Accepted Manuscript Synthesis and electrochemical performance of WS2 nanosheet for thermal batteries Shengnan Guo, Hao Guo, Xueying Wang, Yongping Zhu, Ming Yang, Qing Zhang, Ying Chu, Jianyong Wang PII: DOI: Reference:
S0167-577X(19)30580-4 https://doi.org/10.1016/j.matlet.2019.04.030 MLBLUE 26012
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Received Date: Revised Date: Accepted Date:
17 March 2019 7 April 2019 8 April 2019
Please cite this article as: S. Guo, H. Guo, X. Wang, Y. Zhu, M. Yang, Q. Zhang, Y. Chu, J. Wang, Synthesis and electrochemical performance of WS2 nanosheet for thermal batteries, Materials Letters (2019), doi: https://doi.org/ 10.1016/j.matlet.2019.04.030
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Synthesis and electrochemical performance of WS2 nanosheet for thermal batteries Shengnan Guoa,b, Hao Guoc, Xueying Wang b,*, Yongping Zhub,*, Ming Yanga,b, Qing Zhanga,b, Ying Chub, Jianyong Wangc a
University of Chinese Academy of Sciences, Beijing 100039, China
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
State Key Laboratory of Advanced Chemical Power Sources, Guizhou Meiling Power Sources Co. Ltd., Zunyi, Guizhou 563003, China
AUTHOR INFORMATION Corresponding author E-mail: Xueying Wang*, Email: [email protected]
Yongping Zhu*, Email: [email protected]
Abstract Cathode materials that have better thermal stability, larger specific capacity and higher conductivity can efficiently improve the electrochemical performance of the batteries. In this work, we synthesized tungsten disulfide (WS2) nanosheet and investigated its applicability as cathode material for thermal battery. The WS2 is thermally stable up to 1200 °C. The open-circuit voltage is 1.43 V versus Li-B anode. At a current density of 150 mA cm-2, the Li-B/WS2 single cell exhibits a specific capacity of 334.7 mA h g-1 with a cut-off voltage of 1.0 V. The end discharge products are proved to be W and Li2S. This work provides a beginning for further applying WS2 as a promising cathode material for long-life thermal batteries. Key words: Tungsten disulfide; Thermal battery; High-capacity cathode; Thermal properties; Nanocrystalline materials. 1. Introduction Thermal batteries are a type of primary reserve battery utilizing inorganic salts as electrolyte. The electrolyte is non-conductive at ambient temperature and has a high specific conductance when melted. On the basis of this uniqueness, thermal batteries mainly used for military applications due to their high power density, long shelf life and good reliability. The most commonly utilized cathode material is pyrite (FeS2) for its advantages of abundant nature resources and stable discharge performance(1). However, the relatively low decomposition temperature and high impedance of the discharge intermediate limit its application to short-life thermal batteries. Cobalt disulfide (CoS2) is a good replacement for FeS2 in long-life thermal batteries for its lower electronic resistivity and higher thermal stability. The drawback is that cobalt is too expensive to be used largely (2). To solve the intrinsic problems of these cathode materials, several strategies have been adopted: (a) reducing the particle size(3, 4) or cell thickness(5) to take fully utilization of the active materials, (b) 2
assembling with conductive materials(6, 7) to improve the conductivity, and (c) synthesizing bimetallic sulfide(8, 9) to take advantage of the synergistic effects. These strategies are insufficient for the rapid development of the military applications. Therefore, high performance thermal batteries with novel and better cathode materials are urgently demanded. Recent years, transition-metal dichalcogenides (TMDs) have attracted intensive interest due to their unique layer-structure analogue to graphite, in which atoms connected by strong covalent bonds in-plane and weak van der Waals forces between layers(10). This feature endows them with kinetically favorable performances and great potential for energy storage and conversion. Among them, MoS2 and WS2 gain most attentions and have been well studied in details of their physicochemical characteristics and applications(11-14). Compared to MoS2, WS2 has a higher intrinsic conductivity and a larger interlayer spacing (6.18 Å), which seems to have more advantages in battery applications. The utilization of MoS2 as cathode for thermal batteries has been reported(15). Therefore, we postulated that WS2 may be a more suitable cathode material for high-performance thermal batteries, which has not been studied in detail yet. Herein, WS2 was synthesized by a facile solid-state reaction of metal W and sublimed sulfur powders. The as-synthesized WS2 nanosheet exhibited an excellent thermal stability and a high specific capacity of 334.7 mA h g-1 at a current density of 150 mA cm-2. The possible discharge mechanism of the Li-B/WS2 batteries was also being investigated. The analysis results indicated that metallic W and Li2S were the end products of the discharge. 2. Experimental The WS2 sample was prepared by using solid-state reaction method. First, the metal W (Aladdin, 99.98 %) and sublimed sulfur (Aladdin, 99.95 %) powders were mixed with atomic ratio of 1:3 in a stainless steel reactor. The reactor was vacuumed and backfilled with pure argon and then sealed 3
and heated in a resistance furnace at 200 °C for 4 hours. After cooling down, the block was ground and then calcined in a tube furnace with flowing argon at 700 °C for 2 h to remove the residual sulfur. The crystal structure of the as-synthesized sample was examined by X-ray diffraction (XRD, Panalytical X’Pert PRO, Holland) at a rate of 15° min-1 from 5° to 90° (2θ). The surface morphology was observed by field-emission scanning electron microscopy (FE-SEM, JSM-7800F (Prime), Japan) with an accelerating voltage of 10 kV. The thermal properties were tested by thermogravimetry (TGA, Seiko TG-DTA6300, Japan) under a nitrogen flow with a ramp rate of 10 °C min-1. The assembly of single cells and the test procedures were performed in an argon-filled dry box with the water and oxygen concentration under 5 ppm. The seperator (2 g) was a mixture of 50 wt. % MgO binder and 50 wt. % LiF-LiCl-LiBr electrolyte (22 wt.% LiF–9.6 wt.% LiCl–68.4 wt.% LiBr). The cathode was made by mixing 50 wt. % WS2 nanosheet (1.7 g) and 50 wt. % separator. The cathode and separator powders were pressed into a pellet of 54 mm in diameter under pressures of 5-10 MPa, separately. Then the cathode pellet, the separator pellet and Li-B alloy anode sheet (1.4 g) were placed layer by layer and pressed into a Li-B/LiF-LiCl-LiBr/WS2 single cell. The cell was activated by internal pyrotechnic source composed of Fe and KClO4 powder. The discharge process was maintained in a high temperature thermostat at 500 °C. Galvanostatic discharging was done using a battery tester (Land CT2001, China). 3. Results and discussion The XRD pattern of the as-synthesized WS2 sample is presented in Fig. 1a. All the diffraction peaks can be indexed to a pure 2H-phase WS2 (JCPDS no. 84-1398) with P63/mmc space group. It is worth noting that the diffraction peaks of the (002), (004), (006) and (008) plane are relatively 4
sharper, in which the peak intensity of the (002) plane is significantly higher than that of other peaks (inset of Fig. 1a), indicating the layer structure of the WS2 sample. SEM image (inset of Fig. 1a, at 50,000x) further confirms the nanosheet morphology of the WS2 sample, which is about 1μm in diameter and 10~30 nm in thickness.
