Highly Crystalline Layered VS2 Nanosheets for All-Solid-State Lithium

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High Crystalline Layered VS2 Nanosheets for All-Solid-State Lithium Batteries with Enhanced Electrochemical Performances Liangting Cai, Qiang Zhang, Jean Pierre Mwizerwa, Hongli Wan, Xue-Lin Yang, Xiaoxiong Xu, and Xiayin Yao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18798 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Applied Materials & Interfaces

High Crystalline Layered VS2 Nanosheets for AllSolid-State Lithium Batteries with Enhanced Electrochemical Performances † ‡

Liangting Cai , , Qiang Zhang

‡,§

, Jean Pierre Mwizerwa

‡,§

, Hongli Wan

‡,§

*,†

, Xuelin Yang ,

Xiaoxiong Xu‡, Xiayin Yao*,‡ †

College of Materials and Chemical Engineering, China Three Gorges University, 8 Daxue Road,

Yichang, Hubei 443002, P. R. China ‡

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences,

Ningbo, Zhejiang 315201, P. R. China §

University of Chinese Academy of Sciences, Beijing 100049, P.R. China

KEYWORDS: layered VS2 nanosheets, high crystallinity, all-solid-state lithium batteries, electrochemical performances, electrochemical reaction kinetics

ACS Paragon Plus Environment

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ABSTRACT: All-solid-state lithium batteries employing inorganic solid electrolytes have been regarded as an ultimate solution to safety issues because of their features of no leakage as well as incombustibility, and are expected to achieve higher energy densities owing to their simplified structure. Two dimensional transition metal dichalcogenides exhibit great potential in energy storage devices due to their unique physical and chemical characteristics. In this work, 50-nmthick high crystalline layered VS2 (hc-VS2) nanosheets are prepared by a solvothermal method and

their

electrochemical

performances

are

evaluated

in

Li/75%Li2S-24%P2S5-

1%P2O5/Li10GeP2S12/hc-VS2 all-solid-state lithium batteries. At 50 mA g-1, the hc-VS2 nanosheets show a high reversible capacity of 532.2 mAh g-1 after 30 cycles. Moreover, stable discharge capacities maintain at 436.8 and 270.4 mAh g-1 at 100 and 500 mA g-1 after 100 cycles, respectively. The superior rate capability and cycling stability are ascribed to the better electronic conductivity and well-developed layered structure. In addition, the electrochemical reaction kinetics and capacity contributions were analyzed via cyclic voltammetry measurements at different scan rates. 1. Introduction Lithium-ion secondary batteries have been considered as clean and highly efficient energy storage devices owing to their merits of high energy density, long lifespan and low selfdischarge. In order to meet the ever-increasing demand for their broad applications, including portable electronics, electric vehicles and large-scale energy storage systems, lithium-ion rechargeable batteries have been extensively studied and developed in the past few decades.1 However, commercial organic liquid electrolytes based lithium-ion batteries generally suffer from combustion and explosion safety issues due to inner short-circuit or/and thermal runaway.2 In addition, traditional lithium transition-metal oxide and phosphate cathodes as well as graphite


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anode delivery low reversible capacities, which have encountered the bottlenecks of energy density for lithium-ion batteries. Therefore, the development of batteries with more reliable safety and higher energy densities has become much more critical.3 Recently, all-solid-state lithium batteries have captured much attention due to their excellent safety and reliability. Among various solid electrolytes, sulfide solid electrolytes, such as Li7P3S11 (17 mS cm-1)4 and Li10GeP2S12 (12 mS cm-1)5, exhibit high ionic conductivities, which have approached or exceeded the ionic conductivities of organic liquid electrolytes at room temperature. Besides, the wide thermodynamically stable electrochemical windows enable them to combine with 5 V class positive electrodes, and further to improve the energy densities of solid-state batteries.6 Moreover, the nonflammable inorganic solid electrolytes are employed as conductive layer to replace the organic liquid electrolytes and separators, which could completely eliminate the possible fire hazards and electrolytes leakage. The high mechanical strength of sulfide solid electrolytes could be easily achieved by a cold pressing process owing to their inherent softness.7 However, large interfacial resistances between lithium transition-metal oxide cathodes and sulfide solid electrolyte usually cause poor electrochemical performance in solid-state batteries resulting from space-charge layer8 and/or element diffusion9. For purpose of suppressing the formation of space-charge layer, an ion-conducting and electron-insulating oxide film were generally introduced into the interface between oxide cathodes and sulfide solid electrolytes.10-13 The rate performances of solid-state batteries could be greatly improved with the decrease of interfacial resistance. Furthermore, the element diffusions were observed, and generally leading to inferior electrochemical performances.14-16 Besides, the limited theoretical specific capacity of lithium transition-metal oxide still can’t satisfy the requirement of high energy density.


