Evoking surface‐driven capacitive process through sulfur implantation into nitrogen‐coordinated hard carbon hollow spheres achieves superior alkali metal ion storage beyond lithium

Owing to the specific merits of low cost, abundant sources, and high physicochemical stability, carbonaceous materials are promising anode candidates for K+/Na+ storage, whereas their limited specific capacity and unfavorable rate capability remain challenging for future applications. Herein, the sulfur implantation in N‐coordinated hard carbon hollow spheres (SN‐CHS) has been realized for evoking a surface‐driven capacitive process, which greatly improves K+/Na+ storage performance. Specifically, the SN‐CHS electrodes deliver a high specific capacity of 480.5/460.9 mAh g−1 at 0.1 A g−1, preferred rate performance of 316.8/237.4 mAh g−1 at 5 A g−1, and high‐rate cycling stability of 87.9%/87.2% capacity retention after 2500/1500 cycles at 2 A g−1 for K+/Na+ storage, respectively. The underlying ion storage mechanisms are studied by systematical experimental data combined with theoretical simulation results, where the multiple active sites, improved electronic conductivity, and fast ion absorption/diffusion kinetics are major contributors. More importantly, the potassium ion hybrid capacitor consisting of SN‐CHS anode and activated carbon cathode deliver an outstanding energy/power density (189.8 Wh kg−1 at 213.5 W kg−1 and 9495 W kg−1 with 53.9 Wh kg−1 retained) and remarkable cycling stability. This contribution not only flourishes the prospective synthesis strategies for advanced hard carbons but also facilitates the upgrading of next‐generation stationary power applications.


| INTRODUCTION
5][6] Contrarily, electrochemical capacitors are characterized by high power density (2-5 kW kg −1 ) and long cycle life (over 1 × 10 5 cycles) but poor energy density due to the electrochemical double-layer ion storage mechanism. 7,8or the aim of satisfying the energy density and power density demands, lithium-ion hybrid capacitors (LIHCs) are established by combining faradaic and non-Faradaic electrodes. 9][12][13][14] However, the sluggish ion storage process on anode electrode caused by the larger K + /Na + radius (1.38/0.98Å vs. 0.68 Å of Li + ) results in the mismatching of kinetics between electrodes and unsatisfied electrochemical performance for KIHCs. 9Therefore, designing anode materials with rapid surface-driven capacitive potassium storage mechanism can improve its cycling stability and specific capacity, and will contribute to the overall performance of KIHCs.
Benefiting from the specific merits of low cost, abundant natural resources, and high physicochemical stability, carbonaceous materials have drawn tremendous attention and successfully applied in various energy storage devices as a series of promising anode candidates. 15,16However, when applied as potassium ion storage anodes, the cycling capability of carbonaceous materials is unsatisfactory due to the substantial volume changes resulting from layered structure degradation and electrode/electrolyte interface instability. 17Besides, the sluggish ion diffusion kinetics in the solid phase caused the poor rate performance, which makes potassium ion storage in carbonaceous materials a great challenge.It has been attested that heteroatom doping and advanced porous nano-architecture designing can introduce more defects, improve the electronic/ionic conductivity, and shorten the ion diffusion path, which is highly conductive to the surface-driven capacitive process. 18,19For instance, Mitlin et al. 20 demonstrate that sulfur doping combined with graphene nanobox morphology can reach fast charging and high reversible capacity for potassium ion storage.Yang et al. 21prove that the well-designed continually and precisely tunable pores in carbonaceous material facilitate stable cycling performance and high capacity for potassium ion storage.Although this inspiring progress has been achieved, there are still some remained challenging tasks.First, the synthesis strategies for more architectures with fundamental insight studies in the K + /Na + storage mechanism are still warranted.Besides, it is still a challenging task to obtain a combined K + /Na + storage performance, for example, high capacity, high rate, and long cycling performance.Moreover, the insufficient fast charge capability of the anode for K + /Na + storage exacerbates the kinetic mismatching between electrodes in hybrid capacitors, making it urgent to develop an outstanding carbonaceous anode with excellent kinetics.
