In recent years, reversible redox activities of both transition-metal (TM) cations and oxygen anions were found to be feasible in a number of Li-excess transition-metal oxides, enabling significant enhancement in charge storage capacity of lithium-ion battery (LIB) cathodes. [1-3] Most high-capacity cathode materials reported in the literature are either layer-structured similar to the well-studied Li- and Mn-rich (LMR) oxides, or cation-disordered rock-salts. For the layered LMR, repeated cycling at high voltages is known to cause significant voltage and capacity fade, hysteresis, and impedance rise. Understanding the origin of these issues has been on-going for well over a decade, and various degradation mechanisms have been proposed. [4-5] Likewise, obtaining high capacity in cation-disordered rock-salt oxides often comes at the expense of cycling stability that becomes progressively worse with either deeper oxidation of oxygen at higher potential or extended cycling involving oxygen redox. It is unclear, however, what properties and/or processes may have caused the performance issues in this newer class of cathode materials with 3D Li migration pathways.

In this presentation, we use cation-disordered Li_1.3Nb_0.3Mn_0.4O₂(LNMO), a compound recently reported to have an impressive discharge capacity of ca. 300 mAh/g at 60 ^oC [6], as a baseline system to investigate the chemical and structural origins of performance deterioration. We show that extensive reduction of the redox active TM occurred both in the bulk and on the surface of the cycled oxide particles. In contrast to what was reported on the layered LMR, TM reduction was not accompanied by phase transition due to cation site migration. We further propose a cathode degradation mechanism and explore design strategies to balanced capacity and stability. As both Li and TM cations share the same crystallographic sites in cation-disordered rock-salts, the oxide chemistry was manipulated to influence the contribution of TM and O redox.The effect of cation and/or anion substitutions in stabilizing the oxide cathodes will be discussed.

References

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Acknowledgment

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.