Lithium Metal Battery Using LiFe _0.5Mn _0.5PO ₄ 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 LiFe _0.5Mn _0.5PO ₄ mixed olivine cathode in a lithium metal cell. The IL solution, namely N-butyl- N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr ₁₄TFSI) 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 Pyr ₁₄TFSI–LiTFSI medium above the room temperature. The Li/Pyr ₁₄TFSI–LiTFSI/LiFe _0.5Mn _0.5PO ₄ 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.