In this contribution, the sulfur implantation in N-coordinated hard carbon hollow spheres (denoted as SN-CHS) with the evoked surface-driven process has been prepared by using 2-mercaptopyridine as pore swelling and doping agent, which holds a promising prospect as high-performance anode material for K + /Na + storage.Specifically, For K + storage, the SN-CHS electrodes deliver a high specific capacity of 480.5 mAh g −1 at 0.1 A g −1 with a capacity retention of 90.0% after 500 cycles.Moreover, it also holds preferred rate performance (a specific capacity of 316.8 mAh g −1 at 5 A g −1 ) and high-rate cycling stability (87.9% capacity retention after 2500 cycles at 2 A g −1 ).Besides, the SN-CHS electrodes deliver a remarkable specific capacity of 460.9 mAh g −1 at 0.1 A g −1 and highrate cycling stability with a capacity retention of 87.2% after 1500 cycles at 2 A g −1 for Na + storage.The potassium ion storage mechanisms are studied by ex situ X-ray photoelectron spectroscopy (ex situ XPS), density functional theory (DFT) calculation, electrochemical impedance spectroscopy (EIS) analysis, and galvanostatic intermittent titration technique (GITT) measurements results, where the multiple active sites, the improved electronic conductivity together with the enhanced ion absorption and diffusion kinetics are responsible for the enhanced ion storage performance.More importantly, the KIHCs consisting of SN-CHS anode and activated carbon cathode (AC) deliver an outstanding energy/power density (189.8Wh kg −1 at 213.5 W kg −1 and power density of 9495 W kg −1 with 53.9 Wh kg −1 ) and remarkable cycling stability.This work not only flourishes the promising synthesis strategies for advanced hard carbons but also facilitates the upgrading of next-generation stationary power applications based on earth-abundant sodium/ potassium.

| Synthesis of SN-CHS, CHS, and AC
Typically, 3.0 mL ammonia aqueous solution (32 wt %) was dropped into 80 mL mixed solution (absolute ethanol/deionized water with volume ratio = 7:1) and stirred for 30 min at room temperature.Thereafter, 2.8 mL tetraethyl orthosilicate was added and continually stirred for 10 min and 0.4 g resorcinol was then added followed by vigorously stirring for 30 min followed by adding 0.56 mL formaldehyde solution and continually stirred for 24 h at room temperature.The resorcinol/ formaldehyde-wrapped silica spheres (denoted as silica@RF spheres) were obtained by centrifugation, washed with water and ethanol several times, and air-dried at 80°C overnights.Then the silica@RF spheres were carbonized at 700°C (2°C/min) for 5 h under an argon atmosphere to obtain silica@carbon spheres.After that, 200 mg silica@carbon spheres were mixed with 10 mL 1 M/L NaOH solution and the mixture was vigorously stirred for 4 h at 70°C in an oil bath.Then, the carbon hollow spheres (denoted as CHS) were obtained by centrifugation, washed with water and ethanol several times, and air-dried at 80°C for several hours.
To synthesize SN-CHS, the as-obtained CHS was added into the solution consisting of 444.0 mg 2mercaptopyridine and 30 mL deionized water and stirred for 6 h followed by centrifugation and vacuum-dried for several hours.Then the as-obtained sample was carbonized at 500°C (2°C/min) for 2 h under an argon atmosphere to obtain SN-CHS.
Typically, a 20 mL Teflon-lined stainless-steel autoclave filled with 200 mg cotton and 15 mL deionized water was firstly heated to 200°C for 12 h in an air-drying oven and cooled down to room temperature.After centrifugation and washing with absolute ethanol several times, and air-dried for several hours, the black-brown hydrochars were collected.After that, the mixture consisting of hydrochars, melamine, KHCO 3 (mass ratio = 1:2:4), and 15 mL deionized water was continuously stirred in a 60°C oil bath until completely dried.The mixture was then carbonized at 850°C (2°C/min) for 1 h in N 2 atmosphere.The as-obtained material was washed with 1 M HCl and absolute ethanol to pH 7 and air-dried at 80°C overnight to obtain AC.

