In recent years, the escalating global energy demand and environmental concerns have driven intensive research into sustainable energy storage systems. Among these, sodium-ion batteries have emerged as a promising alternative to lithium-ion batteries due to the abundant natural reserves of sodium and its similar electrochemical properties to lithium. However, the development of high-performance anode materials for sodium-ion batteries remains a critical challenge. Graphite, commonly used in lithium-ion batteries, exhibits poor sodium storage capability owing to the larger ionic radius of Na+ (1.02 Å) compared to Li+ (0.76 Å) and weak interactions with carbon layers. Hard carbon materials, characterized by their disordered structure and tunable porosity, have shown great potential as anodes for sodium-ion batteries, offering high reversible capacity and stable cycling performance. In this study, we present a novel bifunctional activation strategy to prepare pitch-based hard carbon anodes with optimized microstructure for enhanced sodium-ion storage.
Coal tar pitch, a byproduct of coal processing, is an inexpensive and carbon-rich precursor for carbon materials. However, direct carbonization of pitch typically results in graphitized soft carbon with limited sodium storage capacity due to excessive hydrogen content and strong π-π interactions among aromatic molecules. To address this, we employ potassium citrate monohydrate (K3C6H5O7·H2O) as a bifunctional activating agent. During thermal treatment, the decomposing gas products consume excess hydrogen, promoting solid-state pyrolysis and inhibiting the formation of ordered microcrystals. Simultaneously, the solid residues (potassium salts) act as pore-forming agents, creating abundant closed nanopores during high-temperature carbonization. This approach enables the fabrication of hard carbon with highly disordered, porous sheet-like structures, which are advantageous for sodium-ion insertion and extraction.
The electrochemical performance of sodium-ion batteries heavily relies on the anode material’s ability to facilitate rapid ion diffusion and provide sufficient active sites. The bifunctional activation method allows precise control over the carbon microstructure by adjusting the mass ratio of the activating agent to the pitch precursor. We investigate various ratios and their effects on the structural properties and sodium storage behavior. The optimized hard carbon sample exhibits a high initial coulombic efficiency of 81.5%, a reversible specific capacity of 214.2 mAh·g−1 at 0.1 A·g−1, and excellent rate capability with 116.7 mAh·g−1 at 5 A·g−1. Moreover, it demonstrates remarkable cycling stability, retaining 75.1% of its capacity after 2000 cycles at 1 A·g−1. These findings highlight the effectiveness of our strategy in developing low-cost, high-performance anodes for sodium-ion batteries.
The synthesis process involves mixing coal tar pitch with potassium citrate monohydrate at mass ratios of 1:1, 2:1, and 4:1, followed by heat treatment at 500°C for 2 hours under an argon atmosphere. The intermediate products are then washed with dilute HCl and deionized water to remove impurities. Finally, high-temperature carbonization at 1300°C for 2 hours yields the hard carbon samples, denoted as HC-x-1300 (where x represents the mass ratio). For comparison, directly carbonized pitch (DC-1300) is also prepared. The materials are characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and nitrogen adsorption-desorption analysis. Electrochemical tests are conducted in coin cells with sodium metal as the counter/reference electrode, glass fiber separators, and an electrolyte of 1 M NaPF6 in a mixture of ethylene carbonate and diethyl carbonate.
The structural analysis reveals significant differences between the activated and non-activated samples. SEM images show that DC-1300 possesses a dense, layered morphology, whereas HC-2-1300 displays a porous sheet-like structure with numerous polygonal pores. TEM further confirms that DC-1300 has long-range ordered graphitic layers with an interlayer spacing of approximately 0.345 nm, while HC-2-1300 exhibits a turbostratic structure with short-range order and an expanded interlayer spacing of 0.372 nm. This expansion is crucial for accommodating sodium ions in sodium-ion batteries. XRD patterns indicate that the (002) diffraction peak of HC-x-1300 samples shifts to lower angles compared to DC-1300, signifying larger d-spacings and reduced graphitization. The calculated d002 values are summarized in Table 1.
