Rechargeable Na/Na_0.44MnO₂ cells with ionic liquid electrolytes containing various sodium solutes

Orthorhombic Na_0.44MnO₂ with wide structural tunnels for sodium ion transport is synthesized. Butylmethylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (BMP-TFSI) ionic liquid (IL) with various Na solutes, namely NaBF₄, NaClO₄, NaTFSI, and NaPF₆, is used as an electrolyte for rechargeable Na/Na_0.44MnO₂ cells. The cell with NaClO₄-incorporated IL electrolyte exhibits superior charge-discharge performance due to it having the lowest solid-electrolyte-interface resistance and charge transfer resistance at both the Na and Na_0.44MnO₂ electrodes. The IL electrolyte shows high thermal stability and is suitable for use at an elevated temperature. At 75 °C, the measured capacity of Na_0.44MnO₂ in the IL electrolyte with NaClO₄ is as high as 115 mAh g^-1 (at 0.05 C), which is close to the theoretical value (121 mAh g^-1). Moreover, 85% of this capacity can be retained when the charge-discharge rate is increased to 1 C. These properties are superior to those of a conventional organic electrolyte.