Lithium sulfur batteries (Li-S) have a 3-5 fold higher theoretical energy density than state-of-the-art lithium-ion batteries. In this work, tris(trimethylsilyl)borate (TMSB) is reported for the first time as an effective electrolyte additive to enhance the electrochemical performance of a Li-S battery. With 1% TMSB additive, the sulfur composite cathodes show good cycling stability and outstanding rate capability, and deliver a highly reversible capacity of 1423 mA h g-1 even at 10C. TMSB participates in the formation of a surface layer on sulfur-pyrolyzed polyacrylonitrile (S@pPAN) cathodes, which accelerates lithium ion diffusion and stabilizes the interface.