The relentless pursuit of higher energy density, longer cycle life, and lower cost continues to drive innovation in lithium-ion battery technology. The anode material plays a pivotal role in determining these key performance metrics. While graphite remains the dominant commercial anode due to its excellent cycling stability and low working potential, its theoretical capacity is limited to 372 mAh g-1. This ceiling has spurred extensive research into alternative high-capacity materials like silicon, tin, and various metal oxides. However, these alternatives often suffer from severe volume expansion during lithiation/delithiation, leading to rapid capacity fading and posing significant challenges for practical implementation. Consequently, refining carbon-based materials remains a highly viable pathway, with a focus on enhancing capacity, rate capability, and sustainability.
Biomass-derived carbon materials have emerged as promising candidates for next-generation lithium-ion battery anodes. Their appeal lies in their natural abundance, low cost, diverse structures, and inherent sustainability. Various precursors, such as wood, coconut shells, rice husks, and peanut shells, have been converted into porous, disordered carbons. These materials often exhibit capacities higher than theoretical graphite due to additional lithium storage mechanisms, including capacitive storage on surfaces and within pores, and insertion into disordered interlayer spaces. However, these disordered structures typically come with drawbacks: low initial Coulombic efficiency (ICE) caused by irreversible reactions with extensive surface areas, significant voltage hysteresis, and poor electronic conductivity. To overcome these limitations, enhancing the graphitic order within biomass carbon is a crucial strategy. A highly graphitic structure provides stable and efficient pathways for electron transport and well-defined interlayer spaces for lithium-ion intercalation, which is essential for achieving high performance in a lithium-ion battery.
In this work, we present a detailed study on the transformation of ironwood, a dense and durable hardwood, into a highly graphitic carbon material for use as an anode in a lithium-ion battery. The process involves a two-step thermal treatment: initial carbonization at moderate temperatures to create a carbon skeleton, followed by high-temperature graphitization at 2800°C to induce crystallographic ordering. We systematically investigate the evolution of the material’s structure, morphology, and surface chemistry with increasing treatment temperature. The electrochemical performance is then rigorously evaluated, with a focus on reversible capacity, cycling stability, rate capability, and interfacial kinetics. Our findings demonstrate that the ironwood-derived graphite material exhibits electrochemical properties comparable to commercial synthetic graphite, while offering the significant advantages of a low-cost and renewable precursor. This work provides a comprehensive framework for understanding the structure-property relationships in biomass-derived graphitic anodes and highlights their potential for sustainable energy storage.
1. Experimental Methodology
1.1. Material Synthesis
The synthesis of ironwood-derived graphite anode material was carried out through a controlled thermal treatment process. Ironwood blocks were first subjected to a purification step by soaking in a dilute hydrochloric acid (HCl) solution. This critical step aims to remove soluble inorganic salts and impurities residing within the natural microchannels of the wood, which could otherwise catalyze undesirable reactions or create defects during high-temperature treatment. The acid-treated wood was then thoroughly washed with deionized water and dried in an oven at 60°C for 12 hours.
The thermal processing consisted of two distinct stages:
Stage 1: Carbonization. The dried ironwood precursor was placed in a tube furnace under a continuous flow of inert gas (argon or nitrogen). The temperature was raised from room temperature to various target carbonization temperatures (700°C, 1000°C, 1200°C, and 1650°C) at a controlled heating rate of 3°C min-1. The sample was held at the target temperature for 1 hour to ensure complete carbonization, a process where volatile organic compounds are driven off, leaving behind a predominantly carbonaceous char with a highly disordered, turbostratic structure.
Stage 2: Graphitization. The sample carbonized at 1650°C was selected for further high-temperature treatment. This material was transferred to a specialized graphitization furnace. Under a protective argon atmosphere, the temperature was again ramped at 3°C min-1 to an ultra-high temperature of 2800°C and maintained for 1 hour. This extreme thermal energy provides the necessary activation for carbon atoms to rearrange into a thermodynamically stable, layered graphitic structure.
The materials are labeled as IW-T, where ‘IW’ stands for Ironwood and ‘T’ represents the final treatment temperature (e.g., IW-1200, IW-2800).
