The rapid development of energy storage technologies has positioned lithium-ion batteries (LIBs) as a dominant solution for portable electronics and electric vehicles. However, concerns regarding the limited geographical distribution and rising cost of lithium resources have spurred significant interest in alternative chemistries. Sodium-ion batteries (SIBs) have emerged as a compelling candidate for large-scale stationary storage due to the natural abundance and even geographical spread of sodium. Sharing a similar “rocking-chair” operational principle with LIBs, where ions shuttle between the cathode and anode, SIBs present a potentially more sustainable and cost-effective pathway.
Nevertheless, the development of high-performance SIBs faces material-level challenges, particularly at the anode. The commercial graphite anode used in LIBs exhibits poor thermodynamic stability with sodium ions, leading to negligible capacity. This incompatibility stems from the larger ionic radius of Na+ (1.02 Å) compared to Li+ (0.76 Å) and a higher redox potential (Na/Na+: -2.71 V vs. Li/Li+: -3.04 V). Consequently, a critical research focus is the development of anode materials specifically tailored for efficient sodium ion storage. Among various candidates, carbon-based materials stand out due to their structural tunability, resource availability, and relatively low cost.
Asphalt, a by-product of petroleum refining or coal processing, is an excellent precursor for carbon materials due to its high carbon content, rich aromatic structure, and low cost. However, carbon derived from the direct pyrolysis of asphalt typically develops a highly graphitized structure with tightly stacked graphene layers. The interlayer spacing (d002) in such materials is usually around 0.335–0.340 nm, which is significantly smaller than the estimated critical spacing required for reversible Na+ intercalation (>0.37 nm). This results in very limited sodium storage capacity, often below 35 mAh g-1. Therefore, strategic modifications to control the microstructure and morphology of asphalt-derived carbons are essential to unlock their potential as high-performance anodes for the sodium-ion battery.
This article reviews the progress in engineering asphalt-based carbon anodes for the sodium-ion battery, focusing on two primary modulation strategies: structural regulation and morphological design. We delve into the technical approaches, underlying mechanisms, and resulting electrochemical performance enhancements. The advantages and limitations of each method are analyzed, and future research directions are outlined to guide the development of next-generation anode materials for the sodium-ion battery.
1. Structural Regulation Strategies
Structural regulation aims to modify the intrinsic arrangement of carbon atoms and the local electronic environment to create favorable sites for sodium ion storage. For asphalt-derived carbons, the primary goal is to inhibit excessive graphitization, expand the interlayer distance, and introduce defects or active sites.
1.1 Pre-oxidation Treatment
Pre-oxidation is a straightforward and scalable method to alter the carbonization pathway of asphalt. The process typically involves heating asphalt in air at moderate temperatures (e.g., 250–350°C) before high-temperature carbonization. The mechanism involves the introduction of oxygen-containing functional groups (e.g., -COOH, -OH, C=O) onto the aromatic molecules of the pitch.
During subsequent high-temperature treatment, these functional groups induce cross-linking reactions between molecular planes via the formation of C-O-C bonds. This cross-linking creates a three-dimensional network that physically hinders the parallel alignment and growth of graphitic domains during carbonization. Furthermore, the decomposition of these oxygen groups at high temperatures releases gases (CO, CO2), generating micropores and defects within the carbon matrix. The combined effect is a significant increase in structural disorder and interlayer spacing (often to 0.38–0.42 nm), transforming the material from a graphitic soft carbon into a disordered hard carbon suitable for sodium-ion battery anodes.
The degree of pre-oxidation is crucial. Insufficient oxidation leads to inadequate cross-linking, allowing partial graphitic ordering to persist. Excessive oxidation can fracture the aromatic backbone, drastically reducing carbon yield and introducing excessive defects that promote continuous electrolyte decomposition, thereby lowering the initial Coulombic efficiency (ICE) and cycle stability. An optimal temperature window (e.g., 300–350°C) balances these effects. The performance of pre-oxidized asphalt-derived carbons in the sodium-ion battery is summarized in Table 1.
