SnS2 nanoparticles embedded in sulfurized polyacrylonitrile composite fibers for high‐performance potassium‐ion batteries

Potassium‐ion batteries (PIBs) have garnered significant attention as a promising alternative to commercial lithium‐ion batteries (LIBs) due to abundant and cost‐efficient potassium reserves. However, the large size of potassium ions and the resulting sluggish reaction kinetics present major obstacles to the widespread use of PIBs. Herein, we present a simple method to ingeniously encapsulate SnS2 nanoparticles within sulfurized polyacrylonitrile (SPAN) fibers (SnS2@SPAN) for serving as a high‐performance PIB anode. The large interlayer spacing of SnS2 provides a fast transport channel for potassium ions during charge–discharge cycles, while the one‐dimensional SPAN skeleton offers massive binding sites and shortens the diffusion path for potassium ions, facilitating faster reaction kinetics. Additionally, the excellent ductility of SPAN can effectively accommodate the large volume changes that occur in SnS2 upon potassium‐ion insertion, thereby enhancing the cyclic stability of SnS2. Benefiting from the above advantages, the SnS2@SPAN composites exhibit impressive cyclability over 500 cycles at 4 A g−1, with a capacity retention rate close to 100%. This study provides an effective approach for stabilizing high‐capacity PIB anode materials with large volume variations.

[6][7] Therefore, developing new battery systems based on sustainable charging carriers is required as an alternative to LIBs. [8,9]Potassium-ion batteries (PIBs) have attracted extensive research interest due to the abundant and cost-effective potassium reserves, and their analogous "rocking-chair" energy storage mechanism to LIBs. [10][13] Moreover, in organic electrolytes, the smaller Stokes radius of K + (3.6 Å) compared with Li + (4.8 Å) enables higher ion conductivity, enhancing the overall performance of PIBs. [14][17] Various anode materials, such as carbonaceous materials, [18][19][20] metal alloys, [21][22][23] metal oxides, [24,25] and metal sulfides, [26][27][28][29] have been extensively studied.[32][33] Specifically, SnS 2 stands out due to its abundant sources, low cost, high chemical stability, nontoxicity, and high theoretical specific capacity, making it a well-explored anode material for PIBs. [34,35]Moreover, SnS 2 has a typical layered structure, comprising covalently bonded S─Sn─S trilayers separated by relatively large van der Waals interactions.[38][39] In addition, the relatively weak Sn─S bonds in SnS 2 contribute to enhanced transformation kinetics compared with SnO 2 . [40]nfortunately, pure SnS 2 suffers from poor electronic conductivity and large volume variation during cycling.These issues result in the pulverization and shedding of the active material, leading to a decline in electrochemical performance when used as an anode material for PIBs. [41,42]In response to the above challenges, various strategies have been devised, including the construction of nanostructures, hybridization with conductive matrices, and a combination of the above two strategies.Reducing the size of SnS 2 can effectively shorten the ion/electron transport distance, [43] while hybridizing SnS 2 with an elastic and conductive matrix is capable of improving its conductivity and mitigating volume changes. [44]However, most current conductive matrices have limited potassium storage capacity, thus compromising the overall performance.Therefore, there is a need for elastic and conductive matrices with higher potassium storage capabilities.
In this work, we propose a simple and feasible method for synthesizing SnS 2 @sulfurized polyacrylonitrile (SPAN) composites by encapsulating ultrasmall SnS 2 nanocrystals in SPAN fibers via a coaxial electrospinning technique, followed by a one-step vulcanization process.The combination of ultrasmall SnS 2 nanocrystals with SPAN fibers accelerates the kinetic process and mitigates the bulk effect during potassium-ion insertion and extraction, thereby significantly improving the electrochemical performance of SnS 2 .Furthermore, density functional theory (DFT) calculations suggest that the SPAN skeleton is capable of absorbing SnS 2 and its discharge product, hence preventing the loss of active materials and maintaining structural stability.As expected, the SnS 2 @SPAN fibers exhibit high reversible capacities (641.5 mAh g −1 at 0.2 A g −1 ), excellent rate performance (145 mAh g −1 at 3 A g −1 ), and superior cycling stability (117.5 mAh g −1 after 500 cycles at 4 A g −1 ).Moreover, the assembled full cell delivers a high capacity of 100.5 mAh g −1 after 100 cycles at 3 A g −1 , demonstrating its viability for practical applications.

