English

• Sodium ion batteries (SIBs) and potassium ion batteries (PIBs) are considered to be the potential alternative to lithium ion batteries (LIBs) for grid-scale energy storage systems due to the similar electrochemical behavior with Li, low-cost, abundant Na/K resources and appropriate redox potential [1-6]. However, compared with Li⁺, Na⁺/K⁺ have the larger ionic radius (1.02 Å for Na⁺ and 1.38 Å for K⁺) and atomic weight, which lead to high electrostatic drag in host lattice and tremendous volume variations of anodes during the cycles, thus resulting in sluggish ionic transport kinetics and unsatisfactory cycling stability [7-10]. Therefore, it is important to explore the anodes with high electrochemical performances to promote the further development of SIBs/PIBs. Carbon materials are regarded as ideal anodes due to the raw material availability, low-cost, toxicity and long cycle stability [11]. Although SIBs/PIBs possess a similar storage mechanism with LIBs, the commercial graphite anode is not suitable for SIBs/PIBs, Na⁺ intercalation in graphitic results into NaC₆₄-GIC with poor capacity of 35 mAh/g, while K⁺ intercalation in graphite could leads to 61% volume variations [12, 13]. Meantime, the small interlayer space (0.34 nm) of graphite makes the ion insertion/de-insertion more complicated, leading to sluggish Na⁺/K⁺ transport kinetics and poor cycling performance [14, 15]. Therefore, exploring high-performance carbon materials is an urgent challenge for SIBs/PIBs.

  Disordered carbon anodes, consist of randomly oriented graphite nanodomains, disordered carbon layers and abundant active sites, are the most promising candidates for SIBs and PIBs [16, 17]. Even though the high capacities were achieved, amorphous carbons suffer from low conductivity and poor structural stability, which strongly limit their capacities and cycle stability. To address these issues, efforts have been used to improve the electrochemical performance by optimize the microstructure of carbon materials. Among them, heteroatom doping (such as N, O and S) is an effective stagey to improve the Na⁺/K⁺ insertion/de-insertion behavior in carbon lattice [18-20]. Especially, large size atoms doping (such as S and P atoms) could significantly enlarge the interlayer space of carbon materials, which are kinetically favorable for ion storage [21, 22]. Recent studies indicate that incorporation of multiple heteroatoms doping is more beneficial to improve the electrochemical activity and the reaction kinetics [23, 24]. Therefore, optimize the heteroatoms species and content is worthy for improve the capacities in SIBs and PIBs. Furthermore, construction of unique carbon nanoarchitecture with good rigidity can be buffer volume change to maintain the structural stability [25]. Among the synthesis methods for carbon materials, template synthesis routes can precise tunable the microstructure and pore size of carbon materials by modulating template size [26]. Meantime, carbon materials can be also activated or graphitized by the functional template [27, 28]. Nevertheless, it is challenging to synthesis carbonaceous materials together with the small yet sufficient reaction space, short ion diffusion pathways and abundant active sites. Therefore, it is imperative to develop a reasonable route to fabricate of carbon nanostructures is highly in demand.

  Herein, we develop an efficient method to construct of nitrogen, sulfur co-doped hollow carbon nanospheres (NS-HCS) by in situ growth of polydopamine on nano-Ni(OH)₂ template with subsequent sulfur doping process. During the formation process, the produced Ni nanospheres play as the hard template and catalyst for the formation of hollow carbon nanosphere and ordered graphite microcrystalline structure, while the sulfur doping process can enlarge the interlayer space and create more defects. Benefit from the dual heteroatoms doping and unique nanostructure, two storage mechanisms of surface adsorption and interlayer insertion coexist in NS-HCS electrode, and the abundant active sites increase the surface-dominated Na⁺/K⁺ storage. As a result, NS-HCS electrode delivered a high capacity of 302 mAh/g at 50 mA/g and long cycling stability of 150 mAh/g for nearly 5000 cycles at a high current of 2.5 A/g in SIBs, as well as excellent K⁺ storage performance (the high capacity retention rate of 94.8% for the last 500 cycles at 1.0 A/g).

