In the pursuit of sustainable energy storage solutions, the development of efficient and cost-effective battery systems is paramount. As fossil fuels face depletion and environmental challenges, alternative technologies like the sodium-ion battery have gained significant attention. The sodium-ion battery, often seen as a complementary technology to the lithium-ion battery, offers advantages due to sodium’s abundance and lower cost. However, identifying suitable anode materials for sodium-ion batteries remains a critical research area. Hard carbon materials derived from biomass precursors present a promising avenue due to their low cost, sustainability, and tunable electrochemical properties. In this study, we explore the use of a fast-growing wood, balsa wood, as a precursor for hard carbon anodes in sodium-ion batteries. We investigate how carbonization temperature influences the material’s structure and its subsequent electrochemical performance in sodium-ion batteries. Through comprehensive characterization and electrochemical analysis, we aim to establish clear structure-activity relationships that guide the optimization of biomass-derived hard carbons for high-performance sodium-ion battery applications.
The selection of balsa wood is strategic; it is one of the fastest-growing trees, reaching maturity in just 4-5 years, making it an excellent sustainable resource. Its low density and porous nature suggest it may carbonize effectively without requiring extremely high temperatures. We employed a straightforward one-step carbonization process under an inert atmosphere at three different temperatures: 900°C, 1100°C, and 1300°C. The resulting hard carbon materials were labeled as BS900, BS1100, and BS1300, respectively. These materials were then comprehensively characterized to understand their morphological and structural evolution with temperature.
Scanning electron microscopy revealed that the carbonized materials largely retained the inherent wood morphology, consisting of a loose, stacked assembly of fibrous and layered structures. This porous architecture is beneficial for ion transport within a sodium-ion battery electrode. More profound changes were observed at the microstructural level. X-ray diffraction analysis showed that the (002) peak position shifted to higher angles with increasing carbonization temperature. Using the Bragg equation, we calculated the interlayer spacing (d002):
$$2d\sin\theta = \lambda$$
where λ is the X-ray wavelength (0.154056 nm for Cu Kα). The calculated d-spacing decreased from approximately 0.391 nm for BS900 to 0.379 nm for BS1300. This contraction is typical as carbon structures become more ordered at higher temperatures. Nitrogen adsorption-desorption measurements provided insights into the textural properties. The specific surface area (SBET) was calculated using the Brunauer-Emmett-Teller (BET) theory:
$$\frac{1}{v[(P_0/P)-1]} = \frac{c-1}{v_m c} \left( \frac{P}{P_0} \right) + \frac{1}{v_m c}$$
where v is the adsorbed volume, P/P0 is the relative pressure, vm is the monolayer capacity, and c is the BET constant. The surface area showed a non-monotonic trend: BS1100 exhibited the largest SBET of 38.8 m²/g, compared to BS900 and BS1300. Pore size distribution analysis via 2D-Non-Local Density Functional Theory (2D-NLDFT) indicated the development of micropores around 0.9 nm in BS1100, contributing to its higher surface area. At 1300°C, these micropores seemed to collapse or shrink further, while larger mesopores (3-11 nm) did not effectively transform, leading to a reduced surface area for BS1300. Raman spectroscopy further elucidated the structural disorder. The intensity ratio of the D band (~1350 cm⁻¹, disorder-induced) to the G band (~1580 cm⁻¹, graphitic) (ID/IG) increased from BS900 to BS1100, indicating a rise in structural defects (e.g., sp³ hybridized carbon, edges). The ID/IG ratio for BS1300 was similar to BS1100, suggesting defect concentration plateaued at higher temperatures. We summarize these key structural parameters in the table below:
