage plateau region (Figure 2b). Subsequently, Cao's group [51] provided systematic validation of this mechanism by preparing HC from microcrystalline cellulose (MCC). In situ XRD measurements revealed reversible variations in interlayer spacing within the low‐voltage plateau region, indicating that this region corresponds to the intercalation of Na⁺ ions between graphitic layers to form sodium–carbon compounds (NaC_x). At the same time, a decrease in defect concentration was shown to reduce the sodium storage capacity in the slope region, leading to the conclusion that the high‐potential slope region primarily arises from Na⁺ adsorption at defect sites and heteroatoms in HC. Moreover, Cao et al. [52] conducted a comparative study of HC and graphite, which further demonstrated that the sodium storage mechanism of HC parallels the lithium storage mechanism of graphite. Specifically, the high‐voltage sloping region originates from Na⁺ adsorption at defect sites, while the low‐voltage plateau region corresponds mainly to Na⁺ intercalation/deintercalation within graphitic layers. Collectively, these findings provide compelling evidence supporting the “Adsorption–Intercalation” model of HC.

2.1.3. “Adsorption–Filling” Mechanism

With the in–depth investigation of sodium storage mechanisms, Huang et al. [53] proposed that sodium storage in HC follows an “Adsorption–Filling” mechanism rather than intercalation. In this model, the high‐voltage sloping region corresponds to the adsorption of Na⁺ on the surface or defect sites of HC, while the low‐voltage plateau region is associated with Na⁺ filling into nanopores (Figure 2c). Using natural cotton as a precursor, they prepared HC microtubes via a one‐step carbonization process. Ex situ TEM analysis revealed no discernible changes in the interlayer spacing of graphitic domains before and after discharge, suggesting that Na⁺ does not intercalate into graphitic layers during the electrochemical process. In contrast, defect sites, carbon layer edges, and nanovoids became blurred after sodium storage, confirming Na⁺ storage at these locations. Ex situ XPS further showed that Na⁺ stored in the low‐voltage plateau region exhibited higher binding energy, while Na⁺ at defect sites and carbon edges had lower binding energy. These findings support the interpretation that the slope region corresponds to Na⁺ adsorption on carbon layer surfaces and defect sites, whereas the low‐voltage plateau region corresponds to Na⁺ filling within nanopores. Xu et al. [54] provided additional evidence by demonstrating that filling sulfur into the pores of HC eliminated the low‐voltage plateau, thereby confirming its correlation with the pore‐filling process. Their work also showed that HC exhibits similar sodium‐ion storage behavior in both ether‐ and ester‐based electrolytes, further indicating the absence of intercalation into graphitic layers. Moreover, with increasing carbonization temperature, the concentration of defects and heteroatoms in HC decreases, leading to a reduction in capacity within the slope region. This observation reinforces the conclusion that the slope region originates from Na⁺ adsorption at defect sites or heteroatoms. Collectively, these studies provide strong support for the “Adsorption–Filling” mechanism of sodium storage in HC anodes.

2.1.4. “Adsorption–Intercalation–Filling” Mechanism

Apart from the aforementioned three models, some researchers have proposed that the sodium storage mechanism of HC is a combination of multiple processes. In 2015, Ji et al. [55] introduced the “Adsorption–Intercalation–Filling” mechanism. In this model, the sloping capacity (1.0–0.2 V) originates from Na⁺ adsorption at the edges and defect sites of graphitic microcrystals, the plateau capacity (0.2–0.05 V) arises from Na⁺ intercalation into graphitic layers, and the deep plateau capacity (<0.05 V) corresponds to Na⁺ filling in nanopores formed by the interlacing of graphitic microcrystals (Figure 2d). To validate this mechanism, they investigated the sodium storage behavior of sucrose‐derived HC using galvanostatic intermittent titration technique (GITT). The results showed that the Na⁺ diffusion coefficient in the high‐voltage sloping region is higher than that in the low‐voltage plateau region. This phenomenon was attributed to the fact that Na⁺ adsorption readily occurs at the edges and defect sites of graphitic layers, whereas intercalation requires overcoming electrostatic repulsion. Thus, the slope region was assigned to Na⁺ storage at edges or defects, while the low‐voltage plateau region was attributed to Na⁺ intercalation between graphitic layers. Notably, a sudden increase in the Na⁺ diffusion coefficient near 0.05 V suggested that the range of 0–0.05 V corresponds to Na⁺ filling in nanopores.

More recently, Zhang et al. [56] conducted an in‐depth study on tea‐derived HC using in situ XRD and ex situ Raman analyses. Their results further substantiated the adsorption–intercalation–filling mechanism by providing direct insights into the structural evolution of HC during Na⁺ storage. With the advancement of characterization techniques, a consensus has emerged that the high‐voltage sloping region mainly arises from Na⁺ adsorption on surfaces and defect sites, whereas the low‐voltage plateau region is associated with Na⁺ intercalation, filling, or a combination of both. Nevertheless, these interpretations still require further verification through more advanced characterization methods and systematic experimental studies.

2.2. Evaluation Metrics of the Microstructure of Hard Carbon

The performance of HC anode of SIBs is governed by a complex interplay between their structural characteristics and electrochemical behavior [57]. To systematically evaluate and optimize HC, a comprehensive system of structural and electrochemical metrics has been established. This section provides a detailed framework for assessing HC performance, first outlining the critical structural parameters that govern Na⁺ storage sites, followed by the key electrochemical indicators that determine practical applicability. The microstructure of HC is the key factor determining its sodium storage behavior [58], mainly including carbon microcrystallite, graphitization degree, pore structure, defect density, heteroatom, and et al. These structural features synergistically regulate the transport pathways, active site distribution, and interfacial stability of Na⁺.

2.2.1. Evolution of Carbon Microcrystallites and Graphitization Degree

The degree of graphitization reflects the structural order and the evolution of carbon microcrystallites in HC. It is not a single independent parameter but an overarching structural state that manifests directly through specific spatial metrics, including interlayer spacing (d₀₀₂), along with stacking height (L _c) and lateral size (L _a).

As the structural manifestation of graphitization, the interlayer spacing (d₀₀₂) is a key factor determining the Na⁺ storage capacity and intercalation kinetics of HC. An appropriate d₀₀₂ facilitates the insertion and extraction of Na⁺, influencing both the plateau and slope capacities [59]. Generally, low‐graphitized HC features a more disordered structure with a larger d₀₀₂ value (0.37–0.40 nm), which allows Na⁺ to overcome interlayer van der Waals forces, thereby enhancing the plateau capacity while providing abundant edge/defect sites for slope storage [60, 61]. Based on experimental and theoretical studies, a minimum interlayer spacing of 0.37 nm has long been regarded as the threshold for thermodynamically favorable Na⁺ intercalation [50]. Interestingly, recent work by Li et al. [62] challenged this consensus by demonstrating that HC derived from petroleum coke enables Na⁺ intercalation at d₀₀₂ as low as 0.345 nm. Conversely, highly graphitized HC provides highly ordered layers that facilitate rapid electron transport, but its tightened interlayer spacing and lack of defects severely restrict Na⁺ intercalation and adsorption, leading to diminished capacity [63]. However, excessive structural disorder and massive expansion (d₀₀₂ > 0.42 nm) may weaken interlayer bonding, potentially causing structural collapse during cycling and deteriorated capacity retention.

The evolution of these microcrystallites is typically quantified through a combination of X‐ray diffraction (XRD) and Raman spectroscopy. The interlayer spacing (d₀₀₂) is calculated using the Bragg equation (nλ = 2d sinθ) [64, 65], where n is the diffraction order (usually 1), λ is the incident X‐ray wavelength (0.15406 nm for Cu Kα radiation), d is the interlayer spacing, and θ is the diffraction angle corresponding to the (002) reflection. For HC, the (002) peak generally appears between 23° and 26°(2θ) and is highly sensitive to stacking order, with larger interlayer spacings shifting the peak toward lower angles. Moreover, the degree of structural ordering is inversely proportional to the full width at half maximum (FWHM) of this peak; that is, a narrower FWHM indicates higher graphitization and crystallite growth. Similarly, in Raman spectroscopy, the G band intensity and the I_D/I_G ratio serve as reliable indicators, where a stronger G band denotes a more ordered, highly graphitized structure [66].

To achieve precise control over the graphitization degree, researchers have employed a range of strategies involving carbonization temperature and precursor selection. High‐temperature treatment promotes atomic rearrangement and the growth of graphitic domains, inevitably reducing the interlayer spacing. Precursor intrinsic properties also play a decisive role. Fossil‐based precursors (e.g., pitch, petroleum coke) possess high aromaticity and low heteroatom content, making them highly graphitizable. In contrast, precursors such as biomass and resins are rich in heteroatoms, oxygen‐containing functional groups, and amorphous chains. These features sterically hinder the alignment of graphitic layers during pyrolysis, inherently yielding HC with a low degree of graphitization and an increased interlayer spacing, which is more suitable for the insertion of Na⁺.

2.2.2. Pore Structure

The hierarchical pore structure of HC is a critical factor that not only determines its physical properties but also significantly governs sodium ion storage behavior through size‐dependent interactions. As illustrated in Figure 3, the pore structure of HC consists of micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm), as defined by IUPAC [67]. Among these, micropores play a dominant role in sodium storage, while mesopores and macropores can respectively provide additional active sites for Na⁺ and mitigate volume changes.

