Nitrogen-doped and partially graphitized coal-based hard carbon materials for high-performance sodium-ion storage in SIBs

Abstract

Hard carbon materials are widely regarded as highly promising anode materials in sodium-ion batteries (SIBs) due to their distinctive disordered structure and high capacity. However, their practical applications are greatly limited due to their high cost, low conductivity and unsatisfactory sodium storage performances. In this work, we propose a cost-effective method for the preparation of nitrogen-doped and partially graphitized coal-based hard carbons (NHCs) as high-performance anode materials for SIBs. The resulting NHCs possess the structural advantages of a mesoporous structure, nitrogen doping, and homogeneous graphite domains, which facilitate rapid kinetics and electron transfer. When applied to anodes in SIBs, the NHCs delivered a high reversible capacity of 291.45 mAh g⁻¹, a high initial coulombic efficiency of 88.1%, and an impressive cycling stability of 132.7 mAh g⁻¹ even after 500 cycles at 1 A g⁻¹. Overall, the coal-based hard carbon materials prepared using this straightforward yet effective method have the potential for large-scale energy-related applications in terms of economical precursors, simple synthesis process and excellent storage-energy performances.

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Introduction

Intermittent solar and wind energy storage requires a low-cost and large-scale energy storage system.^1–5 Lithium-ion batteries have been widely used as power sources for electronic products. However, the low reserves and uneven distribution of lithium metal limit the application of lithium-ion batteries in large-scale energy storage systems.^6–10 To address this issue, due to the greater geographical abundance of sodium than that of lithium, sodium-ion batteries have been widely explored as a potential alternative to lithium-ion batteries.^11–13 Although the electrochemical principles of sodium-ion batteries and lithium-ion batteries are similar, the larger ionic radius of Na⁺ makes it impossible for traditional commercial graphite anodes to capture Na⁺ into the interlayers.^14–16 Hard carbon materials have been widely recognized as ideal anode materials for sodium-ion batteries due to their high sodium storage capacity and low sodium insertion potential.^17–20 However, the selection and treatment of precursors remain the core challenges so far.

To the best of our knowledge, coal, as an important energy source in China, has unique advantages such as rich resources, wide distribution, low price, and high carbon content, making it a potential precursor for anode materials of sodium-ion batteries.^21–24 However, the hard carbon materials prepared by directly carbonizing coal usually have an intrinsically disordered microstructure, resulting in poor electrical conductivity, which decays their rate performance.²⁵ To address this issue, Wei et al. developed a hard carbon-soft carbon nanocomposite by pyrolyzing a mixture of biomass and coal gangue.²⁶ Moreover, through a catalyst-assisted synthesis technique, the graphitization degree of hard carbon could be regulated, thus improving its electrical conductivity.^27,28 This unique hard carbon material composed of graphite domains and disordered structures is expected to enhance the electrochemical performance and charge storage capacity.^29,30 In addition, by doping heteroatoms (such as N, O, and F) into hard carbons, active sites can be introduced and electron transfer can be promoted, thereby significantly enhancing the Na⁺ storage capacity of hard carbon.^31–34 Defects and active sites introduced by nitrogen-doped hard carbons can promote the diffusion kinetics and storage of Na⁺.³⁵ For example, Sun et al. used coal as the raw material and directly carbonized it in an ammonia atmosphere to develop a new type of nitrogen-doped microporous carbon. As an anode for sodium-ion batteries, its discharge capacity at 200 mAh g⁻¹ still remained at 190 mAh g⁻¹ even after 500 cycles.³⁶ However, the current synthesis strategies for producing hard carbons with graphite domains and heteroatom doping usually involve multiple reaction steps and harsh reaction conditions.^13,37–39 Therefore, in practical applications, there is an urgent need to develop an efficient and concise synthesis process for preparing hard carbon materials with graphite domains and heteroatom doping.

In this study, we developed a simple strategy to prepare nitrogen-doped and partially graphitized hard carbons (NHCs) by using urea as both the nitrogen source and the catalyst, in combination with anthracite coal, under simple annealing conditions. Through a simple annealing method, nitrogen elements and graphite domains can be variably introduced into the NHCs, and the graphitization of NHCs is optimized. The prepared NHCs exhibited excellent performance in line with expectations. At a current density of 30 mAh g⁻¹, they achieved a high reversible capacity of 291.45 mAh g⁻¹ and a high initial coulombic efficiency of 88.1%. Meanwhile, they featured outstanding rate performance and a long cycle life. These performance features fully demonstrate that NHCs have great application prospects as advanced anode materials for sodium-ion batteries.

