Quantitative Decoupling of Oxygen‐Redox and Manganese‐Redox Voltage Hysteresis in a Cation‐Disordered Rock Salt Cathode

Abstract

AbstractPronounced voltage hysteresis in Li‐excess cathode materials is commonly thought to be associated with oxygen redox. However, these materials often possess overlapping oxygen and transition‐metal redox, whose contributions to hysteresis between charge and discharge are challenging to distinguish. In this work, a two‐step aqueous redox titration is developed with the aid of mass spectrometry (MS) to quantify oxidized lattice oxygen and Mn3+ /4+ redox in a representative Li‐excess cation‐disordered rock salt—Li1.2Mn0.4Ti0.4O2 (LMTO). Two MS‐countable gas molecules evolve from two separate titrant‐analyte reactions, thereby allowing Mn and O redox capacities to be decoupled. The decoupled O and Mn redox coulombic efficiencies are close to 100% for the LMTO cathode, indicating high charge‐compensation reversibility. As incremental Mn and O redox capacities are quantitatively decoupled, each redox voltage hysteresis is further evaluated. Overall, LMTO voltage hysteresis arises not only from an intrinsic charge‐discharge voltage mismatch related to O redox, but also from asymmetric Mn‐redox overvoltages. The results reveal that O and Mn redox both contribute substantially to voltage hysteresis. This work further shows the potential of designing new analytical workflows to experimentally quantify key properties, even in a disordered material having complex local coordination environments.