Separating O-Redox and Mn-Redox Voltage Hysteresis in a Cation-Disordered Rock-Salt Cathode

Abstract

Pronounced voltage hysteresis is commonly associated with oxygen redox in Li-excess cathode materials. However, they often involve coexisting transition-metal and oxygen redox, whose contributions to hysteresis are challenging to distinguish. In this work, a two-step aqueous redox titration was developed with the aid of mass spectrometry (MS) gas analyzer to quantify two coexisting solid-phase analytes, namely oxidized oxygen and Mn3+/4+ in a representative Li-excess cation-disordered rock salt (DRX) material. Two MS-countable gas molecules evolve from two separate titrant-analyte reactions, which allows decoupling Mn and O redox capacities. As incremental redox capacities are quantitatively decoupled, each redox voltage hysteresis can be further evaluated through deconvoluted energy efficiency and overvoltages. Overall, a low DRX operating efficiency arises not only from quasi-static voltage hysteresis of O redox, but also from asymmetric Mn-redox overvoltages. The results reveal that O redox is not always the only culprit for a low energy efficiency and slow kinetics, but instead discharging overvoltage, associated with transition-metal redox, can significantly drag down overall operating efficiency. This work further shows the potential of designing new analytical workflow to experimentally diagnose various DRX materials and inform improvement direction.