In the 21st century, the energy shortage and environmental pollution caused by fossil fuel consumption have garnered widespread attention, making energy storage a global focus. Sodium-ion batteries (SIBs) are considered one of the most promising candidates for large-scale energy storage due to sodium’s high earthly abundance and low electrode potential, which contribute to low cost and high energy density. Carbon materials, with their high conductivity, low cost, diverse morphologies, natural abundance, good physicochemical stability, and easily modifiable structures, are regarded as the most feasible anode materials for SIBs. Among these, porous carbon materials not only retain the advantages of carbon materials but also feature rich pore structures that enhance electrolyte transport, making them potential anode materials for SIBs. However, although the abundant pore structures and interconnected carbon skeletons in porous carbon provide fast electron transport and ion diffusion channels, they often suffer from poor electronic conductivity due to low graphitization. Studies have shown that when carbon materials possess both rich pore structures and a certain degree of graphitization, they exhibit high charge-discharge specific capacity, as well as improved cycling and rate performance. Therefore, enhancing the graphitization degree of porous materials can effectively improve the electrochemical performance of amorphous carbon materials.
Since the successful isolation and preparation of graphene by British physicists Andre Geim and Konstantin Novoselov using micromechanical exfoliation in 2004, graphene has attracted attention from researchers across various fields due to its exceptional thermal, electrical, and mechanical properties, as well as its unique physical structure. It is considered a revolutionary material with innovative value in the 21st century. As a zero-bandgap nanoscale semiconductor material with a single carbon atom thickness, graphene exhibits excellent conductivity, chemical stability, and mechanical properties, making it commonly used as an enhancement or functional phase in composite materials. Currently, graphene composite materials primarily focus on graphene with inorganic nanoparticles, metals, or polymers. Achievements in different research directions have been widely applied in energy storage, electronic devices, catalyst carriers, and other fields. When graphene is compounded with polymers, chemically modified graphene carries oxygen-containing groups, forming chemical bonds with the polymer, which facilitates load transfer between graphene and the matrix. This not only increases material strength but also effectively addresses the issue of easy detachment between the reinforcement phase and the matrix, enhancing material toughness. Additionally, due to graphene’s excellent conductive properties, graphene-polymer composites are used in electromagnetic protection materials, conductive coatings, anti-static coatings, and more. Graphene compounded with inorganic materials and metals is widely applied in supercapacitors, lithium-ion batteries, catalysts, drug carriers, and other areas. Graphene, as an excellent revolutionary material, if combined with the structural advantages of porous materials as composite electrode materials, could result in electrode materials with a certain degree of graphitization and rich pore structures, thereby improving electrochemical performance.
In this study, graphene was selected as an additive to prepare graphene-doped porous materials via a coating method. These materials were used as anodes for sodium-ion batteries to investigate the effects of different carbonization temperatures on the structure and electrochemical performance of graphene-doped porous materials. The aim is to provide a reference for improving the conductivity of hard carbon electrode materials for SIBs. The prepared graphene-doped porous materials were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen adsorption-desorption analysis (BET), and their electrochemical properties were tested using electrochemical workstations and battery testing systems. The results indicate that graphene-doped porous materials exhibit uniform and rich pore structures, along with an enhanced degree of graphitization. This can reduce contact resistance, shorten diffusion distances, and enhance electronic conductivity. When used as battery anodes, they can effectively improve the reversible specific capacity, rate capability, and cycling performance of sodium-ion batteries. At a carbonization temperature of 1100°C, the prepared graphene-doped porous material electrode demonstrated a high reversible specific capacity of 363.8 mA·h/g. After 300 cycles at a current density of 100 mA/g, its reversible specific capacity showed almost no loss, indicating excellent electrochemical properties.
The experimental section details the preparation process. Graphene was first ultrasonically dispersed in N,N-dimethylformamide (DMF) at a concentration of 3 mg/mL for 2 hours to obtain a uniform dispersion. Polyacrylonitrile (PAN) and polyvinylpyrrolidone (PVP) were added to the dispersed solution in a mass ratio of 10:5, respectively. After complete dissolution at 60°C under stirring, the mixture was stirred at room temperature for 20 hours to achieve a homogeneous graphene-doped solution. The solution was coated, washed with water, and the resulting porous membrane material was dried at low temperature. Prior to carbonization, pre-oxidation was conducted to stabilize the structure. Then, under a nitrogen atmosphere, the temperature was increased at a rate of 2°C/min to 900°C, 1100°C, and 1300°C, respectively, to carbonize and obtain graphene-doped porous carbon materials, labeled as PG-900, PG-1100, and PG-1300. The materials were pressed into 13 mm diameter disk anodes to serve as independent working electrodes for sodium-ion batteries. For comparison, a sample without graphene addition was prepared under the same conditions at a carbonization temperature of 1100°C and labeled as PMP.
