The ever-growing demand for portable electronics and large-scale grid energy storage has propelled lithium-ion batteries (LIBs) to the forefront of energy technology. However, concerns regarding the limited and uneven global distribution of lithium resources have intensified the search for sustainable alternatives. Sodium-ion batteries (SIBs) have re-emerged as a compelling candidate due to the natural abundance, low cost, and chemical similarity of sodium to lithium. While the working principles are analogous, a fundamental challenge arises in identifying suitable electrode materials, particularly anodes, as the larger ionic radius of Na+ (1.02 Å vs. 0.76 Å for Li+) hinders its reversible intercalation into standard graphite, the workhorse of LIB anodes.
In conventional ester-based electrolytes, graphite exhibits a meager reversible capacity of approximately 35 mAh/g for sodium storage. This poor performance stems from the inability to form stable sodium-graphite intercalation compounds (Na-GICs) like LiC6 within the narrow interlayer spacing of graphite (≈0.335 nm). To overcome this limitation, researchers have turned to modified carbon structures. Among these, materials derived from graphite oxide (GO) have shown exceptional promise. By chemically oxidizing graphite, the layer spacing is expanded, and oxygen-containing functional groups are introduced, creating more accessible sites for Na+ storage. Subsequent reduction processes yield reduced graphene oxide (RGO), which balances improved conductivity with an optimized structure for sodium-ion battery applications. This article delves into the research progress of graphite oxide-based anodes, focusing on synthesis methods, sodium storage mechanisms, electrochemical performance, and the enhancing effects of heteroatom doping.
Fundamental Concepts of Graphite Oxide-Based Materials
The journey from inert graphite to a high-performance sodium-ion battery anode involves precise chemical and structural modifications. Understanding this family of materials is crucial.
Graphite
Graphite possesses a layered structure with strong in-plane covalent (sp2) bonding and weak van der Waals forces between layers. Its theoretical capacity for lithium is 372 mAh/g (forming LiC6). For sodium, the formation of analogous compounds like NaC6 is thermodynamically unfavorable due to the larger ion causing significant lattice strain. The theoretical capacity for sodium in graphite is thus very low, often cited as below 35 mAh/g in carbonate electrolytes.
Graphite Oxide (GO) and Graphene Oxide
Graphite oxide is produced by treating graphite with strong oxidizers in acidic media (e.g., Hummers’, Brodie’s, or Staudenmaier methods). This process introduces oxygenated functional groups (epoxy, hydroxyl, carboxyl) between the carbon layers, which not only expand the interlayer distance (d002) to 0.7-0.9 nm but also make the material hydrophilic. When this oxidized material is subjected to ultrasonic exfoliation in a solvent, single or few-layer sheets known as graphene oxide (GO) are obtained. The expanded interlayer spacing and functional groups in GO/RGO are key to enabling sodium-ion insertion.
Reduced Graphene Oxide (RGO)
While GO provides the necessary spacing, its high oxygen content severely compromises electronic conductivity. Reduction—via thermal annealing, chemical agents (hydrazine, NaBH4), or electrochemical methods—partially removes these oxygen groups, restoring electrical conductivity to form RGO. Critically, the reduction process can be controlled to retain a desirable, expanded interlayer spacing (typically 0.36-0.44 nm) and a certain density of defects and residual functional groups, which are active sites for sodium storage in sodium-ion batteries.
Sodium Storage Mechanisms and Electrochemical Performance
Unraveling the Storage Mechanism
The electrochemical profile of RGO anodes in a sodium-ion battery typically features a sloping region above 0.1 V (vs. Na/Na+) and a low-potential plateau near 0 V. The exact mechanism corresponding to these features has been extensively studied.
