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ReseaRch aRticle

Cubic Spinel XIn2S4 (X = Fe, Co, Mn): A New Type of Anode Material for Superfast and Ultrastable Na-Ion Storage

Dong Yan, Ke Li, Yaping Yan, Shaozhuan Huang, Yew Von Lim, Yang Shang, Daliang Fang, Lay Kee Ang, and Hui Ying Yang*

DOI: 10.1002/aenm.202102137

1. Introduction

Li-ion batteries (LIBs) have been the pri-mary source of energy storage for elec-tronic, equipment, and grid storage.[1–4] However, the shortage of Li resources and the increasing costs have brought concerns for the future development of LIBs. As the highly prospective substitute for LIBs, Na-ion batteries (NIBs) have attracted much attention in recent years due to the advantages of low cost and abundance of Na, as well as their electrochemical simi-larities.[5–8] Moreover, compared with Li ion, Na ion shows the smaller Stokes radius, which enables better ionic diffu-sion kinetics in the electrolyte.[9] Neverthe-less, the ionic radius of Na ion (1.02 Å) is larger than that of Li ion (0.76 Å), resulting in the relatively sluggish reaction kinetics and giving rise to severe mechanical strain within the host structure during (de)sodia-tion processes.[10] Therefore, it is desirable to develop rational electrode materials for high-performance NIBs.

Recently, more and more efforts are beginning to converge in the study of

bimetallic sulfides as NIB anodes due to their great diversity, rich redox active sites, abundant ionic diffusion channels, high specific capacities, and existing unique synergies.[11–13] Remark-ably, in stark contrast to monometallic sulfides, each compo-nent within the bimetallic sulfides would possess essential and unique functions, stimulate synergies, and thereby contribute to overall performance enhancement.[13–16] For example, Kang and co-workers developed the CoMoS3 as NIB anode, which manifested higher electronic/ionic conductivities and more abundant redox active sites than the monometallic sulfides.[14] Bimetallic Co6Ni3S8 as superior NIB anode was reported by Yang and co-workers,[15] in which the better rate capability and more stable cycling were displayed in contrast to the mono-metallic Co9S8.[15] These encouraging works have proved the advantages of bimetallic sulfides in comparison to the corre-sponding monometallic sulfides as NIB anodes. However, it is challenging to utilize bimetallic sulfides for achieving the out-standing electrochemical performances especially for the supe-rior rate and cycling capabilities.

Among the various bimetallic sulfides, indium-based bime-tallic sulfides showed high performance potentials in various energy storage applications such as the Li-ion, Na-ion, Li–S,

Bimetallic sulfides are prospective candidates as Na-ion battery (NIB) anodes owing to their abundant redox active sites and high specific capacities, while the rate and cycling performances are limited due to their severe mechanical strain and sluggish reaction kinetics. Herein, for the first time, a series of cubic spinel XIn2S4 (X = Fe, Co, Mn) anodes is proposed for superfast and ultrast-able Na-ion storage. The FeIn2S4 delivers the best electrochemical performance among them with the amazing results especially for its superfast charging (9–13 s per charge with ≈300 mAh g−1 input) and ultrastable cycling capabili-ties (25 K cycles with ultralow 0.000801% capacity loss per cycle). Further density functional theory calculations and experimental results indicate that Fe substitution at tetrahedral sites of cubic spinel In2.77S4 can significantly enhance its electrical conductivity, Na-ion binding kinetics, and pseudocapaci-tive contributions, reduce its Na-ion diffusion barrier, and relieve its mechanical strain. Moreover, the inherent In skeleton at octahedral sites can induce a complementary reaction to stabilize the conversion-type Fe-based components. Furthermore, the FeIn2S4-based full cells with excellent electrochemical per-formance are assembled. This work demonstrates the feasibility of enhancing the synergies within bimetallic sulfides to realize remarkable electrochemical performance.