  The synthesis strategy for NS-HCS is detailed in Fig. 1. Firstly, a polydopamine layer was deposited on the surface of Ni(OH)₂ nanoparticles (Fig. S1 in Supporting information) by the self-polymerization reaction of dopamine. Then, the as-prepared precursor was fully mixed with thiourea, and subsequently carbonized under Ar atmosphere. During the carbonization process, along with the carbonization of PDA, Ni metal and H₂O gas are in–situ generated from the decomposition of Ni(OH)₂. XRD pattern can confirm the formation of Ni metal (Fig. S2 in Supporting information). Figs. S2c and d show the TEM images of NS-HCS with residual Ni particles by partial etching of Ni. It can be seen that Ni nanoparticles are evenly distributed in the carbon nansospheres. Under the process, Ni spheres play as the hard template and catalyst for the formation of hollow carbon nanosphere and ordered graphite microcrystalline structure, while H₂O gas plays a crucial role in the formation of nanopores on the surface of carbon nanospheres. At the same time, sulfur and nitrogen elements are introduced into carbon lattice with the decomposition of thiourea. Sulfur doping can expand the interlayer distance for fast ion transport, while the presence of N atoms would provide more active sites to adsorb Na⁺/K⁺ [29]. After the Ni template was removed, the N/S co-doped hollow bubble structure carbon nanospheres (NS-HCS) with rich mesoporous structure was obtained. The obtained NS-HCS exhibits 3D network consist of porous carbon nanospheres, which is conducive to improve the electronic transport and shorten the ion diffusion distance during charge/discharge process.

  Figure 1

  

  Figure 1.  Schematic diagram of the synthesis route of NS-HCS.

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  The morphology of NS-HCS and N-HCS are displayed in Fig. 2. NS-HCS replicate the morphology of the Ni(OH)₂ template, which shows a network by a large number of hollowbubble carbon nanospheres with the size of about 20 nm (Figs. 2a and b). TEM image of NS-HCS further confirm the 3D structure assembled by the carbon nanospheres with the diameter of 20-25 nm. The high-resolution TEM images of NS-HCS and N-HCS displays abundant microcrystalline structure, which can be ascribed to the catalytic graphitization of by the produced Ni metal. The hollow carbon spheres of NS-HCS show a shell thickness of 5-7 nm with the interlayer spacing of 0.39 nm, which was larger than the 0.36 nm wall thickness of N-HCS (Figs. 2c and d). The expanded interlayer spacing is associated with the introduction of sulfur atoms, the short-range microcrystalline carbon with the expanded interlayer spacing is favorable for fast Na⁺/K⁺ diffusion into the carbon layers (Fig. 2f) [30]. The energy-dispersive spectroscopy (EDS) element mapping of NS-HCS confirms that the S and N elements are homogeneously distributed in the bubble carbon (Fig. 2e).

  Figure 2

  

  Figure 2.  (a) SEM image and (b, c) TEM images of NS-HCS. (d) TEM image of N-HCS. (e) The elemental mappings of NS-HCS. (f) Schematic diagram of carbon layer of NS-HCS. (g) XRD pattern and (h) Raman spectra of the N-HCS and NS-HCS. (i) Pore size distribution of N-HCS and NS-HCS (inset: Nitrogen adsorption-desorption isothermal curves of NS-HCS).

