In the evolving landscape of energy storage systems, sodium-ion batteries have emerged as a premier candidate for next-generation applications, primarily due to their excellent safety profile, cost-effectiveness, and the abundance of sodium resources. As the sole commercially viable anode material for sodium-ion batteries, hard carbon offers distinct advantages such as high initial coulombic efficiency and superior cycling stability. Its disordered structure, characterized by defects, functional groups, open and closed pores, and enlarged interlayer spacing (typically 0.37–0.40 nm), provides ample sites for sodium ion storage, which is crucial for achieving high capacity and rate performance in sodium-ion batteries. The synthesis of hard carbon involves various modification treatments applied to diverse precursors, including mineral-based, biomass-derived, and synthetic macromolecular materials. Among these modification strategies, pre-oxidation stands out as a simple, efficient, and widely adopted pretreatment method that plays a pivotal role in tailoring the microstructure of hard carbon, thereby enhancing its electrochemical performance in sodium-ion batteries.
Pre-oxidation, which can be conducted via gas-phase, liquid-phase, or solid-phase methods, introduces oxygen-containing functional groups into the precursor’s molecular structure. This process inhibits graphitization during subsequent carbonization, promotes the formation of a more disordered carbon architecture, and ultimately optimizes the hard carbon for use in sodium-ion batteries. However, the complex molecular structures of different precursors lead to varied oxidation mechanisms, and a clear understanding of these structural evolution processes is essential for designing high-performance hard carbon anodes. In this article, I will explore the structural transformations that occur during the pre-oxidation of various hard carbon precursors and delve into the mechanisms by which pre-oxidation influences hard carbon microstructure. By examining mineral, biomass, and synthetic macromolecular precursors, I aim to provide a comprehensive overview of how pre-oxidation contributes to the preparation and modification of hard carbon for sodium-ion batteries, offering insights for selecting appropriate pre-oxidation methods based on molecular structure.
The historical context of carbon materials reveals that pre-oxidation has been instrumental in their development and application. For instance, in the early 20th century, activated carbon was produced through gas activation in oxidizing atmospheres like air, CO2, or H2O at 400–900°C, creating porous structures for adsorption. Later, in the 1930s, expanded graphite was fabricated via chemical or electrochemical oxidation followed by thermal expansion, which increased interlayer spacing through the volumetric effect of oxygen functional groups. In the 1950s, carbon fibers, such as polyacrylonitrile-based, pitch-based, and rayon-based fibers, relied on pre-oxidation to stabilize linear molecular chains into ladder structures, preventing melting during carbonization. Similarly, for hard carbon anodes in sodium-ion batteries, pre-oxidation serves as a critical step to induce cross-linking and disorder, which are vital for sodium ion storage. The efficacy of pre-oxidation depends on the precursor type, and I will discuss each category in detail.
Mineral-based precursors, such as coal and pitch, are inherently prone to graphitization due to their aromatic-rich structures. Pre-oxidation disrupts this tendency by introducing oxygen functional groups that enhance cross-linking, thereby suppressing graphitization and fostering a disordered hard carbon structure suitable for sodium-ion batteries. For coal, air or liquid oxidants like H22O2 can effectively incorporate oxygen, leading to increased interlayer spacing and defect density. In one study, air-oxidized coal-derived hard carbon exhibited an interlayer spacing (d002) of 0.38 nm and an ID/IG ratio of 3.36, compared to 0.376 nm and 2.946 for non-oxidized coal, indicating enhanced disorder. The oxygen content rose from 6.6% to 17.5% after pre-oxidation, which reduced irreversible capacity loss in sodium-ion batteries. Moreover, the oxidation reactivity varies with coal composition; vitrinite-rich coal, with higher aliphatic content, shows greater oxidation activity, resulting in hard carbon with more ultra-micropores and better sodium storage performance. The evolution of functional groups during coal pre-oxidation can be represented by reactions such as the formation of carbonyl groups: $$ \text{R-CH}_3 + \text{O}_2 \rightarrow \text{R-COOH} + \text{H}_2\text{O} $$ where R represents the aromatic backbone. This process enhances cross-linking, as evidenced by XPS analysis showing increased -C(O)O- content up to 29.41% in oxidized coal.