| Materials characterization
Field-emission scanning electron microscopy (FESEM, JSM-6700F) and transmission electron microscopy (TEM, JOEL JEM-2010; Talos F200X) were utilized to characterize the morphology and structure of the materials.XPS (ESCALAB 250 X-ray photoelectron spectrometer) was carried out to study the surface chemical state of the materials.The surface area and pore size distribution of the materials were calculated through N 2 adsorption-desorption using a Quantachrome instrument (Micrometritics, ASAP 2020) with Brunauer-Emmett-Teller methods and quenched solid DFT model.

| Electrochemical measurements
The electrochemical performance of the materials was tested by assembling CR2016 coin cells in an argon-filled glove box (O 2 /H 2 O < 0.5 ppm).The slurry made up of active materials, carbon black, and sodium carboxymethylcellulose (mass ratio = 7:2:1) in an adequate amount of deionized water was spread on the copper discs (D = 12 mm) and vacuum-dried for 10 h to prepare anode electrodes.The loading mass of active material was controlled between ~0.8 and 1.2 mg for each electrode.Metallic potassium/sodium foil was employed as counter and reference electrodes; 1.0 M potassium bisfluorosulfonylimide (KFSI) in ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio = 1:1) and 1.0 M NaClO 4 in propylene carbonate (PC) with 5 vol% fluoroethylene carbonate were used as electrolyte, and the glass fiber (GF/F, Whatman) was used as separator.
Cyclic voltammetry (CV) and EIS (100 mHz-100 kHz, amplitude of a.c.signal = 5 mV) measurements were performed on a CHI660E electrochemical workstation.The activation energy (E a ) was calculated by the Arrhenius equation 22 : where A is the temperature-independent coefficient, R is the gas constant, T is the absolute temperature, n is the number of electrons transferred, and F is the Faraday constant.
Cycling, rate, and GITT performance were tested on a Netware BTS-610 battery test system.The diffusion coefficient (D) is then calculated using Fick's second law: The current (100 mA g −1 ) pulse (τ 0 , s) and relaxation duration were set as 600/7200 s; m B , M B, and V M are the mass (g), molar mass (g mol −1 ), and molar volume (mL mol −1 ) of the materials; and S represents the active surface area (cm 2 ) of the working electrodes.ΔE S (V) is the potential variations between two adjacent states and ΔE T (V) is the potential change upon pulse current.The density of the sample was obtained by the following equation: where ρ (g cm −3 ), V total (cm 3 g −1 ), and ρ carbon are density, total pore volume, and density of carbon (2 g cm −3 ), respectively.

| Fabrication of the KIHCs
The uniform slurry consisting of active materials, additive carbon black, and polyvinylidene fluoride (mass ratio = 8:1:1) in an adequate amount of N-methylprolinodone was coated on the aluminum discs (D = 16 mm) and vacuumdried at 80°C overnight to prepare cathode electrodes.The anode and cathode were precycled (200 mA g −1 ) for 5 cycles in half-cell configuration, respectively, to activate the electrodes, and the KIHCs were then assembled using the precycled electrodes, electrolyte (1.0 M KFSI in EC/ DEC, volume ratio = 1:1), separator (GF/F, Whatman).The mass ratio of the anode/cathode was fixed to be 1:1, 1:2, and 1:3.Cycling and rate performance of the KIHCs were measured between 0.1 and 4.2 V.The specific energy and power density were calculated according to the following equations: where E (Wh kg −1 ) and P (W kg −1 ) are the energy and power density, respectively.I (A g −1 ) and t (s) are the current density and time duration for a full discharge process after excluding IR drops.

