In recent years, the demand for large-scale, safe, and reliable clean energy storage devices has surged, with sodium-ion batteries emerging as a focal point in energy storage research. As a potential successor to lithium-ion batteries, sodium-ion batteries offer distinct advantages due to the abundance of sodium resources in the Earth’s crust—approximately 440 times more plentiful than lithium—and their lower cost. However, the commercialization of sodium-ion batteries faces challenges, particularly in developing suitable anode materials. The larger ionic radius of sodium ions compared to lithium ions makes it difficult for conventional graphite anodes, with their limited interlayer spacing of 0.335 nm, to accommodate sodium intercalation. Moreover, sodium-graphite intercalation compounds are thermodynamically unstable. In contrast, hard carbon materials have garnered significant attention as promising anode candidates for sodium-ion batteries due to their numerous defects, rich pore structures, appropriate sodium intercalation potentials, high theoretical capacity (around 300 mAh/g), low cost, and good safety profile. Despite these advantages, hard carbon anodes still suffer from issues such as low initial Coulombic efficiency (ICE), poor rate capability, and unsatisfactory cycling stability in practical applications. In this article, I will explore the structural characteristics of hard carbon and delve into various调控 strategies, including pore structure regulation, heteroatom doping, and hard-soft (hard) carbon composites, to enhance the performance of sodium-ion batteries. I will also incorporate tables and mathematical formulas to summarize key findings and provide insights for future commercialization.
The unique properties of hard carbon stem from its amorphous structure, which differs markedly from graphitic carbon. Initially described by Franklin through studies on coal carbonization, hard carbon is characterized as non-graphitizing carbon that retains a disordered arrangement even at high temperatures up to 3000°C. This is attributed to the release of small molecules such as H2, CH4, CO, and CO2 during pyrolysis, along with processes like dehydrogenation, condensation, and isomerization, leading to a rigid, cross-linked structure with defects, micropores, and oxygen-containing functional groups. The widely accepted “house of cards” model proposed by Dahn illustrates hard carbon as composed of small, curved graphene sheets, disordered amorphous regions, and nanopores. These graphene sheets are stacked locally but randomly oriented over larger scales, with an expanded interlayer spacing of approximately 0.38 nm compared to graphite’s 0.335 nm. This structure creates abundant closed and open pores, vacancies, and edge defects, which are crucial for sodium ion storage. The sodium storage mechanism in hard carbon is complex and debated, often involving a combination of adsorption, intercalation, and pore-filling processes. Typically, the charge-discharge profile shows a sloping region at higher potentials attributed to adsorption on defect sites and a plateau region at lower potentials associated with sodium insertion into nanopores. The capacity can be expressed as:
$$ C = C_{\text{sloping}} + C_{\text{plateau}} $$
where \( C_{\text{sloping}} \) represents the capacity from adsorption and \( C_{\text{plateau}} \) from pore filling. Understanding this mechanism is essential for optimizing hard carbon anodes for sodium-ion batteries.
To address the limitations of hard carbon anodes, researchers have developed various structural调控 strategies. These approaches aim to tailor the pore architecture, introduce heteroatoms, or create composite materials, thereby improving ICE, rate performance, and cycling stability. In the following sections, I will discuss each strategy in detail, supported by experimental data and theoretical insights.
Pore Structure Regulation
Pore structure plays a pivotal role in determining the electrochemical performance of hard carbon anodes in sodium-ion batteries. Pores can be classified as open or closed, and by size as micropores (<2 nm), mesopores (2–50 nm), or macropores (>50 nm). Open pores facilitate better electrolyte penetration and sodium ion diffusion, while closed pores may contribute to irreversible sodium trapping and low ICE. Regulation of pore structure can be achieved through precursor selection, carbonization conditions, and templating methods.
For instance, Jing et al. used natural wood as a precursor, subjecting it to chemical treatment and high-temperature carbonization to transform closed pores into open, hierarchical pore structures. This design shortened sodium ion diffusion paths, resulting in excellent rate capability. The hard carbon delivered a capacity of 186 mAh/g at 0.1 A/g and maintained 238 mAh/g after 20 cycles at 0.05 A/g. Similarly, Xiao et al. demonstrated that slower heating rates during carbonization reduce porosity and defect formation, leading to higher ICE. At a heating rate of 0.5°C/min, an ICE of 86.1% was achieved, about 7% higher than at 5°C/min. This is because lower porosity minimizes solid electrolyte interphase (SEI) formation on internal surfaces.
