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

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 10053−10063

Highly Crystalline Layered VS2 Nanosheets for All-Solid-State Lithium Batteries with Enhanced Electrochemical Performances Liangting Cai,†,‡ Qiang Zhang,‡,§ Jean Pierre Mwizerwa,‡,§ Hongli Wan,‡,§ Xuelin Yang,*,† Xiaoxiong Xu,‡ and 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 ‡

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 they are expected to achieve higher energy densities owing to their simplified structure. Two-dimensional transition-metal dichalcogenides exhibit a great potential in energy storage devices because of their unique physical and chemical characteristics. In this work, 50 nm thick highly 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, hc-VS2 nanosheets show a high reversible capacity of 532.2 mAh g−1 after 30 cycles. Moreover, stable discharge capacities are maintained 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. KEYWORDS: layered VS2 nanosheets, high crystallinity, all-solid-state lithium batteries, electrochemical performances, electrochemical reaction kinetics

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 self-discharge. 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-electrolytebased lithium-ion batteries generally suffer from combustion and explosion safety issues because of inner short circuit or/and thermal runaway.2 In addition, traditional lithium transitionmetal oxide and phosphate cathodes as well as graphite anodes deliver low reversible capacities, which have encountered the bottleneck 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 garnered much attention because of 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 approach or exceed the ionic conductivities of organic liquid electrolytes at room © 2018 American Chemical Society

temperature. In addition, the wide thermodynamically stable electrochemical windows enable them to combine with 5 V class positive electrodes and further to improve the energy densities of all-solid-state lithium batteries.6 Moreover, nonflammable inorganic solid electrolytes are employed as a conductive layer to replace organic liquid electrolytes and separators, which could completely eliminate the possible fire hazards and leakage of electrolytes. 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 electrolytes usually cause poor electrochemical performance in all-solidstate lithium batteries resulting from a space-charge layer8 and/ or element diffusion.9 For suppressing the formation of a spacecharge layer, an ion-conducting and electron-insulating oxide film was generally introduced into the interface between oxide cathodes and sulfide solid electrolytes.10−13 The rate performances of all-solid-state lithium batteries could be greatly Received: December 10, 2017 Accepted: March 2, 2018 Published: March 2, 2018 10053

DOI: 10.1021/acsami.7b18798 ACS Appl. Mater. Interfaces 2018, 10, 10053−10063

Research Article

ACS Applied Materials & Interfaces

Scheme 1. (a) Schematic Diagram of the All-Solid-State Lithium Battery and (b) Preparation Processes of hc-VS2 and lc-VS2 Nanosheets

employed as cathodes in all-solid-state lithium batteries. hc-VS2 nanosheets exhibit better electrochemical performances than those of lc-VS2 nanosheets because of their well-developed layered structure. Li/75% Li 2 S-24% P 2 S 5 -1% P 2 O 5 / Li10GeP2S12/hc-VS2 all-solid-state lithium 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.

improved with the decrease in interfacial resistance. Furthermore, element diffusions were observed, generally leading to inferior electrochemical performances.14−16 In addition, the limited theoretical specific capacity of lithium transition-metal oxides still cannot satisfy the requirement of high energy density. Transition-metal sulfide materials, such as NiS,17 Co9S8,18 and FeS,19 have been used as cathodes in all-solid-state lithium batteries because of their superior interfacial compatibility and stability. Their similar compositions and chemical potentials to sulfide solid electrolytes can alleviate a highly resistive spacecharge layer. Moreover, 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 WS2,23−25 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, transitionmetal dichalcogenides are considered as promising candidates for energy storage because of 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 the 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 ions without serious structural distortion. Moreover, the 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. However, to the best of our knowledge, there is no prior literature about VS2 cathode in all-solid-state lithium batteries. Herein, highly crystalline VS2 (hc-VS2) nanosheets were successfully prepared by a facile solvothermal process. Similarly, low crystalline VS2 (lc-VS2) nanosheets were synthesized via a one-step cetyltrimethyl ammonium bromide (CTAB)-assisted solvothermal method. Both hc-VS2 and lc-VS2 were further