| Regulation Strategy | Preparation Method | Reversible Capacity (Current Density) | ICE | Rate Capability | Cycling Stability |
|---|---|---|---|---|---|
| Pre-oxidation | THFI fraction oxidized at 350°C, carbonized at 1400°C | 309.7 mAh g-1 (30 mA g-1) | 90.96% | 209.8 mAh g-1 (1 A g-1) | 205.3 mAh g-1 after 500 cycles at 1 A g-1 |
| Pre-oxidation | Petroleum pitch oxidized at 300°C (0.25°C/min), carbonized at 1400°C | 325.5 mAh g-1 (20 mA g-1) | 77.7% | 264 mAh g-1 (0.5 A g-1) | 186.5 mAh g-1 after 500 cycles at 1 A g-1 |
| Pre-oxidation | Pitch oxidized at 300°C, carbonized at 1400°C | 300.6 mAh g-1 (30 mA g-1) | 88.6% | – | 279.9 mAh g-1 after 200 cycles at 30 mA g-1 |
| Pre-oxidation (Additive) | Pitch/Mg(NO3)2·6H2O composite carbonized at 1400°C | 277.8 mAh g-1 (0.1C) | 80.2% | – | 271.4 mAh g-1 after 200 cycles at 0.1C |
| Pre-oxidation (Additive) | Zinc Gluconate/pitch composite pre-annealed at 400°C, carbonized at 1200°C | 320 mAh g-1 (30 mA g-1) | – | 204 mAh g-1 (5 A g-1) | 250 mAh g-1 after 1000 cycles at 1 A g-1 |
The addition of oxidizing agents (e.g., Mg(NO3)2, dilute HNO3) during pretreatment can further promote the “solid-state carbonization” pathway, effectively consuming residual hydrogen that would otherwise facilitate melting and reorganization, leading to superior performance in the sodium-ion battery.
1.2 Soft/Hard Carbon Composites
This strategy leverages the complementary properties of soft carbon (derived from graphitizable precursors like asphalt) and hard carbon (derived from non-graphitizing precursors like resins or biomass). Hard carbons typically offer larger interlayer spacing and more defects, providing high sodium storage capacity (~300 mAh g-1) but suffer from low electronic conductivity and high surface area leading to low ICE. Soft carbons possess good conductivity but low capacity due to their ordered structure. Creating a composite aims to synergize conductivity with ample active sites.
The composite can be formed by co-carbonizing asphalt with a hard carbon precursor (e.g., phenolic resin, lignin). During pyrolysis, molecular-level interactions or cross-linking at the interface can inhibit the graphitization of the asphalt component while introducing conductive graphitic domains into the hard carbon matrix. This results in a material with optimized electronic percolation paths and enhanced structural stability. The performance of such composites in the sodium-ion battery is shown in Table 2.
| Regulation Strategy | Preparation Method | Reversible Capacity (Current Density) | ICE | Rate Capability | Cycling Stability |
|---|---|---|---|---|---|
| Soft/Hard Composite | Bamboo-derived hard carbon coated with pitch, carbonized | 335.3 mAh g-1 (30 mA g-1) | 86.8% | 274.5 mAh g-1 (1.5 A g-1) | ~305.8 mAh g-1 after 150 cycles at 300 mA g-1 |
| Soft/Hard Composite | Pitch/Phenolic resin mixture carbonized at 1000°C | 349.9 mAh g-1 (50 mA g-1) | 60.9% | 202.1 mAh g-1 (10 A g-1) | 249.3 mAh g-1 after 2500 cycles at 1 A g-1 |
| Soft/Hard Composite | Esterification coupling of oxidized pitch with glucose, carbonized | 328.5 mAh g-1 (20 mA g-1) | 92.08% | 102.2 mAh g-1 (1 A g-1) | 250.6 mAh g-1 after 500 cycles at 0.2 A g-1 |
The success of this approach hinges on achieving intimate interfacial contact and optimizing the ratio between the soft and hard carbon phases to balance capacity, conductivity, and ICE for the sodium-ion battery.
1.3 Heteroatom Doping
Introducing heteroatoms (e.g., N, S, P) into the carbon lattice is a powerful method to tailor the electronic structure, induce defects, and expand interlayer spacing. Doping creates active sites for sodium ion adsorption and can enhance electronic conductivity, improving both capacity and rate performance of the sodium-ion battery anode.
1.3.1 Nitrogen Doping: Nitrogen atoms can integrate into the carbon matrix as pyridinic N (N-6), pyrrolic N (N-5), or graphitic N (N-Q). N-6 and N-5 species create defects and edge sites that enhance capacitive sodium storage, while N-Q improves electronic conductivity. Doping is often achieved by carbonizing pitch in the presence of nitrogen sources like NH3 or urea.
1.3.2 Sulfur Doping: Sulfur has a larger covalent radius (1.02 Å) than carbon. Its incorporation can significantly expand the interlayer spacing. Furthermore, sulfur atoms, especially in the form of thiophene-S, can modify the local charge density and participate in reversible Na-S bonding, contributing to extra capacity. The capacity contribution from sulfur doping (CS) can be conceptually related to the doping level and the associated redox reactions.