1.2. Material Characterization
A suite of characterization techniques was employed to probe the physicochemical properties of the prepared materials.
Morphology and Structure: The surface morphology and microstructure were examined using Scanning Electron Microscopy (SEM). The crystalline structure and degree of graphitization were analyzed by X-ray Diffraction (XRD) using Cu Kα radiation. The interlayer spacing (d002) was calculated from the (002) diffraction peak using Bragg’s law:
$$ d_{002} = \frac{\lambda}{2 \sin \theta} $$
where λ is the X-ray wavelength (0.15406 nm for Cu Kα) and θ is the Bragg angle. The crystallite size along the c-axis (Lc) was estimated using the Scherrer equation:
$$ L_c = \frac{K \lambda}{\beta \cos \theta} $$
where K is the shape factor (~0.9), and β is the full width at half maximum (FWHM) of the (002) peak in radians.
Raman Spectroscopy: This technique was used to assess the structural disorder and graphitic quality. The intensity ratio of the D band (~1350 cm-1, associated with disordered carbon or defects) to the G band (~1580 cm-1, associated with the in-plane vibration of sp2-bonded carbon) is defined as the R value (R = ID/IG). A lower R value indicates a higher degree of graphitic order.
Porosity Analysis: The specific surface area and pore structure were determined by N2 adsorption-desorption isotherms measured at 77 K. The Brunauer-Emmett-Teller (BET) method was applied to calculate the specific surface area (SBET). The pore size distribution (PSD) was derived from the adsorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) model for mesopores (2-50 nm). The total pore volume (Vtotal) was estimated at a relative pressure (P/P0) of ~0.99.
Surface Chemistry: Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) were utilized to identify the functional groups and elemental composition (particularly C and O) on the carbon surface.
1.3. Electrochemical Measurements
To evaluate the performance as a lithium-ion battery anode, CR2032 coin cells were assembled in an argon-filled glovebox with oxygen and moisture levels below 0.1 ppm.
Electrode Preparation: The working electrode was fabricated by mixing the active ironwood-derived carbon material, conductive carbon black (Super P), and sodium carboxymethyl cellulose (CMC) binder in a weight ratio of 8:1:1 with deionized water to form a homogeneous slurry. The slurry was coated onto a copper foil current collector and dried under vacuum at 120°C for 12 hours. The mass loading of the active material was typically in the range of 1.0-1.5 mg cm-2.
Cell Assembly: Lithium metal foil was used as the counter/reference electrode. A Celgard polypropylene membrane served as the separator. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume).
Electrochemical Tests:
• Cyclic Voltammetry (CV): Conducted on an electrochemical workstation at a scan rate of 0.1 mV s-1 within a voltage window of 0.01-3.0 V vs. Li+/Li.
• Galvanostatic Charge-Discharge (GCD): Performed on a battery testing system at various current densities within the same 0.01-3.0 V window. Specific capacity values are calculated based on the mass of the active carbon material.
• Electrochemical Impedance Spectroscopy (EIS): Measured on the electrochemical workstation over a frequency range from 100 kHz to 0.01 Hz with an AC amplitude of 5 mV. The resulting Nyquist plots were fitted with equivalent electrical circuits to extract kinetic parameters.
2. Structural and Morphological Evolution
2.1. Morphology Analysis
The morphological transformation of ironwood through thermal treatment is profound. The carbonization process at temperatures up to 1650°C primarily preserves the macro-scale skeleton of the original wood but converts it into a brittle, glassy carbon char. The SEM images reveal a relatively dense and non-porous external morphology at these stages. In stark contrast, the material subjected to 2800°C graphitization undergoes a significant structural change. The resulting IW-2800 sample exhibits a more open and疏松 structure. This morphological evolution is attributed to the intense thermal stress and atomic reorganization during graphitization. Micro-cracks and voids form, creating a network that is highly beneficial for a lithium-ion battery anode. This porous, interconnected framework facilitates deeper penetration of the electrolyte, drastically reduces the diffusion path length for lithium ions, and provides ample void space to accommodate volume changes during cycling, thereby enhancing structural stability.