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Transition metal sulfide materials, such as NiS17, Co9S818 and FeS19, have been served as cathode in all-solid-state lithium batteries because of their superior interfacial compatibility and stability. Their similar compositions and chemical potentials with sulfide solid electrolytes can alleviate highly resistive space-charge layer. Besides, transition metal sulfide cathodes exhibit high reversible capacity and moderate discharge voltage plateau, indicating high energy density. Furthermore, the serious capacity fade caused by polysulfide shuttle effect in lithium ion batteries can be eliminated in solid-state batteries. Among various transition metal sulfide cathodes, two-dimensional transition metal dichalcogenides, such as MoS2,

20-22

WS223-25 and TiS2,26-27 have aroused increased interest

because of their superior physical and chemical properties, which have been widely applied in the field of electrocatalysis, chemical sensing, and field emission. In particular, transition metal dichalcogenides are considered as the promising candidates for energy storage due to their unique characteristics attributed to the two dimensional layered structure, excellent electronic conductivity and high specific surface areas.28 Vanadium disulfide (VS2) is a representative member of transition metal dichalcogenide family with a hexagonal structure, which consists of metal V sandwiched between two S layers as S-V-S. These three layers stack together by the weak Van der Waals force with an interlayer spacing of 5.76 Å, forming a two-dimensional layered structure. This large interlayer spacing enables fast intercalation/deintercalation of lithium ion without a serious structural distortion. Moreover, excellent electrical conductivity and metallic property facilitate charge transfer. High specific surface areas could provide abundant reaction sites as well as increase the utilization of active materials.29 These superiorities indicate that metallic layered VS2 is a promising cathode material for lithium ion batteries.


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However, to the best of our knowledge, there is no prior literature about VS2 cathode in all-solidstate lithium batteries. Herein, high crystalline VS2 (hc-VS2) nanosheets were successfully prepared by a facile solvothermal process. Similarly, the low crystalline VS2 (lc-VS2) nanosheets were synthesized via a one-step dexadecyl trimethyl ammonium bromide (CTAB)-assisted solvothermal method. Both hc-VS2 and lc-VS2 were further employed as cathodes in solid-state batteries. And hc-VS2 nanosheets exhibit better electrochemical performances than lc-VS2 nanosheets due to its welldeveloped layered structure. Li/75%Li2S-24%P2S5-1%P2O5/Li10GeP2S12/hc-VS2 solid-state batteries can deliver reversible capacities of 436.8 and 270.4 mAh g-1 at 100 and 500 mA g-1 after 100 cycles, respectively.

2. Experimental 2.1 Chemicals Sodium orthovanadate (Na3VO4, 99.9%), thioacetamide (TAA, CH3CSNH2, ≥ 99%), dexadecyl trimethyl ammonium bromide (CTAB, C19H42BrN, 99%), diethylene glycol (C4H10O3, ≥ 98%), and ethanol (C2H6O, ≥ 99.5%) were purchased from Aladdin. Deionized water was obtained from a Millipore system. 2.2 Preparation of VS2 nanosheets Metallic layered hc-VS2 nanosheets were prepared by a solvothermal method. Typically, 2.4 g sodium orthovanadate was completely dissolved into the mixed solution of 35 mL diethylene glycol and 45 mL deionized water by magnetic stirring. Afterwards, the corresponding amount of thioacetamide was added into the aforementioned solution and vigorously stirred for 2 hours.