2. EXPERIMENTAL SECTION 2.1. Chemicals. Sodium orthovanadate (Na3VO4, 99.9%), thioacetamide (TAA, CH3CSNH2, ≥99%), cetyltrimethyl 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 of sodium orthovanadate was completely dissolved in a mixed solution of 35 mL of diethylene glycol and 45 mL of deionized water by magnetic stirring. Afterward, the corresponding amount of thioacetamide was added to the aforementioned solution and vigorously stirred for 2 h. Subsequently, the obtained transparent solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave and maintained at 160 °C for 20 h. After that, the obtained black precipitates were washed with deionized water and ethanol by centrifugation several times. Finally, hc-VS2 nanosheets were collected after a freeze-drying process. lc-VS2 nanosheets were prepared using the above-mentioned method with the assistance of 20 mol % CTAB. 2.3. Characterization of Materials. The structures of the assynthesized 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 was obtained from a microscopic Raman spectrometer (Raman, Renishaw inVia-reflex; Renishaw) using a 532 nm laser. The surface chemical compositions were tested by an X-ray photoelectron spectrometer (XPS, Axis UltraDLD; Kratos) using a Mg target. The morphologies and microstructures were observed with a field emission scanning electron microscope (FESEM, S-4800; Hitachi) using an operating voltage of 8 kV with an energy-dispersive X-ray spectroscopy (EDX) detector and a high-resolution transmission electron microscope (HRTEM, JEOL2100; FEI). The Brunauer−Emmett−Teller (BET) specific surface areas were determined by a nitrogen adsorption apparatus (ASAP 2020M; Micromeritics). 2.4. Cell Assembly and Electrochemical Performance Tests. All-solid-state lithium batteries employing hc-VS2 and lc-VS2 nanosheets as active materials were developed 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) and 75% Li2S-24% P2S5-1% P2O5 (50 mg) were pressed together in a poly(tetrafluoroethylene) mold under 240 MPa successively. Then, the above-mentioned composite 10054


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Figure 1. (a) XRD patterns, (b) Raman spectra, and XPS spectra for (c) O 1s and V 2p and (d) S 2p with peak fitting analysis of hc-VS2 and lc-VS2 nanosheets.

solvothermal process. Then, CTA+-VO43− ion pairs form a connection of CTAB and VS2 nanosheets.33 The adsorption of the surfactant CTAB on those nanosheet surfaces can eliminate part of the dangling bonds and reduce their surface free energies and finally inhibit further growth along the surface and develop into a sheetlike morphology.34 Note that the cationic surfactant CTAB has a significant impact on the crystallinity and morphology of the VS2 nanosheets, which were confirmed by the following XRD and SEM results. The compositions and structures of the as-obtained VS2 were determined by XRD, Raman, and XPS measurements. As shown in Figure 1a, these well-defined diffraction peaks of hcVS2 in the XRD pattern are in accordance with the hexagonal phase of vanadium disulfide (JCPDS card No. 89-1640) with the space group of P3̅m1. These main peaks at 2θ = 15.38, 31.05, 32.06, 35.74, 45.23, and 57.15° correspond to the diffractions from (001), (002), (100), (011), (012), and (110) planes, respectively. However, there are only two broadened diffraction peaks with low intensities for lc-VS2 at 35.74 and 57.15°, which are in accordance with the lattice planes of (011) and (110), respectively, indicating the low-crystallinity nature of lc-VS2. In addition, the diffraction peak from (001) at 15.38° is almost invisible, which demonstrates the short periodicity along the c-direction and the poor crystallinity of lc-VS2. No other diffraction peaks of impurity were observed in the XRD patterns. To further confirm the structures of these two as-obtained VS2, the Raman spectra were recorded in the range of 100−500 cm−1. As shown in Figure 1b, these two characteristic bands located at 141 and 193 cm−1 could be ascribed to the vibration dispersion of 1T-VS2.35 The other 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

cathodes were distributed on the Li10GeP2S12 side uniformly under 240 MPa. Finally, metal lithium foil with a diameter of 10 nm was pressed on the opposite side under 360 MPa. The structure of the allsolid-state lithium battery is schematically elucidated in Scheme 1a. At room temperature, the tetragonal phase, Li10GeP2S12, and the 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. The electrochemical windows for both solid electrolytes are 4.0 V. The detailed processing methods of Li10GeP2S1230 and 75% Li2S-24% P2S5-1% 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 multichannel potentiostat electrochemical workstation (Solartron 1470E) between 0.5 and 3.0 V (vs Li/Li+) at various scan rates. The testing of charge/discharge and cyclic performances was carried out on a multichannel battery test system (LAND CT-2001A; Wuhan Rambo Testing Equipment Co., Ltd.) in the voltage range of 0.5−3.0 V (vs Li/Li+). The mechanism of capacity change was predicted via electrochemical impedance spectroscopy (EIS) measurements, which were conducted between 106 and 10 Hz with the amplitude of 15 mV. All test processes were carried out in a dry Ar-filled glovebox at ambient temperature.