- Interlayer Spacing (d002): This is the most critical parameter. Density Functional Theory (DFT) calculations indicate that for effective Na+ intercalation without co-intercalation, the interlayer spacing should be between 0.44 nm and 0.60 nm. An optimal d-spacing around 0.43-0.45 nm, as commonly achieved in expanded RGO, allows for reversible Na+ insertion/extraction.
$$ E_{form} = f(d_{002}) $$
Where a larger \( d_{002} \) up to an optimum reduces the formation energy \( E_{form} \) of Na-GICs, making the process more favorable. - Defects and Functional Groups: The sloping capacity is widely attributed to the pseudocapacitive adsorption/desorption of Na+ on defect sites (vacancies, edges) and the reversible redox reactions with residual oxygen functional groups (e.g., C=O + Na+ + e– ↔ C-ONa).
- Low-Voltage Plateau: The capacity below 0.1 V is associated with Na+ insertion into the expanded graphitic interlayers and, as evidenced by in-situ TEM studies, the pore-filling and even quasi-metallic Na cluster formation in nanoscale cavities.
Thus, the total capacity \( C_{total} \) of an RGO anode in a sodium-ion battery can be conceptually described as:
$$ C_{total} = C_{intercalation} + C_{adsorption/redox} + C_{pore-filling} $$
where each contribution is influenced by the material’s structure, d-spacing, and defect density.
Electrochemical Performance of Tailored Materials
The performance of graphite oxide-based anodes is highly dependent on the precursor and the specific oxidation/reduction pathway. The following table summarizes key achievements.
| Material & Synthesis Key | Key Structural Feature | Electrochemical Performance in SIB | Ref. |
|---|---|---|---|
| HRGO300 (H2O2-etched, 300°C reduced) | Partially reduced, porous, d ≈ 0.434 nm | 365 mAh/g @ 0.1 A/g; 131 mAh/g @ 10 A/g; 81% retention after 3000 cycles @ 2 A/g | [64] |
| EG-1h (Controlled oxidation & reduction) | Expanded graphite, d ≈ 0.43 nm, long-range order | 284 mAh/g @ 20 mA/g; 73.9% retention after 2000 cycles @ 100 mA/g | [15] |
| ERGO (Electrochemically oxidized & annealed) | Green synthesis, tunable oxygen groups | 268 mAh/g @ 100 mA/g; 60% retention after 2000 cycles @ 500 mA/g | [41] |
| BF-rGO (Boric acid assisted reduction) | B-O-C groups, enlarged d-spacing, high defect density | 280 mAh/g; 89.4% retention after 5000 cycles @ 1 A/g; 153 mAh/g @ 400 mA/g | [66] |
| sRGO vs. rRGO (Slow vs. rapid thermal reduction) | rRGO has looser stacking and more open structure | rRGO: ~372 mAh/g @ 100 mA/g, superior cycling over sRGO | [52] |
| Reduced Graphite Oxide (from GO cake) | Larger d-spacing, more ordered than RGO from exfoliated GO | Superior Li/Na storage kinetics and capacity compared to RGO from GO dispersion | [67] |
The data highlights several critical trends for high-performance sodium-ion battery anodes:
- Optimized d-spacing is paramount: Materials with a d002 around 0.43 nm consistently show high reversible capacity, confirming the intercalation mechanism.
- Defect engineering is beneficial: Controlled introduction of pores and retention of some functional groups enhance slope capacity and rate capability by shortening ion diffusion paths and providing redox-active sites.
- Synthesis method dictates properties: Electrochemical oxidation offers a greener route. The reduction speed and temperature profoundly affect stacking density and oxygen content, directly impacting sodium-ion battery performance. For instance, annealing GO above 500°C in inert gas often leads to layer re-stacking and capacity decay for sodium, whereas it benefits LIB performance.
Heteroatom Doping for Enhanced Performance
Doping RGO with heteroatoms (N, S, B, P) is a powerful strategy to further improve the electrochemical properties of sodium-ion battery anodes. Doping modifies the electronic structure, creates more active sites, and can further expand the interlayer spacing.
