Understanding the Mn Dissolution Mechanism in Rock Salt-Type Li4Mn2O5 Cathodes

Abstract

Herein, we apply a suite of synchrotron and lab scale X-ray techniques to both the cathode and the separator harvested from pristine, charged, or cycled lithium half-cells containing the disordered rock salt (DRX) material Li₄Mn₂O₅, in order to understand Mn dissolution processes throughout charging and discharging. Previous research has hypothesized two concurrent effects that may drive Mn dissolution in cells during cycling: acid-induced disproportionation of Jahn-Teller active Mn³⁺ and structural rearrangement of the cathode lattice. Through depth probing of the Mn oxidation state in both the cathode and separator via soft X-ray absorption spectroscopy (XAS), hard XAS, and X-ray photoelectron spectroscopy (XPS) at progressive states of charge, as well as extended X-ray absorption fine structure (EXAFS) analysis of the local Mn environment, we conclude that the primary driving force of Mn dissolution is high-voltage structural rearrangement. Mn dissolution is, additionally, a main source of capacity fade in Li₄Mn₂O₅ DRX cells.