For pitch, a by-product of petroleum or coal industry, pre-oxidation is essential to prevent melting and foaming during carbonization. Gas-phase oxidation in air at temperatures around 300°C introduces oxygen functional groups like -C-O-C- and -C(O)O-, which promote three-dimensional cross-linking. This inhibits the alignment of aromatic layers and leads to the development of closed pores and ultra-micropores in the resulting hard carbon, crucial for plateau capacity in sodium-ion batteries. The structural parameters of pitch-derived hard carbon can be summarized in a table:
| Sample | d002 (Å) | Lc (nm) | La (nm) | ID/IG | SBET(N2) (m2/g) | SBET(CO2) (m2/g) | fa | rSAXS (nm) |
|---|---|---|---|---|---|---|---|---|
| TS-1400 | 3.79 | 1.03 | 3.47 | 1.23 | 2.8 | 155.6 | 0.52 | 0.80 |
| THFS-1400 | 3.82 | 1.04 | 3.43 | 1.32 | 4.1 | 205.9 | 0.55 | 0.83 |
| THFI-1400 | 3.88 | 0.99 | 3.27 | 1.41 | 13.6 | 399.3 | 0.80 | 1.02 |
Here, THFI (tetrahydrofuran-insoluble) pitch, with higher aromaticity and polar oxygen groups, exhibits greater oxidation activity, leading to hard carbon with larger d002, higher ID/IG, and increased microporosity—key for sodium ion storage in sodium-ion batteries. The pre-oxidation mechanism for pitch involves radical reactions where oxygen attacks alkyl side chains or aromatic rings, forming cross-linked structures. For example, the oxidation of methyl groups can be expressed as: $$ \text{Ar-CH}_3 + \text{O}_2 \rightarrow \text{Ar-COOH} + \text{CO}_2 $$ where Ar denotes an aromatic ring. Liquid-phase oxidation using nitric acid or H2O2 offers an alternative, allowing for selective introduction of carboxyl groups at the C6 position in cellulose-based precursors, which I will discuss later. Overall, pre-oxidation of mineral precursors enhances hard carbon disorder, pore connectivity, and sodium storage capacity, making it vital for sodium-ion battery applications.
Biomass-derived precursors, such as cellulose, starch, and lignin, naturally contain oxygen in their structures, but pre-oxidation further modifies their molecular arrangement to optimize hard carbon formation for sodium-ion batteries. Cellulose, a polysaccharide with high crystallinity due to hydrogen bonding, undergoes pre-oxidation in air to break hydrogen bonds and introduce cross-linking via ether and ester groups. At 300°C, air-oxidized cellulose yields hard carbon with a high specific surface area (555.23 m2/g from CO2 adsorption) and improved pore connectivity (fa = 0.92), compared to argon-treated samples. The evolution of functional groups during cellulose pre-oxidation can be tracked using FTIR, showing peaks at 1660–1680 cm−1 for carbonyl groups. The reaction for ether bond formation can be represented as: $$ \text{2R-OH + O}_2 \rightarrow \text{R-O-R + H}_2\text{O} $$ where R is the glucose unit. This cross-linking extends carbon layers (La up to 4.11 nm) and creates curved pores, beneficial for sodium ion diffusion in sodium-ion batteries. Alternatively, liquid-phase oxidation with H2O2 or TEMPO reagent selectively oxidizes C6 hydroxyl groups to carboxylate, reducing specific surface area and improving initial coulombic efficiency in sodium-ion batteries. For instance, TEMPO-oxidized cellulose-derived hard carbon shows a decrease in SBET from 586 to 126 m2/g, raising ICE from 25% to 72%.