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Subsequently, the obtained transparent solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 160 oC for 20 hours. After that, the obtained black precipitates were washed with deionized water and ethanol by centrifugation for several times. Finally, the hc-VS2 nanosheets were collected after a freeze drying process. The lc-VS2 nanosheets were prepared using the above-mentioned method with the assistance of 20 mol% CTAB. 2.3 Materials characterizations The structures of as-synthesized samples were characterized by X-ray diffraction (XRD, D8 Advance, Bruker) with Cu Kα radiation (λ = 1.54178 Å) at a voltage of 40 kV. The metal-sulfur vibration mode were obtained from microscopic Raman spectrometer (Raman, Renishaw in Viareflex, Renishaw) using 532 nm laser. The surface chemical compositions were tested by X-ray photoelectron spectrometer (XPS, AXIS ULTRADLD, Kratos) using Mg target. The morphologies and microstructures were observed with field emission scanning electron microscope (FESEM, S-4800, Hitachi) using an operating voltage of 8 kV with an energydispersive X-ray spectroscopy (EDS) detector and high resolution transmission electron microscope (HRTEM, JEOL2100, FEI). The Brunauer-Emmett-Teller (BET) specific surface areas were determined by the nitrogen adsorption apparatus (ASAP 2020M, Micromeritics). 2.4 Cells assembly and electrochemical performance tests All-solid-state lithium batteries employing hc-VS2 and lc-VS2 nanosheets as active materials were constructed to investigate their electrochemical performances. In general, VS2 nanosheets, Li10GeP2S12 solid electrolytes and Super P were mixed homogeneously by manual grinding in an optimal weight ratio of 45:50:5. Bilayer solid electrolytes consisting of Li10GeP2S12 (100 mg)


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and 75%Li2S-24%P2S5-1%P2O5 (50 mg) were pressed together in a polytetrafluoroethylene mold under 240 MPa successively. Then, above-mentioned composite cathodes were distributed on Li10GeP2S12 side uniformly under 240 MPa. Finally, metal lithium foil with diameter of 10 nm was pressed on the opposite side under 360 MPa. The structure of solid-state battery is schematically elucidated in Scheme 1a. At room temperature, the tetragonal phase Li10GeP2S12 and orthorhombic phase 75%Li2S-24%P2S5-1%P2O5 electrolytes possess ionic conductivities of 8.27×10-3 and 1.54×10-3 S cm-1, respectively. And the electrochemical windows for both solid electrolytes are 4.0 V. The detailed processing methods of Li10GeP2S1230 and 75%Li2S-24%P2S51%P2O531 solid electrolytes can be found elsewhere. The electrochemical behaviors of hc-VS2 and lc-VS2 nanosheets were examined by means of cyclic voltammetry (CV). The CV curves were recorded on the multi-channel potentiostat electrochemical workstation (Solartron 1470E) between 0.5 V and 3.0 V (vs. Li/Li+) under various scan rate. The charge/discharge and cyclic performances testing were carried out on a multichannel battery test system (LAND CT-2001A, Wuhan Rambo Testing Equipment CO., Ltd.) in voltage range of 0.5-3.0 V (vs. Li/Li+). The mechanism of capacity change were predicted via electrochemical impedance spectroscopy (EIS) measurements which was conducted between 106 HZ and 10 HZ with the amplitude of 15 mV. All test processes were carried out in dry Ar-filled glove box at ambient temperature.

3. Results and discussion Scheme 1. (a) Schematic diagram of the solid-state battery and (b) the preparation processes for hc-VS2 and lc-VS2 nanosheets.


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Scheme 1b demonstrates the formation of hc-VS2 and lc-VS2 nanosheets. Typically, a great amount of crystalline nuclei generate in the early stage of reaction. With increasing the reaction time, these nuclei aggregate into nanoparticles with different sizes. In order to decrease the surface free energies, the larger nanoparticles grow up further at the expense of the smaller nanoparticles based on Ostwald ripening process.32 Subsequently, numerous nanosheets gradually form and stack or self-assembly into microflowers. Finally, layered hc-VS2 nanosheets are obtained. For lc-VS2 nanosheets, cationic surfactant CTAB and inorganic precursor VO43form the CTA+-VO43- ion pairs during the initial stage of CTAB-assistant solvothermal process. Then, the CTA+-VO43- ion pairs form connection of CTAB and VS2 nanosheets.33 The adsorption of the surfactant CTAB on those nanosheets surfaces can eliminate part of the dangling bonds and reduce their surface free energies, and finally inhibit the further growth along the surface and develop into sheet-like morphology.34 Note that the cationic surfactant CTAB has significant impacts on the crystallinity and morphology of VS2 nanosheets, which were confirmed by the following XRD and SEM results.


