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

Abstract

Pronounced 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 Mn 3+ /4+ redox in a representative Li-excess cation-disordered rock salt-Li 1.2 Mn 0.4 Ti 0.4 O 2 (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.

A Critical Analysis of Chemical and Electrochemical Oxidation Mechanisms in Li-Ion Batteries

“…This explanation is consistent with the existing literature on oxygen in LIB positive electrodes. It is well-known that oxygen redox occurs within many oxide positive electrodes during battery charging and discharging. − Both experimental and theoretical studies have observed peroxides or “peroxo-like” oxygen dimers in the electrode bulk. − These dimers have been suggested as intermediates that can eventually lead to neutral O 2 evolution. More recently, Genreith-Schriever et al performed ab initio molecular dynamics simulations on LiNiO 2 electrodes and showed that peroxide dimers form on the electrode surface, which is more directly related to oxygen loss and reactions with electrolyte molecules.…”

“…It is well-known that oxygen redox occurs within many oxide positive electrodes during battery charging and discharging. 39 − 43 Both experimental and theoretical studies have observed peroxides or “peroxo-like” oxygen dimers in the electrode bulk. 44 − 46 These dimers have been suggested as intermediates that can eventually lead to neutral O 2 evolution.…”

Dialing in the Voltage Window: Reconciling Interfacial Degradation and Performance Decay for Cation-Disordered Rocksalt Cathodes

“…On the 5th discharge, all cells show one broad reductive peak centered around 4.1 V vs Li/Li + , which is considered to correspond to intermixed Mn 3+/4+ and oxygen redox processes. 24 The differential capacity profiles for the six cells on the 5th cycle closely resemble one another, with minor differences at the ends of the voltage windows due to the differences in cut-off voltages. The similarity of the 5th cycle differential capacity profiles for the different cells demonstrates that initial cell-to-cell variation is small.…”

Nonintrusive thermal-wave sensor for operando quantification of degradation in commercial batteries

“…The graphite electrodes were cut into 16 cm 2 pieces, rinsed with dimethyl carbonate (Gotion), and dried under vacuum at room temperature to remove the residual electrolyte from the surface and the pores. Then, the dried graphite electrode was placed into a custom-made three-neck glass vessel 46 . The vessel was attached to the mass spectrometry apparatus and purged with continuous Ar to remove any residual contaminants inside the vessel and the line.…”