| Sample | d002 (nm) | BET Surface Area (m2·g−1) | Pore Volume (cm3·g−1) | Initial Coulombic Efficiency (%) | Reversible Capacity at 0.1 A·g−1 (mAh·g−1) |
|---|---|---|---|---|---|
| DC-1300 | 0.345 | 3.3 | 0.005 | 75.6 | 113.5 |
| HC-1-1300 | 0.376 | 155.5 | 0.098 | 82.2 | 195.0 |
| HC-2-1300 | 0.372 | 164.8 | 0.084 | 81.5 | 214.2 |
| HC-4-1300 | 0.370 | 378.9 | 0.214 | 79.4 | 188.1 |
Raman spectroscopy provides insights into the defect density and disorder of the carbon materials. The intensity ratio of the D band (∼1350 cm−1) to the G band (∼1590 cm−1), ID/IG, is higher for HC-x-1300 samples than for DC-1300, indicating greater structural disorder and more defects, which are beneficial for sodium storage in sodium-ion batteries. Nitrogen adsorption-desorption isotherms reveal that all samples exhibit Type I curves, suggesting a microporous nature. The BET surface areas and pore volumes increase with the amount of activating agent, as shown in Table 1. However, excessive activation (HC-4-1300) leads to an overly high surface area, which may promote irreversible side reactions and reduce the initial coulombic efficiency.
The electrochemical performance of these hard carbon anodes is evaluated in sodium-ion batteries. Cyclic voltammetry (CV) curves at a scan rate of 0.25 mV·s−1 display characteristic redox peaks near 0.01–0.1 V, corresponding to the insertion and extraction of sodium ions. The first cycle shows irreversible reduction peaks between 0.3–0.6 V due to solid electrolyte interphase (SEI) formation. Compared to DC-1300, HC-2-1300 exhibits smaller irreversible peaks and better overlap in subsequent cycles, reflecting superior reversibility. The sodium-ion diffusion coefficient (DNa+) is calculated from the peak current (Ip) versus square root of scan rate (v1/2) using the Randles-Sevcik equation:
$$I_p = 2.69 \times 10^5 A n^{3/2} C_0 D_{Na^+}^{1/2} v^{1/2}$$
where A is the electrode area, n is the number of electrons transferred, and C0 is the sodium-ion concentration. The calculated DNa+ for HC-2-1300 is 2.34 × 10−7 cm2·s−1, which is an order of magnitude higher than that of DC-1300 (1.46 × 10−8 cm2·s−1). This enhanced kinetics contributes to the excellent rate performance of HC-2-1300 in sodium-ion batteries.
Galvanostatic charge-discharge (GCD) profiles at 0.1 A·g−1 consist of a sloping region above 0.1 V and a plateau region below 0.1 V. The sloping region is associated with sodium adsorption on defect sites, while the plateau corresponds to sodium insertion into graphitic layers and pore filling. The capacity contributions from these regions are analyzed for each sample, as summarized in Table 2. HC-2-1300 delivers the highest total reversible capacity, with a significant plateau capacity of 94.7 mAh·g−1 more than DC-1300. This improvement is attributed to the expanded interlayer spacing and abundant closed nanopores, which facilitate efficient sodium storage.
| Sample | Slope Capacity (mAh·g−1) | Plateau Capacity (mAh·g−1) | Total Reversible Capacity (mAh·g−1) |
|---|---|---|---|
| DC-1300 | 68.3 | 45.2 | 113.5 |
| HC-1-1300 | 102.5 | 92.5 | 195.0 |
| HC-2-1300 | 119.5 | 94.7 | 214.2 |
| HC-4-1300 | 98.4 | 89.7 | 188.1 |
Rate capability tests are conducted at current densities ranging from 0.1 to 5 A·g−1. HC-2-1300 maintains capacities of 214.2, 192.2, 176.5, 162.5, 147.2, and 116.7 mAh·g−1 at 0.1, 0.2, 0.5, 1, 2, and 5 A·g−1, respectively. When the current density returns to 0.1 A·g−1, the capacity recovers to 195.7 mAh·g−1, demonstrating good reversibility. In contrast, DC-1300 suffers from rapid capacity decay at high rates. The superior rate performance of HC-2-1300 is linked to its porous structure and fast sodium-ion diffusion, which are critical for high-power applications of sodium-ion batteries.