FIGURE 3.

Schematic illustration of pore structure in hard carbon and the distinct functions in Na⁺ storage.

Furthermore, open pores (connected to the material surface and accessible to electrolyte penetration) and closed pores (isolated within the carbon matrix and inaccessible to bulk electrolyte) within the HC structure exhibit distinct functionalities. Open pores promote electrolyte penetration into the carbon matrix, facilitating ion transport and enhancing the surface adsorption of Na⁺, which are crucial for the slope capacity of HC [68]. In contrast, closed pores are primarily responsible for plateau capacity by restricting the entry of solvated Na⁺ [69], thereby facilitating desolvation and the formation of metal‐like sodium clusters.

Given the critical and complex interplay between pore size, distribution, and sodium storage mechanisms, accurate pore characterization requires a combination of advanced techniques. N₂ adsorption–desorption isotherms remain a standard approach for probing pores accessible to N₂ molecules, yielding key parameters such as specific surface area, pore volume, and pore size distribution via the Barrett–Joyner–Halenda (BJH) method. CO₂ adsorption‐desorption serves as a complementary technique for ultramicropores, since CO₂ molecules (kinetic diameter ≈ 0.33 nm) exhibit higher diffusion in sub‐nanometer pores, enabling more accurate analysis of closed ultramicropores. Furthermore, small‐angle X‐ray scattering (SAXS) provides unique insights into both open and closed pores, including those inaccessible to gas adsorption methods.

By integrating these complementary techniques, precise quantification of pore structures in HC can be achieved. Above all, appropriate pore structures are essential for enhancing sodium storage performance. However, excessive porosity increases specific surface area, accelerates electrolyte decomposition, and lowers the ICE. Therefore, achieving a proper balance in pore size, distribution, and openness is critical for optimizing the electrochemical performance of HC.

2.2.3. Defect Density

Defects, including edge sites, vacancies, and lattice distortions, are the primary sources of surface active sites in HC, directly contributing to slope capacity by facilitating Na⁺ adsorption [70, 71]. The density and distribution of these defects strongly influence surface chemistry and electronic conductivity, thereby affecting sodium storage kinetics. Accurate quantification of defect density is thus essential.

Raman spectroscopy is the most widely applied technique, where the intensity ratio of the D band (∼1350 cm^{− 1}, disordered carbon/defects) to the G band (∼1580 cm^{− 1}, sp² carbon), denoted as I_D/I_G, serves as an indicator of defect density [72]. Higher ID/IG values correspond to higher defect levels. X‐ray photoelectron spectroscopy (XPS) can provide additional insights via C 1s spectra, particularly the sp²/sp³ hybridization ratio [73], while high‐resolution TEM (HRTEM) offers direct atomic‐scale visualization of lattice distortions and edge sites. Together, these techniques allow comprehensive defect characterization, laying the groundwork for targeted defect regulation.

Defect density can be tuned through carbonization conditions, precursor selection, and post‐treatment strategies. Low‐temperature carbonization (<1000°C) preserves more defects by suppressing graphitization, whereas high‐temperature treatment (>1300°C) promotes atomic rearrangement and reduces defect density [74]. Biomass‐derived precursors typically yield HC with higher defect concentrations due to their heteroatom content and irregular polymer structures, while fossil–based precursors generate more ordered frameworks [75]. Post‐treatments such as HNO₃ oxidation can also introduce oxygen‐functionalized defects, enhancing Na⁺ adsorption sites.

Notably, an appropriate defect density is crucial [76]: moderate defects enhance slope capacity and Na⁺ transport kinetics, whereas excessive defects accelerate electrolyte decomposition and reduce ICE. Thus, precise regulation is vital to balance storage capacity with electrochemical stability.

2.2.4. Heteroatom

Heteroatom doping (e.g., N, P, S, O) profoundly influences sodium storage by modifying the electronic structure, surface chemistry, and carbon framework configuration of HC. Different electronegativities and atomic radii of heteroatoms create polar sites, defects, and lattice distortions, thereby optimizing Na⁺ adsorption, ion diffusion, and electron transport [77]. For example, nitrogen doping introduces pyridinic and pyrrolic N sites that enhance Na⁺ adsorption [78]; phosphorus doping promotes charge redistribution [79]; sulfur doping provides polar sites and pseudocapacitive contributions [80], while oxygen doping increases active sites through functional groups [81]. Synergistic multi–element doping often surpasses the effect of single‐element doping.

At the atomic scale, heteroatoms enter the HC lattice via substitutional doping (replacing C atoms in the sp² lattice) or interstitial doping (occupying vacancies/interlayer spaces). Substitutional doping modulates electronic density around adjacent carbons [82], while interstitial doping expands interlayer spacing and introduces defects [83]. Both mechanisms enhance Na⁺ accessibility by lowering adsorption energy barriers and improving ion transport.

Nevertheless, excessive doping may introduce instability and accelerate electrolyte decomposition, while insufficient doping provides limited improvement. Thus, precise control of doping type, content, and chemical state is essential for balancing capacity, kinetics, and stability.

2.3. Evaluation Metrics of the Electrochemical Performance of Hard Carbon

The electrochemical performance of HC is the ultimate measure of its practical applicability in SIBs, directly determining its viability in energy storage applications. This performance is primarily evaluated through critical metrics, including specific capacity, initial Coulombic efficiency (ICE), rate capability, and cycle stability, all of which critically govern the electrode's efficiency and long‐term reliability. Intrinsically, these electrochemical parameters are inherently linked to the dynamic structural evolution of HC during electrochemical cycling. As illustrated in Figure 4, the fundamental principle of “structure determines performance” underpins this relationship: critical microstructural features directly modulate ion transport kinetics, charge‐transfer efficiency, and structural stability, thereby dictating the macroscopic electrochemical behavior of HC.

FIGURE 4.

Schematic illustration of the structure‐performance relationship of HC.

2.3.1. Specific Capacity

Specific capacity with a unit of mAh g^{− 1} represents the charge stored per unit mass of active material and is fundamental for evaluating HC performance. It consists of slope capacity and plateau capacity, both determined by the microstructural features of HC. The slope capacity arises from Na⁺ adsorption on defects, heteroatom sites, and internal pore surfaces, correlating with defect density, heteroatom content, and specific surface area [84]. The plateau capacity results from Na⁺ intercalation into graphitic layers and filling of closed micropores, depending on interlayer spacing, graphitization degree, and closed pore structure [85].

Optimizing the total specific capacity requires balancing slope and plateau contributions. Excessive slope capacity can reduce ICE due to side reactions, whereas overly dominant plateau capacity may restrict ion transport if graphitic layers are too ordered. Also, the microstructural engineering is essential to maximize the synergy between slope and plateau capacities.

2.3.2. Initial Coulombic Efficiency (ICE)

Initial Coulombic efficiency (ICE) is defined as the ratio of charge to discharge capacity in the first cycle, reflecting irreversible Na⁺ loss and directly impacting energy efficiency [86]. Low ICE reduces effective capacity and increases costs for sodium supplementation. Primary sources of ICE loss include SEI formation, Na⁺ trapping at defects, and side reactions at the electrode‐electrolyte interface [87]. Defects and functional groups with high binding energy exacerbate irreversible Na⁺ consumption.

Currently, comprehensive approaches to improving ICE mainly include the following aspects: (i) Surface modification: this involves optimizing the precursor or applying pre/post‐treatments to reduce surface defect density, minimize the specific surface area (SSA), and suppress irreversible electrolyte decomposition [88]. (ii) Electrolyte engineering: beyond intrinsic material design, regulating the electrolyte formulation, such as optimizing solvent/salt systems and employing functional additives, is a highly effective approach to passivating the anode surface. Advanced electrolyte engineering facilitates the formation of a robust, thin, and inorganic‐rich solid electrolyte interphase (SEI), which significantly reduces continuous Na⁺ consumption during initial cycle [89, 90, 91]. (iii) Pre‐sodiation techniques: this strategy involves physically or chemically introducing active sodium into the anode before cell assembly, directly compensating for the irreversible Na⁺ loss during SEI formation [92].

Notably, electrolyte engineering is highly complementary to microstructural design. While structural optimization determines the availability of active sites and surface reactivity, electrolyte formulation governs interfacial chemistry and SEI stability. Therefore, synergistic optimization of material structure and electrolyte composition is essential for achieving high ICE and practical electrochemical performance.

2.3.3. Rate Performance

Rate capability is used to evaluate HC's reversible capacity under varying current densities, reflecting the ion transport and charge‐transfer kinetics in HC. Na⁺ diffusion in HC is primarily governed by interlayer spacing, pore structure, and defect distribution. Hierarchical pores enable rapid ion transport: micropores provide storage, while meso/macropores shorten diffusion paths. Defects and edges offer additional adsorption sites but may hinder diffusion if excessive. The rate capability can be investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic intermittent titration technique (GITT).

Optimizing rate performance relies on synergistic strategies: tailoring interlayer spacing, constructing hierarchical pores, regulating defects, doping heteroatoms, and electrolyte optimization.