D. Yan, K. Li, Y. Von Lim, Y. Shang, D. Fang, H. Y. YangPillar of Engineering Product Development (EPD)Singapore University of Technology and Design (SUTD)Singapore 487372, SingaporeE-mail: yanghuiying@sutd.edu.sgY. YanInternational Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of EducationInstitute of Microscale OptoelectronicsShenzhen UniversityShenzhen 518060, ChinaS. HuangKey Laboratory of Catalysis and Energy Materials Chemistry of Ministry of EducationSouth-Central University for NationalitiesWuhan 430074, ChinaL. K. AngScience, Mathematics, and Technology (SMT)Singapore University of Technology and Design (SUTD)Singapore 487372, Singapore

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.202102137.

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and Li–air batteries, owing to their excellent electrochemical and catalytic activities.[17–22] For instance, indium-based bime-tallic sulfides showed the high specific capacities in LIB[20] due to the alloying reactions between Li and indium.[23] However, it usually brings the severe volume change to the active materials with resultant poor cycling performance.[20] While the indium belongs to the electrochemical inactive metal in NIB, and the indium-based components from CuInS2 anode control a revers-ible Na-ion (de)intercalation process, as evidenced in our pre-liminary work.[21] This can help to excite a complementary working mechanism together with the conversion-dominated Cu-based component to stabilize cycling with high capacity con-tribution. However, the electrochemical performances of pure CuInS2 as NIB anode are still limited (capacity <600 mAh g−1 at low rate; maximum rate ≤5 A g−1; capacity retention ≤73% during 1000 cycles) mainly due to its own limitations such as the relatively low electrochemical activity with approximate four-electron transfer reaction, relatively poor kinetics, and tetragonal structure with small unit cell volume (≈342 Å3). Furthermore, Liu and co-workers reported a hexagonal ZnIn2S4/carbon anode in NIB, which showed a high capacity of 560 mAh g−1 at 3.2 A g−1, but a poor capacity retention of 62% after 500 cycles.[22] Therefore, there remain challenges for realizing high capacity while delivering the ultrastable and superfast Na-ion storage. Further development of other novel indium-based bimetallic sulfides with the more appropriate structures for Na-ion storage is highly urgent.

Owing to the huge unit cell volume (≈1200 Å3), excellent electrical conductivity, high structure symmetry, and stable structure,[24–27] cubic spinel XIn2S4 (X = divalent metal ion) is the promising candidate. Generally, the cubic spinel XIn2S4 is derived from its counterpart of cubic spinel In2.77S4 based on an isomorphous substitution approach.[25–27] The iso-morphous substitution can realize the replacement of the indium atoms at tetrahedral sites with other metal atoms without changing the previous structure.[25–27] Apparently, this approach can develop an isostructural series of XIn2S4 and provide a powerful strategy for optimizing its physical and chemical properties via accommodating different metal atoms. Therefore, according to its huge performance poten-tials and high flexibility in the property modulation, the cubic spinel XIn2S4 is a viable option as NIB anodes. It is worth-while to conduct the in-depth investigation on cubic spinel XIn2S4 anode through replacing different metal atoms at its tetrahedral sites for optimizing Na-ion storage performances. Nevertheless, comprehensive research on cubic spinel XIn2S4 as NIB anode are yet to be reported.

Herein, an isomorphous series of cubic spinel XIn2S4 (X = Fe, Co, Mn) as a new type of NIB anodes was developed through a self-template route. The FeIn2S4 anode presented the most excellent performances among them with the high spe-cific capacity (608.3 mAh g−1 at 500 mA g−1), superior rate per-formance (305.8 mAh g−1 at 100 A g−1), and ultrastable cycling capability (80% capacity retention after 25  000 cycles). In par-ticular, the superior rate property enables a superfast charging feature of the cell, which can finish one charging within 9–13 s with a high capacity input of ≈300 mAh g−1. The comprehen-sive electrochemical performances transcend most of other reported advanced NIB anodes. The mechanisms accounting

for these excellent electrochemical performances are mainly due to the significant optimization effect on electrochemical performances induced by the X substitution at the tetrahedral sites of cubic spinel In2.77S4, which were unveiled through the substantial in/ex situ characterizations and density functional theory (DFT) calculations. It was revealed that Fe replacement at tetrahedral sites could most significantly boost the electro-chemical kinetics of In2.77S4 among all samples, and effectively relieve its mechanical strain. In addition, the pristine indium at octahedral sites can effectively cause a reversible Na-ion (de)intercalation process to excite a complementary working mech-anism together with the conversion-controlled Fe-based species, which is beneficial for the stable cycling without compromising its high reversible capacity. Furthermore, a high-performance full cell based on the FeIn2S4 anode and Na3V2(PO4)3/carbon cathode was developed to indicate its prospect for further application.