Long-term cycling stability is assessed at 1 A·g−1 over 2000 cycles. HC-2-1300 retains 122.0 mAh·g−1 with a capacity retention of 75.1% and near-100% coulombic efficiency. DC-1300, however, shows significant capacity fading. The enhanced cycling stability of HC-2-1300 stems from its robust microstructure, which minimizes volume changes and maintains electrical conductivity during repeated sodium insertion/extraction cycles. These results underscore the potential of bifunctional activated pitch-based hard carbon as a durable anode material for sodium-ion batteries.
To further understand the sodium storage mechanism, we analyze the relationship between capacity and scan rate using the power law: $$i = a v^b$$ where i is the current, v is the scan rate, and b is an exponent that indicates the charge storage behavior (b = 0.5 for diffusion-controlled processes, b = 1 for capacitive processes). For HC-2-1300, the b-value calculated from the cathodic peak is 0.72, suggesting a mixed contribution from diffusion-controlled insertion and surface-controlled capacitive effects. This hybrid mechanism explains the high rate capability and capacity of the material in sodium-ion batteries.
The optimization of the activating agent ratio is crucial for balancing structural disorder and porosity. Excessive activation (as in HC-4-1300) increases surface area but may lead to more SEI formation and lower initial coulombic efficiency. The ideal ratio (2:1 in this study) yields a hard carbon with moderate surface area, expanded interlayer spacing, and sufficient closed pores, maximizing sodium storage performance. We propose that the bifunctional activation strategy can be extended to other carbon precursors for designing advanced anodes for sodium-ion batteries.
In conclusion, we have developed a facile and effective bifunctional activation method to prepare pitch-based hard carbon anodes for sodium-ion batteries. By using potassium citrate monohydrate as an activating agent, we achieve control over the carbon microstructure, resulting in materials with high disorder, expanded interlayer spacing, and tailored porosity. The optimized hard carbon (HC-2-1300) exhibits outstanding electrochemical properties, including high reversible capacity, excellent rate capability, and long cycling stability. This work provides a promising route for low-cost, high-performance anode materials in sodium-ion batteries, contributing to the advancement of sustainable energy storage technologies.
Future research could focus on scaling up the synthesis process, exploring other bifunctional agents, and integrating these anodes into full sodium-ion battery cells. Additionally, in-depth studies on the sodium storage mechanism using in-situ characterization techniques would further optimize the material design. The versatility of this approach may also benefit other energy storage systems, such as potassium-ion batteries or supercapacitors, highlighting its broad applicability.
The development of efficient sodium-ion batteries is pivotal for grid-scale energy storage and electric vehicles, given the abundance and low cost of sodium resources. Our findings demonstrate that pitch-based hard carbon, derived from industrial byproducts, can be transformed into high-value anode materials through strategic activation. This aligns with circular economy principles and reduces reliance on critical materials. As research on sodium-ion batteries progresses, we anticipate that such carbon materials will play a significant role in commercializing this technology.
Moreover, the electrochemical performance of sodium-ion batteries depends not only on the anode but also on cathode materials, electrolytes, and cell engineering. Future work should consider the compatibility of our hard carbon anodes with various cathodes, such as layered oxides or polyanionic compounds, to develop balanced sodium-ion battery systems. The optimization of electrolyte formulations to enhance SEI stability and ion conductivity could further improve the overall performance.
In summary, the bifunctional activation strategy presented here offers a scalable and cost-effective method to produce high-quality hard carbon for sodium-ion battery anodes. By leveraging the unique properties of potassium citrate monohydrate, we have unlocked the potential of coal tar pitch, turning a waste product into a valuable energy storage material. This approach paves the way for sustainable battery technologies that meet the growing demands for renewable energy integration and electrification.