| Computational details
Our DFT calculations used the Vienna ab initio simulation package 23,24 with the projected augmented wave method. 25he kinetic energy cutoff and the plane-wave basis set, for the Kohn-Sham one-electron states is set to be 500 eV.The geometric optimizations and electronic structure are calculated by using the Perdew-Burke-Ernzerhof exchangecorrelation potential within the generalized gradient approximation. 26A graphene sheet (PC) was modeled using a 6 × 7 supercell with 84 atoms.The defect graphene (DC) is modeled by deleting six C atoms of 6 × 7 graphene supercell.According to our experimental results, one pyridinic N atom, one pyrrolic N atom, one graphitic N atom, and one substituted sulfur atom doped defect-graphene to model the S/N co-doped graphene (SN-DC). 27Brillouin zone integration is used with a grid of 2 × 2 × 1 Monkhorst-Pack 28 k-mesh for geometry optimizations and 4 × 4 × 1 k-mesh for the electronic structure calculations.The size of all geometry structures is kept fixed, but atomic positions are fully relaxed with the convergence criteria of energy and force set at 10 −4 eV and 0.03 eV/Å, respectively.The vacuum thickness along the z axis is set to 15 Å, which is large enough to avoid interaction between periodic images.The adsorption energy (E ads ) was performed using the following equation: where E CX and E C are the total energy of the system after/before the K absorption, respectively.E K is the energy per atom in bulk metal.
The charge density difference (Δρ) was obtained using the equation: Δρ = ρ (graphene+X) − ρ graphene − ρ X , where ρ (graphene+X) , ρ graphene , and ρ X are the total charge densities of K adsorbed graphene, graphene, and K, respectively.The charge density difference quantifies the redistribution of electron charge due to the interaction between K and graphene.

| RESULTS AND DISCUSSIONS
Figure 1A illustrates the synthesis process for CHS and SN-CHS, which can be briefly described as follows: the monodispersed silica@RF spheres are first synthesized by using resorcinol and formaldehyde solution as precursors, which are then carbonized at 700°C under argon atmosphere to prepare silica@carbon spheres.The CHS is then obtained after a facile etching procedure, and the SN-CHS is synthesized by annealing the mixture consisting of 2-mercaptopyridine and CHS.The detailed synthesis parameters are given in the Section 2. The morphology and microstructure of the as-synthesized silica@carbon spheres, CHS and SN-CHS are characterized by FESEM and TEM measurements.As shown in Figure 1B, the silica@carbon spheres feature regular spherical particles with relatively smooth surface, which has a mean diameter of 200-400 nm.The uniformity of the spheres allows them to self-assemble orderly, which is further confirmed by the 2D hexagonal arrays in the TEM image (inset of Figure 1B).The monodispersed CHS are synthesized after the etching procedure (Figure 1C), which are well inherited in the size and morphology of silica@carbon spheres.
The hollow structure of CHS is then characterized by a TEM image (inset of Figure 1C), which consists of a uniform carbon shell (~35 nm in thickness) that envelops a void center.After annealing with 2-mercaptopyridine, the SN-CHS is endowed with numerous micropores in the carbon shell, accompanied by well-maintained hollow sphere morphology and microstructure (Figure 1D,E).The enriched porous structure is conductive to shorten the diffusion path and improve the ion storage kinetics, thus favoring the enhanced K + /Na + storage properties.Figure 1F suggests the carbon shell holds amorphous characteristics, implying that there are plenty of defects in the carbon layer.
Figure 1G displays the elemental mapping analysis, demonstrating the homogeneous dispersion of C, N, and S elements throughout the sample.The morphology of the control sample prepared with different amounts of 2-mercaptopyridine (2 and 6 mmol) has also been investigated (Supporting Information: Figure S1), which reveals the morphology and microstructure of carbon hollow spheres gradually destroyed with the content of 2-mercaptopyridine increasing, which are aroused by the excessive decomposition of the SN-CHS during annealing process with the assistance of more byproduct generated from the increased 2-mercaptopyridine.