2. Results and Discussion

XIn2S4 (X = Fe, Co, Mn) was synthesized via a two-step hydro-thermal method. First, the In2.77S4 was obtained through a hydrothermal method. Second, the synthesized In2.77S4 powder was served as a self-sacrificing template precursor to derive XIn2S4. In the second step, thioacetamide, metal salt precur-sors (Fe2+, Co2+, Mn2+), and the collected In2.77S4 were mixed with ethanol to conduct a hydrothermal reaction. Wherein, the thioacetamide reacted with the X2+ radicals to form the X2+S. Subsequently, the reaction between X2+S and In2.77S4 leads to the formation of XIn2S4. Their formation mechanisms are illustrated in Figure 1a,b. The In2.77S4 displays the cubic spinel structure (Figure 1a), and the In atoms occupied the tetrahedral and octahedral sites.[25] When it reacted with the X2+S, the X2+ would take up the tetrahedral sites (Figure 1b), resulting in the formation of XIn2S4.[25] The refinements for X-ray diffraction (XRD) profiles of the as-prepared In2.77S4 and XIn2S4 (X = Fe, Co, Mn) were performed by employing the cubic spinel models including the In2.77S4 (JCPDS No. 01-088-2495; Figure  1c), FeIn2S4 (JCPDS No. 00-033-0613; Figure  1d), MnIn2S4 (JCPDS No. 01-079-1014; Figure 1e), and CoIn2S4(JCPDS No. 03-065-7271; Figure  1f). The refinement results show the reasonable fit with the low Chi2 values around 2,[28] confirming the pure In-based cubic spinel structure of the as-prepared samples without other impurities. Their unit cell volumes from the refinement results are around 1200 Å3 (Table S1, Supporting Information), and the large unit cell volumes are beneficial for Na-ion diffusion and storage applications.[29] Morphologies of the In2.77S4 and XIn2S4 were revealed through the scanning electron microscopy (SEM; Figure 1g–j) and transmission elec-tron microscopy (TEM; Figure  1k–n), displaying the plate-like morphology. The abundance of plate-like configuration with 2D feature is in favor of offering abundant electrochemical channels and active sites for rapid ionic activation and diffu-sion.[30,31] Interplanar distance of all samples at 0.32  nm cor-responding to the (311) plane was further confirmed via the high-resolution TEM (HRTEM) (insets in Figure  1k–n). As a comparison, the interplanar distance of XIn2S4 is slightly smaller than that of In2.77S4 owing to the larger ionic radius

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of In than those of Fe, Mn, and Co. Energy dispersive spec-troscopy (EDX; Figure S1, Supporting Information) results verify the uniform distribution of X, In, and S elements in the XIn2S4 with the X:In:S ratio at around 1:2:4. The nitrogen adsorption–desorption tests show that all samples exhibit the IV-type isotherms with mesoporous feature (Figure S2, Supporting Information). The In2.77S4, FeIn2S4, MnIn2S4, and CoIn2S4 show the specific surface areas of 34.2, 41.5, 70.1, and 53.6 m2 g−1, respectively.Figure 2a displays the cyclic voltammetry (CV) profiles of

the FeIn2S4 at 0.1  mV s−1. In the first cathodic scan, the peak at around 0.7  V is the initial sodiation process and the solid electrolyte interface (SEI) layer formation, and the peak at 0.5  V belongs to the succeeding sodiation. In the subsequent anodic scan, the anodic peaks at 0.8, 1.1, 1.3, and 1.5  V corre-spond to the stepwise desodiation reactions. After the first scan,