| Characterization of SnS 2 @SPAN
The synthesis process of the SnS 2 @SPAN is illustrated in Figure 1.Initially, a solution containing polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), and N,Ndimethylformamide (DMF) was used as the outer-axis precursor solution.Meanwhile, an inner-axis precursor solution was formulated by dissolving SnCl 2 •2H 2 O and styrene-acrylonitrile (SAN) in DMF.Subsequently, coaxial electrostatic spinning was performed to prepare precursor nanofibers, referred to as PMMA-PAN@SnCl 2 -SAN.Lastly, the SnS 2 @SPAN fibers were obtained through a one-step sulfurization method by annealing the precursor fibers with sulfur at 450°C.The morphology of the SnS 2 @SPAN and SnS 2 samples was characterized using scanning electron microscopy (SEM).Figure 2A shows that the diameter of the SnS 2 @SPAN fiber is ~500 nm with a large number of tiny SnS 2 nanoparticles embedded within the fibers.The SEM images of pure SnS 2 are shown in Figure S1.The synthesized SnS 2 exhibits a twodimensional sheet-like structure with a thickness ranging from a few dozen to several tens of nanometers.The detailed microstructure of SnS 2 @SPAN was further characterized by transmission electron microscopy (TEM).As depicted in Figure 2B, the SnS 2 nanoparticles are uniformly confined with the SPAN fibers, consistent with the SEM result.
A high-resolution TEM image of the SnS 2 @SPAN composite is presented in Figure 2C.The average layer spacing of SnS 2 crystals is determined to be ~0.59nm, corresponding to the (001) crystal plane of SnS 2 . [45]The large interlayer spacing promotes the diffusion of K + ions and provides enough space for K storage, thereby enhancing the electrochemical performance of SnS 2 .The selected area electron diffraction pattern displayed in Figure 2D exhibits a couple of bright circles, representing the (001), ( 011), (110), and (210) planes of SnS 2 (JCPDS:23-677).Furthermore, energy-dispersive X-ray spectroscopy (EDS) mapping confirms the uniform distribution of Sn, S, C, and N elements within the SnS 2 @SPAN composite (Figure 2E).The crystal structure of the SnS 2 @SPAN composites was examined using X-ray diffraction.As shown in Figure 3A, the diffraction peaks of SnS 2 @SPAN match well with the hexagonal phase of SnS 2 (JCPDS:23-677), indicating the successful synthesis of SnS 2 crystals. [46]he structural properties of SnS 2 @SPAN and SnS 2 were further analyzed using Raman (Figure 3B).The narrow diffraction peak at 311 cm −1 is attributed to the A 1g vibrational mode of SnS 2 in the composite.Peaks at 179 and 365 cm −1 are associated with C─S bonding, while the peak at 465 cm −1 is attributed to the S─S bond, confirming the presence of SPAN.49][50] X-ray photoelectron spectroscopy was performed to study the surface chemical composition and electronic valence states of SnS 2 @SPAN.As shown in Figure 3C, the C 1s spectrum shows three characteristic peaks at 286.0, 283.8, and 284.8 eV, which correspond to the C─S/C─N, C─Sn, and C─C/C═C bonds, respectively.[53] The S 2p spectrum in Figure 3D can be deconvoluted into four peaks located at 161.6, 162.8, 163.5, and 164.7 eV.The former two peaks correspond to S 2p 3/2 and S 2p 1/2 , while the latter two peaks are related to S─S and C─S, respectively. [54]As shown in Figure 3E, the Sn 3d spectrum exhibits a typical pair of Sn 3d 3/2 and Sn 3d 5/2 peaks at 494.9 and 486.5 eV, respectively, suggesting the successful synthesis of SnS 2 rather than SnS.