The surface chemical states of CHS and SN-CHS are further ascertained by the XPS technique.As can be seen in Supporting Information: Figure S2, the SN-CHS is mainly composed of C, S, and N with the estimated atomic percentage being 94.2%, 1.4%, and 4.3%, respectively, while the CHS only consists of C elements.In the high-resolution C 1s spectra (Figure 2A SN-CHS, respectively, 20 which indicates the successful doping of S/N heteroatoms into carbon matrix.As for CHS, only C-C/C=C (284.9 eV), C-O (286.9 eV), and O-C=O (288.8 eV) can be obtained, respectively.As shown in Figure 2B, the high-resolution S 2p spectra of SN-CHS show two dominant peaks at 163.9 and 165.1 eV along with a small peak at 169.6 eV, corresponding to the S atoms on C-S-C 2p 3/2 , C-S-C 2p 1/2 , and C-SO x -C, respectively. 29The N 1s high-resolution spectra (Figure 2C) can be deconvoluted into pyridinic N (398.7 eV), pyrrolic N (399.8),graphitic N (401.0eV), and N-O (402.4 eV), respectively. 30he abundant heteroatom doping in the carbon matrix can manipulate electronic structure for improved ion attraction, which is conductive to enhancing the surface capacitive process and specific capacity. 18,31,32Figure 2D,E exhibits the N 2 adsorption-desorption isotherms for the SN-CHS and CHS, where a larger specific surface area of 903.6 m 2 g −1 can be obtained for SN-CHS than that of CHS (206.1 m 2 g −1 ), with showing similar type IV isotherm of both samples.Moreover, SN-CHS shows a higher total pore volume of 2.2 cm 3 g −1 than that of CHS (0.89 cm 3 g −1 ) with the pore size located at 4.6 nm.The enlarged specific surface area and total pore volume of SN-CHS are aroused by the release of H 2 S byproduct during the annealing process, 33 which demonstrates the 2-mercaptopyridine could act as pore swelling agent for the increased specific surface area and pore volume in carbon materials.The abundant porosity endows SN-CHS with more buffering space to avoid structural deterioration, more exposed active sites, and ion transfer channels, thus leading to enhanced cyclability and fast ion/electron transportation.The Raman spectra analysis of SN-CHS and CHS are carried out subsequently.
As shown in Figure 2F, the D band (located at 1347 cm −1 , owing to the A 1g vibration mode of sp 2 carbon rings with defects) and the G band (located at 1600 cm −1 , owing to the E 2g vibration mode of sp 2 carbon atoms) represent a disordered and ordered carbon structure, respectively, 34 where the intensity ratios of the D and G band (I D /I G ) are usually used to estimate defect densities. 17The I D /I G of SN-CHS was measured to be 4.2, which is 3.5 times higher than that of CHS (I D /I G = 1.2).The higher I D /I G value of the SN-CHS can be ascribed to its enlarged specific surface area and total pore volume that enriches more exposed defects and edges, which can supply more active sites for surface-driven capacitive ion storage and thus realize enhanced potassium ion storage capabilities. 35he potassium storage performance of the SN-CHS electrode is then investigated in a half-cell configuration using K metal as the counter and reference electrode.The CV curves of the SN-CHS anode (Figure 3A) show that the prominent irreversible peak appears at 0.4 V and 0.01 V during the initial cathodic scan, which could be ascribed to the formation of solid electrolyte interface (SEI) and the intercalation of potassium ion into carbon layers. 36,379][40] Subsequently, the welloverlapped cycles demonstrate the good reversibility of SN-CHS for potassium ion storage.As the charge/ discharge profiles displayed in Figure 3B, a high specific capacity of 480.5/905.6 mAh g −1 with a coulombic efficiency of 53.1% can be obtained during the initial discharge/charge process.2][43][44] The subsequent charge/discharge curves display the pseudolinear voltage response with a plateau located at ~1.5 V, which corresponds to the depotassiation process of K + .Besides, the coulombic efficiency gradually rises up to 98.0% (10th cycle), indicating the fast kinetic properties and the excellent rate/cycling performance.Figure 3C exhibits an outstanding cycling performance of the SN-CHS electrode delivering a specific capacity of 445.7 mAh g −1 (10th cycle) and maintaining 401.2 mAh g −1 after 500 cycles, corresponding to a remarkable capacity retention of 90%.Besides, the SN-CHS electrodes also exhibit outstanding rate performance by delivering specific capacities of 479.6, 438.8, 403.4,373.1, 340.3, and 316.8 mAh g −1 at the current densities of 100, 200, 500, 1000, 2000, and 5000 mA g −1 , respectively (Figure 3D), and showing well-maintained shapes of charge/discharge profiles (Figure 3E).
