Deconvolution of Intermixed Redox Processes in Ni-based Cation-Disordered Li-Excess Cathodes
2020
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.
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