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  The enlarged interlayer spacing in NS-HCS was further confirmed by X-ray diffraction (XRD) measurement (Fig. 2g). A broad peak at ~25° corresponding to the carbon (002) diffraction. Compared to N-HCS, the (002) peaks for NS-HCS are shifted to lower angle, indicating the enlarged interlayer distance by the S-doping. A slight shift of (002) peak was also observed with the increased temperature from 650 ℃ to 850 ℃, implying the change process of interlayer space and graphitization degree (Fig. S3 in Supporting information) [31]. Raman spectra of NS-HCS and N-HCS show the peaks located at ~1350 cm⁻¹ and ~1580 cm⁻¹, corresponding to the D-bands (disordered or defective carbon band) and G-bands (crystalline graphite band), respectively (Fig. 2h) [32]. The relative intensity ratio (I_D/I_G) of NS-HCS was calculated to be 1.227, which is higher than that of N-HCS with a ratio of 0.92, indicating a higher disordered degree of NS-HCS. Meantime, the G band became sharpened with temperature increasing to 850 ℃, indicating high graphitic at high relative temperature, which can be conducive to the catalytic action caused by the Ni template (Fig. S4 in Supporting information). From the N₂ absorption/desorption analysis (Fig. 1i), NS-HCS shows a typical IV isotherm with H3 hysteresis loops, implying the similar mesoporous structure. and NS-HCS exhibits a higher specific surface area (484.96 m²/g) than that of N-HCS. Furthermore, the pore size distribution of the samples is centered at range of 1.8-5 nm. As a result, the large surface area and abundant pore structure of NS-HCS can ensure fast ion infiltration and increase the contact area with electrolyte, thus leading to the improved electrochemical performance.

  The X-ray photoelectron spectroscopy (XPS) spectra were investigated to understand the chemical compositions of NS-HCS (Fig. 3, Figs. S5 and S6 in Supporting information). The peaks of NS-HCS located at about 284, 400, 553 and 163 eV can be ascribed to C 1s, N 1s, O 1s and S 2p, respectively, suggesting the successful incorporate of sulfur in NS-HCS (Fig. S5). The high-resolution C 1s spectrum for NS-HCS (Fig. 3a) was deconvoluted into five peaks located at 284.53, 284.80, 285.57, 286.58, and 287.40 eV, representing C-S, C-C, C-N, C-O and C=O groups, respectively, confirming the heteroatom-doping state in carbon [33]. The high-resolution XPS O 1s spectrum shows three types of binding at about 530.62, 532.20 and 533.66 eV, corresponding to the O-C, O=C, O-H groups, respectively (Fig. 3b). The N and S contents for NS-HCS are 4.04 at% and 1.3 at%, respectively. The high-resolution S 2p spectrum shows the binding energies at 163.94 eV for S 2p_3/2 and 165.18 eV for S 2p_1/2, respectively, which can be attributed to the C-S-C covalent bond, which is responsible for the fast kinetics and high reversible capacity (Fig. 3c) [34, 35]. The peak centered at 167.92 eV is assigned to C-SO_x-C. The high-resolution N 1s spectra can be decomposed into four peaks, corresponding to pyridinic-N (N-6, 398.46 eV), pyrrolic-N (N-5, 400.08 eV), graphitic-N (N-Q, 401.02 eV), and N-oxide groups (N-O, 402.30 eV) (Fig. 3d). N doping in carbon lattice can effectively enhance the electronic conductivity, especially the suspended edge nitrogen such as pyridine-N and pyrrolic-N, which have high electrochemical activity [36, 37]. As displayed in Fig. 3e, NS-HCS possesses a relatively high ratio of pyridinic-N (36.90%) and pyrrolic-N (22.14%) after further doping process, the increased pyridinic-N and pyrrolic-N doped sites would be beneficial for ion adsorption. The increased content of pyridinic-N and pyrrolic-N can be ascribed to the incorporation of sulfur. The sulfur doping in carbon lattice can induce more defect sites in the nitrogen-doped carbon skeleton, while graphitic-N can be converted into pyridinic-N or pyrrolic-N at the edge sites of defects [31]. As displayed in Fig. 3f, the existence of nitrogen and sulfur groups can create more defects and enlarge the interlayer distance in carbon lattice, leading to the improved ion storage performance for NS-HCS.

  Figure 3

  

  Figure 3.  Fine and fitted XPS spectra of NS-HCS: (a) C 1s, (b) S 2p, (c) O 1s and (d) N 1s. (e) N distribution. (f) Imaginary diagram of carbon layer in NS-HCS.