Experimental

Preparation of NHCs

Nitrogen-doped and partially graphitized hard carbon (NHC) materials were prepared using Yulin bituminous coal obtained from Shaanxi, China, as a precursor. Prior to carbonization, 5 g of the raw coal was subjected to ball milling at 100 rpm for 12 hours. Thereafter, the material was washed with 10% (mass fraction) hydrofluoric acid and 5 M HCl at 60 °C for 12 hours each, with the objective of removing impurities. The resulting black powder was physically mixed with different proportions of urea. The solid mixture was then placed in a high-temperature tube furnace in a nitrogen atmosphere and heated to 1300 °C at a rate of 2 °C per minute for 2 hours. The resulting product was dissolved in a 2 M HCl solution and subjected to ultrasonication for 30 minutes. It was then washed three times with distilled water to obtain nitrogen-doped, partially graphitized hard carbons, which were subsequently dried at 70 °C overnight. The resulting samples were designated as NHC-1, NHC-2, NHC-3, and NHC-4, in the ascending order of proportion. For purposes of comparison, the sample pyrolyzed directly at 1300 °C without the addition of urea was designated as HC.

Results and discussion

The synthetic route for NHCs is shown in Fig. 1. First, deashed bituminous coal and urea were ball milled and mixed in a ball mill tank to obtain a solid-phase mixture of coal and urea. The comprehensive industrial and elemental analyses of the bituminous coal and deashed bituminous coal are meticulously documented in Table S1. A small amount of metallic ash remained in the coal sample after acid washing. The introduction of metallic ash elements was proved to effectively increase the degree of graphitization, resulting in outstanding sodium storage performance.⁴⁰ High-temperature carbonization treatment was carried out on the solid-phase mixture, with urea as both the catalyst and the nitrogen source to modulate the carbon structure and increase the Na⁺ adsorption sites. Subsequently, the carbonization products were purified by acid washing to remove the excess catalyst, and thus, the NHCs were obtained. The FT-IR results (Fig. S1) indicated that acid washing could effectively remove the inorganic mineral components in the raw coal, while most of the signals of the organic functional groups weakened or disappeared after high-temperature carbonisation, with some oxygen-containing groups remaining. The detailed morphology and microstructure of HC and NHC-2 (NHC-2 was the best sample for the proportion of urea used, and the remaining proportions were NHC-1, NHC-3 and NHC-4) were investigated by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). The FESEM images (Fig. 2a and Fig. S2, S3) demonstrated the microstructure of HC catalyzed by different proportions of urea. It is evident that the NHCs display a more smooth and porous structure as the proportion of urea increases, which is conducive to the storage of Na⁺ and enhances the wettability of the electrolyte to the anode. This distinctive structure is caused by the catalysis and etching that occur during the decomposition of urea. According to the present study, the specific catalytic pathway of urea during the coal carbonization stage is as follows. First, urea decomposes upon heating to yield products with reduction and alkaline properties, which react with oxygen-containing functional groups in the coal molecular structure, thereby reducing defects in the carbon skeleton. As the temperature increases, nitrogen atoms are incorporated into the carbon lattice, altering the electronic structure of the carbon layers and lowering the energy barrier for carbon atom rearrangement and ordering. At elevated temperatures, the decomposition products of urea react with amorphous carbon, promoting the rearrangement of carbon atoms via sp² hybridization into larger aromatic lamellae. This process enhances the degree of graphitization, thereby improving the electrical conductivity and structural stability of the material.^41–43 The TEM images (Fig. 2b and Fig. S2) further confirmed this smoother and more porous microstructure. The HRTEM image (Fig. 2c) demonstrated the coexistence of graphite domains and disordered carbon domains in NHC-2, while the HC samples exhibited a disordered pseudo-graphitic structure (Fig. S2). The uniform distribution of C, N and O in NHC-2 (Fig. 2d–g) indicated that the nitrogen atom was successfully doped. These results collectively suggested that nitrogen and graphite structural domains were formed in the NHCs.

Fig. 1 Schematic of the preparation of NHCs.