Fig. 1. (a) XRD pattern and SEM image, (b) TGA results. Fig. 1b shows the TGA results of the as-prepared WS2 nanosheet. It can be seen that the overall weight loss up to 1200 °C is less than 2 wt. %, indicating the excellent thermal stability of WS2. This character is very beneficial for thermal batteries. When the internal pyrotechnic source is ignited, the inner temperature of the battery exceeds 1000 °C for a short period of time(16), which is higher than the decomposition temperatures of FeS2 (550 °C) and CoS2 (650 °C)(2). The fugitive sulfur produced will lead to serious thermal runaway, which may destroy the battery. This problem can be totally resolved when WS2 is adopted as cathode. Moreover, cathode materials with higher thermal stability allow higher operating temperature, which enables faster transportation of lithium ions and longer lifetime. Therefore, the extremely thermal stable WS2 is superior to the existing FeS2 and CoS2 cathodes. The Li-B/WS2 battery was discharged at a constant current density of 150 mA cm-2. The pulse mode test was to load a set of decreasing pulse current (Ipulse) densities within 2 s, and caused changes in the voltage (ΔUpulse) correspondingly. The total polarization (R) can be calculated by the 5
Fig. 2. Electrochemical performances of the Li-B/WS2 single cell: (a) open-circuit discharge, (b) discharge curve at 150 mA cm-2, (c) pulse discharge, (d) total polarization. Fig. 2a shows that the open circuit voltage (OCV) is about 1.43 V. Fig. 2b exhibits the discharge performance of single cell at 150 mA cm-2. The initial discharge voltage is about 1.39 V following a long voltage plateau. The specific capacity is 334.7 mA h g-1 with the cut-off voltage of 1.0 V (75% of the initial voltage), which is higher than the 303 mA h g-1 of CoS2 (at 500 °C, 100 mA cm-2)(18). Therefore, the larger specific capacity enables WS2 a promising candidate for long-life thermal batteries. Fig. 2c presents the details of the pulse model. The total polarization of the Li-B/WS2 single cell (Fig. 2d) gradually increases from 10.8 mΩ to 11.2 mΩ with the increasing current densities up to 2 A cm-2, showing better conductivity than 14 mΩ of the Li-Al/FeS2 cell(7). To investigate the discharge mechanism of the Li-B/LiF-LiCl-LiBr/WS2 thermal battery system, the cathode materials after deeply discharge was examined by ex-situ XRD, as shown in Fig. 3. 6
Besides the peaks of the graphite buffer and the separator compositions, there are clear characteristic peaks of element W, Li2S and Li2O. Li2O is the reaction product of Li2S with moisture during the test, for the smell of H2S. Thus, W and Li2S should be the final products of the discharge processing. Combined with the flat discharge cure shown in Fig. 2b, the electrochemical mechanism can be conclude as a conversion reaction:
Fig. 3. XRD pattern of the cathode after discharge. 4. Conclusions In summary, we have synthesized WS2 nanosheet and investigated its thermal stability and electrochemical properties. The decomposition temperature of WS2 is higher than 1200 °C. When employed in thermal batteries, the higher operating temperature enables higher ionic conductivity of the single cell. The OCV of the Li-B/WS2 single cell is 1.43 V. The high specific capacity of 334.7 mA h g-1 is obtained at 150 mA cm-2. The end discharge products are W and Li2S, while the intermediate products need to be further investigated. Acknowledgements The work was supported by the Innovation Fund of Chinese Academy of Sciences [CXJJ-16M253] and Opening Project of State Key Laboratory of Advanced Chemical Power Sources [SKL-ACPS007]. 7
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1. WS2 is used as cathode material for thermal batteries. 2. The thermal decomposition temperature of WS2 is higher than 1200 °C . 3. The conductivity of WS2 cathode is higher than FeS2 for thermal batteries. 4. The specific capacity of Li-B/LiF-LiCl-LiBr/WS2 single cell is 334.7 mA h g-1 at 150 mA cm-2.
conflicts of interest none