the curves exhibit the well reproducible profiles, implying the high reversibility and stable kinetics. Cycling performances of XIn2S4 and In2.77S4 at 500 mA g−1 are shown in Figure 2b. The FeIn2S4, MnIn2S4, CoIn2S4, and In2.77S4 exhibited the high ini-tial Coulombic efficiencies (ICEs) of 91.8%, 85.8%, 87.1%, and 83.9%, respectively. The high ICE is due to the excellent com-patibility and few side reactions at the interface between the electrode and DME-based electrolyte, resulting in the formation of thin SEI layer with low irreversible sodium loss. Their CEs increased to 99% after the first cycle, indicating the excellent electrochemical reversibility. After 70 cycles, FeIn2S4, MnIn2S4, CoIn2S4, and In2.77S4 showed the reversible capacities of 608.3, 572.4, 558.4, and 501.3 mAh g−1, respectively. Notably, the bime-tallic XIn2S4 possessed the higher capacity versus the mono-metallic In2.77S4, and the FeIn2S4 exhibited the highest capacity among them after cycling. This is mainly due to the superior

Figure 1. Structure information of In2.77S4 and XIn2S4 (X = Fe, Co, Mn): a,b) Scheme illustration for the formation mechanism. c–f) Rietveld refined XRD patterns. g–j) SEM images. k–n) TEM and HRTEM images.

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electrochemical kinetics of bimetallic sulfides,[11–13] and the best optimization effect of Fe substitution. Corresponding charge–discharge curves of XIn2S4 and In2.77S4 at 500 mA g−1 are shown in Figure 2c and Figure S3 in the Supporting Information, dis-playing the sloping feature, and thereby indicating the strong pseudocapacitive behavior of all samples.[6] This is in favor of endowing them with the fast kinetics and stable cycling.[6,32,33] Furthermore, the long-term cycling performances of FeIn2S4 and In2.77S4 at 500  mA g−1 are compared in Figure S4 in the Supporting Information. The pristine In2.77S4 showed a low reversible capacity of 344.2 mAh g−1 after 300 cycles with a poor

capacity retention of 54%. As comparison, the FeIn2S4 displayed a much improved capacity retention of 81% with a higher revers-ible capacity of 522.8 mAh g−1 after 500 cycles. Then, the rate performances of XIn2S4 and In2.77S4 are compared in Figure 2d. The XIn2S4 showed the better rate performance than the In2.77S4, and the FeIn2S4 exhibited the best rate capability among them with the highly reversible capacities of 669.1 mAh g−1 at 0.1 A g−1, 648.1 mAh g−1 at 0.2 A g−1, 638.4 mAh g−1 at 0.5 A g−1, 625.7 mAh g−1 at 1 A g−1, 619.5 mAh g−1 at 2 A g−1, 602.7 mAh g−1 at 5 A g−1, 582.4 mAh g−1 at 10 A g−1, 545.4 mAh g−1 at 20 A g−1, 473.0 mAh g−1 at 50 A g−1, and 305.8 mAh g−1 at 100 A g−1. It is

Figure 2. Electrochemical performances of In2.77S4 and XIn2S4 (X = Fe, Co, Mn): a) CV curves of FeIn2S4 at 0.1 mV s−1. b) Cycling performances of In2.77S4 and XIn2S4 at 500 mA g−1. c) Charge–discharge profiles of the FeIn2S4 at 500 mA g−1. d) Rate capabilities of In2.77S4 and XIn2S4. e) Charge–discharge curves of the FeIn2S4 at the various current densities. f) Comparison of the electrochemical performances among the FeIn2S4 and other reported supe-rior NIB anodes, where the SCL means the specific capacity at low rate, HR indicates the highest rate, ICE implies the initial Coulombic efficiency, and CLPC is the capacity lose per cycle for long-term cycling. g) Cycling performances of XIn2S4 at 20 A g−1 after the activation process. h) Long-term cycling evaluation of FeIn2S4 at 100 A g−1 after the activation process, and the inset displays the corresponding time–voltage curves.