2.3.4. Cycle Performance

Cycle stability indicates the ability of HC to retain capacity over prolonged charge–discharge cycles. It depends on the structural integrity and interfacial reversibility, including forming a uniform and flexible SEI to prevent repeated electrolyte decomposition, constructing a robust HC framework to maintain structural integrity, resist volume‐induced collapse, keep electronic pathways, minimize the side reactions, and prevent electrolyte degradation. Therefore, general strategies such as SEI engineering, surface coatings with Al₂O₃ or TiO₂ layers, and structural regulation with controlled graphitization and porous structure [93].

2.4. The Intrinsic Structure‐Mechanism‐Performance Relationship

Overall, the sodium storage behavior of HC can be understood from a unified microstructure‐mechanism‐performance perspective. Specifically, different microstructural features govern distinct sodium storage mechanisms, which in turn determine the electrochemical performance. Closed micropores are primarily responsible for Na⁺ storage via the filling mechanism, contributing significantly to the low‐voltage plateau capacity. In contrast, defects and heteroatom functional groups provide active sites for Na⁺ adsorption, dominating the sloping capacity. Meanwhile, an enlarged interlayer spacing facilitates Na⁺ intercalation/deintercalation and improves ion transport kinetics, thereby influencing both plateau capacity and rate capability.

Importantly, these structural parameters are intrinsically coupled and must be carefully balanced. For example, excessive defects and high specific surface area may enhance slope capacity but lead to severe irreversible Na⁺ loss and low ICE. Similarly, highly ordered graphitic structures improve electronic conductivity but limit the availability of storage sites. Therefore, achieving high‐performance HC requires a rational design that balances pore structure, defect density, and graphitic ordering to optimize both capacity and efficiency.

3. The Preparation Route and Modification Strategies of Resin‐Based Hard Carbon Anode Material

3.1. The Overview of Resin as a Precursor

Resin is a type of high molecular weight polymer with a complex cross‐linking molecular structure and high carbon content [94, 95]. Resins are divided into natural resins and synthetic resins. Natural resins are mainly extracted from plants, secretions from animals, or minerals. Although natural resins are environmentally friendly, their low carbon content (compared with synthetic resins) and complex composition limit their application in large‐scale industrial production [87, 96, 97, 98]. Therefore, synthetic resin, especially phenolic resins, has become the primary precursor for resin‐based HC [99, 100, 101, 102]. This section will first introduce the widely used phenolic resins as precursors for the fabrication of HC, then describe less‐toxic green resins that have been explored to prepare HC.

3.2. The Synthesis of Phenolic Resin and Derived Hard Carbon via Simple Carbonization

Phenolic resins are primarily synthesized through a condensation polymerization reaction by using a variety of phenolic and aldehyde as reactants [103, 104, 105]. In addition, various monomers can be utilized to modify the backbone structure and spatial configuration of the polymer by using different derivatives. Chen et al. synthesized the resin by using the most fundamental phenol and formaldehyde under alkaline conditions (Figure 5a) [103]. Moreover, Guo et al. also synthesized phenolic resins using resorcinol and different aldehydes as reactants (Figure 5b) [106]. As shown in Figure 5c, Zhang et al. utilized resorcinol and methanal as raw materials to form phenolic resins after curing at a certain temperature [107]. These studies all demonstrated the diversity of monomers for synthesizing phenolic resins, which were further used to produce HCs.

FIGURE 5.

Methods for synthesizing phenolic resins using (a) formaldehyde‐phenol. Reproduced with permission [103]. Copyright 2024, Elsevier, (b) resorcinol‐aldehyde. Reproduced with permission [106]. Copyright 2025, Elsevier, and (c) resorcinol‐formaldehyde. Reproduced with permission [107]. Copyright 2020, Springer Nature. The relationship between the electrochemical properties of HCs derived from (d) resorcinol‐formaldehyde. Reproduced with permission [108]. Copyright 2015, Elsevier and (e) formaldehyde‐phenol.Reproduced with permission [109]. Copyright 2015, Elsevier after carbonization at different temperatures.

In the early stages, researchers generally obtained HC by carbonizing resins and then modified the HC by controlling the carbonization temperature. For example, Wang et al. reported the synthesis of phenolic resin‐based HC microspheres via a hydrothermal method using phenol, formaldehyde, and sodium hydroxide as reactants (Figure 5d) [108]. And they investigated the effects of different carbonization temperatures (800°C, 1000°C, 1250°C, and 1500°C) on the structure and electrochemical properties of the prepared HC. All PF‐HCSs exhibited a perfect spherical structure with a wide particle size distribution (1–8 micrometers). The report suggested that 1250°C is the optimal carbonization temperature, as the interlayer spacing is maximized, which is favorable for the reversible intercalation and deintercalation of Na⁺. Meanwhile, as shown in Figure 5e, Hasegawa et al. prepared HC by directly carbonizing resorcinol‐formaldehyde resin gels at different temperatures (800–3000°C) [109]. The experimental results showed that as the carbonization temperature increases, the interlayer spacing decreases, which in turn increases the energy barrier for Na⁺ intercalation. The HC anode carbonized between 1600°C and 2500°C exhibited high reversible capacity and high ICE (92%).

3.3. The Modification Route of Phenolic Resin‐Based Hard Carbon

Although HC can be effectively prepared by simply carbonizing phenolic resins, temperature control alone is insufficient to fully satisfy the high electrochemical performance requirements of HC. Therefore, scientists have begun to explore modification methods for HC, optimizing the electrochemical properties and expanding their application scope by adjusting porosity [110], cross‐linked density [111], doping heteroatom [112], etc.

3.3.1. Pore Regulation

Heat treatment is a relatively simple method to regulate the pore structure. Wang et al. synthesized phenolic resin according to a two‐step carbonization strategy and optimized the closed pore structure by regulating the pre‐annealing temperature in Figure 6a [113]. The report indicated that the optimal HC material featured a larger closed pore size and volume, and verifies the “adsorption‐intercalation‐filling” mechanism. In addition, “templating method” is widely used in the modification of HC. Li et al. introduced C60 into the phenolic resin precursor and synthesized HC using a one‐step carbonization method (Figure 6b) [114]. The modified material achieved a plateau capacity of 268 mAh g⁻¹, higher than that of the original sample (200 mAh g⁻¹). This highlights that C60 increased the Na⁺ storage sites by modulating the closed pore structure of HC as a template. Furthermore, Lin et al. first proposed constructing a closed structure into phenolic resin precursors by taking advantage of the steric hindrance effect. By grafting aromatic groups, they increased the internal free volume and backbone rigidity of the precursor, which facilitated the formation of abundant closed pores during the carbonization and enhanced the performance of HC in SIBs (Figure 6c) [115]. In summary, the storage sites for Na⁺ can be increased by reasonably enhancing the number of closed pores, thereby improving the electrochemical performance of HC.

FIGURE 6.

(a–c) The schematic diagram of the modification of phenolic resin‐based hard carbon through pore regulation. Reproduced with permission [113]. Copyright 2024, Elsevier. Reproduced with permission [114]. Copyright 2025, American Chemical Society. Reproduced with permission [113, 114, 115]. Copyright 2024, Wiley. The modification of phenolic resin‐based hard carbon by (d) N/S co‐doping. Reproduced with permission [112]. Copyright 2024, IOP Publishing. and (e) N/S co‐doping.Reproduced with permission [116]. Copyright 2023, Elsevier. Adjusting the crosslinking degree by (f) changing the molar ratio of raw material. Reproduced with permission [117]. Copyright 2024, OAE Publishing and (g) mixing with pitch. Reproduced with permission [118]. Copyright 2022, Wiley. (h,i) The schematic diagram of synthesis and modification of green resin. Reproduced with permission [119]. Copyright 2021, Elsevier. Reproduced with permission [120]. Copyright 2025, Wiley.

3.3.2. Heteroatom Doping

To date, enhancing Na⁺ storage capacity has been a target of many studies. Doping HC with heteroatoms like B, N, P, and S is a common strategy. It can increase interlayer spacing, improve electronic conductivity, and introduce more defects, thereby boosting reversible capacity [121, 122]. Zhang et al. employed thiourea to simultaneously introduce nitrogen and sulfur heteroatoms, producing N/S co‐doped HC microspheres in one step [112]. This strategy incorporated both N and S, aiming to enlarge interlayer spacing, increase defects and active sites, and thereby enhance the HC Na⁺ storage capacity (Figure 6d). Similarly, Sun et al. employed melamine as a heteroatom source to fabricate N/O co‐doped HC microspheres and systematically investigated the influence of dual N/O doping coupled with microstructure disordering in SIBs [116]. The report demonstrated that the incorporation of N and O introduced additional defects and functional groups, furnishing abundant active sites for reversible Na⁺ adsorption and intercalation. Moreover, the dopants disrupted the graphite layer stacking and enlarged the interlayer spacing, thereby facilitating rapid Na⁺ diffusion (Figure 6e).