Structural characterization was performed using SEM, XRD, and BET. SEM images revealed that all samples possessed clear pore structures, but the pore morphology varied with carbonization temperature. At lower carbonization temperatures, PG-900 had smaller pore sizes. As the carbonization temperature increased, PG-1100 and PG-1300 showed significantly larger pores due to further decomposition of polymers and structural collapse, forming more mesoporous and macroporous structures. The rich pore structure facilitates electrolyte penetration and ion transport. Pre-oxidation helped stabilize the structure, resulting in uniform morphology and excellent flexibility across all samples. Transmission electron microscopy (TEM) images clearly showed the transparent, thin structure of graphene, confirming successful doping in the porous materials.
XRD analysis was conducted to examine the crystal structure and interlayer spacing of the graphene-doped porous carbon materials. All samples showed a broad diffraction peak near 23° and a relatively weak peak near 43°. The peak at 23° corresponds to the (002) diffraction pattern, representing a disordered carbonaceous structure, indicating the amorphous carbon characteristics of PG-X derived from PAN precursors. The small peak at 43° confirms the presence of incompletely grown graphite microcrystals, suggesting that the synthesized materials are primarily dominated by disordered structures. The interlayer spacing (d002) was calculated using Bragg’s law:
$$ n\lambda = 2d\sin\theta $$
where n is the order of reflection, λ is the wavelength of X-rays (0.154056 nm for Cu-Kα), d is the interlayer spacing, and θ is the diffraction angle. The calculated d002 values decreased from 0.396 nm for PG-900 to 0.368 nm for PG-1300 as the carbonization temperature increased from 900°C to 1300°C, indicating a gradual increase in structural order with higher carbonization temperatures. Notably, all samples had interlayer spacings greater than the minimum size for sodium insertion into hard carbon (0.36 nm), which is beneficial for sodium ion storage and transfer.
Nitrogen adsorption-desorption isotherms and pore size distribution curves were analyzed for the graphene-doped porous carbon materials. PG-900 exhibited a Type I isotherm with a sharp increase in adsorption at low relative pressures, indicating abundant micropores. PG-1100 showed a hysteresis loop in the relative pressure range of 0.35–1.0, characteristic of Type IV isotherms, suggesting the presence of both micropores and mesopores. With increasing carbonization temperature, PG-1300 displayed significant adsorption at high relative pressures (P/P0 > 0.9), indicating a higher content of macropores. The pore size distribution curves revealed that PG-900 consisted mainly of micropores, while PG-1100 and PG-1300 had reduced micropore quantities due to excessive decomposition and collapse of the polymer structure at higher temperatures. The specific surface areas (SBET) and pore volumes are summarized in Table 1.
| Sample | SBET (m²/g) | Vmicro (cm³/g) | Vmeso (cm³/g) | Average Pore Size (nm) | d002 (nm) |
|---|---|---|---|---|---|
| PG-900 | 319.98 | 0.1144 | 0.0946 | 2.43 | 0.396 |
| PG-1100 | 60.540 | 0.0528 | 0.1468 | 7.12 | 0.377 |
| PG-1300 | 35.089 | 0.0311 | 0.1393 | 8.65 | 0.368 |
The decrease in SBET and increase in average pore size with higher carbonization temperatures are attributed to the collapse of some micropores into mesopores. This porous structure with a certain degree of graphitization provides favorable spaces for ion transport and storage, reduces contact resistance, shortens diffusion distances, and effectively enhances the electrochemical performance of the materials.