Fig. 2 (a) SEM, (b) TEM and (c) HRTEM images of NHC-2, and (d–g) elemental mapping images of NHC-2.

In order to further study the microstructure of the NHCs, X-ray diffraction (XRD) and Raman spectroscopic measurements were systematically carried out. As shown in Fig. 3a and S4, the XRD patterns of HC and NHCs showed a distinct broad peak at 24° and a weak peak near 43° corresponding to the (002) and (100) planes of graphitic carbon, respectively, confirming the formation of graphitic domains in NHCs.⁴⁴ It is noteworthy that as the percentage of urea increases (from NHC-1 to NHC-4), the (002) peak shifts to a higher angle accordingly. This change indicates a gradual decrease in the lattice spacing of the (002) plane together with an increase in the degree of graphitisation. The microcrystalline structural parameters are mainly d₀₀₂ (layer spacing), L_a (crystal length) and L_c (crystal thickness), which were calculated for quantitative analysis according to the Bragg equation (d₀₀₂ = nλ/2 sin θ) and the Debye-Scherrer equation (L(nm) = kλ/β cos θ), where λ is the wavelength of the X-rays used (0.154 nm), β is the half peak width of the (100) and (002) crystal planes, and θ is the corresponding diffraction angle. For the classic graphitized carbon materials, the k values were 1.84 and 0.90 for the (100) and (002) crystal planes, respectively, and the specific calculated detailed values are shown in Table S2. The calculated values of d₀₀₂ for HC, NHC-1 to NHC-4 were 0.381, 0.380, 0.378, 0.375 and 0.369 nm, which are in agreement with the results of TEM image shown in Fig. 2c. The slight right shift of the (002) peak of NHC-2 compared to the HC suggested that the urea-catalysed process increased the degree of graphitisation of the coal-sourced HC. Correspondingly, d₀₀₂ decreased from 0.381 to 0.378 nm. In particular, L_a and L_c increased with the increase in urea consumption, and this result is also evidence of the increase in the degree of graphitisation of the NHC samples.³⁷ To further investigate the effect of urea on the microcrystalline structure of HC in detail, Raman testing was carried out. As shown in Fig. 3b, the Raman spectra of NHC-2 reflect the D-band characteristic peak (≈1350 cm⁻¹), representing defective or disordered carbon, and the G-band characteristic peak (≈1580 cm⁻¹), representing the in-plane sp² orbital structure of the graphitic carbon, which proves the coexistence of graphitic and disordered carbon.⁴⁵ Moreover, the main peaks can be divided into D4, D1(D), D3, D2 and G-band peaks (Fig. S5), corresponding to positions near 1200, 1360, 1500, 1620 and 1590 cm⁻¹, respectively.⁴⁶ The I_D/I_G relative intensity ratios of HC and NHC-2 were 1.11 and 0.98, respectively. The significant decrease in the I_D/I_G values with the addition of urea indicates a decrease in the degree of disorder of the carbon materials, suggesting that urea effectively promotes the graphitisation of HC. The Raman spectrum of the comparative sample is shown in Fig. S6, in which the I_D/I_G values progressively decrease from 1.06 to 0.98, 0.93, and 0.78 with the increase in the content of urea, indicating a gradual reduction in material defects, an increase in structural ordering, and an enhanced graphitization degree. This observation is consistent with the result from the XRD patterns.

Fig. 3 (a) XRD patterns, (b) Raman spectra, (c) N₂ adsorption/desorption isotherms (inset: pore size distributions) and (d, e and f) high-resolution XPS spectrum of C 1s, N 1s and O 1s of NHC-2.

