Ultra-Stable Cycling of High Capacity Room Temperature Sodium-Sulfur Batteries Based on Sulfurated Poly(acrylonitrile)

Abstract

We report on a room temperature (RT) sodium-sulfur (Na−S) battery based on a sodium anode, a sulfurated poly(acrylonitrile) (SPAN) cathode and an electrolyte containing sodium tetrakis(hexafluoroisopropyloxy) borate (Na[B(hfip)4]; hfip=hexafluoroisopropoxide) in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC) and fluoroethylene carbonate (FEC). The hfip anion as a weakly coordinating anion (WCA) provides high anodic stability, high ionic conductivity, and superior electrochemical performance in carbonate-based solvents. The Na-SPAN cell exhibits an initial discharge capacity of 1360 (Formula presented.) and a remarkable reversible capacity of 1072 (Formula presented.) after 1000 cycles at 3 C (C=C-rate, 5.025 (Formula presented.)) with an insignificant average capacity decay of less than 0.021 % per cycle. A careful choice of the discharge cut-off potential (DCP) reveals that a DCP of 0.2 V allows for stable cycling for more than 500 cycles while a DCP of 0.5 V results in a constant capacity decay. The excellent cycle stability at a DCP of 0.2 V is likely to be caused by the high conversion of the SPAN-bound sulfur into Na2S.