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worthwhile to highlight one of the most outstanding findings is that the FeIn2S4 showed the remarkable highly reversible capacity around 300 mAh g−1 at the ultrahigh current density of 100 A g−1, which is unrivalled among any of present and past works for anode materials in NIBs. Subsequently, a capacity of 653.8 mAh g−1 could be reverted as the current density returned to 100  mA g−1, suggesting its outstanding rate capability and superior stability. As observed from the corresponding charge–discharge profiles (Figure  2e; Figure S5, Supporting Informa-tion), the increase in current density (from 0.1 to 20 A g−1) only causes a small variation in overpotential for XIn2S4, further demonstrating the superior electrochemical kinetics.[34] At the ultrahigh current densities of 50 and 100 A g−1, the FeIn2S4 exhibited the lowest overpotential among all samples, implying its best electrochemical kinetics at high rates. In view of the high capacity contribution of FeIn2S4 at 100 A g−1, its cycling stability at this stage was further carried out (Figure  2h). The FeIn2S4 manifested the highly reversible capacity of 222.3 mAh g−1 after 25  000 cycles with ultralow 0.000801% capacity loss per cycle. Additionally, it exhibited the fast charging ability which can finish one charging within 9–13 s with high capacity input of ≈300 mAh g−1 (Figures S6 and S7, Supporting Information).

Furthermore, the cycling performances of XIn2S4 at 20 A g−1 are compared in Figure 2g and Figure S8 in the Sup-porting Information. After 1500 cycles, FeIn2S4, MnIn2S4, and CoIn2S4 showed the reversible capacities of 455.2, 261.1, and 327.7 mAh g−1 with the capacity retentions of 80%, 72%, and 78%, respectively, indicating their remarkable performance sta-bilities. In summary, the FeIn2S4 as NIB anode exhibited the most excellent electrochemical performances among all sam-ples, demonstrating the most effective optimization effects of Fe substitution on the In-based structure. The comprehensive electrochemical performances of FeIn2S4 surpass most of other reported high-performance NIB anodes (Figure  2f; Figure S9 and Table S2, Supporting Information),[35–45] indicating its huge potential and prospects in the establishment of the supe-rior NIB anode with the superfast and ultrastable charging capabilities.

To reveal the role of X substitution at the tetrahedral site on Na-ion storage, DFT calculations and kinetics analyses were conducted. DFT calculations were first carried out to reveal and compare the X substitution effect (Figure 3). As shown in the density of states (DOS) curves (Figure  3a–d), the FeIn2S4 exhibited the strongest highly intensified energy states near

Figure 3. DFT calculations of In2.77S4 and XIn2S4 (X = Fe, Co, Mn): a–d) Density of states (DOS) of In2.77S4 and XIn2S4. e–h) Top views of the charge density difference for the binding between inserted Na ion and In2.77S4, or XIn2S4. i) Binding energies between the inserted Na ion and In2.77S4, or XIn2S4. j) Comparison for the Na-ion diffusion energy barrier of In2.77S4 and XIn2S4. k) Strain energy–volume deformation curves of NaIn2.77S4 and NaFeIn2S4. l) Comparison for the interior stress between NaxIn2.77S4 and NaxFeIn2S4 versus the different Na concentrations per unit cell.

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Fermi energy level among all samples, indicating the signifi-cantly enhanced electrical conductivity of the In-based spinel structure after Fe substitution. The high electrical conductivity is beneficial for improving the electrochemical reversibility and charge transfer kinetics. The binding reactions between Na ion and In2.77S4 or XIn2S4 were further simulated (Figure S10, Supporting Information). Obvious charge redistribution occurred after the binding reaction with strong electron deple-tion around the inserted Na region (Figure 3e–h). The FeIn2S4 showed the most obvious charge redistribution phenomenon, indicating the much improved Na-ion binding reaction kinetics of the In-based structure after Fe substitution (Figure 3i). This could benefit the energetically favorable Na-ion binding within the spinel structure instead of its self-clustering, in favor of improving the reaction kinetics. Next, the Na-ion diffusion processes within the In2.77S4 and XIn2S4 structures were simu-lated (Figure S11, Supporting Information). As a comparison, the lowest Na-ion migration energy barriers could be obtained after Fe substitution (Figure  3j), which is beneficial for the Na-ion diffusion kinetics, thus contributing to the excellent rate capability of FeIn2S4. To understand the role of Fe substitution for relieving the mechanical strain within the In-based spinel structure, the strain energies were calculated by simulating the different volume deformations for the sodiated In2.77S4 and FeIn2S4 (Figure 3k,l; Figure S12, Supporting Information), which are fitted by the quadratic function curves (details in Table S3 in the Supporting Information). As seen, the curve of NaFeIn2S4 displays a much smaller slope than that of the NaIn2.77S4 (Figure 3k), indicating that the generated stress level in the sodiated FeIn2S4 is much lower versus the counterpart of the sodiated In2.77S4. For instance, the Na0.5FeIn2S4 exhibits