Nitrogen (N) Doping
Nitrogen, with a similar atomic size to carbon, incorporates into the carbon lattice as pyridinic-N, pyrrolic-N, and graphitic-N. This doping enhances conductivity, increases the density of states near the Fermi level, and provides additional defect sites with strong binding affinity for Na+. N-doped 3D graphene aerogels, for example, combine a porous conductive network with active N sites, delivering high capacity (288 mAh/g) and excellent rate performance (152 mAh/g at 5 A/g).
Sulfur (S) Doping
Sulfur has a larger covalent radius (102 pm) than carbon (77 pm). Its incorporation significantly expands the carbon interlayer distance and introduces reversible redox-active C-Sx-C sites. S-doped graphene can achieve remarkably high capacities (~380 mAh/g) with excellent cycling stability. The larger d-spacing and the favorable redox reactions between S and Na contribute to its outstanding performance in sodium-ion batteries.
Boron (B) Doping
Boron, having one less electron than carbon, acts as an electron acceptor (p-type doping). DFT studies predict that boron-doped graphene (e.g., BC3) can offer a very high theoretical capacity (up to 762 mAh/g for Na) by adsorbing Na on both sides of the sheet. The average sodiation voltage is calculated to be around 0.44 V, which is suitable for avoiding Na plating. Boron doping also improves electronic conductivity and Na+ diffusion rates.
The effects of different dopants can be summarized as follows:
| Dopant | Primary Effects on Carbon Matrix | Benefits for SIB Anode | Typical Capacity Enhancement |
|---|---|---|---|
| Nitrogen (N) | n-type doping; creates defect sites; enhances electronic conductivity. | Increased active sites for Na+ adsorption; improved rate capability. | ~250-350 mAh/g |
| Sulfur (S) | Significantly expands interlayer spacing; introduces redox-active centers. | Enables intercalation; provides additional faradaic capacity; excellent cycling. | ~350-400 mAh/g |
| Boron (B) | p-type doping; modifies charge distribution. | High theoretical capacity; suitable working potential; fast kinetics (predicted). | Very high predicted (~760 mAh/g) |
Conclusion and Future Perspectives
Graphite oxide-based materials, particularly engineered RGO and its doped derivatives, have demonstrated tremendous potential as high-capacity, stable, and rate-capable anodes for sodium-ion batteries. The core achievement lies in successfully expanding the graphite interlayer spacing to accommodate Na+ ions, coupled with strategic defect and doping engineering to enhance storage sites and kinetics. However, for widespread commercialization in sodium-ion battery technology, several challenges must be addressed.
- Cost-Effective and Green Synthesis: Scaling up the production of high-quality GO/RGO while minimizing the use of harsh chemicals and energy-intensive steps is crucial. Electrochemical oxidation and low-temperature reduction methods show promise for greener production routes for sodium-ion battery components.
- Precise Structure-Property Control: Future work must focus on precisely correlating specific structural parameters (e.g., exact d-spacing distribution, type and concentration of defects/dopants) with electrochemical outcomes in sodium-ion batteries. Advanced in-situ characterization and machine learning could guide the synthesis of materials with optimized properties.
- Enhancing Initial Coulombic Efficiency (ICE): Like many carbon anodes, RGO-based materials often suffer from low ICE due to irreversible reactions forming the solid electrolyte interphase (SEI) and with residual functional groups. Pre-sodiation, electrolyte engineering, and surface coating are essential strategies to improve this key metric for practical sodium-ion batteries.
- Development of Composite Structures: Integrating RGO with other high-capacity active materials (e.g., metal sulfides, phosphides, or alloying materials) to form composite anodes can combine the benefits of conductive, buffering graphene networks with high theoretical capacity phases, paving the way for next-generation sodium-ion battery anodes.
In conclusion, the transformation of graphite into functional graphite oxide-based anodes represents a significant leap forward for sodium-ion battery technology. Through continued research into scalable synthesis, mechanistic understanding, and advanced material design, these carbon-based anodes are poised to play a pivotal role in enabling cost-effective and sustainable energy storage solutions.