Highly Reversible Local Structural Transformation Enabled by Native Vacancies in O2-Type Li-Rich Layered Oxides with Anion Redox Activity.

A novel O2-phase LiNi[□Mn]O cathode with native vacancies (denoted as "□") was delicately designed. By a combination of noninvasive Li pj-MATPASS NMR and electron paramagnetic resonance measurements, it is unequivocally shown that the reservation of native vacancies enables the fully reversible local structural transformation without the formation of Li in the Li layer (Li) in LiNi[□Mn]O during the initial and subsequent cycling. In addition, the pernicious in-plane Mn migration that would result in the generation of trapped molecular O is effectively mitigated in LiNi[□Mn]O. As a result, the cycle stability of LiNi[□Mn]O is significantly enhanced compared to that of the vacancy-free LiNiMnO, showing an extraordinary capacity retention of 102.31% after 50 cycles at a rate of 0.1C (1C = 100 mA g). This study defines an efficacious strategy for upgrading the structural stability of O2-type Li-rich layered oxide cathodes with reversible high-voltage anion redox activity.