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  The sodium storage behavior of NS-HCS was measured in a sodium-ion half-cell, as presented in Fig. 4. Fig. 4a shows the cyclic voltammetry (CV) curves of NS-HCS at 0.1 mV/s, an obvious broad reductive peak appeared at 0.9 V in the first cycle, corresponding to the formation of solid-electrolyte interface (SEI) films with the decomposition of the internal electrolyte. While the reversible reductive peak at 0.1 V could be attributed to the insertion and deintercalation of Na⁺ in small carbon domains [38]. The following CV curves show the rectangular-shaped behavior, implying a good cycling stability of NS-HCS electrode. The CV curves of N-HCS, NS-HCS-650 and NS-HCS-850 electrode shows the similar electrochemical behavior, implying the same reaction mechanism (Fig. S7 in Supporting information). The first cycle CV curves of these electrodes shows a wider cathodic peak near 1.0 V than that of NS-HCS electrode, which proves that more interface side reactions originating from the decomposition of electrolyte. Fig. 4b and Fig. S7 present the galvanostatic discharge/charge (GDC) profiles in the first three cycles at 0.05 A/g. As shown in Fig. 4b, the initial discharge and charge capacities are 949.6 and 279.4 mAh/g, respectively, corresponding to the initial Coulombic efficiency (ICE) of 29.4%. The low value of ICE can be attributed to the electrolyte decomposition on the inner/outer surface of carbon nanospheres. The subsequent cycles are almost overlap, implying the good cycle stability of NS-HCS. Furthermore, the smooth sloping charge/discharge curves confirms its capacitive-dominated storage behavior [11, 39].

  Figure 4

  

  Figure 4.  Electrochemical performance of the electrodes versus Na/Na⁺. (a) CV curves of NS-HCS at 0.1 mV/s. (b) GDC curves of NS-HCS. (c) The cycling stability of N-HCS-650, N-HCS, N-HCS-850, NS-HCS-650, NS-HCS and NS-HCS-850 electrodes at 0.1 A/g. (d) Cycling performance of NS-HCS and N-HCS samples at 0.1 A/g. (e) Rate capability of NS-HCS and N-HCS samples. (f) Comparison with the reported carbonaceous materials for SIBs. (g) Long-term cycling performance of the NS-HCS electrodes at 0.1 and 2.5 A/g.

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  Fig. 4c and Fig. S8 (Supporting information) compare the cycling stability of N-HCS-650, N-HCS (750 ℃), N-HCS-850, NS-HCS-650, NS-HCS (750 ℃) and NS-HCS-850 electrodes at 0.1 A/g. Apparently, the NS-HCS electrode shows the highest capacity, and the capacities of the S-doped carbon materials were always higher than the corresponding sulfur-free carbon materials. Fig. 4d further compares the cyclic performance of NS-HCS and N-HCS at 0.1 A/g. The capacity of NS-HCS electrode maintains 274.8 mAh/g after 100 cycles with the Columbic efficiency of nearly 100%. In contrast, the reversible capacity of N-HCS electrode only reach 203.9 mAh/g after 100 cycles. Fig. 3e compares the rate performance of the NS-HCS and N-HCS for SIBs. Obviously, NS-HCS electrode displays high capacities of 324.8, 262.3, 176.6, 142.6, 108.9, 94.9, 90.4 and 83.9 mAh/g at current densities of 0.05, 0.1, 0.5, 1.0, 3.0, 5.0, 10.0 and 20.0 A/g, respectively, showing better rate that that of N-HCS electrode. Meanwhile, NS-HCS has higher rate performance than NS-HCS-850 and NS-HCS-650 (Fig. S9 in Supporting information). When the current density turns back to 0.1 and 0.05 A/g, the reversible capacity of 281.6 and 313.6 mAh/g can return, implying the good reversibility. The rate capability of NS-HCS is also superior to those of the reported carbon-based materials, indicating a promising application in SIBs (Fig. 4f) [32, 40-46]. The long cycling performance of NS-HCS electrode was further investigated at 0.1 and 2.5 A/g. The capacity is maintained at 254 mAh/g after 500 cycles at 0.1 A/g. To further explore the deep reason for the excellent cycle stability, the morphology of cycled NS-HCS electrode is shown in Fig. S11 (Supporting information). The sphere-shape nanostructure of NS-HCS is well maintained, indicating the structural stability of NS-HCS, thus ensures excellent cycle stability. Even at a higher current density of 2.5 A/g, a capacity of 149.5 mAh/g can be maintained after 5000 cycles with the Coulombic efficiency of nearly 100%, implying excellent cycle stability (Fig. 4g). The cycle stability of NS-HCS is also superior to those of the reported carbon-based materials, indicating a promising application in SIBs (Table S1 in Supporting information).