2.2. Crystalline Structure (XRD Analysis)
XRD provides definitive evidence of the progression from disordered carbon to graphitic carbon. The samples carbonized at lower temperatures (IW-700 to IW-1650) display very broad and weak diffraction peaks. The (002) peak, if discernible, is centered around 2θ ≈ 22-25°, corresponding to a large interlayer spacing (d002 > 0.35 nm) typical of turbostratic, non-graphitic carbon. This indicates a highly disordered stacking of graphene-like layers.
The XRD pattern of the IW-2800 sample marks a dramatic transition. It shows a sharp and intense (002) diffraction peak at 2θ ≈ 26.3° and a smaller (100)/(101) peak at ~42.2°. Using Bragg’s law, the d002 spacing is calculated to be 0.338 nm. This value is very close to the ideal interlayer distance of graphite (0.335 nm) and significantly smaller than that of disordered carbons. The sharpness of the peak indicates a large crystalline domain size along the c-axis (Lc). This highly ordered, layered structure is the foundation for its application as an intercalation-based anode in a lithium-ion battery, as it provides well-defined and stable pathways for lithium ion insertion and extraction.
| Sample | d002 (nm) | FWHM(002) (°) | Lc (nm) | R (ID/IG) |
|---|---|---|---|---|
| IW-700 | 0.387 | Very Broad | < 2 | ~1.05 |
| IW-1200 | 0.365 | ~10 | ~8 | ~0.92 |
| IW-1650 | 0.352 | ~5 | ~16 | ~0.65 |
| IW-2800 | 0.338 | ~0.5 | > 50 | 0.29 |
2.3. Defect and Graphitic Order (Raman Analysis)
Raman spectroscopy further quantifies the structural perfection. All samples show the characteristic D and G bands. The R value (ID/IG) systematically decreases with increasing treatment temperature, as summarized in Table 1. The high R values (~1.05-0.65) for carbonized samples confirm their disordered nature with abundant defects and edges. The IW-2800 sample, however, exhibits a remarkably low R value of 0.29. An R value significantly less than 1 is a clear signature of a well-developed graphitic structure with a low concentration of in-plane defects. Furthermore, the appearance of a distinct 2D band (~2700 cm-1) in the IW-2800 spectrum, which is sensitive to the stacking order of graphene layers, corroborates the formation of a multi-layered graphitic system. This high degree of graphitization is crucial for achieving high electronic conductivity, which is a key requirement for power performance in a lithium-ion battery.
2.4. Porosity and Surface Area (BET Analysis)
The N2 adsorption-desorption isotherms reveal the evolution of the porous structure. The carbonized samples (e.g., IW-1200) show a combination of Type I and Type IV isotherms, indicating the presence of both micropores (<2 nm) and mesopores. The graphitized IW-2800 sample displays a distinct Type IV isotherm with a pronounced H3-type hysteresis loop, which is associated with slit-shaped pores formed by the aggregation of plate-like particles—consistent with a graphitic material.
The BET surface area generally decreases after ultra-high temperature treatment because graphitization tends to “heal” small micropores and collapse some of the pore structure. However, the pore size distribution (PSD) plot for IW-2800 reveals a fascinating and advantageous feature: a bimodal distribution of mesopores centered at approximately 2.24 nm and 3.2 nm. These pores are not remnant from the precursor but are created during graphitization due to the rearrangement and stacking of graphitic domains. Such a mesoporous structure is highly desirable for a lithium-ion battery anode. It provides a high accessible surface area for rapid charge transfer at the electrode/electrolyte interface while maintaining a structure that is not excessively reactive (which would lower ICE). The mesopores act as ion-buffering reservoirs and short diffusion channels, significantly enhancing rate performance.
| Sample | SBET (m² g⁻¹) | Vtotal (cm³ g⁻¹) | Primary Pore Size (nm) | Isotherm Type |
|---|---|---|---|---|
| IW-1200 | ~450 | ~0.25 | Micropores (<2), Broad Meso | I + IV |
| IW-2800 | ~85 | ~0.15 | 2.24, 3.2 (Mesopores) | IV (H3) |
The total pore volume can be related to the density (ρ) of the electrode and its theoretical capacity (Ctheo) for a porous material considering both intercalation and pore-filling contributions, though it’s a complex relationship often described empirically.