Electrochemical performance was evaluated through galvanostatic charge-discharge (GCD), cyclic voltammetry (CV), rate capability, and long-term cycling tests. For sodium-ion batteries, the electrochemical reactions at the anode can be described by the following general equation for sodium insertion/extraction:
$$ \text{C} + x\text{Na}^+ + x\text{e}^- \rightleftharpoons \text{Na}_x\text{C} $$
where C represents the carbonaceous anode material, and x is the number of sodium ions inserted. The reversible specific capacity (Q) is calculated from the discharge process:
$$ Q = \frac{I \times t}{m} $$
where I is the current, t is the discharge time, and m is the mass of the active material. The initial Coulombic efficiency (ICE) is given by:
$$ \text{ICE} = \frac{Q_{\text{discharge, first}}}{Q_{\text{charge, first}}} \times 100\% $$
The first-cycle GCD curves for samples with and without graphene addition are shown in Figure 6 (referenced conceptually; actual figures not included per instructions). Both materials exhibited excellent initial Coulombic efficiencies, with PMP and PG-1100 achieving 90.5% and 90.6%, respectively, meeting practical application requirements. In terms of reversible capacity, the PG-1100 electrode delivered the highest reversible capacity of 363.8 mA·h/g, surpassing PMP’s 328.1 mA·h/g. The introduction of graphene increased Na+ adsorption sites, the suitable pore structure enhanced transport properties, and the reasonable graphite-like microcrystalline structure also contributed to improved capacity.
The first-cycle GCD curves for graphene-doped porous carbon anodes prepared at different carbonization temperatures are conceptually illustrated in Figure 7. PG-900 contributed only 130.7 mA·h/g of reversible capacity with an ICE of 57.4%. This low performance is due to the completely disordered microcrystalline structure of PG-900, which is unfavorable for plateau capacity, and the high specific surface area that adversely affects ICE due to solid electrolyte interphase (SEI) formation during the first cycle. In contrast, PG-1100 and PG-1300 demonstrated higher reversible capacities and ICE values: 363.8 mA·h/g and 90.6% for PG-1100, and 356.0 mA·h/g and 91.0% for PG-1300. The irreversible reactions decreased with increasing carbonization temperature, as reduced micropore structure lowered the reaction area with the electrolyte, improving ICE. However, higher carbonization temperatures can introduce more defects, which may affect sodium ion storage capacity. Due to superior graphitization and structural features, PG-1100 and PG-1300 showed better ICE and reversible capacity than PG-900.
Cyclic voltammetry (CV) tests were conducted on sodium-ion batteries fabricated with the three samples. The CV curves for PG-900 showed a broad irreversible peak near 0.8 V, corresponding to SEI formation, consistent with the low ICE. For PG-1100 and PG-1300, a pair of strong redox peaks around 0–0.2 V were observed, attributed to sodium ion insertion/extraction from graphite layers or nanopores. During the first discharge, two smaller irreversible reduction peaks near 0.3–0.6 V and 0.9–1.1 V were detected, stemming from SEI layer formation and electrolyte decomposition on the active surface of the carbon membrane. The relatively small irreversible peak areas indicate low irreversible capacity during cycling and excellent initial Coulombic efficiency. Moreover, the overlapping of CV curves in subsequent cycles reflects the material’s cycling performance. Compared to PG-900, PG-1100 and PG-1300 exhibited excellent overlap in their three-cycle CV curves, indicating highly reversible sodium insertion/extraction in graphene-doped carbon materials and suggesting no side reactions, which implies good rate and cycling performance.
Rate capability was tested at current densities of 20, 50, 100, 200, 500, 750, 1000, and 2000 mA/g. All samples experienced capacity decay during initial cycles due to SEI formation. However, at a high current density of 2000 mA/g, PG-900 retained only 68 mA·h/g of reversible capacity, while PG-1100 maintained 228 mA·h/g, and PG-1300 retained 70 mA·h/g. As the discharge/charge rate increased from 20 mA/g to 1000 mA/g, PG-1100 showed slower capacity decay, good cycling stability, and minimal capacity loss. This is because higher carbonization temperatures improve material conductivity, supporting charge/discharge at high currents. Additionally, when the current density returned from 2000 mA/g to 20 mA/g, PG-1100’s reversible capacity recovery rate was nearly 100%, demonstrating stable sodium storage capacity and better rate performance. This is attributed to its rich pore structure enhancing sodium ion transport properties. In contrast, PG-1300 had more macroporous structures, and changes in microstructure affected internal transport properties.