  To understand the Na-ion storage characteristics in NS-HCS, kinetic analysis based on the CV measurement at different scanning rates was conducted. Fig. 5a displays the CV curves of NS-HCS at scan rates of 0.1-2.0 mV/s, all CV curves display similar shapes and to cover an enlarged area range with the increasing scanning rate. For Na-ion storage, the quasirectangular shape in the high potential region demonstrating capacitive processes, while the reversible sharp cathodic peak in the low potential region indicate diffusion-controlled faradaic processes even at high scanning rates. According to the power-law, the storage mechanism can be determined as follow (Eqs. 1 and 2) [5];

  Figure 5

  

  Figure 5.  (a) CV curves of NS-HCS at various scan rates from 0.1 mV/s to 2.0 mV/s in SIBs. (b) linear relationships between log(i) and log(v). (c) Contribution ratio of the NS-HCS and N-HCS electrodes at various scan rates. (d, e) Na-ion diffusion coefficient of the electrodes during sodiation and desodiation process, respectively. (f) Schematic illustration for the ion transfer process of NS-HCS electrode.

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  (1)

  (2)

  where a and b are adjustable parameters; i and ν are peak current and scan rate, respectively. Fig. 5b shows log(i) versus log(v) plots at different discharge potentials. Peak 2 and peak 3 have higher b values of 0.904 and 0.923, respectively, indicating the surface capacitive dominated electrochemical behavior owing to the abundant surface defects in NS-HCS. On the contrary, peak 1 show the b values of 0.676, demonstrating that sodium storage at the lower potential is mainly diffusion-controlled. The capacitive contribution can be further calculated using Eq. 3 [47, 48]:

  (3)

  here, k₁ and k₂ are constants, i and v are the current (A) at a fixed potential and scan rate. As shown in Fig. 5c and Fig. S10 (Supporting information), the capacitive contribution of the capacity is about 80.2% at 2.0 mV/s, and the capacity contribution incremental with the increasing of scan rate. These high-proportion capacitive contributions reveal fast sodium adsorption/desorption process. The galvanostatic intermittent titration technique (GITT) was further used to evaluate the kinetics and their diffusion coefficients (Figs. 5d and e, Fig. S11 in Supporting information). Obviously, the NS-HCS exhibited a smaller overpotentials than that of N-HCS, indicating the better diffusion kinetic. Furthermore, NS-HCS electrode displays the larger Na⁺ diffusion coefficients (D_Na⁺) than that of N-HCS electrode (Figs. 5d and e). During the discharge process, D_Na⁺ during discharge of both electrodes decreases with voltage drop. This suggests that other Na⁺ need to overcome a repulsive force that already attached to the active sites during the sodiation process [49]. We believe that the accelerated diffusion might be associated with the expanded interlayer distance and increased active sites of NS-HCS, which is beneficial for the fast sodium storage at high rates. Moreover, the electrical impedance spectroscopic (EIS) of the NS-HCS electrode before and after 200 cycles were measured (Fig. S13 in Supporting information). Compared with the fresh electrode (Fig. S13a), the charge transfer resistance of NS-HCS electrode decreases after 200 cycles, the improved conductivity could contribute to the enhanced ion storage properties with the increased cycles. Furthermore, the slope of the Z′-ω^−1/2 (ω = 2πf) curves in the low-frequency region is significantly reduced after 200 cycles (Fig. S13b). This is mainly due to the deep penetration of the electrolyte and the decrease of the internal impedance of NS-HCS electrode. The equivalent circuit model and calculated parameters are shown in Fig S12c (Supporting information). The cycled NS-HCS electrode has a lower charge transfer resistance (R_ct, 88.63 Ω) and electrolyte resistance (R_s, 11.09 Ω) than that of fresh electrode, further indicating a decrease in polarization after cycles [50]. Fig. 5f is a schematic diagram of the ions storage in NS-HCS electrode. The two storage mechanisms of surface adsorption and interlayer insertion coexist in NS-HCS electrode. At the same time, the hollow structure accelerates the ions transport and resists the negative impact of ion insertion/deintercalation on the integrity of the electrode structure. Therefore, synergistic control of microstructure, heteroatoms doping and graphitic is an efficient strategy to enhance Na⁺/K⁺ storage performance.