2.5. Surface Chemistry (FTIR and XPS)
FTIR spectra show a progressive simplification of surface functional groups with increasing temperature. In lower-temperature carbonized samples, distinct peaks corresponding to C=O stretching (~1700 cm-1), C=C stretching (~1600 cm-1), and C-O stretching (~1200 cm-1) are visible. These oxygen-containing functional groups can contribute to irreversible capacity through side reactions with the electrolyte. In the IW-2800 sample, these peaks are vastly diminished or absent, indicating that the extreme heat has effectively removed most of the unstable surface functionalities. XPS analysis confirms this trend, showing a strong decrease in the atomic percentage of oxygen from >10% in IW-1200 to <2% in IW-2800. This “cleaning” of the surface is a major factor in achieving a high initial Coulombic efficiency in the resulting lithium-ion battery anode, as it minimizes parasitic reactions during the first cycle.
3. Electrochemical Performance as a Lithium-Ion Battery Anode
3.1. Electrode Kinetics and Interface Analysis (EIS)
Electrochemical Impedance Spectroscopy (EIS) offers deep insights into the kinetic barriers within the lithium-ion battery cell. The Nyquist plots for all samples consist of a depressed semicircle in the high-to-medium frequency region and an inclined line in the low-frequency region.
High-Frequency Intercept (Re): Represents the ohmic resistance of the electrolyte, separator, and contacts. This value is similar for all cells.
Semicircle (Rct): The diameter of the semicircle corresponds to the charge-transfer resistance (Rct) at the electrode/electrolyte interface. A clear trend is observed: Rct decreases substantially with increasing treatment temperature. The IW-2800 electrode exhibits the smallest semicircle. This low Rct is a direct consequence of its high electronic conductivity (from graphitization) and favorable mesoporous structure that ensures excellent electrolyte wetting and a large electroactive surface area. The simplified equivalent circuit model is Re + Qdl/Rct + W, where Qdl is a constant phase element representing the double-layer capacitance and W is the Warburg element for semi-infinite diffusion. The impedance can be modeled as:
$$ Z = R_e + \frac{R_{ct}}{1 + (j \omega R_{ct} C_{dl})^\alpha} + \sigma \omega^{-1/2}(1-j) $$
where $\alpha$ is an exponent related to the capacitance dispersion and $\sigma$ is the Warburg coefficient.
Low-Frequency Warburg Region: The slope of the line is related to the solid-state diffusion coefficient (DLi+) of lithium ions within the bulk electrode material. The IW-2800 electrode shows a steeper slope compared to the carbonized samples, indicating faster ion diffusion kinetics. This is attributed to the ordered graphite layers providing unobstructed 2D diffusion channels for Li+ and the mesopores facilitating rapid access to these channels.
3.2. Reaction Mechanism (Cyclic Voltammetry)
CV curves elucidate the electrochemical reactions occurring during cycling. For disordered carbon anodes (IW-700 to IW-1650), the CV profiles are characterized by broad, sloping redox humps without distinct peaks. This indicates a combination of intercalation into disordered layers, adsorption on defect sites, and pore filling—processes that occur over a wide potential range.
The CV of the IW-2800 electrode is distinctly different and resembles that of natural graphite. In the first cathodic (discharge) scan, a sharp reduction peak below 0.25 V appears, corresponding to the staged intercalation of lithium ions into the graphite layers to form LiC6. A broad irreversible peak around 0.6-0.9 V is also observed, which is associated with the formation of the solid electrolyte interphase (SEI) on the freshly exposed graphitic surface. In the subsequent anodic (charge) scan, a sharp peak near 0.1-0.3 V and a broader peak around 0.55-0.65 V are visible, representing the de-intercalation of lithium from the staged compounds. From the second cycle onward, the CV curves overlap almost perfectly, demonstrating exceptional electrochemical reversibility and stability of the SEI layer. The potential difference between the main reduction and oxidation peaks is small, indicating low polarization and high efficiency for this biomass-derived graphite in a lithium-ion battery.