3. RESULTS AND DISCUSSION Scheme 1b demonstrates the formation of hc-VS2 and lc-VS2 nanosheets. Typically, a great amount of crystalline nuclei generates in the early stage of reaction. With increasing reaction time, these nuclei aggregate into nanoparticles with different sizes. To decrease the surface free energies, the larger nanoparticles grow further at the expense of the smaller nanoparticles on the basis of Ostwald ripening process.32 Subsequently, numerous nanosheets gradually form and stack or self-assemble into microflowers. Finally, layered hc-VS2 nanosheets are obtained. For lc-VS2 nanosheets, cationic surfactant CTAB and inorganic precursor VO43− form CTA+VO43− ion pairs during the initial stage of the CTAB-assisted 10055


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Figure 2. (a−c) SEM images of hc-VS2 nanosheets under different magnifications, (d) EDX spectra and elemental mapping images of hc-VS2 nanosheets for V and S, and (e) TEM and (f) HRTEM images of hc-VS2 nanosheets. The inset in (f) is the corresponding Selected area electron diffraction (SAED) pattern.

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, respectively, which demonstrates that there is no further oxidation of V4+ for these two samples.36,38 The S 2p peaks at 161 and 164 eV are associated with 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 a hexagonal phase have been successfully synthesized by a solvothermal process.

The morphologies and microstructures of hc-VS2 and lc-VS2 were observed by FESEM and HRTEM, as shown in Figures 2 and 3. The homogeneous and monodispersed hc-VS2 microflowers are composed of numerous nanosheets with a thickness of about 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, lc-VS2 nanosheets with the thickness of 30 nm self-assemble into flowerlike microstructure and agglomerate (Figure 3a−c). Moreover, the EDX spectra and elemental mapping images of hc-VS2 and lc-VS2 microflowers indicated that V and S distributed homogeneously in an atomic ratio of about 1:2 (Figures 2d and 3d). 10056


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

SAED image of lc-VS2 displays diffraction rings with few diffraction spots, which is consistent with the XRD results. The nitrogen adsorption−desorption isotherm curves of hcVS2 and lc-VS2 nanosheets are displayed in 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 hc-VS2 nanosheets show a pseudotype-IV curve with an H3 hysteresis loop that originates from the layer stacked VS2 nanosheets.40 Moreover, N2 adsorption−desorption isotherms of lc-VS2 nanosheets show a pseudotype-IV curve with an H2 hysteresis loop. This behavior is related to the slit pores and mesopores, which result from the interspaces among loosely stacked VS2 nanosheets. A capillary condensa-

The TEM images of hc-VS2 and lc-VS2 nanosheets are shown in Figures 2e and 3e. Clearly, hc-VS2 nanosheets with the thickness of 50 nm stack together. In contrast, lc-VS2 nanosheets are thinner than hc-VS2, which agrees well with the SEM observation. To further confirm the microstructures of hc-VS2 and lc-VS2, HRTEM and SAED measurements were performed (Figures 2f and 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 hc-VS2 nanosheets possess good crystallinity. However, only a small domain shows the lattice fringes for lcVS2, indicating that lc-VS2 nanosheets have intrinsically low crystallinity with few small crystalline regions. In addition, the 10057


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sweep, two peaks at 2.22 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 onward, the smaller overpotential and welloverlapped CV curves indicate the smaller electrode polarization and excellent reversibility of electrochemical reaction. The CV curves of lc-VS2 nanosheets are similar to those of hcVS2, which suggests that both have an identical electrochemical reaction process. Furthermore, lc-VS2 nanosheets show a lower intensity than that of hc-VS2, indicating the lower reversible capacity. The galvanostatic discharge−charge profiles of Li/75% Li2S24% 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 all-solid-state lithium batteries employing hcVS2 nanosheets exhibited initial discharge and 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 the solid electrolyte as well as the 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 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. 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 all-solid-state lithium batteries. Clearly,

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

tion step at high relative pressure demonstrates the presence of large pores. Moreover, a hysteresis loop at a higher pressure may reflect the interparticle structure among 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 makes it possible to obtain a more uniform mixture. Moreover, the abundant void space among adjacent nanosheets could accommodate the volume changes and prevent structural distortion. To reveal the reaction mechanisms and electrochemical performances of as-prepared VS2 microflowers, all-solid-state lithium batteries employing hc-VS2 and lc-VS2 nanosheets as cathode materials were assembled. Cyclic voltammograms of all-solid-state lithium 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

Figure 5. CV curves of (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 (first cycle, black lines; second cycle, red lines; third cycle, blue lines). 10058


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Figure 6. (a) Cyclic performances and (b) charge−discharge profiles of all-solid-state lithium batteries using hc-VS2 and lc-VS2 nanosheets cycled at 50 mA g−1 between 0.5 and 3.0 V (vs Li/Li+). Charge−discharge curves of the batteries using (c) hc-VS2 and (d) lc-VS2 nanosheet electrodes at the 20th cycle at different current densities.