1.3.3 Phosphorus Doping: Phosphorus has low electronegativity and strong electron-donating ability. P-doping can create abundant defects, widen the d-spacing, and generate electron holes in the carbon matrix, significantly boosting electronic conductivity.
1.3.4 Co-doping: Co-doping with multiple heteroatoms (e.g., N/S, N/O/S) can create synergistic effects. Different atoms induce various types of defects and electronic modifications, leading to superior comprehensive properties for the sodium-ion battery anode. The total capacity (Ctotal) of a heteroatom-doped carbon can be considered as a sum of contributions from intercalation (Cinter), defect/edge adsorption (Cads-defect), and heteroatom-induced reactions (Chetero):
$$C_{total} \approx C_{inter} + C_{ads-defect} + C_{hetero}$$
Table 3 summarizes the electrochemical performance of heteroatom-doped asphalt-based carbons in the sodium-ion battery.
| Doping Element | Preparation Method | Reversible Capacity (Current Density) | ICE | Rate Capability | Cycling Stability |
|---|---|---|---|---|---|
| N | Coal pitch with NaCl template carbonized in NH3/Ar at 1000°C | 270 mAh g-1 (100 mA g-1) | 66.0% | 124 mAh g-1 (10 A g-1) | 176 mAh g-1 after 1000 cycles at 0.5 A g-1 |
| S | Pre-oxidized pitch with MgSO4 carbonized at 700°C | 574.5 mAh g-1 (0.2 A g-1) | 66.5% | 187.7 mAh g-1 (10 A g-1) | 149.2 mAh g-1 after 800 cycles at 10 A g-1 |
| P | Coal pitch with H3PO4 carbonized at 900°C | 251 mAh g-1 (100 mA g-1) | 67.8% | 178 mAh g-1 (1 A g-1) | 201 mAh g-1 after 200 cycles at 100 mA g-1 |
| N, O, S | Pitch powder with urea & MgO template carbonized at 700°C | 352.7 mAh g-1 (0.2 A g-1) | 36.0% | 236.4 mAh g-1 (1 A g-1) | 140 mAh g-1 after 900 cycles at 5.0 A g-1 |
2. Morphological Regulation Strategies
Morphological engineering focuses on designing the macro-/nanostructure of the carbon material to shorten ion diffusion pathways, improve electrolyte wetting, and accommodate volume changes, thereby enhancing the kinetic performance and stability of the sodium-ion battery anode.
2.1 Constructing Porous Structures
Porous carbons provide a high surface area for capacitive sodium adsorption and interconnected channels for rapid ion transport. The pore size distribution significantly influences the storage mechanism: micropores (<2 nm) contribute primarily to surface-driven capacity, while mesopores (2-50 nm) facilitate ion diffusion, critical for high-rate performance in a sodium-ion battery.
Common methods include template-assisted carbonization (using hard templates like MgO, CaCO3, or NaCl) and chemical activation (using KOH, H3PO4). Template methods offer good control over pore size. For instance, using a nanosized CaCO3 template can yield interconnected mesoporous soft carbon with enhanced kinetics. A major challenge with high-surface-area porous carbons is their low ICE due to excessive SEI formation. Strategies like surface coating with a thin layer of soft carbon or designing partially closed pores can mitigate this issue. The sodium ion diffusion time (τ) through a porous electrode can be approximated by:
$$\tau \propto \frac{L^2}{D_{eff}}$$
where \(L\) is the diffusion length and \(D_{eff}\) is the effective diffusion coefficient, which is greatly increased by a well-interconnected porous network. Table 4 lists the performance of porous asphalt-derived carbons.
| Morphology Strategy | Preparation Method | Reversible Capacity (Current Density) | ICE | Rate Capability | Cycling Stability |
|---|---|---|---|---|---|
| Porous Structure | Mesophase pitch with CaCO3 template carbonized at 800°C | 331 mAh g-1 (30 mA g-1) | 45.0% | 53 mAh g-1 (10 A g-1) | 103 mAh g-1 after 3000 cycles at 500 mA g-1 |
| Porous Structure | Pitch + H3PO4 pre-oxidized, washed, carbonized at 1600°C | 416.1 mAh g-1 (0.1C) | 89.7% | ~40 mAh g-1 (1C) | ~360 mAh g-1 after 50 cycles at 0.1C |
| Porous Structure | Emulsified pitch spheres oxidized, sulfur-crosslinked, carbonized at 600°C | 423.8 mAh g-1 (0.1 A g-1) | 53.5% | 125.2 mAh g-1 (5.0 A g-1) | 222.5 mAh g-1 after 2000 cycles at 1 A g-1 |
| Porous + Coating | Deeply pre-oxidized pitch carbonized, then coated with soft carbon layer | 311.9 mAh g-1 (50 mA g-1) | 82.9% | 109.7 mAh g-1 (5 A g-1) | 223.0 mAh g-1 after 1000 cycles at 1 A g-1 |
2.2 Carbon Nanofiber Membranes
Electrospinning is a powerful technique to fabricate self-standing, flexible carbon nanofiber (CNF) membranes directly from pitch/polymer blends. These membranes feature a three-dimensional interconnected network of one-dimensional fibers, which provides short diffusion paths for ions and electrons, excellent mechanical flexibility, and direct use as binder-free electrodes for the sodium-ion battery.