3.3. Cycling Performance and Reversible Capacity
Galvanostatic charge-discharge cycling provides the most direct measure of performance. The voltage profiles for carbonized materials are sloping, consistent with their disordered structure. Their reversible capacities are modest, typically below 250 mAh g-1 at low current density, and they often suffer from significant capacity fade due to unstable SEI and irreversible trapping of lithium in disordered pores.
The IW-2800 electrode delivers a dramatic improvement. Its voltage profile displays the characteristic plateaus of graphite staging near 0.2 V vs. Li+/Li during discharge and corresponding plateaus during charge. At a current density of 0.05 A g-1, the IW-2800 electrode achieves a reversible capacity of approximately 340-350 mAh g-1. This value is close to the theoretical capacity of graphite and is maintained with outstanding stability over prolonged cycling. After 100 cycles, the capacity retention exceeds 98%, with a reversible capacity steadfastly remaining at 340.4 mAh g-1. The Coulombic efficiency quickly rises to over 99.5% after the first few cycles and remains near 100% thereafter. This performance rivals that of high-quality commercial synthetic graphite and underscores the success of the graphitization process in creating a stable, high-capacity anode material for a lithium-ion battery from a sustainable precursor.
| Sample | 1st Discharge Capacity (mAh g⁻¹) | 1st Charge Capacity (mAh g⁻¹) | Initial Coulombic Efficiency (ICE, %) | Reversible Capacity at 100th cycle (mAh g⁻¹) | Capacity Retention (%) |
|---|---|---|---|---|---|
| IW-700 | ~580 | ~205 | ~35.3 | < 150 | < 75 |
| IW-1200 | ~450 | ~214 | ~47.6 | ~180 | ~84 |
| IW-1650 | ~400 | ~260 | ~65.0 | ~235 | ~90 |
| IW-2800 | ~370 | ~345 | ~93.2 | 340.4 | > 98.5 |
The ICE is a critical parameter for full-cell energy density. The IW-2800’s high ICE of ~93% is a result of its low specific surface area (reducing SEI formation sites) and clean, stable surface chemistry.
3.4. Rate Capability
The power performance of a lithium-ion battery anode is tested under increasing current densities. The IW-2800 electrode demonstrates remarkable rate capability. When the current density is increased stepwise from 0.05 A g-1 to 0.1, 0.2, 0.5, and even 1.0 A g-1, the electrode delivers stable capacities. At a high rate of 1.0 A g-1, it can still provide a capacity of around 200 mAh g-1. Most importantly, when the current density is switched back to 0.05 A g-1, the capacity recovers almost completely to its original value, demonstrating excellent structural resilience and kinetic reversibility. This superb rate performance is a synergistic result of its high electronic conductivity (graphitic network), fast ion diffusion (ordered layers and mesopores), and stable interfacial properties. The capacity (C) at a given current density (i) often follows a power-law relationship with overpotentials related to charge transfer and diffusion, but the key metric is the high capacity retention (Crate/Clow) achieved by IW-2800.
4. Discussion: The Synergy of Structure and Performance
The exceptional electrochemical performance of the ironwood-derived graphite (IW-2800) as a lithium-ion battery anode can be understood as the culmination of optimized structural features achieved through high-temperature graphitization.
1. The Graphitic Framework for Efficient Intercalation: The sharp XRD peak and low Raman R value confirm a highly ordered graphitic structure with a d002 spacing of 0.338 nm. This is the perfect host for the reversible intercalation of lithium ions via a staging mechanism. The ordered lattice minimizes structural strain during ion insertion/extraction, leading to exceptional cycling stability. The high crystallinity also ensures superior bulk electronic conductivity, lowering the overall cell impedance and enhancing rate performance.
2. Engineered Porosity for Enhanced Kinetics: Unlike non-porous synthetic graphite, the IW-2800 material retains a beneficial mesoporous network (2.24 nm and 3.2 nm) created during graphitization. These mesopores are not defects but engineered features that provide:
• Shortened Ion Diffusion Paths: They allow electrolyte to percolate deep into the electrode particle, drastically reducing the solid-state diffusion distance for Li+ to reach the graphitic layers.