As the current density is set back to 100 mA g −1 , the SN-CHS electrodes deliver a specific capacity of 452.2 mAh g −1 .Moreover, the SN-CHS electrodes also deliver superior rate performance when compared with other control samples (Supporting Information: Figure S3) and recently reported works (Figure 3F and Supporting Information: Table S1), further confirming its remarkable K + storage capability.As an important indicator for hybrid capacitors, the high-rate cycling performance of SN-CHS electrodes is then measured at 2000 mA g −1 (Figure 3G).Excitingly, the SN-CHS electrodes show a stable cycling performance by delivering a specific capacity of 312.5 mAh g −1 at 10th cycle and 274.7 mAh g −1 at 2500th cycle, corresponding to a capacity retention of 87.9%, demonstrating satisfactory fast kinetics for K + storage when compared with reported cycling performance (Supporting Information: Table S1).As for the CHS electrodes, the much lower specific capacities of 260.2, 227.2, 210.0, 193.5, 169.5, and 147.4 mAh g −1 can be obtained at 100, 200, 500, 1000, 2000, and 5000 mA g −1 , respectively, and much inferior high-rate cycling performance with only 126.0 mAh g −1 maintained after 1400 cycles.
The ion storage characters are further investigated according to the CV curves measured at different scan rates from 0.1 to 0.9 mV s −1 (Figure 3H).Based on the power law between current response (i) and scan rate (v) raised by Dunn and coworkers (Equation 8), 45 the total stored charge during CV measurement can be quantitatively separated into diffusion-controlled region and surface-driven capacitive controlled region: where a and b are defined as adjustable parameters, and k 1 v and k 2 v 1/2 represent the surface-driven capacitive and diffusion-controlled charge storage process, respectively.Accordingly, the surface-driven capacitive contributions of SN-CHS electrodes for the total charge at various scan rates are calculated, where a ratio of 77.9% can be obtained at 0.9 mV s −1 , which is higher than that of CHS at all scan rates (Figure 3I).The high capacitive contribution features rapid mass transportation and ion storage kinetics, which is ascribed to the well-developed hierarchical porous structure and abundant S/N doped heteroatoms, further conductive to the enhanced rate and cycling properties.
Besides excellent potassium storage performance, the SN-CHS can also act as a highly promising host for sodium ion storage, as indicated in Figure 4.The CV curves of the SN-CHS anode are firstly measured (Figure 4A), where the peaks appear at 0.8 and 0.01 V, which can be ascribed to the formation of SEI and the intercalation of Na + into carbon layers, respectively. 36,379][40] The following cycles overlapped well, indicating the good reversibility of SN-CHS for potassium ion storage.
Figure 4B displays the charge/discharge curves, where a high specific capacity of 460.9/947.82][43][44] The successive curves overlapped well with each other with a pseudo-linear voltage response behavior.The plateaus situated at ~1.8 V can be ascribed to the desodiation process of Na + , which is similar to that of potassium storage.Figure 4C displays an outstanding cycling performance of SN-CHS electrodes at 100 mA g −1 with a capacity retention of 82.5% (335.7 mAh g −1 at 100th cycle vs.406.8mAh g −1 at 10th cycle).As shown in Figure 4D, the SN-CHS electrodes also deliver excellent rate performance by showing specific capacities of 481.7, 432.8, 397.2, 361.8, 315.3, and 237.4 mAh g −1 at the current densities of 100, 200, 500, 1000, 2000, and 5000 mA g −1 , respectively, with well-maintained shapes of charge/discharge profiles (Figure 4E), and show superior rate performance when compared recently reported works (Figure 4F and Supporting Information: Table S2). 46,47Moreover, the SN-CHS electrodes exhibit excellent high-rate cycling performance by showing a capacity retention of 87.2% (292.7 mAh g −1 at 1500th vs. 323.8mAh g −1 at 10th cycle), demonstrating excellent cyclability for Na + storage when compared with reported cycling performance (Supporting Information: Table S2).In comparison, the CHS electrodes deliver much inferior rate (240.7,209.8, 168.6, 154.4,136.2, and 108.4 mAh g −1 at the current densities of 100, 200, 500, 1000, 2000, and 5000 mA g −1 , respectively) and high-rate cycling performance (125.0 mAh g −1 remained after 1300 cycles) Similarly, the Na + storage characters are further characterized based on the CV curves at scan rates range from 0.1 to 0.9 mV s −1 (Figure 4H).Compared with CHS electrodes, higher capacitive contributions of SN-CHS electrodes can be obtained at various scan rates (Figure 4I), suggesting the Na + storage capability is remarkably enhanced.