The process typically involves electrospinning a solution containing pitch and a spinning agent (e.g., polyacrylonitrile-PAN, polyvinylpyrrolidone-PVP), followed by stabilization and carbonization. Additives like Bi(NO3)3 or NH3 treatment can be incorporated to further regulate microstructure (induce pores, dope nitrogen) during carbonization. The fibrous morphology ensures good electrolyte penetration and strain accommodation during sodiation/desodiation cycles. However, the low tap density of such membranes remains a challenge for achieving high volumetric energy density in practical sodium-ion battery cells. Their performance is summarized in Table 5.
| Morphology Strategy | Preparation Method | Reversible Capacity (Current Density) | ICE | Rate Capability | Cycling Stability |
|---|---|---|---|---|---|
| CNF Membrane | Coal pitch/PVP/Bi(NO3)3 electrospun, stabilized, carbonized at 1200°C | 216.3 mAh g-1 (0.1 A g-1) | ~80.1% | 75.6 mAh g-1 (2.0 A g-1) | 125.3 mAh g-1 after 1000 cycles at 1.0 A g-1 |
| CNF Membrane | Coal tar pitch/PAN electrospun, pre-oxidized, carbonized with NH3 treatment | 345 mAh g-1 (0.1 A g-1) | ~53.4% | 156 mAh g-1 (5 A g-1) | 235 mAh g-1 after 10000 cycles at 1 A g-1 |
3. Conclusion and Future Perspectives
Asphalt stands as a highly promising, low-cost precursor for carbon anode materials in the sodium-ion battery. Through deliberate structural and morphological engineering—such as pre-oxidation, composite formation, heteroatom doping, porosity control, and nanofiber fabrication—the inherent limitations of directly pyrolyzed pitch carbon can be effectively overcome. These strategies successfully expand interlayer spacing, introduce beneficial defects, create abundant active sites, and optimize ion/electron transport pathways, leading to significantly enhanced reversible capacity, rate capability, and cycling stability.
However, several scientific and technological challenges must be addressed to advance these materials towards commercialization in the sodium-ion battery:
1. Fundamental Understanding: A more quantitative and mechanistic understanding of the structure-property relationship is needed. Advanced in situ characterization techniques (e.g., in situ XRD, XPS, TEM) coupled with computational modeling (DFT, MD simulations) should be employed to precisely elucidate the sodium storage mechanisms (intercalation, pore-filling, adsorption on defects/heteroatoms) under real operating conditions. The evolution of microstructure during synthesis, especially the role of cross-linking and pore formation/closure, requires deeper investigation.
2. Multi-objective Optimization and Synergistic Design: Future work should focus on integrated strategies that simultaneously address multiple performance metrics. For instance, combining pre-oxidation with targeted heteroatom doping and controlled pore engineering could yield carbons with high capacity, high ICE, and excellent kinetics. The trade-offs between different properties (e.g., surface area vs. ICE, capacity vs. tap density) must be systematically studied and optimized for practical sodium-ion battery applications.
3. Scalable and Sustainable Manufacturing: Research must bridge the gap between lab-scale synthesis and industrial production. This involves developing simpler, greener, and more cost-effective processes. Examples include exploring self-templating methods, using waste biomass as co-precursors or doping sources, optimizing continuous pre-oxidation processes, and scaling up electrospinning or other fiber production techniques. The use of abundant and recyclable templates (e.g., NaCl) is advantageous.
4. Full-cell Performance and Practical Evaluation: Ultimately, the performance of these engineered asphalt-based carbons must be validated in full sodium-ion battery cells paired with appropriate cathodes and electrolytes. Studies on electrode formulation (binder, conductive agent), electrode density, and long-term cycling under realistic conditions are essential to assess their true potential for commercial sodium-ion battery technology.
In conclusion, the field of asphalt-derived carbon anodes for the sodium-ion battery is rich with opportunities. By continuing to innovate in material design and grounding development in fundamental insights and practical considerations, these low-cost carbon materials can play a pivotal role in enabling the widespread adoption of sustainable sodium-ion battery technology for large-scale energy storage.