3.3.3. Control the Crosslinking Degree of Resin Precursors

Controlling the crosslinking degree of resin precursors has been proven to be an effective strategy for modifying both microstructure and microscopic properties of the derived HC [123, 124]. As shown in Figure 6f, Lu et al. synthesized phenolic resin precursors with different crosslinking degrees by adjusting the molar ratio of formaldehyde to 3‐aminophenol [117]. The precursors with a higher crosslinking degree formed more ordered graphite structures during the carbonization process, which enhances the molecular strength and structural stability of the derived HC. The factors collectively contributed to the high reversible capacity and excellent performance of HC in SIBs. In addition, Zhang [27] et al. demonstrated the effectiveness of optimizing the microstructure and electrochemical performance of HC by employing a pre‐oxidation strategy to tailor the crosslinking degree of precursors. It was observed that with the extension of pre‐oxidation time, the crosslinking degree of phenolic resin was increased, leading to a reduction in the specific surface area of HC, and an increase in ICE. Higher crosslinking degrees typically require more complex processes. To achieve high performance while reducing costs, Yin [118] et al. proposed an innovation method of preparing highly crosslinked composite HC by mixing low‐cost pitch with resin and adjusting the proportion of pitch and the carbonization temperatures. They found that an appropriate amount of pitch not only promoted the formation of a short graphitic layer but also optimized the interlayer spacing, thereby enhancing the Na⁺ storage performance of HC (Figure 6g).

3.4. Exploration of Green Resin

Although researchers have successfully demonstrated the tunability of the phenolic resin system in Na⁺ storage through various strategies, the HC precursors highlighted in these studies are predominantly derived from phenol and formaldehyde, which are highly toxic chemicals to humans and the environment. Consequently, green resins have been explored by alternative approaches [23, 113, 125, 126]. For example, Beda et al. utilized phloroglucinol and glyoxylic acid as less‐toxic raw materials to form phenolic resins by a simple sol‐gel method [98]. Similarly, Wang et al. prepared resorcinol‐glyoxylic acid green resin within a shorter time by the hydrothermal method, overcoming the two major industrial barriers of long process time and high toxicity (Figure 6h) [119]. The modification strategies of green resin are similar to the approaches described above. Fan et al. cured epoxy phenol novolac resin with maleic anhydride and then carbonized it. Owing to the high crosslinking density provided by the epoxy groups, the one‐step process yields HC featuring low specific surface area and abundant closed pores [125]. Duan et al. blended o‐cresol aldehyde epoxy resin with zinc acetate (Figure 6i) [120]. The zinc acetate not only promoted radical crosslinking but also acted as ZnO nanotemplates, which favors semi‐closed ultramicropores with a size of 0.3–0.8 nm. Meanwhile, Zhou et al. synthesized HC by esterification crosslinking between 3,4,9,10‐perylene‐tetracarboxylic acid‐dianhydride (PTCDA) and phenolic resin [94]. The introduction of PTCDA enhanced the thermal stability and structural disorder of the resin via esterification crosslinking, while expanding the interlayer spacing, achieving a reversible capacity of 308.7 mAh g⁻¹ and ICE of 77.9%.

In summary, resin‐based HC offers exceptional tunability through diverse modification routes, including pore regulation, heteroatom doping, and so on, which effectively optimize the critical structural parameters. To provide a comprehensive overview, the electrochemical performance of various resin‐derived HC anodes in some recent works is summarized in Table 1.

TABLE 1.

Summary of resins from different monomers and electrochemical performance of their derived HC.

Monomers	Electrolyte	Capacity (mAh g−1)	ICE (%)	Rate performance (mAh g−1)	Capacity retention (mAh g−1)	Refs.
tri‐aminophenol formaldehyde	1 M NaPF6 in DME	351@0.03A g−1	83	263@2 A g−1	254.5 (2000th)@2 A g−1	[113]
Phenol Paraformaldehyde	1 M NaPF6 in DME	286.34@0.02 A g−1	85.05	236.05@2 A g−1	184.99 (2500th)@2 A g−1	[124]
p‐phenylenediamine phloroglucinol	1 M NaPF6 in EC: DMC (1:1)	375.8@0.03 A g−1	72.3	303.5@2 A g−1	—	[127]
phenolic resin	1 M NaPF6 in DME	∼400@0.05 A g−1	89.3	219@4 A g−1	208 (1000th)@2 A g−1	[128]
3‐aminophenol	1.0 M NaPF6 in Diglyme	349.3@0.02A g−1	81.3	221.6@2 A g−1	329.3(100th)@0.1 A g−1	[129]
benzoxazine	1 M NaPF6 in DME	325.7@0.05A g−1	90.6	78.9@15 A g−1	317.0(100th)@0.1 A g−1	[130]
epoxy resin	1.0 M NaPF6 in Diglyme	327.9@0.03A g−1	89.9	278.2@1 A g−1	∼300(1000th)@0.5 A g−1	[131]
phenolic resin	1.0 M NaPF6 in Diglyme	353.0@0.05A g−1	85.64	252.5@1 A g−1	—	[132]
tri‐aminopheno	1 M NaPF6 in DME	396@0.03A g−1	84.2	283.51@1 A g−1	248.53(3000th)@2 A g−1	[133]

4. The Modification Strategies for Biomass‐Based Hard Carbon

4.1. The Overview of Biomass Precursor

Biomass materials, such as bamboo, coconut shells, and agricultural waste, have become important sources for fabricating HC due to their renewability, low cost, and tunable structure [134, 135]. The biomass‐based HC usually provides rich Na⁺ storage sites and achieves a higher specific capacity [4]. However, there are still several challenges that need to be addressed, such as improving the ICE, cycling stability, and industrial production of biomass‐based HC [136, 137]. Therefore, researchers have mainly focused on modification strategies such as pyrolysis condition control, pre‐treatment, and doping to optimize the electrochemical performance [4].

4.2. The Modification Route of Biomass‐Based Hard Carbon

4.2.1. Pyrolysis Control

The control of pyrolysis conditions is of great significance in the preparation process of biomass‐based HC. Pyrolysis conditions, including temperature, atmosphere, heating rate, and retention time, have an important impact on the microstructure and Na⁺ storage performance of HC.

4.2.1.1. Pyrolysis Temperature

The pyrolysis processes of biomass are very complex. Initially, the primary components (e.g. cellulose, hemicellulose, and lignin) undergo crosslinking at a low temperature (200–300°C). Subsequently, the crosslinked polymer structures are subjected to carbonization at intermediate temperatures (400–600°C). Finally, during the high‐temperature graphitization stage (800–1200°C), the carbon structure gradually evolves into HC. This material features a disordered matrix containing nanoscale pores and defects that enable efficient sodium storage and impart distinctive electrochemical properties [138, 139]. In general, HC starts to form pseudo‐graphitic microcrystalline structure with enhanced electronic conductivity and structural orderliness above 1000°C [140]. Tang et al. found that as the pyrolysis temperature increased from 1100°C to 1700°C, the crystallinity of the natural wood‐derived HC had a decreased interlayer spacing and significantly increased content and size of closed pores (Figure 7a) [141]. These properties collectively enhanced the reversible capacity and cycling of HC. The Pei's team found that the tea residues derived HC displayed a higher plateau capacity in the low voltage region, a higher reversible capacity, and excellent cycling performance by operating the pyrolysis at 1400°C [142].

FIGURE 7.

The modification of biomass‐derived hard carbon by (a) pyrolysis temperature control. Reproduced with permission [141]. Copyright 2023, Springer Nature. (b) Relationship between biochar surface area and pyrolysis rate. Reproduced with permission [143]. Copyright 2025, High Education Press. Exploring the influence of different pyrolysis rates on (c, d) surface area. Reproduced with permission [144]. Copyright 2021, Elsevier. Reproduced with permission [145]. Copyright 2022, Elsevier, and (e) electrochemical performance. Reproduced with permission [146]. Copyright 2018, Wiley. The modification of biomass‐derived HC by (f–g) pyrolysis atmosphere control. Reproduced with permission [147]. Copyright 2019, KIM Publishing. Reproduced with permission [148]. Copyright 2024, Wiley.

Of course, the temperature can't be increased indiscriminately to enhance the graphitization degree of HC. Excessive carbonization temperature leads to structural collapse and contraction, as well as a reduction in the total surface area and pore volume. Also, high temperature is detrimental to cost reduction and usually requires a high‐quality instrument [4, 149, 150]. For example, Sharma et al. employed a flame‐pyrolysis route, systematically varying the temperature to convert castor and sesame‐derived liquids into HC microspheres. The sample carbonized at 1200°C exhibited the best performance. Upon further elevating the temperature to 1400°C, the microspheres underwent secondary growth, leading to a diminished specific surface area and a collapsed or shrunken pore network that severely hindered Na⁺ diffusion pathways [151].

4.2.1.2. Pyrolysis Rate

The pyrolysis rate plays a decisive influence on both the structure and performance of HC. Based on the pyrolysis rate, the process can be classified as conventional, fast, and flash pyrolysis. At fast pyrolysis rates, it can yield a substantial fraction of gaseous products. In contrast, traditional slow pyrolysis is primarily employed for biomass‐based HC and can be divided into three sequential stages [4]. During the first stage (room temperature to 200°C), moisture and readily volatile species are expelled. The second stage (200–500°C) is characterized by intensive devolatilization and decomposition. The final stage (>500°C) involves the degradation of more thermally robust organic degradation [143].