Precise control over pore size distribution can further optimize performance. Yang et al. treated walnut shells with cetyltrimethylammonium bromide and KOH to eliminate micropores while preserving mesopores. The resulting hard carbon, rich in mesopores, exhibited a capacity of 283.7 mAh/g at 20 mA/g, 83% higher than microporous counterparts. Even after 320 cycles at 200 mA/g, a capacity of 189.4 mAh/g was retained. This highlights the importance of mesopores in enabling facile sodium ion transport. Moreover, the concept of “sieve carbon” has been proposed, where pores with entrance diameters smaller than 0.4 nm (below the size of solvated sodium ions) and main body diameters below 2 nm can selectively exclude solvent molecules, promoting reversible sodium cluster formation and enhancing plateau capacity. The relationship between pore size and sodium storage can be modeled using the following equation for diffusion-limited capacity:
$$ C_{\text{diff}} = \frac{z F A \sqrt{D t}}{\delta} $$
where \( z \) is the charge number, \( F \) is Faraday’s constant, \( A \) is the electrode surface area, \( D \) is the diffusion coefficient, \( t \) is time, and \( \delta \) is the diffusion length. By tailoring pore structures, \( D \) and \( \delta \) can be optimized for improved kinetics.
Table 1 summarizes recent studies on pore structure regulation in hard carbon anodes for sodium-ion batteries, showcasing various precursors and their electrochemical outcomes.
| Precursor | Pore Type | Initial Coulombic Efficiency (%) | Rate Performance (Capacity at Specific Current) | Reference Insights |
|---|---|---|---|---|
| Rubberwood Sawdust | Hierarchical Open Pores | 56.0 | 275 mAh/g at 100 mA/g | Bio-oil derived hard carbon with enhanced stability |
| Unburned Charcoal | Low Porosity | 88.0 | 292.3 mAh/g at 50 mA/g | Waste-derived hard carbon with high ICE |
| Lotus Seedpod | Hierarchical Porous | 50.0 | 330.6 mAh/g at 50 mA/g | Natural precursor yielding high capacity |
| Waste Cork | Hierarchical Pores | 81.0 | 360 mAh/g at 30 mA/g | Regulated pore structure for improved Na storage |
| Phenolic Resin with Ethanol | Open Pores from Porogen | 84.0 | 410 mAh/g at 30 mA/g | Solvothermal method for precise pore control |
Heteroatom Doping
Heteroatom doping is a powerful strategy to modify the electronic and structural properties of hard carbon for sodium-ion batteries. By incorporating elements such as nitrogen (N), phosphorus (P), sulfur (S), or oxygen (O) into the carbon matrix, one can enhance electrical conductivity, increase defect density, expand interlayer spacing, and provide additional active sites for sodium ion adsorption. Among these, N-doping is the most extensively studied due to its ability to introduce pyridinic N, pyrrolic N, and graphitic N configurations, each contributing differently to sodium storage.
Gan et al. designed a three-dimensional porous hard carbon doped with N via NH3 treatment during pyrolysis. The material, denoted as 3DPCS-800-N, exhibited a high ICE of 84.5% and a capacity of 437 mAh/g at 20 mA/g. They proposed that pyridinic N radicals (C—N•) provide stable sodium storage sites, as evidenced by X-ray absorption near-edge structure spectroscopy showing reversible changes in C—N• bonds during cycling. The doping effect can be quantified by the change in conductivity \( \sigma \), which follows the relation:
$$ \sigma = \sigma_0 + k [\text{Dopant}] $$
where \( \sigma_0 \) is the intrinsic conductivity and \( k \) is a constant dependent on the dopant type.
Phosphorus doping, with its larger atomic radius, effectively expands the interlayer spacing and creates defects. Yan et al. used PCl3 and C6H12 as phosphorus and carbon sources, respectively, to achieve in situ P-doping with up to 30% P content. The P—(C3) bonds protruded in the carbon lattice, increasing interlayer spacing to 0.38–0.40 nm, and facilitated two distinct sodium ion channels. This material delivered an exceptional capacity of 510.4 mAh/g at 100 mA/g and maintained 397.1 mAh/g at a high current density of 10 A/g, demonstrating superior rate capability.
