Article pubs.acs.org/IECR
Double Transition-Metal Chalcogenide as a High-Performance Lithium-Ion Battery Anode Material Dongyun Chen,†,§ Ge Ji,†,§ Bo Ding,† Yue Ma,† Baihua Qu,† Weixiang Chen,‡ and Jim Yang Lee*,† †
Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore, 119260, Singapore ‡ Department of Chemistry, Zhejiang University, Hangzhou, 310027, China S Supporting Information *
ABSTRACT: Transition-metal dichalcogenides (TMDs) are a recent addition to a growing list of anode materials for the nextgeneration lithium-ion battery (LIB). The actual performance of TMDs is however constrained by their limited electronic conductivity. For example, MoS2, the most studied TMD, does not have adequate rate performance even in the few-layer form or after compounding with nitrogen-doped graphene (NG). WS2, a TMD with a higher intrinsic electronic conductivity, is more suitable for high rate applications but its theoretical capacity is lower than that of MoS2. Hence, we hypothesize that a composition-optimized composite of MoS2, WS2, and NG may provide high capacity concurrently with good rate performance. This is a report on the design and preparation of double transition-metal chalcogenide (MoS2/WS2)-nitrogen doped graphene composites where the complementarity of component functions may be maximized. For example the best sample in this study could deliver a high discharge capacity of 1195 mAh·g−1 at 100 mA·g−1 concurrently with good cycle stability (average of 0.02% capacity fade per cycle for 100 cycles) and high rate performance (only 23% capacity reduction with a 50 fold increase in current density from 100 mA·g−1 to 5000 mA·g−1).
1. INTRODUCTION Lithium ion batteries (LIBs) are the most advanced small rechargeable batteries on the market today. The intended use of LIBs for larger-scale and more demanding applications (e.g., electric vehicles or grid-scale energy storage) would require a significant upgrade of the battery energy density, power density, and cycle life; which are best met with new electrode materials and/or new electrode structures. A wide range of electrochemically active materials other than the standard carbon anode and lithium cobalt oxide cathode in the original Sony design have been examined.1 Among them transition-metal dichalcogenides (TMDs) are a recent addition that has drawn significant interest as a potential substitute for the carbon (graphite) anode. The Li+ storage capacity of TMDs (∼800− 1000 mAh/g) is more than twice of that of the graphite anode (372 mAh/g).2−4 TMDs (e.g., MoS2, WS2) are layered binary compounds consisting of three stacked atomic layers (e.g., S− Mo−S). The 2D layers are held together by van der Waals forces and hence the insertion and extraction of Li+ can be a facile process.5 Several recent reports on layered MoS2-based anodes have shown good performance in terms of the capacity for reversible Li+ storage.6−13 The main issue with MoS2 electrodes is their low intrinsic electronic conductivity which affects their rate performance in LIBs. While the rate performance of MoS2 may be improved by compounding with nitrogen-doped graphene (NG), the result is less than optimal.14,15 In our previous work,15 we have found higher rate performance in WS2, another TMD with the same 2D layered structure but with a higher intrinsic electronic conductivity. The capacity of WS2 is however lower than that of MoS2.8,13,16 Thus, we hypothesize that a compositionoptimized composite of MoS2/WS2 with graphene may provide © 2014 American Chemical Society
the best balance between specific capacity and rate performance and consequently improve the current electrochemical performance of TMD-based electrodes. The interaction between graphene and layered TMDs (MoS2 and WS2) can also be demonstrated with fewer conductivity-related issues to probe the origin of improvement in graphene−TMD composite LIB anodes. This article presents a facile one-pot hydrothermal procedure whereby MoS2 /WS 2−NG (MWG) composites may be synthesized without the need for gaseous reactants. The reduction of TMD precursors to single- or few-layer graphene-like nanosheets occurs in the presence of a surfactant, cetyltrimethylammonium bromide (CTAB). The MoS2 and WS2 contents of the composites can be easily adjusted to vary their performance in reversible Li+ storage. This new composite material was also found to considerably improve the Li+ transport properties of TMDs. Electrochemical measurements confirmed the concurrence of high capacity, good cycle stability, and high-rate performance for LIB applications.
2. EXPERIMENTAL SECTION Materials. All chemicals used were of the analytical grade. Natural graphite powder (