Many studies indicate that an increased pyrolysis rate generally leads to a significant enhancement in the specific surface area of biochar within a certain range (Figure 7b). The pyrolysis experiments using walnut shell as the feedstock demonstrated a steady increase in specific surface from 19.3 to 126.5 m² g⁻¹ as the pyrolysis rate was elevated from 10 to 40°C min⁻¹ (Figure 7c) [144]. Similarly, Guo et al. selected camphor wood as the precursor. They found that within an ultra‐low pyrolysis rate (0.25–5°C min⁻¹), both pore volume and specific surface area exhibited a monotonic decrease with decreasing pyrolysis rate (Figure 7d) [145]. However, numerous studies have reported that the relationship between specific surface area and pyrolysis rate often follows a “increase‐then‐decrease” trend, with an optimal rate that maximizes pore structure development. At low and moderate pyrolysis rates, the accelerated pyrolysis facilitates the volatilization and decomposition of organic components, thereby promoting the formation and expansion of pores and enhancing the specific surface area. But once the pyrolysis rate exceeds a certain critical threshold, this trend may reverse. Chen et al. employed polar wood as the precursor and observed that increasing the pyrolysis rate from 10 to 30°C min⁻¹ led to a rise in specific surface area from 210 to 245 m² g⁻¹, but a further increase to 50°C min⁻¹ resulted in a decline [152]. This phenomenon was attributed to the fact that excessively high pyrolysis rate can induce localized particle melting and pore blockage, thereby hindering the development of an effective specific surface area [153].

For performance, different pyrolysis methods and rates will result in different electrochemical performance. Xiao et al. employed a conventional pyrolysis approach to investigate the effect of pyrolysis rate regulation and revealed a tendency (Figure 7e) [146]. When the pyrolysis rate was decreased from 5 to 0.5°C min⁻¹, structural defects and porosity were largely eliminated, leading to a remarkable enhancement in performance, with the ICE increasing to 86.1% and the capacity reaching 361 mAh g⁻¹. In addition, Zhen et al. introduced spark plasma sintering (SPS) for HC preparation, using an ultrafast pyrolysis rate of 300–500°C min⁻¹ with flash sintering ≤5 min [154], effectively replacing the conventional slow tube furnace process. This approach challenged the notion that slow pyrolysis is necessary for high performance, enabling scalable synthesis of low‐defect HC with high ICE (≈ 90%), reversible capacity (≈ 300 mAh g^{− 1}). Interestingly, samples prepared with heating rates of 100 and 500°C min⁻¹ exhibited nearly identical electrochemical performance. This indicates that, under the ultrafast, high‐pressure SPS conditions, the electrochemical properties are relatively insensitive to the heating rate. In recent years, beyond SPS, other rapid pyrolysis techniques, such as Joule heating [155, 156] and microwave carbonization, have also been extensively employed [157, 158].

In short, the pyrolysis rate dictates a delicate trade‐off between defect control and pore development in biomass‐derived HC. Conventional slow pyrolysis enhances ICE and capacity but suffers from long duration and poor scalability. By contrast, emerging rapid pyrolysis techniques enable fast and efficient synthesis of high‐performance HC, though their practical application will depend on overcoming challenges related to energy consumption and cost‐effectiveness.

4.2.1.3. Pyrolysis Atmosphere

In addition to changing the pyrolysis temperature and pyrolysis rate, in recent years, the regulation of pyrolysis atmosphere has emerged as a crucial strategy to optimize the structural and surface chemistry of biomass‐based HC for SIBs. It could enable the fine‐tuning of porosity, defect density, and functional groups, which in turn affect Na⁺ storage behavior and overall electrochemical performance [4]. Nitrogen (N₂) and argon (Ar) are the most commonly used inert gases. The pyrolysis performance in N₂ and Ar is comparable, whereas differences emerge at elevated temperature. Marino et al. investigated the effect of annealing atmospheres of Ar and N₂, and found there are no significant differences in the performance of almond shells‐derived HC produced at 1000°C [159]. As shown in Figure 7f, Xu et al. obtained similar findings for hornet nest‐derived carbon at the same temperature [147]. However, the HC treated in Ar exhibited higher ICE and better capacity retention compared to N₂ when the annealing temperature increased to 1200–1400°C. Nonetheless, owing to the relatively high cost of Ar, it is considerably less used compared to N₂ [4]. Beyond single gases, researchers have attempted to use mixed‐gas atmospheres for HC synthesis. Zhou et al. employed an Ar/H₂ mixture, in which Ar provided an inert environment to suppress high‐temperature oxidation of sucrose‐derived HC, thus maintaining the structural integrity of the carbon framework [28]. Meanwhile, H₂ acted as an in situ reducing/repairing function, reacting with surface oxygen‐containing functional groups to lower the overall oxygen content.

Moreover, reactive gases, including air, carbon dioxide (CO₂), small molecules such as H₂, H₂O, and CO, can influence the formation mechanisms of HC by modulating reactions at specific stages of the synthesis process. For instance, Figure 7g shows that Zheng et al. introduced CO₂ gas during the synthesis process, which etched the starch‐derived HC microspheres and generated a more abundant closed pore structure [148]. Shen et al. developed a low‐temperature air preheating strategy (210–220°C) that enabled the controlled release of H₂O, CO, and CO₂, elucidating a non‐crosslinking pathway for starch‐to‐HC conversion. This method preserved the spherical morphology of the particles and offered a broadly applicable guideline for biomass‐derived HC design [160]. To investigate the different mechanisms of air and CO₂ on the pyrolysis of biochar, Yu et al. conducted a comparative study on biomass carbonization under different pyrolysis atmospheres: air and CO₂ [161]. Their results revealed that air pyrolysis tended to introduce a higher proportion of inorganic constituents into the resulting biochar, while CO₂ favored the preservation of organic carbon.

In summary, these studies underscore that regulating the pyrolysis atmosphere offers an effective means to tailor the microstructure and surface chemistry of biomass‐derived HC. Nevertheless, this strategy is often accompanied by a reduction in product yield, which in turn elevates the overall production cost [4].

4.2.2. Pre‐Treatment Process

In the preparation process of biomass‐based HC, the pre‐treatment is a crucial step, and it mainly aims to optimize the composition and structure of biomass to enhance the performance during the subsequent carbonization process. There are various pre‐treatment methods, including hydrothermal pre‐treatment, acid washing pre‐treatment, and enzymatic degradation, and so on [162, 163, 164].

4.2.2.1. Hydrothermal Pre‐treatment

Hydrothermal pre‐treatment is a relatively common method. It is not only low‐cost and in line with the concept of sustainable development, but also plays a certain role in enhancing the electrochemical performance. For example, Ma et al. employed a hydrothermal carbonization route as a tunable pre‐treatment process for oak leaves biowaste precursors, followed by high temperature carbonization to produce high‐performance HC anodes (Figure 8a) [165]. It is worth noting that the team discovered that pre‐hydrothermal could modulate and stabilize the properties of the HC, thereby achieving a smooth spherical morphology and generating more defects. These defects exhibited an inhomogeneous surface electrostatic potential and a lower activation energy for Na⁺ adsorption. In turn, this led to the formation of a core‐shell‐like SEI enriched with PF₆ ⁻ and NaF, which accelerated interfacial Na⁺ diffusion kinetics. Similarly, Zhang et al. adopted a method combining low‐temperature sulfuric acid hydrothermal treatment and high‐temperature carbonization. This method not only selectively etched the hemicellulose component of the biomass but also utilized a reconstruction strategy to achieve a high closed pore volume (0.23 cm³ g⁻¹) and small closed pore size (1.8 nm), thereby obtaining excellent electrochemical performance for Na⁺ storage [166].

FIGURE 8.

The modification of biomass‐derived hard carbon by (a) hydrothermal pretreatment. Reproduced with permission [165]. Copyright 2025, Wiley. (b) Elemental composition of biomass ash during pyrolysis. Reproduced with permission [4]. Copyright 2025, Wiley. The modification of biomass‐derived hard carbon by (c) acid washing pre‐treatment. Reproduced with permission [167]. Copyright 2017, Royal Society of Chemistry, (d) enzymatic degradation pretreatment. Reproduced with permission [168]. Copyright 2025, American Chemical Society, (e,f) heteroatom doping. Reproduced with permission [169]. Copyright 2025, Elsevier. Reproduced with permission [170, 171]. Copyright 2022, Elsevier, (g) metal doping. Reproduced with permission [172]. Copyright 2018, American Chemical Society, and (h,i) composite with soft carbon and metal oxides/sulfides. Reproduced with permission. Copyright 2025, Wiley, [173]. Reproduced with permission [174]. Copyright 2024, Elsevier.

4.2.2.2. Acid Washing Pre‐Treatment

Ash elements are intrinsic constituents of biomass raw materials, which will form different compounds such as carbonates, silicates, and phosphates during the biomass pyrolysis process (Figure 8b) [4]. The presence of ash elements significantly affects the reversible capacity and ICE of HC. While some elements in the ash indeed contribute to the performance enhancement of HC, the complexity of ash content poses challenges to the reproducibility required for large‐scale production. For example, HC derived from sunflower seeds, which contained 3.2 wt.% ash, exhibited a reversible capacity of 300 mAh g⁻¹. In contrast, the HC obtained from papaya seeds with an ash content of 6.7 wt.% demonstrates a lower reversible capacity of just 256 mAh g⁻¹ [175]. Therefore, as shown in Figure 8c, Dahbi et al. successfully removed inorganic potassium salt impurities by washing argan shells with HCl before carbonization, thereby increasing the ICE from the original 66.2% to 76.9% and the capacity from 350 to 372 mAh g⁻¹ (the sample carbonized at 800°C) [167].

