Traditional Li-ion positive electrodes such as layered LiTMO₂ (TM = Co, Ni, Mn) store lithium ions and electrons largely by employing the cationic redox of the transition metal (TM), yielding a capacity of ~140 mAh g^-1 upon charging to 4.2 V_Li. Recent advances on Li-overstoichiometric layered materials Li_1+xTM_1−yO₂ (TM = Ti, Ni, Co, Mn, Ru, Ir) offer considerably greater first-cycle capacities than the conventional layered materials LiMO₂ with over than 200 mAh g^-1.^1-4 The extra capacity can only be rationalized through the activation of anionic redox in those chemistries, in addition to conventional cationic redox mechanism. However, by tuning transition metal species, researchers observed different reversibility of the anionic redox process, inducing drastically distinct capacity fade over charge and discharge. ^{1, 3-4} To fully utilize and harvest the extra capacity from oxygen redox process, a clear understanding the redox behavior of those materials is needed. In the past decades, there are several competing hypotheses proposed on the redox process of Li-rich layered materials^5-7, but a governing anionic redox mechanism is still lacking.

In this work, we employed X-ray core-level spectroscopies, coupled with ab-initio calculations to first understand the redox processes and structural evolution of model systems exhibiting oxygen redox. Through this effort, we have presented a holistic picture of the anionic redox mechanism in the context of previously proposed redox schemes put forward by different researchers. Moreover, through leveraging the learning of the redox process of model materials, we systematically tuned the transition metal species in the metal oxygen framework in those Li-rich oxides, which allows us to pinpoint potential electronic-structure-based descriptors to capture the activation and reversibility of the anionic redox process. This study has laid the foundation for future high-throughput screening of new generation high-energy-density positive electrodes for Li-ion batteries.

References

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2.Sathiya, M.; Rousse, G.; Ramesha, K.; Laisa, C.; Vezin, H.; Sougrati, M. T.; Doublet, M.-L.; Foix, D.; Gonbeau, D.; Walker, W., Nature Materials 2013, 12 (9), 827-835.

3.Yabuuchi, N.; Nakayama, M.; Takeuchi, M.; Komaba, S.; Hashimoto, Y.; Mukai, T.; Shiiba, H.; Sato, K.; Kobayashi, Y.; Nakao, A., Nature Communications 2016, 7, 13814.

4.Perez, A. J.; Jacquet, Q.; Batuk, D.; Iadecola, A.; Saubanère, M.; Rousse, G.; Larcher, D.; Vezin, H.; Doublet, M.-L.; Tarascon, J.-M., Nature Energy 2017, 2 (12), 954.

5.Gent, W. E.; Lim, K.; Liang, Y.; Li, Q.; Barnes, T.; Ahn, S.-J.; Stone, K. H.; McIntire, M.; Hong, J.; Song, J. H., Nature Communications 2017, 8 (1), 2091.

6.Seo, D.-H.; Lee, J.; Urban, A.; Malik, R.; Kang, S.; Ceder, G., Nature Chemistry 2016, 8 (7), 692-697.

7.Saubanère, M.; McCalla, E.; Tarascon, J.-M.; Doublet, M.-L., Energy & Environmental Science 2016, 9 (3), 984-991.