Graphene/Co9S8 nanocomposite paper as a binder-free and free-standing anode for lithium-ion batteries

Abstract

Flexible lithium ion batteries with high energy density have recently received tremendous interest due to their potential applications in flexible electronic devices. Herein, we report a simple high energy ball-milling technique together with vacuum filtration to fabricate a highly flexible, conductive, robust and free-standing RGO/Co₉S₈ nanocomposite paper with high conductivity (121 S cm⁻¹), tensile strength (50.4 MPa) and Young's modulus (3.5 GPa) which can be directly used as a free-standing anode for flexible LIBs without binders, conducting agents and metallic current collectors. The free-standing RGO/Co₉S₈ anode with a high mass active material loading of 66.7 wt% Co₉S₈ can deliver a high specific capacity of 1415 mA h g_Co9S8⁻¹ (944 mA h g_electrode⁻¹) and maintain 573 mA h g_Co9S8⁻¹ (382.2 mA h g_electrode⁻¹) after 500 cycles at a current density of 1C (1C = 545 mA g⁻¹). More importantly, the rate capability was improved by introducing RGO. The RGO/Co₉S₈ anode exhibited impressive capacities of 1096.70 mA h g_Co9S8⁻¹ with a capacity recuperability of 69.4% as the current returned to 0.1C. These results demonstrate that the well designed nanocomposite is of great potential as an anode for flexible LIBs. As far as we know, such improved electrochemical performance can be attributed to the nanosized Co₉S₈ particles with a diameter of ∼25 nm homogeneously dispersed on the surface of high conductive graphene sheets that can be obtained owing to the milling impact stress, which enhances surface electrochemical reactivity and shortens the transport length of lithium ions and electrons. What's more, the large specific surface area of the graphene sheet enables the uniform distribution of Co₉S₈ and offers better ability to accommodate volume expansion/shrinkage of Co₉S₈ during repeated charge/discharge cycles.