Starch, another biomass precursor, requires pre-oxidation to prevent melting and foaming during carbonization. Air oxidation at low temperatures promotes dehydration and cross-linking, resulting in spherical hard carbon particles with minimal aggregation. The degree of oxidation influences morphology; longer oxidation times yield more monodisperse spheres, enhancing electrode uniformity in sodium-ion batteries. Lignin, a complex aromatic polymer, undergoes pre-oxidation to modify its cross-linked structure. In air at 150–250°C, lignin experiences cleavage of methoxy and phenolic groups, along with oxidation of alkyl side chains to ester functionalities. This increases cross-linking density, leading to hard carbon with larger d002 (up to 0.39 nm) and more mesopores. The structural evolution during lignin pre-oxidation involves radical reactions, such as: $$ \text{Lignin-OCH}_3 + \text{O}_2 \rightarrow \text{Lignin-COOH + CH}_2\text{O} $$ Hard carbon from oxidized lignin demonstrates reversible capacities up to 340 mAh/g in sodium-ion batteries, highlighting the role of pre-oxidation in enhancing sodium storage performance. Thus, for biomass precursors, pre-oxidation primarily adjusts molecular structure by breaking hydrogen bonds, introducing cross-links, or selective oxidation, which tailors hard carbon porosity and disorder for optimal use in sodium-ion batteries.
Synthetic macromolecular precursors, like phenolic resin, possess inherent cross-linked structures, but pre-oxidation can further enhance their properties for sodium-ion battery anodes. Phenolic resin pre-oxidized in air introduces additional carbonyl groups (-C=O), which improve electrolyte wettability and facilitate the formation of a stable solid electrolyte interface (SEI) in sodium-ion batteries. Studies show that pre-oxidation increases the cross-linking degree, as indicated by thermal stability shifts in TG-DTG curves. For example, pre-oxidized phenolic resin-derived hard carbon exhibits a higher ICE (84.7% vs. 62.5%) and a reversible capacity of 334.3 mAh/g at 20 mA/g in sodium-ion batteries. The oxidation mechanism involves the conversion of methylene bridges to carbonyls: $$ \text{Ph-CH}_2\text{-Ph + O}_2 \rightarrow \text{Ph-COOH + Ph-CHO} $$ where Ph represents a phenyl group. This reaction enhances cross-linking, reduces specific surface area, and minimizes carbon structure decomposition during pyrolysis, contributing to better cycling stability. Liquid-phase oxidation with phosphoric acid on epoxy resin introduces P-C and P-O bonds, providing active sites for sodium ion storage and further boosting capacity in sodium-ion batteries. In summary, pre-oxidation of synthetic precursors mainly augments existing cross-links, reduces surface area, and modifies surface functional groups, all of which benefit the electrochemical performance of hard carbon in sodium-ion batteries.
To consolidate the mechanisms by which pre-oxidation influences hard carbon structure for sodium-ion batteries, I will summarize key points using formulas and tables. Pre-oxidation introduces oxygen functional groups that impact cross-linking, pore structure, microcrystalline size, and surface properties. The dominant functional groups include carbonyls (-C=O), esters (-C(O)O-), and ethers (-C-O-C-), each playing specific roles. For instance, -C(O)O- groups promote three-dimensional cross-linking and pore development, while -C-O-C- groups extend carbon layer dimensions and induce curvature. The general oxidation reaction can be expressed as: $$ \text{Precursor + O}_2 \rightarrow \text{Precursor-O}_x + \text{CO/CO}_2 $$ where the release of CO and CO2 during pyrolysis etches pores, enhancing porosity. The effect of pre-oxidation on hard carbon parameters is summarized below:
| Precursor Type | Key Functional Groups Introduced | Effect on Hard Carbon Structure | Impact on Sodium-Ion Battery Performance |
|---|---|---|---|
| Mineral (e.g., coal, pitch) | -C(O)O-, -C=O | Increased d002, defect density, ultra-micropores | Higher plateau capacity, improved rate capability |
| Biomass (e.g., cellulose, lignin) | -C-O-C-, -COOH | Reduced crystallinity, enhanced pore connectivity | Boosted reversible capacity, better ICE |
| Synthetic (e.g., phenolic resin) | -C=O, -C-O-C- | Lower SBET, stabilized SEI formation | Enhanced cycling stability, higher ICE |
The oxidation reactivity of precursors depends on molecular structure. Aliphatic-rich components (e.g., vitrinite in coal) exhibit higher activity due to easier oxygen attack, leading to more disordered hard carbon. Similarly, precursors with lower crystallinity (e.g., pre-oxidized cellulose) allow more thorough oxidation. The relationship between precursor structure and oxidation efficacy can be modeled using the formula: $$ \text{Oxidation Activity} \propto \frac{\text{Aliphatic Content}}{\text{Aromatic Content}} $$ This highlights that tailoring pre-oxidation conditions based on precursor composition is crucial for optimizing hard carbon for sodium-ion batteries.
