Covalently interlinked graphene sheets with sulfur-chains enable superior lithium-sulfur battery cathodes at full-mass level

Abstract

Sulfur represents a low-cost, sustainable, and high theoretical capacity cathode material for lithium-sulfur batteries, which can meet the growing demand in portable power sources, such as in electric vehicles and mobile information technologies. However, the shuttling effect of the formed lithium polysulfides, as well as their low conductivity, compromise the electrochemical performance of lithium-sulfur cells. To tackle this challenge, a so far unexplored cathode, composed of sulfur covalently bonded directly on graphene is developed. This is achieved by leveraging the nucleophilicity of polysulfide chains, which react readily with the electrophilic centers in fluorographene, as experimental and theoretical data unveil. The reaction leads to the formation of carbon-sulfur covalent bonds and a particularly high sulfur content of 80 mass%. Owing to these features, the developed cathode exhibits excellent performance with only 5 mass% of conductive carbon additive, delivering very high full-cathode-mass capacities and rate capability, combined with superior cycling stability. In combination with a fluorinated ether as electrolyte additive, the capacity persists at approximate to 700 mAh g(-1) after 100 cycles at 0.1 C, and at approximate to 644 mAh g(-1) after 250 cycles at 0.2 C, keeping approximate to 470 mAh g(-1) even after 500 cycles.