In order to assess the intrinsic pore structure, the coal-derived HC and NHC-2 were selected as representative samples for N₂ adsorption and desorption analyses. These analyses consisted of specific surface area (SSA) calculations by comparing the pore size distributions using the Brunauer–Emmett–Teller (BET) technique. As shown in Fig. 3c and S6, both samples exhibited typical type IV isotherms, indicating a predominantly mesoporous structure. This is consistent with the morphology and microstructure obtained from the FESEM and TEM results. The pyrolysis process releases a large number of small molecules within the structural units of bituminous coal, resulting in HC samples with a large pore structure. In Fig. S7 and the inset in Fig. 3c, the pore size distribution of HC, NHC-1, NHC-2, NHC-3 and NHC-4 is illustrated. It can be observed that NHC-2 comprises pores with a diameter in the range of 2 to 30 nm. Due to the porous structure of NHC, the BET specific surface area of the HC, NHC-1, NHC-2, NHC-3, and NHC-4 samples increase progressively from 9.9024 to 17.8940, 19.5124, 29.4677 and 71.6606 m² g⁻¹, indicating that a higher urea content leads to a more developed pore structure in the hard carbon (Table S2), which can be attributed to the decomposition process of urea.^47,48 The copious gas generated by urea decomposition creates a rich pore structure, which increases the specific surface area of the material. This enhancement facilitates electrolyte wetting on the electrode surface and enables rapid sodium-ion transport, thereby improving the rate capability of the battery. In conclusion, urea catalytic treatment can effectively enhance the sodium storage performance of coal-based hard carbon materials.

A series of X-ray photoelectron spectroscopy (XPS) measurements were carried out to gain further insights into the chemical state and elemental composition of NHC-2. Both samples have peaks at 284.5 and 530.8 eV belonging to C and O, respectively, and NHC-2 also has a peak at 399.8 eV belonging to N (Fig. S8).³⁶ As shown in the C 1s XPS spectra (Fig. 3d), the peak of C 1s 284.5 eV could be assigned to C sp² (283.9 eV), C sp³ (284.5 eV), C–N (285.4 eV), C–O (287.3 eV) and –C=O (289. 9 eV), which is similar to the HC fitting results (Fig. S9) except that the extra C–N peaks appeared after N doping. In addition, C sp³ corresponds to carbon interlayer defects, and the smaller the area ratio of A_sp³/A_sp², the fewer the defects.^49,50 The low A_sp³/A_sp² ratio of NHC-2 indicates the reduction of defects, which is attributed to a higher degree of carbonisation. To investigate the influence of urea content on the graphitization of the materials, the C 1s high-resolution XPS spectrum analysis was performed on samples with different doping concentrations, and the results are presented in Fig. S10. The A_sp³/A_sp² ratios of HC, NHC-1, NHC-2, NHC-3 and NHC-4 are 0.5, 0.35, 0.3, 0.16 and 0.14, respectively. The progressively decreasing area ratio indicates the gradual enhancement of graphitization in the hard carbon material with the increase in urea content. The high-resolution N 1s XPS spectra of NHC-2 (Fig. 3e) can be deconvoluted into graphitic-N (404.4 eV), pyrrolic-N (400.8 eV) and pyridinic-N (398.3 eV). Pyridinic-N and pyrrolic-N exhibit a higher chemical activity than graphitic-N. The pyridinic and pyrrolic nitrogen species create numerous structural defects with high surface energy within the carbon framework. These defects serve as excellent adsorption sites for sodium ion storage, directly enhancing the battery capacity. Meanwhile, these defective sites facilitate rapid redox reactions with sodium ions on the electrode surface, leading to pseudocapacitive behavior that enables fast charging and discharging while contributing additional capacity.⁵¹ Therefore, the introduction of nitrogen enhances the rate capability of SIBs.⁵² As shown in Fig. 3f and Fig. S9, the O 1s spectra of NHC-2 and HC display the characteristic peaks at 531.3, 532.4 and 535.7 eV corresponding to C=O, C–O and chemisorbed oxygen functional groups, respectively.⁵³ It is noteworthy that NHC-2 has more chemisorbed oxygen functional groups than that of HC, which may be related to its smooth and loose structure. These results further indicate that nitrogen doping and graphitization optimization can be achieved through urea catalysis.

The electrochemical Na⁺ storage performance of NHC and HC samples was investigated by cyclic voltammetry (CV) and constant current charge/discharge in the range of 0.01 V–3.0 V (vs. Na/Na⁺). As shown in Fig. 4a and Fig. S11, the redox peak pair at ∼0.1 V corresponds to the low-voltage plateau region observed in the charge/discharge curves. However, the sharp reduction peak located near 0.01 V may be due to the embedding of Na⁺ in graphite domains of hard carbon to form Na⁺ intercalation compounds, as previously observed in graphite.²⁷ In addition, a broad reduction peak located at 0.5 V appeared in the first cycle due to the irreversible side reaction of the unstable electrolyte on the electrode surface, leading to the formation of a solid electrolyte interphase (SEI) film, which is consistent with previously reported results.⁴⁵ Compared to these two samples, the CV curve of NHC-2 has the highest peak current intensity at 0.1 V, a smaller area of irreversible peaks and a high degree of overlap in subsequent cycles, suggesting that it has the fastest reaction kinetics and highly reversible sodium storage behavior.