the stress value of 1.01 GPa (Figure 3l; details in Table S3 in the Supporting Information), which is lower than the Na0.5In2.77S4 (1.23 GPa). With increasing the inserted Na atom number from 0.5 to 1, the increased interior stress for NaIn2.77S4 (4.98 GPa) is much higher than that for NaFeIn2S4 (2.36 GPa), demonstrating the effectively relieved mechanical strains within the In-based spinel structure after Fe substitution (Figure  3l; Table S3, Supporting Information). This can help to prompt the stable electrochemical cycling of FeIn2S4 owing to its remarkable strain-relieved feature.

The evaluation of charge storage behavior for XIn2S4 was investigated via CV characterization at different scan rates (Figure 4a; Figures S13 and S14, Supporting Information). The pseudocapacitive contributions of the FeIn2S4 at dif-ferent scan rates were quantified (Figure  4b).[46–49] It exhib-ited the large pseudocapacitive contribution of 87.3% even at the low scan rate of 0.2  mV s−1, suggesting its strong pseu-docapacitive behavior. As a comparison, the FeIn2S4 showed the highest pseudocapacitive contributions at the different scan rates among the XIn2S4 samples (Figure  4c). The high pseudocapacitive response is beneficial for the excellent elec-trochemical kinetics and reversibility, enabling high efficien-cies and superior stability at the fast reaction rates.[4,34,50,51] Moreover, the pseudocapacitive distribution of FeIn2S4 during the whole redox processes was evaluated in Figure  4d. The anodic peak at around 1.2  V is mainly contributed by diffu-sion reaction, and the others are mainly dominated by the pseudocapacitive processes at high rates. Then, the ex situ electrochemical impedance spectroscopy (EIS) analyses were carried out after the different cycles to investigate the kinetics evolution of XIn2S4 (Figure 4e; Figures S15 and S16,

Figure 4. Kinetics analyses of FeIn2S4: a) CV curves at various scan rates. b) Normalized ratio of pseudocapacitive and diffusion contributions at dif-ferent scan rates. c) Comparison of the pseudocapacitive contributions among the XIn2S4 samples. d) CV curve with pseudocapacitive contribution at 1 mV s−1. e) Ex situ EIS spectra after different cycles. f) In situ EIS spectra in the 6th cycle.

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Supporting Information). Results indicate the relatively high resistance of XIn2S4 after the first cycle with decreased resist-ances during the subsequent cycles, mainly due to the SEI layer formation at initial cycles and activation effect after the electrochemical stabilization process.[21] Ultralow charge transfer resistances were observed (≈5 Ω) within 1000 cycles for XIn2S4, demonstrating their outstanding electrochemical kinetics and ultrahigh stability during long-term electrochem-ical cycling. Figure 4f shows the in situ EIS spectra of FeIn2S4 after the electrochemical stabilization. Within one cycle, the ultralow and ultrastable charge transfer resistances were displayed at the different (de)sodiation states, indicating its superfast and robust reaction kinetics during the successive

structure evolution processes. The outstanding electrochem-ical kinetics are the key factors for the superior rate and cycling capabilities of FeIn2S4.

In situ XRD was conducted to observe the structural evolution of FeIn2S4 upon successive (de)sodiation processes during initial two cycles for illustrating its working mechanism (Figure 5a,b). The peak at around 38.9° belongs to the BeO ascribed to the in situ cell window. Peaks at around 23.7°, 27.9°, and 33.7° correspond to the diffraction peaks of FeIn2S4 (JCPDS No. 00-033-0613). With sodiation from OCV state to 0.7 V (state I), the peaks of FeIn2S4 shift to the lower Bragg 2φ angle, indicating expansion of unit cell due to Na-ion intercalation process and formation of Nax–FeIn2S4. During sodiation from 0.7 to 0.6  V (state II),

Figure 5. Working mechanism investigations for FeIn2S4: a) In situ XRD with a 2D contour plot during initial two cycles. b) Selected curves from the in situ XRD results. c1–f1) Ex situ SAED patterns and c2–f2) HRTEM images at the different (de)sodiation states within the first cycle.