Furthermore, pre-oxidation affects microcrystalline parameters. For example, in pitch-derived hard carbon, extended oxidation increases La due to ether-bond-mediated cross-linking, as shown by XPS data where ether content rises from 32% to 50% with longer oxidation time. This correlates with La values from 2.643 nm to 3.172 nm. The interlayer spacing d002 can be expanded through the volumetric effect of oxygen groups, following the relation: $$ d_{002} = d_0 + k \cdot [O] $$ where d0 is the initial spacing, [O] is oxygen content, and k is a constant. Such structural modifications directly influence sodium ion storage mechanisms in sodium-ion batteries, where larger d002 facilitates ion intercalation, and pores provide adsorption sites.
In terms of electrolyte interaction, pre-oxidation introduces surface functional groups that improve wettability, leading to more stable SEI formation. This reduces impedance growth during cycling in sodium-ion batteries. For instance, pre-oxidized phenolic resin hard carbon shows an impedance increase of only 33.8 Ω after 15 cycles, compared to 78.1 Ω for non-oxidized carbon, contributing to capacity retention of 91.9% over 140 cycles. The SEI stabilization can be described by the adsorption of electrolyte components onto oxygenated sites: $$ \text{Surface-O + EC} \rightarrow \text{SEI layer} $$ where EC represents ethylene carbonate in the electrolyte. This underscores the multifaceted role of pre-oxidation in enhancing hard carbon performance for sodium-ion batteries.
Looking ahead, while gas-phase pre-oxidation is commonly used due to its simplicity, it has drawbacks such as high energy consumption and toxic gas emission. Liquid-phase oxidation offers selectivity and uniformity but involves corrosive chemicals. Future innovations could integrate pre-oxidation with advanced techniques like microwave-assisted or electrospinning methods to deepen oxidation efficiency. Characterization tools such as electron paramagnetic resonance (EPR), extended X-ray absorption fine structure (EXAFS), and in situ X-ray diffraction (XRD) can provide deeper insights into oxygen local environments and dynamic structural changes during pre-oxidation. Additionally, techniques like temperature-programmed desorption (TPD) can measure active surface area, while neutron scattering and electrochemical quartz crystal microbalance (EQCM) can probe ion diffusion in SEI layers for sodium-ion batteries. These advancements will enable more precise design of hard carbon anodes, further pushing the boundaries of sodium-ion battery technology.
In conclusion, pre-oxidation is a cornerstone strategy for modifying hard carbon precursors to achieve high-performance anodes in sodium-ion batteries. By introducing oxygen functional groups, it inhibits graphitization, enhances cross-linking, develops porous structures, and improves surface properties, all of which contribute to superior sodium storage capacity, rate capability, and cycling stability. The mechanisms vary with precursor type—mineral, biomass, or synthetic—but the overarching goal is to create a disordered carbon matrix optimized for sodium ion accommodation. As research progresses, refining pre-oxidation methods and understanding their molecular-level impacts will be key to unlocking the full potential of hard carbon in sodium-ion batteries, paving the way for cost-effective and efficient energy storage solutions.