To further understand the potassium storage mechanisms in SN-CHS, as shown in Figure 5A, ex situ XPS analysis of S 2p and N 1s are first performed at pristine, fully potassiation, and fully depotassiation states (cycled at 100 mA g −1 ), respectively.After the SN-CHS electrodes are fully potassiated to 0.01 V, the binding energy of S 2p negatively shifts to a lower value along with two peaks emerging at 163.7 and 164.7 eV that is assigned to the K 2 S compound, 20,29 implying the strong interaction between K and S atoms, thus leading to a lower valence state of the S.After being fully depotassiated to 3.0 V, the intensity of S 2p peaks increases and the binding energy of C-S-C 2p 3/2 and C-S-C 2p 1/2 shift to 164.0 and 165.1 eV, indicating a reversible variation of the S valence state.As for N 1s, the pyridinic N and pyrrolic N species shift to lower binding energy (398.5/399.5 eV) with decreased intensity at a fully potassiated state, which almost recovers to its original state when fully depotassiated, suggesting that these two types of N species in SN-CHS can function as active sites for K + storage.
As shown in Figure 5B and Supporting Information: Figures S4 and S5, the Fermi level shifts to the conduction band with additional S doping before and after K + adsorption, suggesting improved electronic conductivity after S doping, [48][49][50] which further results in improved potassium storage kinetics.As displayed in Figure 5C-E, the absorption energy (ΔD Ea ) for K + can be calculated to be −0.2,−0.33, and −4.9 eV for PC, DC, and SN-DC, respectively, indicating that the S/N co-doping can further facilitate the absorption ability.Moreover, the electron density difference analysis demonstrated that a larger charge accumulation region can be established in SN-DC (Supporting Information: Figure S6), revealing the higher adsorption energy could be attributed to the stronger bond formation.The EIS data at different temperatures are also measured (Figure 5F−G and Supporting Information: Figures S7 and S8), where the values of R ct for SN-CHS decrease upon the temperature increase, which is smaller than CHS at each temperature.The apparent Ea is calculated based on the Arrhenius equation (Equation 1), where the value for SN-CHS (24.9 kJ mol −1 ) is much smaller than CHS (67.6 kJ mol −1 ) and similar results are obtained for Na + storage (Supporting Information: Figure S9), which is responsible for the enhanced capacity and preferred rate performance.As displayed in Figure 5H,I and Supporting Information: Figure S10, the GITT measurement and the overpotential at the corresponding process are represented by potential variations during the relaxation period, where the overpotential for SN-CHS is much smaller than CHS. 51Moreover, the diffusion coefficient of K + for SN-CHS is higher than CHS throughout various potassiation/depotassiation levels, indicating a superior K + diffusion kinetics in SN-CHS with lower K + extraction barriers.
To evaluate the practicability, the KIHCs based on SN-CHS anode and AC (denoted as SN-CHS//AC) are assembled based on the working principle illustrated in Figure 6A, where the potassiation of SN-CHS anode and the adsorption of FSI − ions on the surface of AC cathode during the charge process and that goes conversely for discharge process. 37,52,535][56][57][58] The discharge capacity can be calculated to be 84.9, 65.5, 55.9, 46.7, 40.0, and 32.8 mAh g −1 corresponding to the current density of 0.1, 0.2, 0.5, 1, 2, and 5 A g −1 , respectively.The Ragone plot of the device can then be calculated (Figure 6C) based on Equations 4-6, where a maximum energy density of 189.8 Wh kg −1 can be achieved at the power density of 213.5 W kg −1 and an energy density of 53.9 Wh kg −1 remains even at a power density of 9495 W kg −1 .Moreover, the KIHCs exhibit outstanding cycling stability (Figure 6D), where a capacity retention of 84.0% and an energy retention of 75.0% can be observed after 5000 cycles at the rate 1 A g −1 .Impressively, this obtained energy/power densities and cyclability are superior to most reported values of potassium ion-based hybrid capacitors (Supporting Information: Table S3).
