Unravelling Solid-State Redox Chemistry in Li_1.3Nb_0.3Mn_0.4O₂ Single-Crystal Cathode Material

Recent reports on high capacities delivered by Li-excess transition-metal oxide cathodes have triggered intense interest in utilizing reversible oxygen redox for high-energy battery applications. To control oxygen electrochemical activities, fundamental understanding of redox chemistry is essential yet has so far proven challenging. In the present study, micrometer-sized Li_1.3Nb_0.3Mn_0.4O₂ single crystals were synthesized for the first time and used as a platform to understand the charge compensation mechanism during Li extraction and insertion. We explicitly demonstrate that the oxidation of O^2– to O^n– (0 < n < 2) and O₂ loss from the lattice dominates at 4.5 and 4.7 V, respectively. While both processes occur in the first cycle, only the redox of O^2–/O^n– participates in the following cycles. The lattice anion redox process triggers irreversible changes in Mn redox, which likely causes the voltage and capacity fade observed on this oxide. Two drastically different redox activity regions, a single-phase behavior involving only Mn^3+/4+ and a two-phase behavior involving O^2–/O^n– (0 ≤ n < 2), were found in Li_xNb_0.3Mn_0.4O₂ (0 < x < 1.3). Morphological damage with particle cracking and fracturing was broadly observed when O redox is active, revealing additional challenges in utilizing O redox for high-energy cathode development.