Co-doping with multiple heteroatoms often yields synergistic effects. Huang et al. synthesized N,P-co-doped hard carbon (NPHC) from sagebrush shells and H3PO4. The material featured a three-dimensional hierarchical porous structure and enlarged interlayer spacing, offering dual continuous transport paths for sodium ions. Compared to singly doped NHC and undoped HC, NPHC showed the highest capacity of 336 mAh/g at 25 mA/g. Similarly, Chen et al. prepared N,O,P-co-doped carbon networks (NOP-CN) from starch, urea, and phytic acid. The co-doping further improved conductivity and sodium ion affinity, as confirmed by density functional theory calculations. The capacity reached 341.3 mAh/g at 0.05 A/g, with 119.1 mAh/g retained after 2000 cycles at 5 A/g. The enhanced performance can be attributed to the combined effects of heteroatoms, which modify the charge distribution and reduce diffusion barriers. The sodium adsorption energy \( E_{\text{ads}} \) can be expressed as:
$$ E_{\text{ads}} = E_{\text{total}} – (E_{\text{carbon}} + E_{\text{Na}}) $$
where \( E_{\text{total}} \) is the energy of the doped carbon with adsorbed sodium, \( E_{\text{carbon}} \) is the energy of the doped carbon, and \( E_{\text{Na}} \) is the energy of a sodium atom. Doping typically lowers \( E_{\text{ads}} \), promoting stronger sodium binding.
Table 2 provides an overview of heteroatom doping studies in hard carbon anodes for sodium-ion batteries, highlighting key performance metrics.
| Dopant(s) | Precursor | Initial Coulombic Efficiency (%) | Rate Performance (Capacity at Specific Current) | Key Findings |
|---|---|---|---|---|
| K+ (from KOH) | Coconut Shell | 69.2 | 313.8 mAh/g at 50 mA/g | Expanded interlayer spacing via natural K doping |
| S | Pitch | 56.0 | 488 mAh/g at 50 mA/g | Tunable doping sites for excellent Na storage |
| N, P | Lignin | 82.4 | 336 mAh/g at 25 mA/g | Co-doping enhances capacity and stability |
| N, S, O | Samara (Winged Fruit) | — | 333 mAh/g at 100 mA/g | Hierarchical porous carbon with surface capacitance |
| N (from NH3) | Polymer-derived Carbon | 84.5 | 437 mAh/g at 20 mA/g | Radical sites for extra sodium storage |
Hard-Soft (Hard) Carbon Composite Structures
Combining hard carbon with other carbon materials, such as soft carbon (graphitizable carbon) or another hard carbon variant, is an effective approach to mitigate the shortcomings of pure hard carbon anodes in sodium-ion batteries. Soft carbon, with its higher electrical conductivity and structural order, can complement hard carbon’s disorder and porosity, leading to composites with balanced properties. Similarly, coupling different hard carbon precursors can leverage synergistic interactions during carbonization.
Xue et al. fabricated a composite of porous hard carbon derived from zeolitic imidazolate framework-67 and carbon nanotubes grown from polyvinyl alcohol. This structure provided increased electrode-electrolyte contact area and facilitated sodium ion diffusion, yielding a capacity of 306.8 mAh/g at 500 mA/g and retaining 256.8 mAh/g after 1000 cycles. The composite’s performance surpasses that of individual components, highlighting the benefits of hybridization.
Xie et al. developed a hard-soft carbon composite using filter paper (cellulose-based hard carbon) and mesophase pitch (soft carbon precursor). By adjusting the ratio of hard to soft carbon, they optimized the pore structure; soft carbon tended to block open pores in hard carbon, reducing specific surface area and forming more closed pores. At a 5:2 hard-to-soft carbon ratio, the composite exhibited the best performance, with a capacity retention of 74% after 100 cycles at 150 mA/g. This demonstrates the importance of compositional control in composite design.
Another innovative strategy involves coupling carbonization of mixed precursors to engineer hard carbon structures. Zhang et al. co-carbonized sucrose and phenolic resin, where the interaction between functional groups (e.g., hydroxyl and carboxyl) led to dehydration and cross-linking, resulting in a hard carbon with very low specific surface area (1.54 m2/g). This material achieved a high ICE of 87% and a capacity of 319 mAh/g at 30 mA/g. When paired with an O3—Na0.9[Cu0.22Fe0.30Mn0.48]O2 cathode in a full sodium-ion battery, an initial Coulombic efficiency of 80% and an energy density of 256 Wh/kg were attained. The composite formation can be described by a simple rule of mixtures for capacity:
$$ C_{\text{composite}} = x C_{\text{hard}} + (1 – x) C_{\text{soft}} $$
where \( x \) is the mass fraction of hard carbon, and \( C_{\text{hard}} \) and \( C_{\text{soft}} \) are the capacities of the individual components. However, synergistic effects often lead to deviations from this linear relation.