| Sample | Carbonization Temperature (°C) | d002 spacing (nm) | SBET (m²/g) | ID/IG Ratio |
|---|---|---|---|---|
| BS900 | 900 | 0.391 | ~20.5 | ~2.1 |
| BS1100 | 1100 | 0.385 | 38.8 | ~2.5 |
| BS1300 | 1300 | 0.379 | ~25.3 | ~2.5 |
The electrochemical performance of these hard carbon anodes was evaluated in half-cell sodium-ion battery configurations. The initial galvanostatic charge-discharge profiles at a low current density of 20 mA/g provided immediate insights. For sodium-ion battery hard carbon anodes, the charge (sodium extraction) curve is typically analyzed in two voltage regions: a sloping region above 0.15 V attributed to sodium adsorption/desorption in pores and on defect sites, and a low-voltage plateau below 0.15 V attributed to sodium insertion/extraction into the graphitic interlayers. BS1100 delivered the highest reversible capacity of 276.4 mAh/g at 0.1 A/g. Its capacity distribution showed a balanced contribution from both the sloping and plateau regions, correlating with its moderate interlayer spacing and high surface area. In contrast, BS1300 showed a significantly reduced capacity in the sloping region (>0.15 V) despite having a surface area comparable to BS900. This suggests that the nature of the surface/defects in BS1300, likely more numerous or energetically different as hinted by the Raman data, hinders effective sodium storage via adsorption. The initial Coulombic efficiency (ICE), a critical parameter for practical sodium-ion battery applications, was highest for BS1100 at 72.2%, compared to BS900 and BS1300. This superior ICE is linked to an optimized microstructure that minimizes irreversible sodium consumption during solid electrolyte interphase (SEI) formation.
Rate capability testing is crucial for assessing the power performance of a sodium-ion battery anode. The results were striking. While BS1100 showed the best capacity at low rates, both BS900 and BS1100 maintained significantly higher capacities than BS1300 at high current densities. At 5 A/g, BS900 and BS1100 delivered 132 mAh/g and 143.1 mAh/g, respectively, whereas BS1300 only managed 63.9 mAh/g. To understand this disparity, we employed electrochemical techniques that probe kinetic parameters. Galvanostatic Intermittent Titration Technique (GITT) was used to estimate the apparent sodium-ion diffusion coefficient (DNa+). The diffusion coefficient can be approximated from the potential transient after a current pulse using the following equation for semi-infinite linear diffusion:
$$D = \frac{4}{\pi} \left( \frac{n_m V_m}{S} \right)^2 \left( \frac{\Delta E_s}{\tau (dE_\tau / d\sqrt{t})} \right)^2$$
where nm is the molar amount, Vm is the molar volume, S is the electrode/electrolyte contact area, ΔEs is the steady-state voltage change, τ is the current pulse duration, and dEτ/d√t is the slope of the potential vs. square root of time during the relaxation period. Although absolute values require precise geometric factors, the relative diffusion rates were clearly evident. The GITT profiles and derived diffusion intensity plots showed that BS900 and BS1100 facilitated much faster sodium-ion diffusion throughout the charge/discharge process compared to BS1300. Electrochemical Impedance Spectroscopy (EIS) further complemented these findings. The Nyquist plots for all samples consisted of a semicircle in the high-medium frequency region (associated with charge transfer resistance, Rct) and an inclined line in the low-frequency region (associated with Warburg diffusion). The diameter of the semicircle, representing Rct, was smallest for BS1100 and BS900, indicating more favorable charge transfer kinetics at the electrode/electrolyte interface. BS1300 exhibited a larger Rct. The combined kinetic advantages—faster ion diffusion and lower charge transfer resistance—directly explain the superior rate performance of BS900 and BS1100 in sodium-ion battery tests.
