Charge-discharge performance of Na2/3Fe1/3Mn2/3O2 positive electrode in an ionic liquid electrolyte at 90 °C for sodium secondary batteries

Ionic liquid is a promising electrolyte for sodium secondary batteries because of its negligible volatility, low flammability, and high thermal and electrochemical stabilities. In this study, Na2/3Fe1/3Mn2/3O2 as a positive electrode material was synthesized by a solid-state reaction, and its charge-discharge performance was investigated at 90 °C in the ionic liquid electrolyte Na[FSA]-[C3C1pyrr][FSA] (FSA = bis(fluorosulfonyl)amide; C3C1pyrr = N-methyl-N-propylpyrrolidinium). The Na2/3Fe1/3Mn2/3O2 positive electrode delivered an initial discharge capacity of 227 mAh g−1 at a current rate of 20 mA g−1 in the voltage range of 2.0-4.3 V. This study is the first where the Na2/3Fe1/3Mn2/3O2 positive electrode demonstrated a high reversible discharge capacity, larger than 220 mAh g−1, due to the high operation temperature in the ionic liquid electrolyte.