Intermediate-Temperature Operation of Sodium Secondary Batteries with High Rate Capability and Cyclability Using Ionic Liquid Electrolyte

Abstract

Electrochemical behavior of Na metal is one of the key factors to construct high performance Na secondary batteries. The present study reports temperature dependence of Na metal deposition/dissolution efficiency and morphology of the deposited Na metal in the Na[FSA]-[C2C1im][FSA] ionic liquids (C2C1im+ = 1-ethyl-3-methylimidazolium and FSA− = bis(fluorosulfonyl)amide). The deposition/dissolution efficiency shows the minimum at 298 K and increases by both increasing and decreasing temperature. Optical microscopic analysis reveals that Na metal is grown in a whisker-like form during cathodic polarization at 298 K, whereas smooth deposits are observed at 363 K. Although the deposition form at 273 K is also whisker-like, the thickness and length of each whisker is smaller and shorter, respectively, than those at 298 K. Such differences in morphology affect the Na metal deposition/dissolution efficiencies.