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 °C. (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.

Here, hc-VS2 nanosheet electrode is chosen for evaluating further the cycling stability at 100 mA g−1 and understanding the mechanism of capacity changes using EIS measurements. The all-solid-state lithium batteries employing hc-VS2 maintained a high reversible specific capacity of 436.8 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 is 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 sloped line in the lowfrequency region denoted Warburg resistance (Zw), related to Li-ion diffusion into the bulk electrode. Rct mainly stems from the interfacial resistance between hc-VS2 and Li10GeP2S12 solid electrolytes in the cathode layer.46 The Z′-intercept in the highfrequency range can be ascribed to the Ohmic resistance (Re), which stems from the resistance of the electrode and solid

hc-VS2 nanosheets exhibit a reversible capacity of 532.2 mAh g−1 after 30 cycles at 50 mA g−1, which is higher than that of lcVS2 nanosheets. Figure 6b shows the 10th, 20th, and 30th charge−discharge profiles for hc-VS2 and lc-VS2 nanosheets. As can be seen, hc-VS2 nanosheets exhibit a slightly smaller polarization and a higher reversible capacity than those of lcVS2 nanosheets. After 30 cycles, the charge−discharge curves of hc-VS2 nanosheets overlap well, indicating a better reversible electrochemical reaction and cycle stability in all-solid-state lithium batteries. The rate capability is further investigated, and Figure 6c,d shows the galvanostatic discharge−charge profiles of all-solidstate lithium batteries at various current densities. With the increase of current densities from 50 to 500 mA g−1, the reversible capacities of hc-VS2 gradually decrease from 532.2 to 349.8 mAh g−1. By comparison, lc-VS2 shows a rapid capacity decay with the increasing current densities. Clearly, hc-VS2 displayed a higher reversible capacity than that of lc-VS2 at the same current densities because of the well-developed layered structure. 10059


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gradually broaden with a small shift, indicating the fast electrochemical reaction kinetics and small polarization. The relationship between the peak current (i) and the scan rate (v) can be depicted as a power law: i = avb. The b-value is calculated by the fitting slope of the log(v)−log(i) plot. The bvalue of 0.5 represents a diffusion-controlled behavior (intercalation process) and 1.0 corresponds to a surfacemediated behavior (capacitive process).50 The fitted b-values for four redox peaks are 0.42, 0.52, 0.51, and 0.50, suggesting that the electrochemical reaction kinetics is mainly diffusioncontrolled instead of surface-mediated (Figure 9b). The capacity contribution from diffusion-controlled (k2v1/2) and surface-mediated (k1v) processes could be determined by the following relationship: i = k1v + k2v1/2. As shown in Figure 9c, 75% of the total capacity originates from the capacitive process at a scan rate of 1.0 mV s−1. With the increase in scan rate, the contribution of the capacitive process gradually increases (Figure 9d). Note that this increasing trend represents the contribution ratio rather than absolute amount because pseudocapacitance is an intrinsic characteristic.39,51

electrolyte layers. CPE represents the nonideal capacitance of the double layer.37,47−49 According to the fitted results (Table 1), after 50 cycles, both Ohmic resistance and charge transfer 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

resistance change slightly. However, after 100 cycles, both of them increase further because of the volume and local stress/ strain changes, causing the reversible capacity to fade gradually. The high-rate cyclic performances of hc-VS2 and lc-VS2 nanosheets in all-solid-state lithium batteries were evaluated further 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 demonstrated an excellent cycling stability of all-solidstate lithium batteries because of the well-developed layered structure of hc-VS2 nanosheets. To confirm the good structural stability, the morphology of hc-VS2 nanosheets after the cycling test was observed by SEM. As shown in Figure 8b,c, the hc-VS2 electrode still maintains the sheetlike structure as original morphology after 100 cycles at 500 mA g−1, indicating a good structure stability of hc-VS2 nanosheets during the charge− discharge process. In contrast, lc-VS2 nanosheets exhibited low discharge specific capacity of 145.6 mAh g−1 and poor cycle performances because of the low crystallinity and poor particle dispersion. To evaluate further the electrochemical reaction kinetics of hc-VS2 nanosheet cathodes, CV curves were recorded between 0.5 and 3.0 V (vs Li/Li+) and are displayed in Figure 9a. As the scan rate increases from 0.2 to 1.0 mV s−1, similar redox peaks

