Self-Assembly of Flexible Free-Standing Three-Dimensional Porous MoS2-Reduced Graphene Oxide Film for High-Performance Lithium-Ion Batteries

Abstract

Free-standing and flexible battery electrodes are highly pursued due to its good electrochemical performance and transformation ability. Two-dimensional materials such as transition metal dichalcogenides (TMDs) and graphene with large surface area and good mechanical properties provide a good solution for this problem. The high lithium storage of MoS2 is attractive for achieving high capacity while the high conductivity of graphene is of favorable for good rate capability. The excellent mechanical properties of graphene are able to form a stable structure contributing to long cycle life, flexibility and get rid of substrates [1, 2]. A self-assembly process was developed to produce MoS2/ reduced graphene oxide (MG) hydrogel films in this work. Making use of the ordered structure existed in liquid crystalline graphene oxide (LCGO) dispersion [3], a layer structure with small MoS2 nanosheets sandwiched between large LCGO nanosheets could be easily obtained by simply mixing their dispersion together. A heating process at 70 ℃ was used to accelerate the self-assembly process. The unique layer structure turned into a three-dimensional porous structure during the self-assembly process and preserved after freeze-drying, which could effectively increase the surface area of active materials for high performance. This work develops a simple, straightforward and cost-effective method to fabricate self-assembled, layer-by-layer, free-standing porous MG hydrogel films. The structure displayed excellent electrochemical properties as a lithium-ion battery electrode [4]: a high discharge capacity of 800 mAh g-1 at a current density of 100 mA g-1; and an excellent cycling stability with slightly increased capacity (112.5%) after 500 cycles at a current density of 400 mA g-1. Such enhancement can be ascribed to the synergistic effect between these two components and gradual perfection of the ordered structure. This approach enables the exploitation of the LC order of GO sheets to organize and align 2D MoS2 in-between GO sheets, which may provide a new avenue for the development of 3D porous flexible composite electrode materials with high performance using the unique LCGO.