Anionic Redox in Rechargeable Lithium Batteries

The extraordinarily high capacities delivered by lithium-rich oxide cathodes, compared with conventional layered oxide electrodes, are a result of contributions from both cationic and anionic redox processes. This phenomenon has invoked a lot of research exploring new kinds of lithium-rich oxides with multiple-electron redox processes. Though proposed many years ago, anionic redox is now regarded to be crucial in further developing high-capacity electrodes. A basic overview of the previous work on anionic redox is given, and issues related to electronic and geometric structures are discussed, including the principles of activation, reversibility, and the energy barrier of anionic redox. Anionic redox also leads to capacity loss and structural degradation, as well as voltage hysteresis, which shows the importance of controlling anionic redox reactions. Finally, the techniques used for characterizing anionic redox processes are reviewed to aid the rational choice of techniques in future studies. Important perspectives are highlighted, which should instruct future work concerning anionic redox processes.

Anionic redox plays an important role in obtaining high capacities in lithium-rich layered oxide cathodes. Previous studies on anionic redox and related electronic- and geometric-structure issues, as well as problems induced by anionic-redox and suitable characterization techniques, are reviewed, in order to direct future work on high-capacity electrodes that utilize anionic redox processes and widen the scope of these materials.

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