In summary, we have prepared the SN-CHS with sulfur implantation for evoking a surface-driven process by using 2-mercaptopyridine as S/N sources and a pore swelling agent, which delivers superior electrochemical performance for K + /Na + storage.The underlying ion storage mechanisms are disclosed by combined methods including ex situ XPS, DFT calculation, EIS analysis, and GITT measurements, where the multiple active sites, improved electronic conductivity, and enhanced ion absorption and diffusion kinetics are the major contributors to being conductive to the enhanced ion storage performance.More importantly, the KIHCs combining SN-CHS anode and AC cathode reach outstanding energy/power density (189.8Wh kg −1 at 213.5 W kg −1 and power density of 9495 W kg −1 with 53.9 Wh kg −1 ) and remarkable cycling stability.Such improvements reveal that using 2-mercaptopyridine as the S/N source and pore swelling agent offers a promising design approach toward achieving advanced carbonaceous material for K + /Na + storage.
), the dominant C-C/C=C peak at 284.6 eV, along with C-N/C-S at 285.5 eV, C-O at 286.3 eV, and O-C=O at 288.4 eV can be clearly recognized for F I G U R E 1 Morphology and microstructure of N-coordinated hard carbon hollow sphere (SN-CHS).(A) Illustration of synthesis process for SN-CHS.Field-emission scanning electron microscopy and transmission electron microscopy (TEM) images of (B) precursor, (C) CHS, and (D, E) SN-CHS.(F) High-resolution TEM analysis and (G) elemental mapping results.

F
I G U R E 2 (A) C 1s X-ray photoelectron spectroscopy (XPS) spectra of carbon hollow sphere (CHS) and N-coordinated hard CHS (SN-CHS); (B) S 2p, and (C) N 1s XPS spectra of SN-CHS.(D) Nitrogen adsorption-desorption isotherm.(E) Pore size distribution and (F) Raman spectra of CHS and SN-CHS.

F I G U R E 3
Potassium ions storage performance.(A) Cyclic voltammetry (CV) curves at 0.1 mV s −1 between 0.01 and 3.0 V. (B) Charge/ discharge profiles and (C) cycling stability at 100 mA g −1 .(D) Rate capability and (E) corresponding charge/discharge profiles.(F) Rate capability comparison with reported works.(G) Cycling stability at 2000 mA g −1 .(H) CV curves and (I) capacitive contributions at various scan rates.

F I G U R E 4
Sodium ions storage performance.(A) Cyclic voltammetry (CV) curves at 0.1 mV s −1 between 0.01 and 3.0 V. (B) Charge/ discharge profiles and (C) cycling stability at 100 mA g −1 .(D) Rate capability and (E) corresponding charge/discharge profiles.(F) Rate capability comparison with reported works.(G) Cycling stability at 2000 mA g −1 .(H) CV curves and (I) capacitive contributions at various scan rates.

F
I G U R E 5 K + storage mechanism for N-coordinated hard carbon hollow sphere (SN-CHS) electrodes.(A) Ex situ X-ray photoelectron spectroscopy (XPS) spectra for S 2p and N 1s at pristine and fully potassiated/depotassiated states.Theoretical calculations in different configurations: (B) The band structures of propylene carbonate (PC), defect graphene (DC), and SN-DC before K absorbing.Top view of simulations for one K adsorbed on the (C) PC, (D) DC, (E) SN-DC and corresponding ΔD Ea , respectively.(F) Nyquist plots at various temperatures of SN-CHS, (G) E a calculation; (H) galvanostatic intermittent titration technique (GITT) measurements, and (I) corresponding diffusion coefficients for SN-CHS and CHS electrodes.