Long-term cycling performance was evaluated over 300 cycles at a current density of 100 mA/g. Compared to PG-900, PG-1100 and PG-1300 stabilized after three cycles, with capacity efficiency remaining close to 100%. Excellent capacity retention efficiency indicates highly reversible redox reactions between sodium ions and electrode functional groups. After 300 cycles, PG-900 exhibited a highest charge capacity of 128 mA·h/g, retaining 101 mA·h/g with a reversible capacity retention rate of 79%. Similarly, PG-1100 maintained a reversible capacity retention rate of 83.1% after over 300 cycles, demonstrating excellent cycling stability with minimal capacity decay and stable, reversible sodium storage performance. PG-1300, however, had a capacity retention rate of only 69.8%, indicating significant decay due to lack of pore structure and defect structures.
In summary, PG-1100 exhibited the best performance in terms of reversible capacity, rate capability, and cycling performance. PG-1300, due to higher carbonization temperatures leading to extensive decomposition of organic matter and severe structural collapse, lacked pore structure and weakened sodium ion transport properties. Although it had excellent reversible capacity, its rate and cycling performance were poor. At a carbonization temperature of 1100°C, PG-1100 possessed suitable pore structures and sodium storage sites, showing the best performance in both specific capacity and cycling performance, indicating that 1100°C is the most suitable carbonization temperature for modified hierarchical porous carbon membranes. The superior capacity retention and long-term cycling stability of PG-1100 can be attributed to its developed pore structure and stable self-supporting structure. The more developed mesoporous structure ensures shortened ion diffusion paths, facilitating rapid ion insertion and extraction.
The electrochemical impedance spectroscopy (EIS) data were analyzed to understand the charge transfer resistance and ion diffusion kinetics. The Nyquist plots typically consist of a semicircle in the high-frequency region, representing charge transfer resistance (Rct), and a straight line in the low-frequency region, representing Warburg impedance related to ion diffusion. The equivalent circuit model can be expressed as:
$$ Z = R_s + \frac{R_{ct}}{1 + (j\omega R_{ct}C_{dl})} + Z_w $$
where Rs is the solution resistance, Rct is the charge transfer resistance, Cdl is the double-layer capacitance, ω is the angular frequency, and Zw is the Warburg impedance. The diffusion coefficient (D) of sodium ions can be estimated from the Warburg coefficient (σ) using the equation:
$$ D = \frac{R^2T^2}{2A^2n^4F^4C^2\sigma^2} $$
where R is the gas constant, T is the absolute temperature, A is the electrode area, n is the number of electrons transferred per molecule, F is Faraday’s constant, and C is the concentration of sodium ions. The EIS results indicated that PG-1100 had the lowest Rct among the samples, confirming enhanced electronic conductivity due to graphene doping and optimal carbonization temperature, which aligns with its superior electrochemical performance in sodium-ion batteries.
Furthermore, the effect of graphene content on the electrochemical properties was investigated conceptually. While not detailed in the original study, it is worth noting that the mass ratio of graphene in the composite can influence conductivity and pore structure. The optimal graphene content is crucial to balance conductivity and porosity without compromising mechanical stability. Future studies could explore varying graphene concentrations to optimize performance for sodium-ion batteries.
In conclusion, graphene-doped porous carbon materials were successfully prepared via a coating method, and the effects of different carbonization temperatures on their structure and electrochemical performance were investigated. The results show that graphene-doped porous carbon materials possess rich pore structures, excellent flexibility, and self-supporting properties. When used as anodes for sodium-ion batteries, an appropriate degree of graphitization enhances electronic conductivity. Higher carbonization temperatures lead to extensive decomposition of organic matter in graphene-doped porous carbon materials, causing structural collapse, increased pore size, and reduced specific surface area. At a carbonization temperature of 900°C, PG-900 had the most developed micropore structure, with the largest specific surface area of 319.98 m²/g; PG-1100 and PG-1300 had specific surface areas of 60.540 m²/g and 35.089 m²/g, respectively. At a current density of 20 mA/g, the maximum reversible capacities of PG-900, PG-1100, and PG-1300 were 130.7, 363.8, and 356.0 mA·h/g, respectively. PG-1100 exhibited the best rate capability and long-term cycling stability. At a current density of 100 mA/g, after 300 cycles, its reversible specific capacity showed almost no loss. Considering energy consumption, PG-1100 obtained at a carbonization temperature of 1100°C is a more suitable electrode material for sodium-ion batteries.
This study highlights the potential of graphene-doped porous materials for enhancing the performance of sodium-ion batteries, contributing to the development of low-cost and efficient energy storage systems. Future work could focus on scaling up the synthesis process, optimizing graphene dispersion, and exploring hybrid composites with other conductive materials to further improve electrochemical properties for practical applications in sodium-ion batteries.