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Figure 1. (a) XRD patterns, (b) Raman spectra and XPS spectra for (c) O 1s & V 2p and (d) S 2p with peak fitting analysis of hc-VS2 and lc-VS2 nanosheets. The compositions and structure of as-obtained VS2 were determined by XRD, Raman and XPS measurements. As shown in Figure 1a, these well-defined diffraction peaks of hc-VS2 in XRD pattern are in accordance with hexagonal phase of vanadium disulfide (JCPDS card NO. 891640) with the space group of P3m1. These main peaks at 2θ = 15.38 o, 31.05 o, 32.06 o, 35.74 o, 45.23 o and 57.15 o are correspondence to the diffraction from (001), (002), (100), (011), (012) and (110) planes, respectively. However, there are only two broaden diffraction peaks with low intensity for lc-VS2 at 35.74 o and 57.15 o, which are in accordance with the lattice plane of (011) and (110), indicating the low crystallinity of lc-VS2 in nature. In addition, the diffraction peak of (001) at 15.38 o is almost invisible, which demonstrates that the short periodicity along the c-


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direction and poor crystallinity of lc-VS2. No other diffraction peaks of impurity were observed in XRD patterns. In order to further confirm the structures of these two as-obtained VS2, the Raman spectra were conducted in the range of 100-500 cm-1. As shown in Figure 1b, these two characteristic band located at 141 and 193 cm-1 could be ascribed to the vibration dispersion of 1T-VS2.35 Another two peaks at 284 and 405 cm-1 are assigned to the in-plane (E1g) vibration mode of hexagonal VS2 nanosheets and out-of-plane (A1g) symmetric displacements of sulfur atoms along the c-axis vibration modes, respectively.36-37 The surface chemical compositions of hc-VS2 and lc-VS2 nanosheets were revealed by XPS measurements (Figure 1c,d). The binding peaks at 517 and 524 eV correspond to the spin-orbit splitting of V 2p3/2 and V 2p1/2, which demonstrates that there is no further oxidation of V4+ for these two samples.36, 38 The S 2p peaks of 161 and 164 eV are associated with the S 2p3/2 and S 2p1/2, indicating the presence of S2- in the final products.35, 39 As a result, hc-VS2 and lc-VS2 nanosheets with hexagonal phase have been successfully synthesized by a solvothermal process.


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Figure 2. (a)-(c) SEM images of hc-VS2 nanosheets under different magnification, (d) EDX spectrum and elemental mapping images of hc-VS2 nanosheets for V and S, (e) TEM and (f) HRTEM images of hc-VS2 nanosheets, the inset in (f) is the corresponding SEAD pattern.


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Figure 3. (a)-(c) SEM images of lc-VS2 nanosheets under different magnification, (d) EDX spectrum and elemental mapping images of lc-VS2 nanosheets for V and S, (e) TEM and (f) HRTEM images of lc-VS2 nanosheets, the inset in (f) is the corresponding SEAD pattern.


