Enhanced Cycle Performance in a Binder Free Lithium Sulfur Battery Using Few-Layer Graphene Foam Scaffold

Abstract

A lithium-sulfur (Li-S) battery has a high theoretical specific capacity (1675 mAh g-1 of elemental sulfur) and a high nominal energy density (2500 Wh kg-1 of cell weight), which offers the prospect of a significant energy density improvement compared to the mainstream lithium-ion batteries (150 Wh kg-1). However, the development of Li-S battery technology has been plagued by problems arising from the highly insulating nature of sulfur (5×10-30 S cm-1 at 25 °C), the high solubility of lithium polysulfides in the electrolyte and volumetric expansion of sulfur during lithiation. Here, we demonstrate the design and preparation of few-layer graphene (FLG) foam scaffold for sulfur loading to fabricate cathode structures for lithium-sulfur batteries. In this work, a free-standing FLG monolithic network foam was formed as a negative of a Ni metallic foam template by CVD followed by etching away of Ni. Then, a three-dimensional (3-D) cathode system for lithium sulfur (Li-S) batteries has been synthesised by loading sulfur on to this interconnected network of few-layered graphene (FLG) via a sulfur solution infiltration method. This cathode system did not require any additional binding agents, conductive additives or a separate metallic current collector thus decreasing the weight of the cathode by typically ~20-30 wt%. A Li-S battery with the sulfur/FLG foam cathode shows good electrochemical stability and high rate discharge capacity retention for up to 400 discharge/charge cycles at a high current density of 3200 mA g-1, which due to the FLG foam offers excellent electrical conductivity, an appropriate pore structure for containing the electro-active sulfur and facilitates rapid electron/ion transport. Even after 400 cycles the capacity decay is only ~0.064% per cycle relative to the early (e.g. the 5th cycle) discharge capacity, while yielding an average columbic efficiency of ~96.2%. To further understand the sulfur loading ability of FLG foam, the composites with high content (up to 72 wt %) were also investigated. The composite with high content of sulfur still presented good initial discharge capacity and good cycle stability after 200 cycles. Our results indicate the potential suitability of graphene foam for efficient and high-performance batteries.