| Electrochemical performance
The electrochemical performances of the SnS 2 @SPAN electrode were studied, as presented in Figure 4. Figure 4A shows the initial three cyclic voltammetry (CV) curves of the SnS 2 @SPAN electrode at a scan rate of 0.2 mV s −1 .The broad reduction peak at 0.56 V during the first cathodic scan can be attributed to the conversion reaction of SnS 2 , the alloying reaction of Sn, and the formation of a solid electrolyte interphase (SEI). [55]During the anodic scan, the main peak centered at 1.05 V corresponds to the depotassiation of K x Sn, whereas the subsequent oxidation peak at 2.00 V is related to the depotassiation of K 2 S. [26,56] The highly overlapped CV curves after the first cycle manifest the good reversibility of the SnS 2 @SPAN electrode during charge-discharge cycles.Figure 4B presents the galvanostatic charge-discharge (GCD) profiles of the SnS 2 @SPAN electrode at 0.2 A g −1 .The initial discharge capacity is 1093.7 mAh g −1 and the F I G U R E 3 (A) XRD pattern of the SnS 2 @SPAN.(B) Raman spectra of SnS 2 @SPAN and SnS 2 .XPS spectra of the SnS 2 @SPAN composite: (C) C 1s, (D) S 2p, and (E) Sn 3d.SPAN, sulfurized polyacrylonitrile; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction.reversible charge capacity is 641.5 mAh g −1 , resulting in an initial Coulombic efficiency of 58.7%.The capacity loss at initial cycles can be attributed to the gradual formation of a compact and robust SEI film on the SnS 2 @SPAN electrode, as well as partially irreversible conversion reactions during (de)potassiation processes. [50,57]Notably, a sloping plateau at ~0.8-1.0V is observed upon the first discharge process, and a plateau between 1.5 and 2.0 V is evident in the charging curve, in agreement with the CV curves.
In addition, the SnS 2 @SPAN electrode exhibits decent rate capability (Figure 4C), delivering capacities of 427, 333, 249, 181, and 145 mAh g −1 at current densities of 0.2, 0.5, 1, 2, and 3 A g −1 , respectively.The cyclic stability of SnS 2 @SPAN and SnS 2 electrodes was compared at a current density of 3 A g −1 (Figure S2).The SnS 2 @SPAN exhibits an initial charge capacity of 149.2 mAh g −1 and maintains a capacity of 127.1 mAh g −1 after 800 cycles, signifying its good cycling stability.For comparison, the SnS 2 electrode shows rapid capacity decay, with a charge capacity of 18.5 mAh g −1 after 100 cycles, highlighting the effectiveness of the SPAN framework in improving the performance of SnS 2 .Moreover, the SnS 2 @SPAN electrode also exhibits excellent cyclic stability, maintaining an impressive capacity of 117.5 mAh g −1 at 4 A g −1 for 500 cycles (Figure 4D), accompanied by a remarkable capacity retention rate of ~100%.To understand the excellent cycling stability of the SnS 2 @SPAN electrode, coin cells were disassembled after 100 charge-discharge cycles at 1 A g −1 , and the morphology and elemental distribution of the cycled SnS 2 @SPAN electrode were analyzed using SEM and EDS.As illustrated in Figure S3, the overall fibrous morphology of SnS 2 @SPAN was well maintained, indicating that the SPAN framework effectively accommodates the volume changes of SnS 2 during cycling.Furthermore, the uniform distribution of Sn, S, C, and N elements in the cycled SnS 2 @SPAN electrode suggests the excellent structural stability of the SnS 2 @SPAN electrode.[60][61][62][63][64][65][66][67] We further conducted electrochemical impedance spectroscopy to analyze the kinetics of the SnS 2 @SPAN and SnS 2 electrodes (Figure 4E).The semicircular part in the high-frequency region mainly represents the charge transfer resistance (R ct ), and the slope in the lowfrequency region stands for the diffusion resistance (W) inside the electrode material. [68]The calculated R s values for SnS 2 @SPAN and SnS 2 electrodes are ~4.8 and 2.6 Ω, while their R ct values are ~4322 and 10,301 Ω, respectively, demonstrating improved charge transfer capability in the SnS 2 @SPAN electrode.

| Kinetic analysis
To investigate the potassium storage mechanism of SnS 2 @SPAN electrode materials, CV was recorded at different scan rates ranging from 0.2 to 1.0 mV s −1 .As shown in Figure S4a, the cathodic/anodic peak shifts toward a lower/higher potential when the scan rate increases.The relationship between the peak current (i) and the scan rate (v) can be generally described by the following equations: where a and b are adjustable constants.Notice that the value of b differentiates the storage behavior during the electrochemical reaction. [69]A value of b close to 0.5 indicates that a diffusion-controlled process dominates the electrochemical reactions, whereas a value close to 1.0 suggests that pseudocapacitive behavior governs the electrochemical reactions.As shown in Figure S4b, the b values for peaks 1, 2, and 3 are 0.85, 0.65, and 0.63, respectively, suggesting that a significant proportion of the K storage capacity is governed by diffusion-controlled processes.To further determine the contribution of diffusion-controlled behavior to the overall process, the following equation was employed: where k 1 and k 2 are the constants, and k 1 (V) • v and k 2 (V) • v 1/2 represent the contribution of capacitive and diffusion control processes, respectively. [70]Figure S4c shows that approximately 25% of the overall capacity is attributed to the capacitive process at a scan rate of 0.6 mV s −1 .The contribution of pseudocapacitance to the overall capacities of SnS 2 @SPAN is plotted in Figure S4d.
As the scan rate increases from 0.2 to 1.0 mV s −1 , the pseudocapacitance contribution rises from 16% to 23%, 25%, 27%, and 31% consecutively.These findings indicate that the K + storage in SnS 2 @SPAN electrodes is dominated by diffusion-controlled processes.

