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, which enables 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. In the layered oxides, repeated cycling involving the anionic redox causes significant voltage and capacity fade, hysteresis and impedance rise. Understanding the chemical origin of these issues in LMR has been on-going for well over a decade, and various degradation mechanisms have been proposed. [4-5] The effect of oxygen redox on the newer cation-disordered cathodes, however, has not been investigated. It is unclear whether the performance issues observed in LMR are inherent to anionic redox or they are structure and/or chemistry dependent.

In this presentation, we report the fundamental understanding of oxygen redox chemistry in cation-disordered rock-salts and its effect on cathode performance. The correlation among the extent of oxygen redox, discharge capacity, capacity retention as well as voltage retention was systematically investigated on a model compound, Li_1.3Nb_0.3Mn_0.4O₂ (LNMO), which was recently reported to have an impressive discharge capacity of ca. 300 mAh/g at 60 ^oC. [6] We reveal that the high capacity brought in by oxygen redox in this class of materials comes at the expense of stability (as shown in the Figure), which worsens progressively with either deeper oxidation of oxygen at higher potential or extended cycling. The chemical and structural origins of the performance deterioration were further investigated. Design strategies to balance capacity and stability in this class of oxide cathodes will also 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.