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The morphologies and microstructures of hc-VS2 and lc-VS2 were observed by FESEM and HRTEM, as shown in Figure 2 and Figure 3. The homogenous and monodispersed hc-VS2 microflowers are composed of numerous nanosheets with thickness around 50 nm. The diameter of hc-VS2 microflowers is estimated to be about 5 µm (Figure 2a-c). With the assistance of cationic surfactant CTAB, the lc-VS2 nanosheets with the thickness of 30 nm self-assembly into flower-like microstructure and agglomerate together (Figure 3a-c). Moreover, the EDX spectra and elemental mapping images of hc-VS2 and lc-VS2 microflowers indicated that the V and S distributed homogeneously in atomic ratio about 1:2 (Figure 2d,3d). The TEM images of hc-VS2 and lc-VS2 nanosheets are shown in Figure 2e,3e. Clearly, the hcVS2 nanosheets with the thickness of 50 nm stack together. In contrast, the lc-VS2 nanosheets are thinner than hc-VS2, which is well agree with the SEM observation. In order to further confirm the microstructure of hc-VS2 and lc-VS2, the HRTEM and SAED measurements were performed (Figure 2f,3f). The HRTEM image of hc-VS2 shows a clear interplanar spacing of 0.279 nm, corresponding to the (100) lattice plane. In addition, the SAED result suggests that hexagonal hcVS2 nanosheets possess good crystallinity. However, only a small domain shows the lattice fringes for lc-VS2, indicating that the lc-VS2 nanosheets are intrinsically low crystallinity with few small crystalline region. In addition, the SAED image of lc-VS2 displays diffraction rings with few diffraction spots, which are consistent with the XRD results. The nitrogen adsorption/desorption isotherms curves of hc-VS2 and lc-VS2 nanosheets are displayed in the Figure 4. The BET specific surface areas of hc-VS2 and lc-VS2 nanosheets are 11 and 36 m2 g-1, respectively. N2 adsorption-desorption isotherms of the hc-VS2 nanosheets show pseudo-type-IV curve with H3 hysteresis loop which originates from the layer stacked VS2 nanosheets.40 Moreover, N2 adsorption-desorption isotherms of the lc-VS2 nanosheets show


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pseudo-type-IV curve with H2 hysteresis loop. This behavior is related to the slit pores and mesopores which resulting from the interspaces between loosely stacked VS2 nanosheets. A capillary condensation step at high relative pressure demonstrate the presence of large pores. Moreover, a hysteresis loop at a higher pressure may reflect the interparticle structure between the VS2 microflowers.41 Although the large specific surface area of lc-VS2 benefits the favorable interfacial contacts between solid electrolytes and active materials, the good monodispersity of hc-VS2 make it possible to obtain a more uniform mixture. Meanwhile, the abundant void space between adjacent nanosheets could accommodate the volume changes and prevent the structural distortion.

Figure 4. Nitrogen adsorption-desorption isotherm curves of the hc-VS2 and lc-VS2 nanosheets.


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Figure 5. CV curves of the (a) hc-VS2 and (b) lc-VS2 nanosheets; galvanostatic charge-discharge profiles of (c) hc-VS2 and (d) lc-VS2 nanosheets at 50 mA g-1 (1st cycle, black lines, 2nd cycle, red lines, 3rd cycle, blue lines). To reveal the reaction mechanisms and electrochemical performances of as-prepared VS2 microflowers, solid-state batteries employing hc-VS2 and lc-VS2 nanosheets as cathode materials were assembled. Cyclic voltammograms of solid-state batteries for the first three cycles were carried out in the potential range of 0.5 - 3.0 V at a scan rate of 0.1 mV s-1. As shown in Figure 5a,b, during the initial cathodic sweep, two peaks at 2.22 V and 0.90 V could be ascribed to the complex phase transition in the Li+ intercalation process of layered VS2 according to VS2 + xLi+ + xe- → LixVS2.42-43 The peaks at 1.41 and 2.43 V during anodic scan can be assigned to the Li+ deintercalation and the formation of VS2. From the second cycle onwards, the smaller overpotential and well-overlapped CV curves indicated that the smaller electrode polarization


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and excellent reversibility of electrochemical reaction. The CV curves of lc-VS2 nanosheets are similar to hc-VS2, which suggests that both of them have identical electrochemical reaction process. Furthermore, lc-VS2 nanosheets show lower intensity than hc-VS2, indicating the lower reversible capacity. The galvanostatic discharge-charge profiles of Li/75%Li2S-24%P2S5-1%P2O5/Li10GeP2S12/VS2 all-solid-state lithium batteries for the first three cycles at 50 mA g-1 are shown in Figure 5c,d. The solid-state batteries employing hc-VS2 nanosheets exhibited initial discharge/charge capacities of 728.5 and 472.2 mAh g-1, respectively. The initial Coulombic efficiency is about 65.1%, which is mainly attributed to the poor contact between LixVS2 and solid electrolyte as well as volume change after the first Li-ion intercalation process. Actually, the initial Coulombic efficiency could be improved by coating sulfide electrolyte on the surface of active materials to form an intimate interfacial architecture.18,

44

The lc-VS2 nanosheets delivered a discharge

specific capacity of 648.7 mAh g-1 with a Coulombic efficiency of about 60.0%. The lower discharge capacity and Coulombic efficiency may result from the poor crystallinity and insufficient contact. Furthermore, the voltage plateaus are consistent with the potential of redox peaks in CV curves.