We further analyzed the charge storage mechanism using cyclic voltammetry at various scan rates (ν). For a process dominated by semi-infinite diffusion (e.g., intercalation), the peak current (ip) scales with the square root of the scan rate (ν1/2). For a surface-controlled capacitive process, ip scales linearly with ν. The relationship is often expressed as:
$$i_p = a \nu^b$$
where a and b are constants. A b-value of 0.5 indicates diffusion control, and 1.0 indicates capacitive control. By plotting log(ip) vs. log(ν) for the main reduction peak, we obtained b-values. The b-value for BS1300 was closer to 0.5, indicating its electrochemical process is more heavily limited by sodium-ion diffusion. In contrast, BS900 and BS1100 showed b-values closer to 1, especially at higher scan rates, signifying a greater contribution from faster, surface-controlled processes. This mechanistic insight aligns perfectly with the rate performance: BS1300’s diffusion-limited behavior severely hampers its capacity at high currents in a sodium-ion battery, while the others maintain better performance due to a more capacitive-friendly storage mechanism. We can summarize the kinetic and mechanistic parameters as follows:
| Sample | Relative Na+ Diffusion Rate (from GITT) | Relative Charge Transfer Resistance (from EIS) | b-value (from CV analysis) | Capacity at 5 A/g (mAh/g) |
|---|---|---|---|---|
| BS900 | High | Low | ~0.8 | 132.0 |
| BS1100 | Highest | Lowest | ~0.85 | 143.1 |
| BS1300 | Low | High | ~0.6 | 63.9 |
The long-term cycling stability of a sodium-ion battery anode is another critical metric. At a current density of 1 A/g, BS1100 demonstrated excellent capacity retention of 87.9% after 500 cycles, outperforming both BS900 and BS1300. This robust cyclability can be attributed to its optimal structural integrity. The moderate interlayer spacing provides a stable host for sodium insertion/extraction without excessive volume strain. The relatively high but not excessive defect concentration and surface area likely promote the formation of a stable SEI. In contrast, the more condensed structure of BS1300 and its different defect nature might lead to less reversible sodium storage and potential structural degradation over cycles.
To put the performance of our balsa-derived hard carbon into perspective, we compared BS1100 with a typical commercial or literature hard carbon (denoted here as WHC) often used in sodium-ion battery research. WHC typically has a lower specific surface area and a broader pore size distribution with significant pores in the 33-50 nm range. While the low-rate capacity in the two voltage regions was similar between BS1100 and WHC, BS1100 exhibited a more stable cycling performance with a slower capacity fade at 1 A/g. This suggests that the specific hierarchical porous structure derived from balsa wood, featuring a combination of fibrous macro/mesopores and developed micropores, is particularly effective in maintaining electrode integrity and facilitating sustained ion access during repeated cycling in a sodium-ion battery.
In conclusion, our investigation into balsa wood-derived hard carbons for sodium-ion battery anodes reveals a clear structure-activity relationship governed by carbonization temperature. The carbonization temperature critically tunes the interlayer spacing, specific surface area, pore structure, and defect concentration. For sodium-ion battery applications, an intermediate temperature of 1100°C yielded a material (BS1100) with a balanced set of properties: a moderate interlayer distance (~0.385 nm), a high specific surface area (38.8 m²/g) with developed microporosity, and an optimal level of structural defects. This unique microstructure translates into superior electrochemical performance: high reversible capacity, excellent initial Coulombic efficiency, remarkable rate capability, and stable long-term cycling. The kinetic analyses unequivocally show that BS1100 benefits from high sodium-ion diffusion rates, low charge transfer resistance, and a charge storage mechanism with a significant capacitive contribution, allowing it to outperform materials carbonized at lower or higher temperatures, especially under high-current conditions. This work underscores the potential of fast-growing, sustainable biomass like balsa wood as a low-cost precursor for high-performance hard carbon anodes in sodium-ion batteries. The insights gained into the interplay between carbonization conditions, microstructure, and electrochemical kinetics provide a valuable framework for the rational design of next-generation biomass-derived carbons for advanced sodium-ion battery systems. Future work could explore pretreatment methods or heteroatom doping to further enhance the capacity and initial Coulombic efficiency of such sustainable anodes for the ever-evolving landscape of sodium-ion battery technology.