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the signals of Nax–FeIn2S4 gradually disappear replaced by the appearance of Na2S (≈36.6° and ≈37.6°; JCPDS No. 00-047-0178) and Nay–In6S7 (≈24.0°, ≈35.4°, and ≈38.9°; JCPDS No. 01-080-0126),[21] corresponding to the subsequent conversion process. The conversion reaction contributes high specific capacity, while its induced severe volume change would lead to the capacity decay. Moreover, no characteristic peaks of Fe are observed at this stage mainly due to its small crystalline size or amorphous feature.[34,44,52,53] As sodiation from 0.6 to 0.01 V (state III), the peaks of Nay–In6S7 gradually shift to the lower 2φ angle and come back to the higher angle during the subsequent desodiation process (from 0.01 to 1.2 V; state IV), demonstrating the reversible Na-ion (de)intercalation process. It is reported that the Na-ion (de)intercalation processes usually display the relatively low volume variation, which could significantly sta-bilize the conversion-based components during cycling.[21,54,55]

Therefore, the Nay–In6S7 could be served as the “buffer” to sta-bilize the conversion-type Fe-based species to prompt the stable electrochemical cycling.[21] During desodiation process from 1.2 to 2.5  V (state V), the peaks of Na2S and Nay–In6S7 disappear instead by the reappearance of Nab–FeIn2S4 with the lower crys-tallinity,[34,56] affirming the reversible structure evolution. In the second cycle, identical reaction process was obtained, further confirming the highly reversible and stable reaction mecha-nism during cycling.

Owing to the higher energy intensity and resolution of electron from TEM than those of X-ray from the XRD, ex situ TEM was further carried out to investigate the reaction mecha-nism to confirm the above results. Figure  5c1–f1,c2–f2 shows the ex situ selected area electron diffraction (SAED) and HRTEM results, respectively. At the sodiation state of 0.9  V (Figure  5c1,c2), the information assigning to the sodiated

Figure 6. Electrochemical performances of the as-prepared full cell based on the FeIn2S4 anode and Na3V2(PO4)3/carbon cathode: a) CV curves and b) charge–discharge profiles of the corresponding half cells. c) Charge–discharge curves of the full cell at different current densities. d) Rate and e) cycling performances of the full cell (all electrochemical tests are based on the total mass of the cathode and anode). f–h) Optical images of the motor and LED in the model plane powered by one as-developed pouch cell.

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FeIn2S4 can be detected (JCPDS No. 00-033-0613). As sodia-tion to 0.01  V (Figure  5d1,d2), the characteristic signals of Fe (JCPDS No. 00-001-1252), Na2S (JCPDS No. 00-047-0178) and Nay–In6S7 (JCPDS No. 01-080-0126) are identified, indicating the conversion processes. As desodiation to 0.8 V, the SAED pattern (Figure  5e1) and HRTEM result (Figure  5e2) show the signals of Fe, Na2S, and Nay–In6S7. Besides, the calculated interlayer spacings of Nay–In6S7 get smaller, demonstrating the Na-ion extraction process. As desodiation to 2.5  V (Figure  5f1,f2), the diffraction rings and lattice fringes of Fe, Na2S, and Nay–In6S7 disappear replaced by those of the Nab–FeIn2S4, indicating the highly reversible conversion reaction. From these results, the FeIn2S4 exhibited the stepwise electrochemical process with the synergetic effects between conversion and Na-ion (de)interca-lation reactions, in favor of prompting its stable electrochemical cycling without sacrificing its high specific capacity. Overall, the detailed working processes of FeIn2S4 are summarized as follows:

i) Na-ion intercalation: FeIn2S4 + (x) Na+ + (x) e− → Nax–FeIn2S4ii) Conversion: 3Nax–FeIn2S4 + (10 +y − 3x) Na+ + (10 +y − 3x)

e− → 5Na2S + Nay–In6S7 + 3Feiii) Na-ion intercalation: Nay–In6S7  + (z) Na+  + (z) e−  →