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Figure 6. (a) Cyclic performances and (b) charge-discharge profiles of solid-state batteries using hc-VS2 and lc-VS2 nanosheets cycled at 50 mA g-1 between 0.5 and 3.0 V (vs. Li/Li+). Chargedischarge curves of the batteries using (c) hc-VS2 and (d) lc-VS2 nanosheet electrode at the 20th cycle under different current densities. Figure 6a shows the cyclic performances of hc-VS2 and lc-VS2 nanosheets at 50 mA g-1 in the voltage range of 0.5~3.0 V at room temperature in solid-state batteries. Obviously, the hc-VS2 nanosheets exhibit reversible capacity of 532.2 mAh g-1 after 30 cycles at 50 mA g-1, which are higher than that of lc-VS2 nanosheets. Figure 6b shows the tenth, twentieth and thirtieth chargedischarge profiles for hc-VS2 and lc-VS2 nanosheets. As can be seen, the hc-VS2 nanosheets exhibit slight smaller polarization and higher reversible capacity than lc-VS2 nanosheets. After 30 cycles, the charge-discharge curves of hc-VS2 nanosheets overlap well, indicating a better reversible electrochemical reaction and cycle stability in solid-state batteries.


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The rate capability is further investigated and Figure 6c,d show the galvanostatic dischargecharge profiles of solid-state batteries at various current densities. With the increase of current densities from 50 mA g-1 to 500 mA g-1, the reversible capacities of hc-VS2 gradually decrease from 532.2 to 349.8 mAh g-1. By comparison, the lc-VS2 shows rapid capacity decay with increasing current densities. Clearly, the hc-VS2 displayed a higher reversible capacity than lcVS2 at the same current densities due to the well-developed layered structure.

Figure 7. (a) Cyclic performance of hc-VS2 nanosheets at 100 mA g-1 between 0.5 and 3.0 V (vs. Li/Li+) at 25 oC; (b) Nyquist plots of hc-VS2 nanosheets at 100 mA g-1 after 1st, 50th and 100th cycles. The inset is the equivalent circuit model. Table 1. Fitted results after 1st, 50th and 100th cycles of EIS After 1st cycle

After 50th cycle

After 100th cycle

Sample

Re (Ω)

Rct (Ω)

Re (Ω)

Rct (Ω)

Re (Ω)

Rct (Ω)

hc-VS2

113.9

45.3

146.1

126.5

313.3

263.7

Here, hc-VS2 nanosheet electrode is chosen for further evaluating the cycling stability at 100 mA g-1 and understanding the mechanism of capacity changes using EIS measurements. The solid-state batteries employing hc-VS2 maintained a high reversible specific capacity of 436.8


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mAh g-1 with a capacity retention of 71% at 100 mA g-1 after 100 cycles (Figure 7a). The capacity of hc-VS2 nanosheets increases slightly in the first 50 cycles because of an activation process.45 Figure 7b shows the Nyquist plots of hc-VS2 nanosheets after different cycles. The fitted equivalent circuit model are given in the inset of Figure. 7b. Nyquist plots mainly consist of a compressed semicircle in the middle frequency region, which corresponds to the charge transfer resistance (Rct) and solid electrolyte interface resistance, and a slop line in the low frequency region which denotes as Warburg resistance (Zw), related to the Li-ion diffusion into the bulk electrode. Rct mainly stem from the interfacial resistance between hc-VS2 and Li10GeP2S12 solid electrolytes in cathode layer.46 The Z’-intercept in the high frequency range can be ascribed to the ohmic resistance (Re), which stems from the resistance of electrode and solid electrolyte layers. CPE represents the non-ideal capacitance of the double layer.37,

47-49

According to the fitted results (Table 1), after 50 cycles, both ohmic resistance and charge transfer resistance change slightly. However, after 100 cycles, both of them further increase due to the volume and local stress/strain changes, causing the reversible capacity fade gradually.


