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 degrees C in the ionic liquid electrolyte Na[FSA]-[C(3)C(1)pyrr][FSA] (FSA = bis (fluorosulfonyl) amide; C(3)C(1)pyrr = 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. (C) 2017 Elsevier Ltd. All rights reserved.