4.2.2.3. Enzymatic Degradation Pre‐Treatment

Compared with traditional hydrothermal or acid washing methods, the use of enzymatic degradation can break down the complex components in biomass, such as cellulose, hemicellulose, and lignin, thereby tailoring their properties. This simplifies the process, not only reducing costs but also being milder and more env environmentally friendly. Jiang et al. utilized a variety of bio‐enzymes secreted by Bacillus licheniformis to selectively cleave the chemical bonds in cellulose, hemicellulose, and lignin within lignocellulosic biomass (Figure 8d) [168]. This process partially depolymerized these biopolymers into short‐chain biopolymers, thereby precisely tailoring the molecular composition of HC precursor. The results demonstrated that the basswood‐derived HC achieved a reversible capacity of 366.4 mAg g⁻¹. Notably, even when the current density was increased to 1000, the plateau capacity retention rate remained as high as 74.3%.

4.2.2.4. Post‐Treatment Strategies

In addition to pre‐treatment, post‐treatment strategies have emerged as effective approaches to further optimize the microstructure and surface properties of HC after carbonization. Compared with pre‐treatment, post‐treatment enables more targeted regulation of specific structural features, thereby providing deeper insights into the structure‐performance relationship of HC materials. Recent comprehensive studies and reviews highlight that post‐treatment techniques, such as mechanical processing, thermal reconstruction, activation, surface functionalization, and doping modification, are pivotal for the structural reconstruction of HC, each of which plays a distinct role in modulating sodium storage behavior [176, 177, 178, 179].

Mechanical processing (e.g., ball milling) can reduce particle size and improve ion diffusion kinetics. However, it often introduces excessive defects and opens closed pores, leading to a loss of plateau capacity and reduced ICE [180, 181]. Thermal reconstruction promotes atomic rearrangement, repairs excessive defects, and adjusts pore structure, thereby enhancing electronic conductivity and improving ICE [182]. In some cases, modified flash Joule heating technology can also induce the formation of additional closed nanopores, contributing to increased plateau capacity [183]. Surface coating strategies, such as carbon coating or soft carbon encapsulation, can effectively reduce the exposed surface area and suppress excessive electrolyte decomposition, thereby stabilizing the solid electrolyte interphase (SEI) and ICE [184]. Activation and heteroatom doping further regulate pore structure, electronic properties, and surface chemistry. Activation increases pore volume and accessible surface area, contributing to higher capacity [185], whereas doping introduces additional active sites and modulates charge distribution, enhancing Na⁺ adsorption and diffusion kinetics [176].

Notably, post‐treatment strategies often involve trade‐offs. For example, increasing defect density and surface area can enhance slope capacity but may lead to severe irreversible Na⁺ consumption and low ICE, while excessive structural reconstruction may reduce active sites. Therefore, rational design of post‐treatment processes requires balancing pore structure, defect density, and surface chemistry to optimize both capacity and efficiency. Overall, post‐treatment provides an effective route for decoupling and precisely tuning individual structural parameters, offering valuable guidance for understanding sodium storage mechanisms and improving the practical performance of HC.

4.2.3. Doping and Composite Strategies

In addition to process optimization, heteroatom doping, metal element doping, and composite engineering have emerged as versatile strategies to improve the electrochemical properties of biomass‐derived HC. Despite their differences in implementation, their approaches share a common mechanism: they effectively tailor the carbon microstructure and electronic environment. They also modulate defect density and optimize the SEI, thereby enhancing Na⁺ storage capacity, rate kinetics, and cycling stability [186, 187, 188].

4.2.3.1. Heteroatom Doping

Heteroatom doping, such as N, S, P, B, F, introduces abundant defects and additional Na⁺ storage sites, while simultaneously modulating the electronic structure of carbon, thereby enhancing the capacity, conductivity, and rate performance. For instance, N doping can create pyridinic and pyrrolic configurations that contribute to reversible Na⁺ adsorption. Huang et al. doped carbon derived from waste African redwood with N via co‐pyrolysis with melamine, which introduced abundant pyridinic‐N and pyrrolic‐N sites, and promoted the formation of close pores [170]. As shown in Figure 8e, Ji et al. found that agar‐derived N‐doped carbon with abundant N‐6/N‐5 sites exhibited a defect‐rich structure, facilitating Na⁺ transport. This material delivered a high capacity of 560.5 mAh g^{− 1}, and excellent ICE of 74.9% [169]. And P doping can enlarge interlayer spacing, facilitating ion diffusion. Wang et al. employed a one‐step phosphoric treatment to achieve P‐doping in sisal fiber‐derived HC, which expanded the interlayer spacing and introduced abundant defect sites for Na⁺ storage [189]. This led to significantly improved cycling stability, rate performance, and capacitive behavior compared with undoped carbon. Similarly, He et al. prepared F‐doped microporous carbon nanospheres via annealing and solvothermal treatment [190]. The F element could suppress irreversible Na⁺/K⁺ adsorption and facilitate electrochemical kinetics, while contributing to the formation of stable SEI and improved cycling stability.

Compared with single heteroatom doping, co‐doping strategies are more effective. Because the synergistic interaction between different dopants can better regulate the carbon structure, provide diverse active sites, and enhance both electronic conductivity and ion transport. He et al. found that S/N co‐doped hierarchical porous carbon derived from biomass starch showed enlarged interlayer spacing and a defect‐rich porous structure (Figure 8f) [171]. It could enhance Na⁺ storage and demonstrate high capacity and long‐term cycling, illustrating the advantages of dual heteroatom doping. Ding et al. reported that S/P co‐doped HC derived from oak seeds [191]. The co‐doping improved ion and electron transport, increased active sites, and lowered the Na⁺ nucleation barrier, enabling uniform and reversible Na plating.

In short, heteroatom doping in HC, including single‐element doping and co‐doping, introduces defects, enlarges interlayer spacing, and provides additional Na⁺ storage sites, thereby enhancing capacity, conductivity, and rate performance. Single‐element doping allows targeted tuning of structure and ion adsorption, while co‐doping exploits synergistic effects for improved ion/electron transport and cycling stability [187, 188]. The future challenges include precise control of the type, content, and distribution of dopants to optimize performance and maintain structural integrity over long‐term cycling processes.

4.2.3.2. Metal Doping

Some researchers found that doping HC with trace amounts of metals has been shown to significantly enhance electrochemical kinetics by introducing abundant active centers and favorable electronic energy levels. These metallic species can improve electronic conductivity, facilitate ion transport, and stabilize the electrode structure during cycling [192]. Wu et al. demonstrated that HC derived from coconut endocarp rich in K⁺ can be prepared via a simple and solvent‐free carbonization method (Figure 8g) [172]. The K⁺ doping expanded interlayer spacing, improved ion transport, and enhanced electrochemical performance. Yu et al. prepared Ca²⁺ doped HC derived from tamarind fruits [193]. The introduced Ca²⁺ served to expand the carbon interlayer spacing, while careful control of the calcination temperature preserved the intrinsic porosity, leading to enhanced cycling stability of the resulting material. Furthermore, Huang et al. synthesized Ni/N co‐doped HC nanoshells by doping Ni²⁺ chitosan‐derived carbon sources, ensuring uniform nanographitic domains [194]. The synergistic effect of Ni²⁺, which induced graphitization and N‐doping, which induced defect/electronic modulation, enhanced Na⁺ storage.

4.2.3.3. Composite Strategies

Besides heteroatom and meta doping, combining biomass‐derived HC with other materials has emerged as an effective strategy to further enhance electrochemical performance. HC can be combined with soft carbon to improve electronic conductivity and structural flexibility, or integrated with metal oxides/sulfides to provide additional redox‐active sites, buffer volume changes, and facilitate ion transport [195].

HC offers abundant Na⁺ storage sites but suffers from poor conductivity, while soft carbon (SC) exhibits higher graphitization and faster charge transfer, yet limited capacity [196, 197, 198]. According to the above, Xie et al. reported a composite which exhibited higher capacity and ICE than single phase hard or soft carbon by pyrolysis of a mixture of hard and soft carbon precursors at 1000°C [199]. The optimal HC/SC composite was synthesized by tuning precursors’ ratios, resulting in a reduced loss of SEI‐related Na⁺ while preserving abundant storage sites, enabling adsorption in the high‐voltage sloping region and nanovoid filling in the low‐voltage plateau region. Similarly, Yin et al. synthesized an HC/SC composite using pecan shells as the HC precursor and asphalt as the soft carbon source at 1000°C [200]. The soft carbon effectively sealed excessive open pores in HC, thereby reducing surface area and suppressing side reactions. As a result, the optimal product delivered higher capacity (68.4% improved at 50 mA g⁻¹) along with superior rate performance and cycling stability compared to pure HC. In addition to the previous studies on simple soft carbon, as shown in Figure 8h, Cui et al. enhanced electrochemical performance by coating pine wood‐derived HC with N‐doped soft carbon [173]. The soft carbon coating substantially increased the number of closed pores, while nitrogen doping enhanced Na⁺ adsorption kinetics and provided additional active sites for sodium storage. Consequently, the optimal anode delivered a high reversible capacity of 314 mAh g⁻¹ at 30 mA g⁻¹, with an improved ICE of 85.2%.