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Figure 8. (a) Cycling stability of hc-VS2 and lc-VS2 nanosheets at 500 mA g-1; (b, c) SEM images of hc-VS2 nanosheets after 100 cycles. The high-rate cyclic performances of hc-VS2 and lc-VS2 nanosheets in solid-state batteries was further evaluated at 500 mA g-1, as shown in Figure 8a. After 100 cycles, the discharge capacity retained at 270.4 mAh g-1 after 100 cycles, demonstrating excellent cycling stability of solidstate batteries because of the well-developed layered structure of hc-VS2 nanosheets. To confirm the good structural stability, the morphology of the hc-VS2 nanosheets after cycling test were observed by SEM. As shown in Figure 8b,c, the hc-VS2 electrode still maintain the sheet-like structure as original morphology after 100 cycles at 500 mA g-1, indicating good structure stability of hc-VS2 nanosheets during charge-discharge process. By contrast, the lc-VS2


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nanosheets exhibited low discharge specific capacity of 145.6 mAh g-1 and poor cycle performances because the low crystalline and poor particle dispersion.

Figure 9. (a) CV curves of the solid-state batteries at different scan rates for the second cycle. (b) The fitted lines and log (peak current, i) versus log (scan rate, v) plots at different oxidation and reduction peaks. (c) The CV curve and capacity contribution (shaded area) at 1.0 mV s-1. (d) The capacity contribution at different scan rates (0.2, 0.4, 0.6, 0.8 and 1.0 mV s-1). These results correspond to the solid-state batteries using hc-VS2 nanosheets. To further evaluate the electrochemical reaction kinetics of hc-VS2 nanosheets cathode, CV curves were recorded between 0.5 and 3.0 V (vs. Li/Li+) and displayed in Figure 9a. As the scan rate increase from 0.2 to 1.0 mV s-1, the similar redox peaks gradually broaden with a small shift, indicating the fast electrochemical reaction kinetics and small polarization. The relationship between peak current (i) and scan rate (v) can be depicted as power law: i = avb. The b-value is


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calculated by the fitting slope of the log(v)-log(i) plot. The b-value of 0.5 represents a diffusioncontrolled behavior (intercalation process), while 1.0 corresponds to the surface-mediated behavior (capacitive process).50 The fitted b-values for four redox peaks are 0.42, 0.52, 0.51 and 0.50, respectively, suggesting that the electrochemical reaction kinetics are mainly diffusioncontrolled instead of surface-mediated (Figure 9b). The capacity contribution from diffusioncontrolled (k2v1/2) and surface-mediated (k1v) process could be determined by the relationship: i = k1v + k2v1/2. As shown in Figure 9c, the 75% of the total capacity originates from the capacitive process at a scan rate of 1.0 mV s-1. With the increase of scan rate, the contribution of capacitive process gradually increase (Figure 9d). Note that this increasing trend represents the contribution ratio rather than absolute amount, since the pseudocapacitance is an intrinsic characteristic.39, 51 4. Conclusions Metallic layered VS2 nanosheets were synthesized by a solvothermal method and further employed as active materials in all-solid-state lithium batteries. The hc-VS2 nanosheets delivered a high reversible capacity of 532.2 mAh g-1 at 50 mA g-1 after 30 cycles. After 100 cycles, the solid-state batteries exhibit stable capacity of 436.8 and 270.4 mAh g-1 at 100 mA g-1 and 500 mA g-1, respectively. The superior rate and cyclic performances could be ascribed to the better electronic conductivity and well-developed layered structure. The analysis of electrochemical reaction kinetics revealed that the Li+ storage is mainly based on an intercalation process. Furthermore, the capacity contribution from capacitive process are estimated to be 75% at scan rate of 1.0 mV s-1. Corresponding Author *

Corresponding authors: [email protected] (X. L. Yang) and [email protected] (X.Y. Yao)


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was supported by funding from the Strategic Priority Program of the Chinese Academy of Sciences (Grant No. XDA09010203), National Key Research and Development Program of China (Grant No. 2016YFB0100105), National Natural Science Foundation of China (Grant No. 51772169) and Zhejiang Provincial Natural Science Foundation of China (Grant No. LD18E020004, LY18E020018, LY18E030011) and Youth Innovation Promotion Association CAS (2017342). REFERENCES 1.

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Highly Crystalline Layered VS2 Nanosheets for All-Solid-State Lithium

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