Lithium Metal Battery Using LiFe0.5Mn0.5PO4 Olivine Cathode and Pyrrolidinium-Based Ionic Liquid Electrolyte

Abstract

Ionic liquids (ILs) represent the most suitable electrolyte media for a safe application in high-energy lithium metal batteries because of their remarkable thermal stability promoted by the room-temperature molten salt nature. In this work, we exploit this favorable characteristic by combining a pyrrolidinium-based electrolyte and a LiFe0.5Mn0.5PO4 mixed olivine cathode in a lithium metal cell. The IL solution, namely N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI) dissolving LiTFSI, is designed as viscous electrolyte, particularly suited for cells operating at temperatures higher than 40 °C, as demonstrated by electrochemical impedance spectroscopy. The olivine electrode, characterized by remarkable structural stability at high temperature, is studied in the lithium metal cell using the Pyr14TFSI–LiTFSI medium above the room temperature. The Li/Pyr14TFSI–LiTFSI/LiFe0.5Mn0.5PO4 cell delivers a capacity of about 100 mA h g–1 through two voltage plateaus at about 3.5 and 4.1 V, ascribed to the iron and manganese redox reaction, respectively. The cycling stability, satisfactory levels of the energy density, and a relevant safety content suggest the cell studied herein as a viable energy storage system for future applications.