Unravelling the Chemical and Structural Evolution of Mn and Ti in Disordered Rocksalt Oxyfluoride Cathode Materials Using Operando X-ray Absorption Spectroscopy

Li-rich disordered rocksalt (DRS) cathode materials with naturally abundant resources, high power and energy density have attracted great attention for applications in Li-ion batteries. We have previously investigated the Li2Mn1-xTixO2F (0 <= x <= 2/3, LMTOF) cathode system showing an attractive cycling behavior, which is however limited by poor long-cycle stability and the unclear cation and anion redox activities in the presence of supposedly inactive Ti. In this work, synchrotron operando X-ray absorption spectroscopy (XAS) is performed to study the chemical and structural evolution of Mn and Ti in the cathode compounds during cycling. Selected electrodes with low (x = 1/3) and high (x = 2/3) Ti content are synthesized, and their structural and electrochemical properties are first characterized accordingly. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) are combined to track the change in oxidation states and local coordination environment for the active transition metals. In order to follow the dynamics of the active species, chemometric methods such as principal component analysis (PCA) and multivariate curve resolution (MCR) are applied. Subsequently, the Mn and Ti redox activities are monitored from initial to extended cycles, in combination with ex-situ studies and operando differential electrochemical mass spectrometry (DEMS) to understand the capacity fading and the redox-based degradation processes. The results provide insights into the development of Mn double-redox reactions in the DRS cathodes from initial cycles to prolonged cycling and elucidate the impacts of the reduced Mn redox activity and the increased local ordering on the cycling stability.