Fig. 4 Electrochemical performance of NHC electrodes: (a) CV curves of NHC-2 at 0.5 mV s⁻¹, (b) galvanostatic charge–discharge (GCD) curves during first three turns at a current density of 0.03 A g⁻¹ for NHC-2, (c) GCD curves at a current density ranging from 0.03 A g⁻¹ to 1 A g⁻¹ for NHC-2, (d) GCD curves during first turns at a current density of 0.03 A g⁻¹ for HC and NHC-2, (e) rate performance at a current density ranging from 0.03 A g⁻¹ to 1 A g⁻¹ for HC and NHC-2 and (f and g) long-term cycling performance of HC and NHC-2 at 0.03 and 1 A g⁻¹.

Fig. 4b and Fig. S11, S12 show the first three cycle charge/discharge curves of the five samples. It can be seen from Fig. 4b that the degree of overlap increases in the subsequent cycles except for the first cycle, indicating the high reversibility of sodium storage behavior, which is consistent with the results of the CV curves. The first cycle charge/discharge capacities of HC, NHC-1, NHC-2, NHC-3 and NHC-4 were 230.90/289.35, 260.47/304.28, 291.45/330.81, 279.91/321.36, and 277.58/340.58 mAh g⁻¹, with initial coulombic efficiencies of 79.8%, 85.6%, 88.1%, 87.1%, and 81.5%, respectively. The results indicated that the reversible capacity and initial coulombic efficiency of NHC first increased and then decreased with the increase in urea content. This is due to the fact that carbon graphitization and nitrogen doping are simultaneously enhanced with the increase in urea content. This results in a competitive relationship between the reduction of defects due to graphitization and the increase in defects due to nitrogen doping. Excessive or insufficient defects are detrimental to the sodium ion storage performance, whereas NHC-2 achieves the optimal balance between graphitization degree and nitrogen doping levels. It can be determined that a moderate level of pore structure and graphitization is required to achieve superior sodium storage performance. The charge–discharge curves of NHC-2 and HC at different current densities of 0.03, 0.05, 0.1, 0.2, 0.5 and 1 A g⁻¹ are shown in Fig. 4c and Fig. S11, from which it can be observed that NHC-2 has a higher capacity than that of HC at different current densities. Then, the charge/discharge curves during the first cycles at a current density of 0.03 A g⁻¹ for HC and NHC-2 were recorded (Fig. 4d). In particular, NHC-2 exhibits significantly improved reversibility and higher initial coulombic efficiency than that of HC. According to the literature and experimental results, it can be seen that the addition of urea directly promotes the slope capacity and the plateau capacity.²⁸ This indicates that the sodium storage mechanism of NHC involves slope sodium adsorption from a larger specific surface area and more porous structures, as well as platform sodium insertion optimized from carbon layer graphitization. Interestingly, the delicate balance between the reduction in defects due to the reduction in disorder and the increase in defects due to the increase in specific surface area after graphitisation results in an extremely high initial coulombic efficiency of 88.1%. In addition, NHC-2 exhibited excellent rate performance at different current densities, maintaining a robust capacity of 222.08 mAh g⁻¹ at a current density of 1.0 A g⁻¹ in the ether-based electrolyte. In contrast, the HC had a low capacity of only 148.18 mAh g⁻¹ under the same condition (Fig. 4e). In addition, NHC-2 maintains a stable capacity of 287.07 mAh g⁻¹ at 0.03 A g⁻¹ with a low decay rate of 1.5%. The superior rate performance indicates that NHC-2 has the best ionic and electronic conductivity due to the appropriate d₀₀₂ layer spacing, degree of graphitisation and pore structure, as well as the right amount of defect sites.