  The potassium storage properties of NS-HCS anodes were further investigated, as shown in Fig. 6. The CV curves of PIBs are similar to that of SIBs (Fig. 6a), implying the similar ion transport behavior. It is clearly observed that the cathodic peaksat 0.3 V and 0.6 V in the first cycle disappears in the subsequent cycles, which can be attributed to the formation of SEI layer. Fig. 6b displays the GDC profiles of the NS-HCS electrode for the first three cycles at 0.05 A/g. NS-HCS sample shows typical sloping discharge/charge curves, which indicates the adsorption of K⁺ on the surface of carbon nanospheres [51, 52]. The capacity of NS-HCS electrode decreases in the initial five cycles at 0.05 A/g, and then stabilizes after 10 cycles in 0.1 A/g (Fig. 6c). NS-HCS exhibits high reversible capacities of 232.3 mAh/g. Fig. 6d shows the rate capability of NS-HCS electrode. The rate capability of NS-HCS electrode from 0.05 A/g to 5.0 A/g is 243.2, 200, 145, 114.8, 85.8, 66.1 and 41.6 mAh/g. When the current density is recovered to 0.1 A/g, the capacity returns to 191.4 mAh/g, indicating the excellent stability of the electrode. Additionally, Fig. 6e shows the long-term cyclability of NS-HCS electrode at a high current density of at 1.0 A/g. NS-HCS electrode exhibits robust cycling life by retaining capacity of 108 mAh/g at 1.0 A/g after 700 cycles, with a capacity retention of 94.8% at the last 500 cycles (Fig. 6e). After the initial five activation cycles, the Coulombic efficiencies for NS-HCS are more than 99% at 1.0 A/g, manifesting great electrochemical reversibility. To understand the reaction mechanism of PIBs, the CVs is performed in the range from 0.1 mV/s to 2.0 mV/s (Fig. S13a). The b-value of cathodic/anodic processes can be determined to 0.85 and 0.97, respectively, indicating the capacitive-controlled K⁺ storage process of NS-HCS (Fig. S13b). As given in Fig. S13c, the contribution ratio of surface-controlled capacity is about 49.60% at 0.1 mV/s and gradually reaches 81.16% at 2.0 mV/s, which is highly correlated with the characteristics of its unique structure. Meanwhile, the galvanostatic intermittent titration technique (GITT) was employed to evaluate the K-ion diffusion coefficient (D_K⁺) in NS-HCS (Fig. S13d). The diffusion coefficient profile of NS-HCS shows a slight decrease at 3.0-0.7 V, corresponding to the surface capacitive effects. The diffusion coefficient increases first and then decrease with the insertion of K ions into the graphite layer. The result indicates that the K⁺ storage in NS-HCS is realized by the cooperation of capacitance control and diffusion control. The favourable K⁺ storage kinetics of NS-HCS should be attributed to the Superior edge nitrogen ratio and large interlayer space.