| First-principles calculations
To understand the interaction between SPAN and SnS 2 , as well as its discharge products, we performed first-principles DFT simulations.The adsorption energies of SnS

| Full-cell performance
To show the potential applications of the SnS 2 @SPAN electrode, PIB full cells were assembled by pairing the SnS 2 @SPAN anode with prepotassiated 3,4,9,10perylenetetracarboxylic dianhydride (pPTCDA) cathode.Figure 6A schematically shows the working principle of the assembled SnS 2 @SPAN|pPTCDA full cell.When PTCDA was used as the cathode, it possessed a reversible capacity of 123.9 mAh g −1 at 1 A g −1 (Figure S7). Figure 6B demonstrates that an light emitting diode array can be powered by two PIB full cells connected in series.The GCD curves of the SnS 2 @SPAN|pPTCDA full cell at current densities ranging from 0.1 to 2 A g −1 are displayed in Figure 6C.The SnS 2 @SPAN|pPTCDA full cell exhibits excellent rate performance, maintaining highly reversible capacities of ~323.8, 247.4,206, and 165.2 mAh g −1 at 0.1, 0.5, 1, and 2 A g −1 , respectively (Figure 6D).Upon returning to a current density of 0.1 A g −1 , a capacity of 284.7 mAh g −1 is retained.Notably, the PIB full cell possesses a high capacity of 100.5 mAh g −1 after 100 cycles at 3 A g −1 , further confirming the excellent electrochemical performance of the SnS 2 @SPAN composite (Figure 6E).In conclusion, SnS 2 @SPAN has been successfully prepared through a straightforward coaxial electrospinning technique followed by a one-step vulcanization approach.The resulting one-dimensional structure reduces the diffusion path for K + ions, facilitating their insertion into the SnS 2 nanocrystals.The high sulfur content in SPAN contributes to the enhanced potassium storage capacity, while the excellent ductility of SPAN effectively mitigates the considerable volume changes experienced by SnS 2 during charging-discharging cycles.More importantly, the strong affinity between SnS 2 and SPAN is beneficial for structural stability and electron transfer, as indicated by DFT calculations.By leveraging these advantages, the SnS 2 @SPAN electrode exhibits excellent rate performance and long-term cyclic stability in PIBs.This study offers a new approach for stabilizing tin-based sulfide anodes using an elastic, electron-conductive, and high-capacity matrix.

F
I G U R E 1 Schematic illustrating the synthetic process of SnS 2 @SPAN.SPAN, sulfurized polyacrylonitrile.F I G U R E 2 (A) SEM, (B) TEM, (C) HRTEM images, and (D) SAED pattern of the SnS 2 @SPAN.(E) The EDS elemental mapping of the SnS 2 @SPAN.EDS, X-ray spectroscopy; HRTEM, high-resolution TEM; SAED, selected area electron diffraction; SEM, scanning electron microscopy; TEM, transmission electron microscopy.

F
I G U R E 4 (A) CV curves of the SnS 2 @SPAN electrode at 0.2 mV s −1 .(B) GCD profiles of the SnS 2 @SPAN electrode at 0.2 A g −1 .(C)Rate capability of SnS 2 @SPAN electrode at different current densities between 0.2 and 3 A g −1 .(D) Long-term cycling performance at 4 A g −1 of the SnS 2 @SPAN electrode.(E) Nyquist plots of SnS 2 @SPAN and SnS 2 before cycling.CV, cyclic voltammetry; GCD, galvanostatic charge-discharge; SPAN, sulfurized polyacrylonitrile.

F I G U R E 5
Geometry structure of (A) one SnS 2 molecule, (B) K 2 S molecule, (C) K atom adsorbed on the N-side of the sulfurized polyacrylonitrile (SPAN) fiber, (D) one SnS 2 molecule, (E) K 2 S molecule, and (F) K atom adsorbed on N-doped graphene (NG).(G) Adsorption energies of one SnS 2 molecule, one K 2 S molecule, and one K atom on SPAN fibers and NG.F I G U R E 6 (A) Schematic illustration of the working mechanism of SnS 2 @SPAN|pPTCDA full cell.(B) An LED array powered by the assembled K-ion full cell.(C) GCD curves at different current densities, (D) rate capability, and (E) cyclic performance at 3 A g −1 of SnS 2 @ SPAN|pPTCDA cells.GCD, galvanostatic charge-discharge; LED, light emitting diode; pPTCDA, 3,4,9,10-perylenetetracarboxylic dianhydride; SPAN, sulfurized polyacrylonitrile.
2 , K 2 S and K atoms on SPAN and N-doped graphene (NG) were calculated.The adsorption energy of the SnS 2 molecule on the N-side of SPAN is −3.83 eV (Figure5A), whereas the adsorption energies of the K 2 S molecule and K atom on the N-side of SPAN are −4.47 and −4.60 eV, respectively (Figure5B,C).In contrast, the adsorption energies of the corresponding adsorbates on NG are −2.56,−1.89,and−2.87eV,respectively (Figure5D-F).As shown in Figure5G, the adsorption energies of the SnS 2 molecule, K 2 S molecule, and K atom on the SPAN substrate are much