In addition, HC was also modified by compositing with metal oxides/sulfides, which provide extra redox‐active sites and buffer volume changes, offering a distinct mechanism for performance enhancement. Zhao et al. reported the fabrication of C@MoO composite by coating biomass‐derived carbon spheres with MoO nanoparticles [201]. The nanoscale MoO and resulting core‐shell structure mitigate volume expansion during cycling, while the robust C‐O‐Mo bonds facilitate charge transfer, collectively enhancing cycling stability and rate performance. Shi et al. prepared an N, S co‐doped hard carbon/ZnS composite, using cedarwood wastes as the precursor (Figure 8i) [174]. The composite exhibited a high reversible capacity (369.7 mAh g⁻¹), an excellent rate performance, and a stable cycling (308.3 mAh g⁻¹). The excellent performance was attributed to the synergy between ZnS and N‐doped HC, which provides additional Na⁺ storage sites and enhances the electronic conductivity, respectively.

In summary, combining biomass‐derived HC with SC or metal compounds represents an effective approach to simultaneously improve Na⁺ storage. SC integration reduces surface area, seals pores, and accelerates charge transfer, while metal oxides/sulfides provide additional redox‐active sites and buffer volume changes. Nitrogen or dual heteroatom doping further enhances adsorption kinetics and active site density. To provide a comparative perspective, the precursors and electrochemical performance of biomass‐based HC from recent works are summarized in Table 2. Future efforts could focus on optimizing precursor selection, composite architectures, and doping strategies to maximize capacity, rate performance, and long‐term cycling stability, ultimately guiding the rational design of high‐performance biomass‐derived HC anodes for SIBs.

TABLE 2.

Summary of biomass from different precursors and electrochemical performance of their derived HC.

Precursors	Electrolyte	Capacity (mAh g−1)	ICE (%)	Rate performance (mAh g−1)	Capacity retention (%)	Refs.
walnut shell	1 M NaPF6 in DME	356.2@0.03A g−1	92.8	174.3@2 A g−1	79.8 (450th)@1A g−1	[202]
mangrove wood	1 M NaClO4 in EC: DEC (1:1)	266.6@0.05 A g−1	75	—	80.1 (200th)@0.5 A g−1	[203]
pine wood	1 M NaCF3SO3 in DEG: DME (1:1)	227.1@0.02 A g−1	66.09	63.47@0.4 A g−1	70.56 (100th)@0.05 A g−1	[204]
corn cob enzyme‐hydrolyzed lignin	1 M NaPF6	483.77@0.025 A g−1	83.83	—	70.9 (300th))@1 A g−1	[205]
starches	1 M NaPF6 in EC: DEC (1:1)	333.2@0.03 A g−1	82.5	—	99 (3000th)@0.1 A g−1	[206]
Bamboo pulp board	1 M NaPF6 in DME	338@0.02 A g−1	92.27	140.2@2 A g−1	90.98 (5000th)@0.1 A g−1	[207]
coconut shells	1 M NaPF6 in Dig	326.87@0.05 A g−1	75.17	174.05@1.5 A g−1	89.39 (1000th)@0.5 A g−1	[208]
cassava starch	1 M NaClO4 in EC: DEC (1:1)	451.9@0.1 A g−1	81.5	204@10 A g−1	88.5 (1000th)@0.1 A g−1	[209]
cellulose	1 M NaPF6 in DME	368@0.02 A g−1	90	234@3 A g−1	84.6(2000th)@1 A g−1	[210]

5. The Preparation Route and Modification Strategies of Needle Coke‐Based Hard Carbon Anode Material

5.1. The Overview of Needle Coke as a Precursor

Needle coke, a high‐value industrial byproduct derived from fluidized catalytic cracker decent oil or coal‐tar pitch [211, 212], generally features a unique graphite‐like layered structure, excellent electrical conductivity, and mechanical strength [38]. Benefiting from its cost‐effectiveness and scalable production, needle coke has been regarded as an excellent precursor for producing high‐power carbon anodes for decades in lithium‐ion batteries. However, its application as a precursor for sodium‐ion batteries' HC anodes remains limited. This is primarily because needle coke undergoes high‐temperature calcination during the industrial production process [213], forming a thermally stabilized microstructure that is inherently resistant to pore engineering and interlayer spacing regulation, which are critical parameters for optimizing sodium‐ion storage performance. Therefore, targeted modifications are required to tailor the microstructure of needle coke, thereby fully exploiting its potential as a low‐cost hard carbon precursor for SIBs.

5.2. The Modification Route of Needle Coke‐Based Hard Carbon

To rectify the constraints of narrow interlayer spacing and deficient active sites, researchers have employed a range of modification strategies to enhance the sodium storage properties of needle coke‐based HC, which primarily encompass heteroatom doping, metal catalysis, and surface oxidation. These methods are designed to enlarge carbon layers, create additional storage sites, and optimize the surface chemistry of the needle coke framework, thereby improving overall electrochemical performance.

5.2.1. Heteroatom Doping for Needle Coke‐Based Hard Carbon

The modification strategies of needle coke‐based HC currently mainly focus on heteroatom doping, metal catalytic regulation, and surface oxidation. Doping means introducing other elements or compounds into a matrix to alter its charge and spin distribution as well as the band gap, thereby regulating the surface chemical properties. Sun et al. [39] used S₈ power as sulfur source and etching agent to modify needle coke through a two‐step method of ball milling and carbonization process, thereby successfully preparing thiophene sulfur‐doped needle coke‐based porous carbon (Figure 9a). The optimized SNPC31 sample (a mass ratio of S₈ to needle coke is 3:1) exhibits a high thiophene‐sulfur doping content of 11.95% and consists of rich C─S─C bonds, expanding interlayer spacing from 0.348 to 0.363 nm and increasing specific surface area from 2.1 to 314.5 m² g^{− 1}. Meanwhile, SNPC31 forms the abundant porous structure predominantly around 4 nm (Figure 9b–d), which significantly shortens the ion transport pathway and enhances the electrochemical performance of needle coke‐based hard carbon anodes in SIBs. As shown in Figure 9e,f, the SNPC31 anode delivers high specific capacities of 552.8 mAh g⁻¹ at 50 mA g⁻¹ with the ICE of 55.6%, and demonstrates excellent cycle performance with the specific capacity of 311.6 mAh g^{− 1} at 1 A g^{− 1} with 84.5% capacity retention after 1000 cycles.

FIGURE 9.

(a) Schematic of the preparation process of SNPC. Reproduced with permission [39]. Copyright 2025, Elsevier. (b) SEM image of SNPC31. (c) Pore‐size distribution of NC and SCNPC. (d) High‐resolution XPS spectra of C 1s of SNPC31. (e) Initial charge/ discharge curves of NC and SNPC anodes. (f) Cycling performance of NC and SNPC anodes at 1 A g⁻¹ for 1000 cycles.

5.2.2. Metal Catalysis for Needle Coke‐Based Hard Carbon

Metal catalysis is also an effective method for regulating the surface chemistry of needle coke. Li's group [73] achieved the conversion of C─O to C═O via Fe‐ catalyzed transformation, characterized by the increase of active C═O containing oxygen‐containing functional group (OFGs) and the decrease of inert C─O groups (Figure 10a,b), which directly enhances the sodium storage performance of needle coke‐based hard carbon anodes. As shown in Figure 10c,d, the modified NC‐2‐A exhibits excellent electrochemical performance with the reversible capacity reaching 306 mA h g⁻¹ after 100 cycles at 0.05 A g⁻¹, and the capacity remains at 194 mA h g⁻¹ after 1000 cycles at 2 A g⁻¹. Building upon the systematic exploration of Fe‐based catalytic strategy for OFGs regulation on the surface of needle coke, Li's group further advanced this modification of the bimetallic synergistic effects in their subsequent work [214]. They demonstrated that Fe/Ni bimetallic systems could more effectively promote the conversion from inert C‐O groups to active C═O groups compared to monometallic Fe catalysts. The DFT calculations revealed that the introduction of Ni as a secondary metal significantly increases the active electrons near the Fermi level, and reduces the energy barrier for C═O formation (Figure 10e–g). Compared with the Fe‐monometallic system HC anode, the Fe/Ni HC anode shows a certain improvement in electrochemical performance.

FIGURE 10.