Nay+z–In6S7iv) Na-ion extraction: Nay+z–In6S7  →Naa–In6S7  + (y  +z  −a)

Na+ + (y +z −a) e−

v) Conversion and Na-ion extraction: 5Na2S + Naa–In6S7  + 3Fe → 3Nab–FeIn2S4 + (10 +a − 3b) Na+ + (10 +a − 3b) e−

To further evaluate the application potential of FeIn2S4 as NIB anodes, the full cells were developed via employing the FeIn2S4 as anode and Na3V2(PO4)3/carbon as cathode (Figures S17 and S18, Supporting Information). The full cell exhibited charge/discharge profiles with a distinctive plateau at around 2.7/2.3  V (Figure 6c), corresponding to the charge/discharge platforms between anode and cathode (Figure  6a,b). Besides, there is a low overpotential when increasing the current den-sities (Figure  6c), indicating the excellent electrochemical kinetics. The full cell delivered the initial charge/discharge capacities of 96.2/86.6 mAh g−1 at 100 mA g−1 with a high ICE of 90% (Figure  6d; based on the total mass of cathode and anode). Moreover, it presented the superior rate performances with the discharge capacities of 86.2, 83.2, 80.3, 77.7, and 75.8 mAh g−1 at the current densities of 200, 400, 600, 800, and 1000 mA g−1, respectively (Figure 6d). Upon cycling, a high dis-charge capacity of 70.0 mAh g−1 at 1 A g−1 with capacity reten-tion of 92% and CE of 99% can be obtained after 100 cycles (Figure 6e; Figure S19, Supporting Information), demonstrating the excellent cycling stability. Then, the full cell exhibited the self-discharge rate about 0.00515 V h−1 within 68 hours (details in Figure S20 in the Supporting Information). Remarkably, the full cell showed the high energy densities of 212.7, 202.7, 194.2, 187.5, 180.9, and 176.3  Wh kg−1 at the power densities of 230, 460, 920, 1380, 1840, and 2300 W kg−1, respectively, according to the time–voltage profiles (Figure S21, Supporting Information). With the above performance, a pouch-type full cell based on the FeIn2S4 anode was developed (Figure 6f) to successfully power the light-emitting diode (1.8–2.2 V, 20 mA; Figure 6g) and the motor (>1.5  V, 30  mA; Figure  6h), verifying its great promise for further application.

3. Conclusions

In summary, an isomorphous substitution approach was employed to develop the cubic spinel XIn2S4 (X = Fe, Co, Mn) as the superior NIB anodes. It was unveiled that the Fe substi-tution at the tetrahedral sites of cubic spinel In2.77S4 can excite the optimum effect on Na-ion storage to deliver the amazing electrochemical performances. First, the superstrong pseudo-capacitive behavior, ultralow charge transfer resistance, high electrical conductivity, low Na-ion diffusion barrier, superior Na-ion binding kinetics, and remarkable strain-relieved feature of FeIn2S4 contribute to its superfast and ultrastable Na-ion storage with high capacities, which are the key factors for its outstanding electrochemical performances. In addition, the complementary reaction mechanism of FeIn2S4 can combine the advantages between Na-ion (de)intercalation and conver-sion processes, prompting the stable electrochemical cycling with high capacity. As a result, the FeIn2S4 as NIB anode deliv-ered the amazing electrochemical performances, and its com-prehensive performances are unrivalled among other reported NIB anode materials. Therefore, this work developed a series of new anode materials for NIBs with remarkable electro-chemical performances and provides the deep insights for the further investigation of other superior multi/bimetallic-based anodes for high-performance alkali-ion batteries.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsD.Y. and K.L. contributed equally to this work. This work is supported by Singapore Ministry of Education Academic Research Fund Tier 2 (MOE2019-T2-1-181). The authors gratefully acknowledged the valuable discussions and suggestions given by Prof. Guangzhao Wang and Prof. Dezhi Kong.

Conflict of InterestThe authors declare no conflict of interest.

Data Availability StatementResearch data are not shared.

Keywordsanodes, cubic spinel XIn2S4, Na-ion batteries, operando characterization

Received: July 14, 2021Revised: August 25, 2021

Published online:

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