  Figure 6

  

  Figure 6.  Electrochemical performance of NS-HCS electrodes versus K/K⁺. (a) CV curves. (b) GCD curves 0.05 A/g. (c) Cycle performances at 0.1 A/g. (d) Rate capabilities. (e) Long-term cycling performance at 1.0 A/g.

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  In summary, we realized a nitrogen, sulfur co-doped hollow carbon nanospheres (NS-HCS) material by in situ growth of polydopamine on nano-Ni(OH)₂ template with subsequent sulfur doping process. In this process, polydopamine serves as carbon precursor, thiourea introduces sulfur atoms into the carbon layer and regulates the type distribution of nitrogen, while Ni(OH)₂ acts as sacrificial template and decomposes into Ni spheres at high temperature. During the formation process, the produced Ni nanospheres serve as the template and catalyst to form the hollow nanosphere sturcture and facilitate the graphitization, while the sulfur doping process can enlarge the interlayer space of NS-HCS. The unique structure, abundant structural defects and large interlayer space of NS-HCS enable fast ion transport kinetics. Meanwhile, NS-HCS with partial graphitization can ensure structure stability, while the hollow structure with small sizes could alleviation the stress than that of solid structure during the cycles. When serve as anode in SIBs, NS-HCS anode displays high reversible capacities of 274.7 mAh/g at 0.1 A/g and no capacity decay could be observed after nearly 5000 cycles at 2.5 A/g. NS-HCS electrode also shows extraordinary stable cycle stability with a high capacity retention of 76.5% after 700 cycles at 1.0 A/g. Synergy of Heteroatom doping and hybrid ion storage mechanisms facilitates Na⁺/K⁺ storage in NS-HCS, indicating a promising application as anodes for SIBs and PIBs.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (No. 22165028), the Nature Science Foundation of Gansu Province (No. 20JR10RA108) and the Innovation Fund of Gansu Universities (No. 2020A-013).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.03.062.

  
  
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  Figure 1  Schematic diagram of the synthesis route of NS-HCS.

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  Figure 2  (a) SEM image and (b, c) TEM images of NS-HCS. (d) TEM image of N-HCS. (e) The elemental mappings of NS-HCS. (f) Schematic diagram of carbon layer of NS-HCS. (g) XRD pattern and (h) Raman spectra of the N-HCS and NS-HCS. (i) Pore size distribution of N-HCS and NS-HCS (inset: Nitrogen adsorption-desorption isothermal curves of NS-HCS).

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  Figure 3  Fine and fitted XPS spectra of NS-HCS: (a) C 1s, (b) S 2p, (c) O 1s and (d) N 1s. (e) N distribution. (f) Imaginary diagram of carbon layer in NS-HCS.

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  Figure 4  Electrochemical performance of the electrodes versus Na/Na⁺. (a) CV curves of NS-HCS at 0.1 mV/s. (b) GDC curves of NS-HCS. (c) The cycling stability of N-HCS-650, N-HCS, N-HCS-850, NS-HCS-650, NS-HCS and NS-HCS-850 electrodes at 0.1 A/g. (d) Cycling performance of NS-HCS and N-HCS samples at 0.1 A/g. (e) Rate capability of NS-HCS and N-HCS samples. (f) Comparison with the reported carbonaceous materials for SIBs. (g) Long-term cycling performance of the NS-HCS electrodes at 0.1 and 2.5 A/g.

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  Figure 5  (a) CV curves of NS-HCS at various scan rates from 0.1 mV/s to 2.0 mV/s in SIBs. (b) linear relationships between log(i) and log(v). (c) Contribution ratio of the NS-HCS and N-HCS electrodes at various scan rates. (d, e) Na-ion diffusion coefficient of the electrodes during sodiation and desodiation process, respectively. (f) Schematic illustration for the ion transfer process of NS-HCS electrode.

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  Figure 6  Electrochemical performance of NS-HCS electrodes versus K/K⁺. (a) CV curves. (b) GCD curves 0.05 A/g. (c) Cycle performances at 0.1 A/g. (d) Rate capabilities. (e) Long-term cycling performance at 1.0 A/g.

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