In addition, cycling stability is an important assessing parameter for realizing their practical application of HC materials in SIBs. As such, the cycling performance of HC and NHC-2 was tested at current densities of 0.3 and 1 A g⁻¹. Both HC and NHC-2 had a robust cycle life at 0.3 A g⁻¹ with approximately 90% capacity retention after 100 cycles, but NHC-2 has a higher capacity of 238.24 mAh g⁻¹ than that of HC, which has a relative low capacity of only 209.64 mAh g⁻¹ (Fig. 4f). After 500 cycles at a high current density of 1 A g⁻¹, NHC-2 still delivers a capacity of 132.7 mAh g⁻¹ with a capacity retention of about 60%, whereas HC possesses only 98.63 mAh g⁻¹ with a capacity retention of about 50% (Fig. 4g). After the first few cycles, the coulombic efficiency of NHC-2 at 1 A g⁻¹ was about 99%, combined with a highly stable SEI film and good electrochemical reversibility. All these features make NHC-2 very promising for application in SIBs. To investigate the sodium storage performance of NHC-2 in ester-based electrolytes, batteries were assembled using an ethylene carbonate/diethyl carbonate (EC/DEC) electrolyte, as illustrated in Fig. S13. It can be observed that NHC-2 hard carbon exhibits inferior performance in ester-based electrolytes compared to ether-based electrolytes, underscoring the necessity of compatibility between the hard carbon electrode material and the electrolyte.

To investigate the sodium storage mechanism and reaction kinetics of NHC-2 and HC, CV measurements were performed over a range of scan rates from 0.2 to 2.0 mV s⁻¹ (Fig. 5a and Fig. S14a). It is noteworthy that the redox peak at 0.1 V was slightly shifted to higher voltages with the increase in scan rate, indicating that the electrode has a lower polarization. The relationship between the peak current density and the scan rate (i = av^b, where a is a parameter and b is an adjustable parameter) can be used to determine the charge/discharge mechanism. Typically, a b-value close to 0.5 indicates that the electrochemical reaction is dominated by a diffusion control mechanism, while a value close to 1 indicates that capacitive behavior is not controlled by diffusion.⁵⁴Fig. 5b shows the good linear relationship of NHC-2, with a calculated b-value of 0.42 for the oxidation peak named Peak1 and 0.58 for the reduction peak named Peak2, indicating that the charge storage behaviour in NHC-2 is mainly dominated by diffusion processes. The b values of the oxidation peak and the reduction peak in HC are 0.74 and 0.79, which illustrate that the electrochemical reaction process is jointly controlled by diffusion and capacitive behaviors, and the capacitive process plays a major role in the electrochemical reaction (Fig. S14b). The contribution part of capacitive capacities at specific scan rates was calculated using the following equation:

where k₁v and k₂v^1/2 represent the capacitive and diffusion-controlled processes, respectively. The values of k₁ and k₂ can be obtained from the slope and intercept of the y-axis on the plot of i/v^1/2versus v^1/2. Fig. 5c and Fig. S14c show that the capacitive proportions of NHC-2 and HC are 49.8% and 52.4% at 5 mV s⁻¹. The capacitive charge contribution ratio of NHC-2 and HC at a scan rate of 0.2–2 mV s⁻¹ is shown in Fig. 5d and Fig. S14d, which increases from 33.6% to 61.4% with the increase in the scan rate from 0.2 to 2 mV s⁻¹. The capacitive charge contribution ratio of HC increases from 36.5% to 63.7%. This result indicated that the sodium storage behavior of NHC-2 is dominated by a diffusion-controlled process with a fast kinetic process, which is conducive to rapid ion transport during charging and discharging, thus ensuring its excellent multiplicity performance.

Fig. 5 (a) CV curves of NHC-2 ranging from 0.2 to 2.0 mV s⁻¹. (b) Log (i, current) vs. log (v, scan rate) plots at specific peak currents. (c) Contribution ratio of the capacitive charge at a scan rate of 1 mV s⁻¹ in NHC-2. (d) Contribution ratios of the capacitive charge in NHC-2 at different scan rates.