• Increased Electroactive Interface: They create additional accessible surface area where charge transfer can occur efficiently, explaining the low Rct.
• Buffer Space: They can accommodate minor local volume changes, further promoting structural integrity.
The relationship between pore size and ion transport can be considered in the context of the modified Fick’s law for porous electrodes, where effective diffusivity (Deff) is related to the pore geometry and tortuosity (τ):
$$ D_{eff} = \frac{\epsilon}{\tau} D $$
where ε is the porosity and D is the bulk diffusion coefficient. The bimodal mesoporous structure in IW-2800 likely optimizes ε/τ for fast ion transport.
3. Stable Surface for High Coulombic Efficiency: The removal of most oxygenated functional groups via high-temperature treatment yields a clean, hydrophobic graphitic surface. This minimizes irreversible side reactions with the electrolyte during the first cycle, leading to a thin, stable, and low-resistance SEI layer. This is the primary reason for the high ICE (~93%), which is essential for practical full-cell lithium-ion battery applications where lithium inventory is limited.
4. Cost and Sustainability Advantage: From a commercial perspective, the use of ironwood, a renewable and low-cost biomass waste, as the sole precursor offers a compelling economic and environmental advantage over petroleum- or pitch-based synthetic graphite. While the graphitization step at 2800°C is energy-intensive, it is a standard industrial process for producing high-quality graphite. The overall life-cycle cost and carbon footprint could be favorable when considering the renewable origin of the raw material.
In summary, the IW-2800 material successfully bridges the gap between the high performance of synthetic graphite and the sustainability of biomass carbon. It achieves this by not just being a “carbonized biomass,” but by being transformed into a true graphite through controlled ultra-high temperature treatment, thereby inheriting the best properties of both worlds for lithium-ion battery applications.
5. Conclusion and Future Perspectives
In this comprehensive study, we have demonstrated the successful conversion of ironwood biomass into a high-performance graphitic anode material for lithium-ion batteries. A systematic two-stage thermal treatment—carbonization followed by 2800°C graphitization—proved essential in transforming the disordered carbon skeleton into a highly ordered graphite structure. The resulting material (IW-2800) exhibits an ideal combination of structural attributes: a narrow interlayer spacing (0.338 nm), a low defect density (R=0.29), and a unique bimodal mesoporous network centered at 2.24 nm and 3.2 nm.
Electrochemically, the IW-2800 anode delivers a reversible capacity of ~340 mAh g-1 with outstanding cycling stability (98.5% retention after 100 cycles) and superior rate capability. Its high initial Coulombic efficiency (~93%) and low charge-transfer resistance further confirm its practicality. These properties stem from the synergistic effects of its well-defined graphitic layers enabling efficient lithium intercalation, its mesoporous structure facilitating rapid ion transport, and its clean surface promoting a stable SEI.
This work validates the significant potential of selected biomass precursors, when subjected to appropriate high-temperature processing, to produce graphite-quality anode materials. The use of ironwood points toward a sustainable and cost-effective supply chain for graphite, a critical component in the rapidly expanding lithium-ion battery industry for electric vehicles and grid storage.
Future work can focus on several promising avenues:
1. Process Optimization: Exploring catalysts (e.g., Fe, Co salts) that could lower the required graphitization temperature, thereby reducing energy consumption and cost.
2. Composite Design: Creating composites where the ironwood-derived graphite acts as a conductive, stable matrix for hosting high-capacity materials like nano-silicon or tin, aiming to push capacities beyond 500 mAh g-1 while mitigating volume expansion.
3. Full-Cell Evaluation: Testing the IW-2800 material in practical full-cell configurations paired with commercial cathode materials (e.g., NMC, LFP) to assess real-world energy density, cycle life, and aging mechanisms.
4. Life-Cycle Analysis (LCA): Conducting a detailed cradle-to-gate LCA to quantitatively compare the environmental impact of this biomass-derived graphite with conventional synthetic graphite.
By providing a detailed blueprint for the synthesis, characterization, and electrochemical analysis of biomass-derived graphite, this study contributes a valuable strategy for developing sustainable, high-performance electrode materials that meet the growing demands of the lithium-ion battery market.