Depth-of-Discharge Dependent Capacity Decay Induced by the Accumulation of Oxidized Lattice Oxygen in Li-Rich Layered Oxide Cathode.

More and more basic practical application scenarios have been gradually ignored/disregarded, in fundamental research on rechargeable batteries, e.g. assessing cycle life under various depths-of-discharge (DODs). Herein, although benefit from the additional energy density introduced by anionic redox, we critically revealed that lithium-rich layered oxide (LRLO) cathodes present anomalously poor capacity retention at low-DOD cycling, which is essentially different from typical layered cathodes (e.g. NCM), and pose a formidable impediment to the practical application of LRLO. We systemically demonstrated that DOD-dependent capacity decay is induced by the anionic redox and accumulation of oxidized lattice oxygen (O^n-). Upon low-DOD cycling, the accumulation of O^n- and the persistent presence of vacancies in the transition metal (TM) layer intensified the in-plane migration of TM, exacerbating the expansion of vacancy clusters, which further facilitated detrimental out-of-plane TM migration. As a result, the aggravated structural degradation of LRLO at low-DOD impeded reversible Li⁺ intercalation, resulting in rapid capacity decay. Furthermore, prolonged accumulation of O^n- persistently corroded the electrode-electrolyte interface, especially negative for pouch-type full-cells with the shuttle effect. Once the "double-edged sword" effect of anionic redox being elucidated under practical condition, corresponding modification strategies/routes would become distinct for accelerating the practical application of LRLO.