(a) Schematics of the synthesis process of Fe‐NC hard carbon anodes via metal catalytic regulation. Reproduced with permission [73]. Copyright 2023, Elsevier. (b) The ratio of different OFGs in O 1s spectra and C‐C sp³ in C 1s spectra of Fe‐NC hard carbon anodes. (c) Cycling performances at 0.05 A g⁻¹ for Fe‐NC hard carbon anodes. (d) Long cycling performance of NC‐2‐A of Fe‐NC hard carbon anodes at 2 A g⁻¹. (e) Reaction coordinates for C═O formation from C‐O on x‐NC, TS represents the transition state. (f, g) PDOS for 3d orbitals of metal atoms on x‐NC, the black lines denote the Fermi level. (h) Schematic illustration of the NC‐x synthesis process via surface oxidation. Reproduced with permission [40]. Copyright 2020, Elsevier. (i) XRD patterns of NC‐x. (j) Raman spectra of NC‐x. (k) Rate capability of NC‐x. (l) Cycling performance of NC‐x at 0.05 A g⁻¹. (m) Long‐term cycling test of NC‐2 at 2 A g⁻¹ for 1000 cycles.

5.2.3. Surface Oxidation for Needle Coke‐Based Hard Carbon

Surface oxidation is a way of modifying the chemical and physical properties of carbonaceous materials. Zhao et al. [40] introduced OFGs onto the surface of needle coke through oxidation treatment using concentrated sulfuric acid and potassium permanganate, which significantly increased the adsorption sites (C═O) for sodium ion (Figure 10h). As shown in Figure 10i,j, with the continuous increase of KMnO₄, the interlayer spacing of NC‐x gradually expands to 0.384 nm and the disorder degree of NC‐x also increased. This structural optimization not only provides sufficient space for the smooth intercalation/deintercalation of Na⁺ by expanding the interlayer spacing but also creates more active sites for Na⁺ surface adsorption through the increased defects, thereby collectively laying the structural foundation for enhanced sodium storage performance. Therefore, the resulting material under optimal oxidation (NC‐2) realizes the highest rate capacity of 385 mA h g⁻¹ at 0.05 A g⁻¹, and maintains a capacity of 360 mAh g⁻¹ after 100 cycles at the same current density (Figure 10k,l). Additionally, even at a high current density of 2 A g⁻¹, NC‐2 still retains a reversible capacity of 153 mAh g⁻¹ after 1000 cycles (Figure 10m), which demonstrates that tailoring the surface chemistry of needle coke through oxidation is a highly effective strategy for enhancing its sodium storage capacity.

In summary, modification strategies such as heteroatom doping, metal catalysis, and surface oxidation are effective for regulating the microstructure and surface chemistry of needle coke, which significantly improve Na⁺ storage capacity by introducing more active sites. To provide a comprehensive overview, the electrochemical performances of representative needle coke‐based HC anodes are summarized in Table 3. Although research in this field is currently limited compared to other precursors, needle coke stands out as a promising candidate for large‐scale industrialization due to its inherent low cost and abundant supply.

TABLE 3.

Summary of the electrochemical performance of needle coke‐based HC in recent works.

Name	Electrolyte	Capacity (mAh g−1)	ICE (%)	Rate performance (mAh g−1)	Capacity retention (mAh g−1)	Refs.
SNP31	1 M NaPF6 in DME	552.8@0.05 A g−1	55.6	∼320@2 A g−1	312 (1000th)@1 A g−1	[39]
NC‐2	1 M NaClO4 in PC: EC (1:1) (5% FEC)	1035@0.05 A g−1	42.1	188@2 A g−1	153 (1000th)@2 A g−1	[40]
NC‐2‐A	1 M NaClO4 in PC: VC (1:1) (5% FEC)	768.6@0.05 A g−1	48.6	188@2 A g−1	194 (1000th) @2 A g−1	[73]
Fe/Ni‐NC	1 M NaClO4 in PC: EC (1:1) (5% FEC)	836.9@0.05 A g−1	47.5	222.1@2 A g−1	212.7 (1000th) @2 A g−1	[183]
NCAPS	1 M NaPF6 in Dig	395.6@0.06 A g−1	69.9	195@1.5 A g−1	125 (5000th)@1.5 A g−1	[215]

6. Conclusions and Perspective

6.1. Conclusions

HC is considered the most promising anode for SIBs because of its low‐cost and abundant precursors, and stable cycling performance. Its unique structural features, such as optimized interlayer spacing, pore structure, and tunable surface chemistry, provide efficient pathways for Na⁺ storage and transport, highlighting its strong potential for commercialization. In this review, three representative and industrially viable precursors (resin, biomass, and needle coke) have been systematically analyzed, with particular emphasis on their structure regulation strategies and sodium storage performance. These results indicate that the selection of the precursor is not independent of performance optimization, but fundamentally determines the tunability of critical structural features in HC, thereby directly influencing sodium‐storage mechanisms and electrochemical behavior. Furthermore, different precursor systems exhibit distinct differences in structural controllability, cost, reproducibility, and scalable compatibility (as summarized in Table 4).

1. Resin‐based precursors feature high carbon yields, highly controllable molecular compositions, and excellent reproducibility. Through targeted pore design and heteroatom doping, they deliver the most reproducible electrochemical performances. However, their reliance on expensive and sometimes toxic chemicals raises significant concerns regarding manufacturing costs and long‐term environmental sustainability.

2. Biomass‐based precursors offer hierarchical porosity, abundant inherent defects, and ultra‐low raw material costs. While it is renewable and environmentally friendly, the critical challenges lie in its low carbon yield, excessive open porosity, and poor batch‐to‐batch reproducibility, which severely complicate standardized industrial scalability.

3. Needle Coke‐based precursors benefit from a well‐established industrial chain, needle coke offers superior mechanical robustness, high electrical conductivity, and consistent quality. Nevertheless, its inherently highly graphitizable and stable structure restricts the adjustability of key characteristics, making it particularly challenging to expand interlayer spacing or induce closed pores for enhanced sodium storage capacity.

TABLE 4.

Comparative evaluation of representative industrial precursors for hard carbon production.

Precursor	Source	Microstructure characteristics	Electrochemical performance	Cost	Supply consistency	Environmental sustainability
Resin	Synthetic polymers	Highly tunable; uniform pore structure; controllable defect density; adjustable interlayer spacing	High capacity; relatively high ICE	Moderate ‐High	Moderate ‐High	Moderate ‐Low
Biomass	Natural materials	Rich in defects; variable pore distribution; inherent heteroatoms	Moderate‐high capacity; moderate‐high ICE performance variability	Low	Low ‐Moderate	High
Needle coke	Petroleum/Coal‐derived industrial carbon precursor	Ordered microstructure; low defect density; limited pore development; smaller interlayer spacing	Good conductivity high cycling stability; relatively low capacity	Low	High	Moderate ‐Low

These findings clearly indicate that no single precursor system is universally optimal, and that each exhibits distinct trade‐offs among structural controllability, electrochemical performance, cost, and scalability. Therefore, from an industrial perspective, the future development of HC anodes should not be limited to the optimization of a single parameter, but rather should focus on balancing electrochemical performance with manufacturability, feedstock sustainability, and process economy. In particular, strategies that enable precise yet scalable structural regulation, improved ICE, and environmentally friendly synthesis routes will be critical for the practical deployment of hard carbon in SIBs.

6.2. Perspective

A comparative evaluation of these three precursors from multiple perspectives is summarized in Table 4, exhibiting significant differences in terms of cost, supply stability, environmental sustainability, and electrochemical performance. The fundamental reason for these differences lies in their inherent material properties, which directly determine their respective scalability and application prospects in large‐scale HC anode production.

Resins exhibit a highly tunable and uniform pore structure, controllable defect density, and adjustable interlayer spacing, thereby delivering high reversible capacity and relatively high ICE. However, the relatively higher cost limits their use in ultra‑low‑cost, large‑scale grid storage unless greener resin sources (e.g., lignin‑based or bio‑derived resins) and lower energy synthesis routes are developed. Future scalability efforts should focus on reducing raw material costs through bio‑based alternatives and establishing continuous production processes to enhance yield.

Biomass‑derived HC has the advantages of low cost and high environmental sustainability, but its supply consistency is relatively low due to natural variations in feedstock composition. Biomass is inherently rich in defects and heteroatoms, leading to higher capacity and ICE. These characteristics make biomass‑derived HC the most promising candidate for large‑scale energy storage applications, such as grid‑level batteries and renewable energy integration. However, the main bottleneck to achieving large‐scale production is the instability of raw materials and the low carbon yield of many biomass precursors. To overcome these, future work should establish regional supply chains with standardized pre‑treatment (e.g., hydrothermal carbonization, acid washing) to resolve the consistency issues. With these measures, biomass‑derived HC may become the main anode material for sustainable and cost‐effective SIBs.

As a petroleum/coal‐derived industrial precursor, needle coke leverages mature petrochemical industries to offer low cost and exceptionally high supply consistency. Microstructurally, it naturally forms an ordered structure with low defect density and smaller interlayer spacing. Although limited porosity leads to a relatively low capacity, it ensures excellent electrical conductivity and high cycling stability. Its primary concern remains the relatively lower capacity. Future improvements should focus on increasing capacity through controlled oxidation, heteroatom doping, or co‐carbonization with biomass to introduce more defects and pores, while maintaining the cost and consistency advantages. Additionally, exploring catalytic graphitization at lower temperatures could reduce the energy consumption.

In summary, the three precursor types are not mutually exclusive but rather complementary. Future efforts should focus on their respective advantages, developing recyclable manufacturing technologies for achieving sustainable material recycling.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

All datasets analyzed in this review are cited in the manuscript. Detailed methodologies and raw data can be accessed via the original publications.