Deconvolution of Intermixed Redox Processes in Ni-based Cation-Disordered Li-Excess Cathodes

Cation-disordered rock-salt transition-metal oxides or oxyfluorides (DRX) have emerged as promising cathode materials for Li-ion batteries due to their potential to reach high energy densities and accommodate diverse, lower cost transition-metal chemistries compared to conventional layered oxide materials. However, the intricate local coordination environment in DRX also results in complex electrochemical electron transfer involving parallel mechanisms of transition-metal (TM) redox and oxygen (anionic) redox. Without decoupled and quantitative information of these intermixed redox processes, the origin of irreversibility, voltage hysteresis, and capacity fading is obscured, which impedes the development of strategies to address these issues. Here we deconvolute the mixed redox processes in a Ni-based DRX, Li1.15Ni0.45Ti0.3Mo0.1O1.85F0.15, by combining 18O isotopic enrichment, differential electrochemical mass spectrometry (DEMS), and ex-situ acid titration. The summation of TM-redox and oxygen-redox capacities measured through our approach agrees with the net electron transfer measured by the potentiostat. This study reveals much less Ni oxidation efficiency (59.5%) than its initially designed efficiency (100%) due to competition of oxygen redox, which can occur at potentials as low as 4.1 V (vs. Li/Li+). We propose that the chemical approach presented in this work and its future extension can resolve and quantify various mixed redox processes in different DRX, which allows clear correlations among material design, deconvoluted redox capacities, and battery performance.