Table 3 summarizes key studies on hard-soft (hard) carbon composites for sodium-ion battery anodes, illustrating the diversity of approaches and outcomes.
| Composite Components | Precursors/Method | Initial Coulombic Efficiency (%) | Rate Performance (Capacity at Specific Current) | Notable Features |
|---|---|---|---|---|
| Hard Carbon + Carbon Nanotubes | ZIF-67/PVA thermal decomposition | — | 306.8 mAh/g at 500 mA/g | Porous structure with enhanced diffusion |
| Hard Carbon (Cellulose) + Soft Carbon (Pitch) | Filter paper and mesophase pitch | — | High stability at 150 mA/g | Optimized ratio reduces surface area |
| Hard Carbon from Sucrose and Phenolic Resin | Co-carbonization coupling | 85.0 | 310 mAh/g at 20 mA/g | Low surface area, high ICE |
| Hard Carbon from Lignin and Epoxy Resin | Interpenetrating polymer networks | 82.0 | 316 mAh/g at 30 mA/g | Advanced carbon design for high performance |
| Hard Carbon from Pitch and Lignin | Amorphous carbon blend | 82.0 | 254 mAh/g at 30 mA/g | Low-cost anode with good rate capability |
| Hard Carbon from Polyaniline and Graphene Oxide | Hybrid anode material | 50.0 | 336 mAh/g at 30 mA/g | Nitrogen-doped carbon/graphene hybrid |
Mathematical Modeling and Theoretical Insights
To deepen the understanding of sodium-ion battery performance with hard carbon anodes, mathematical models and equations can be employed. These models help quantify relationships between structural parameters and electrochemical properties. For instance, the capacity contribution from different storage mechanisms can be modeled using a combination of adsorption isotherms and diffusion equations. The Langmuir adsorption model can describe sodium ion adsorption on defect sites:
$$ \theta = \frac{K P}{1 + K P} $$
where \( \theta \) is the fractional coverage, \( K \) is the adsorption constant, and \( P \) is the pressure (or concentration) of sodium ions. For intercalation or pore filling, the Nernst equation relates potential to sodium ion activity:
$$ E = E^0 – \frac{RT}{F} \ln \left( \frac{a_{\text{Na, reduced}}}{a_{\text{Na, oxidized}}} \right) $$
where \( E \) is the electrode potential, \( E^0 \) is the standard potential, \( R \) is the gas constant, \( T \) is temperature, and \( a \) represents activities. Additionally, the diffusion of sodium ions in hard carbon can be described by Fick’s second law:
$$ \frac{\partial C}{\partial t} = D \frac{\partial^2 C}{\partial x^2} $$
where \( C \) is concentration, \( t \) is time, \( D \) is the diffusion coefficient, and \( x \) is the spatial coordinate. Solving this with boundary conditions for porous electrodes yields insights into rate-limiting steps. Empirical models, such as the one for ICE, can be expressed as:
$$ \text{ICE} = \frac{C_{\text{rev}}}{C_{\text{rev}} + C_{\text{irr}}} \times 100\% $$
where \( C_{\text{rev}} \) is reversible capacity and \( C_{\text{irr}} \) is irreversible capacity from SEI formation and sodium trapping. By optimizing pore structure and doping, \( C_{\text{irr}} \) can be minimized.
Conclusion and Future Perspectives
Hard carbon materials hold great promise as anode candidates for sodium-ion batteries, owing to their favorable structural characteristics and cost-effectiveness. Through strategic调控 approaches—pore structure regulation, heteroatom doping, and composite formation—significant improvements in initial Coulombic efficiency, rate capability, and cycling stability have been achieved. However, challenges remain in fully understanding the sodium storage mechanisms, particularly the roles of sloping and plateau regions. Future research should focus on in situ and operando characterization techniques to elucidate these mechanisms and guide material design. Additionally, scalable synthesis methods and integration with high-voltage cathodes are crucial for commercial viability. As the field advances, hard carbon anodes are poised to play a pivotal role in enabling next-generation sodium-ion batteries for large-scale energy storage, contributing to a sustainable energy future. I believe that continued innovation in structural调控, coupled with interdisciplinary collaboration, will unlock the full potential of sodium-ion batteries in the global energy landscape.