4. CONCLUSIONS Metallic layered VS2 nanosheets were synthesized by a solvothermal method and were further employed as active materials in all-solid-state lithium batteries. 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 all-solid-state lithium batteries exhibit stable capacities of 436.8 and 270.4 mAh g−1 at 100 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 Li+ storage is mainly based on an intercalation process. Furthermore, the

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. 10060


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Figure 9. (a) CV curves of the all-solid-state lithium batteries at different scan rates for the second cycle. (b) Fitted lines and log (peak current, i) versus log (scan rate, v) plots at different oxidation and reduction peaks. (c) CV curve and capacity contribution (shaded area) at 1.0 mV s−1. (d) 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 all-solid-state lithium batteries using hc-VS2 nanosheets. (3) Wu, F. X.; Yushin, G. Conversion Cathodes for Rechargeable Lithium and Lithium-ion Batteries. Energy Environ. Sci. 2017, 10, 435− 459. (4) Seino, Y.; Ota, T.; Takada, K.; Hayashi, A.; Tatsumisago, M. A Sulphide Lithium Super Ion Conductor Is Superior to Liquid Ion Conductors for Use in Rechargeable Batteries. Energy Environ. Sci. 2014, 7, 627−631. (5) Kamaya, N.; Homma, K.; Yamakawa, Y.; Hirayama, M.; Kanno, R.; Yonemura, M.; Kamiyama, T.; Kato, Y.; Hama, S.; Kawamoto, K.; Mitsui, A. A Lithium Superionic Conductor. Nat. Mater. 2011, 10, 682−686. (6) Yao, X.; Huang, B.; Yin, J.; Peng, G.; Huang, Z.; Gao, C.; Liu, D.; Xu, X. All-Solid-State Lithium Batteries with Inorganic Solid Electrolytes: Review of Fundamental Science. Chin. Phys. B 2016, 25, No. 018802. (7) Hayashi, A.; Noi, K.; Sakuda, A.; Tatsumisago, M. Superionic Glass-Ceramic Electrolytes for Room-Temperature Rechargeable Sodium Batteries. Nat. Commun. 2012, 3, No. 856. (8) Haruyama, J.; Sodeyama, K.; Han, L.; Takada, K.; Tateyama, Y. Space−Charge Layer Effect at Interface between Oxide Cathode and Sulfide Electrolyte in All-Solid-State Lithium-Ion Battery. Chem. Mater. 2014, 26, 4248−4255. (9) Sakuda, A.; Hayashi, A.; Tatsumisago, M. Interfacial Observation between LiCoO2 Electrode and Li2S−P2S5 Solid Electrolytes of AllSolid-State Lithium Secondary Batteries Using Transmission Electron Microscopy. Chem. Mater. 2010, 22, 949−956. (10) Ohta, N.; Takada, K.; Sakaguchi, I.; Zhang, L.; Ma, R.; Fukuda, K.; Osada, M.; Sasaki, T. LiNbO3-Coated LiCoO2 as Cathode Material for All Solid-State Lithium Secondary Batteries. Electrochem. Commun. 2007, 9, 1486−1490. (11) Uemura, T.; Goto, K.; Ogawa, M.; Harada, K. All-Solid Secondary Batteries with Sulfide-Based Thin Film Electrolytes. J. Power Sources 2013, 240, 510−514. (12) Ohta, N.; Takada, K.; Zhang, L.; Ma, R.; Osada, M.; Sasaki, T. Enhancement of the High-Rate Capability of Solid-State Lithium

capacity contribution from the capacitive process is estimated to be 75% at a scan rate of 1.0 mV s−1.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.L.Y.). *E-mail: [email protected] (X.Y.Y.). ORCID

Xuelin Yang: 0000-0001-5626-701X Xiaoxiong Xu: 0000-0002-8599-4918 Xiayin Yao: 0000-0002-2224-4247 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

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), Zhejiang Provincial Natural Science Foundation of China (Grant Nos. LD18E020004, LY18E020018, and LY18E030011), and Youth Innovation Promotion Association CAS (2017342).

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

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