Fig. 6a shows the electrochemical impedance spectra (EIS) of HC and NHC-2. The two Nyquist plots show a common pattern characterized by a semicircular section coupled to a diagonal line. The smaller the diameter of the semicircle in the high-frequency region, the smaller the corresponding charge transfer impedance (R_ct). The sloping line observed in the low-frequency region is the ion diffusion impedance (Z_w). A larger slope indicates a faster diffuse rate of ions in the electrolyte. The values of R_ct and R_Ω (the intersection of the curve with the horizontal axis) were obtained by equivalent circuit fitting. The minimum value of R_Ω for the NHC-2 sample is only 3.83 Ω, and R_ct is 0.303 Ω. In addition, it can be clearly seen that the slope of NHC-2 is larger in the low-frequency region, reflecting the faster ion diffusion ability. Nitrogen doping infuses additional free electrons into the π-conjugated system of carbon materials, increasing the charge carrier concentration and significantly enhancing the electronic conductivity. This improvement thereby enables batteries to maintain superior performance even under high-rate conditions.⁵⁵

Fig. 6 (a) Nyquist plots after CV activation of HC and NHC-2. (b) GITT potential profiles and calculated Na⁺ diffusion coefficients during (c) discharge and (d) charge processes.

In order to further investigate the kinetic processes of the HC samples, the diffusion kinetics of Na⁺ in HC was investigated using the constant current intermittent titration technique (GITT). Fig. 6b shows the results of HC and NHC-2 at a pulse current of 0.03 A g⁻¹, which were obtained according to the following equation:

where n_m, V_m, τ and S are the moles of the electrode material, the molar volume, the relaxation time and the electrode–electrolyte contact area, respectively. ΔE_s is the potential change caused by the pulse, ΔE_t is the potential change for discharging and charging at constant current, and the Na⁺ diffusion coefficient D was determined using GITT curves. Fig. 6c and d show the curves of D_Na⁺versus voltage during discharging and charging. It can be observed that D_Na⁺ is relatively stable within a range during discharge. When discharged to 0.1 V, D_Na⁺ decreases rapidly and then reverse sequentially, which is consistent with the previously reported results.⁵⁶ The essential reason for this is that Na⁺ ions are initially adsorbed at the defect site as soon as discharge occurs, as well as on the surface and within the layers of the graphene sheet. Subsequently, Na⁺ starts to intercalate the graphene layers and fill the pores. At the same time, the formation of sodium-sodium clusters caused the slower diffusion kinetics than the former. Differently, NHC-2 with more pores, suitable layer spacings and more adsorption sites could promote faster Na⁺ diffusion. During charging, the Na⁺ diffusion rate slows down due to the accumulation of large amounts of sodium in the pores of NHC-2. After that, the diffusion coefficient increases rapidly with the increase in potential.

Conclusions

In summary, we have prepared a kind of nitrogen-doped and partially graphitised coal-based hard carbon (NHC) by pyrolysis of bituminous coal as the raw material and urea as the catalyst via a simple annealing process. The NHCs have the structural advantages of being mesoporous, nitrogen-doped and partially graphitized, which enable rapid kinetics and electron transfer. As an anode for SIBs, NHC-2 exhibited a reversible capacity of 291.45 mAh g⁻¹ at 0.03 A g⁻¹, a high ICE of 88.1%, and a good cycling stability after 500 cycles at a high current density of 1 A g⁻¹. This excellent performance can be attributed to the factors such as the increased specific surface area and suitable defects, as well as the doped heteroatoms. This synthetic strategy of producing partially graphitised and heteroatom-doped hard carbons as advanced anode materials has great potential for advanced energy-storage applications in SIBs.

Author contributions

D. Wang performed the experimental characterization and wrote the manuscript. L. Feng and D. Mo synthesized some of the materials. L. Feng, Y. Zhang and X. Zheng helped to discuss the manuscript. X. Guo and G. Xie supervised, guided and reviewed the work. All authors contributed to the general discussion.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

All relevant data are present within the paper. After the publication of our paper in this journal, these data will be available at https://pubs.rsc.org/en/journals/journalissues/qi#!recentarticles&adv. Access to the related original data and pictures requires the permission of the first author or the corresponding author.

The synthesis process of the materials, Sample characterizations such as SEM, XRD, FTIR, BET, and electrochemical test including CV, GCD and stability test, as well as performance comparison table are provided in the Supplementary Information. See DOI: https://doi.org/10.1039/d5qi01536g.

Acknowledgements

This work was supported by the Key Projects of Intergovernmental International Cooperation in Key R & D programs of the Ministry of Science and technology of China (No. 2021YFE0115800), the National Science Funding Committee of China (No. U20A20250), and the Programs of the Science and Technology of Yulin City (No. CXY-2023-ZX04).

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Footnote

† These authors contributed equally in this work.

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