Lithium Excess-Driven Redox Imbalance Between Cationic and Anionic Redox Causes Electro-Chemo-Mechanical Failure in High-Energy Layered Cathode Materials

Despite the high-energy density of lithium- and manganese-rich layered oxides (LRLOs) provided through the high-voltage anionic redox chemistry, critical issues such as capacity fading, voltage decay, and hysteresis have limited their real-world applications. As an elegant solution, O2-type LRLOs were recently reported to mitigate these voltage issues via reversible transition-metal (TM) migration during the successive charging and discharging; however, these materials still suffer from capacity degradation during long-term cycling. Repetitive mechanical stress and failure, one of the prevailing degradation factors for other layered oxide electrodes, has also been proposed as the origin of the capacity fading even in LRLOs. Nevertheless, in-depth studies on the mechanical behavior in LRLOs, particularly with respect to the extent of high-voltage oxygen redox, remain elusive. Herein, a series of O2-type LRLOs were designed and synthesized to investigate the evolution of the bulk structures as a function of varying oxygen-redox capabilities and to examine their effects on persistent electrochemical degradation. We demonstrate that the generation of bulk microcracks in the particles is strongly coupled with repeated lattice evolution and structural distortions during the high-voltage oxygen redox. These internal cracks lead to the formation of an electrochemically inactive phase near the cracks, which is followed by capacity fading as extended cycling occurs. Based on these results, we propose redox engineering strategy via Ni redox buffer to increase the ratio of cationic to anionic redox activity and thereby mitigate severe lattice disruption, achieving improved capacity retention during long-term cycling. These findings clarify how the oxygen redox in LRLOs is highly correlated to the mechanical degradation (and consequently capacity fading) and provide a general design principle towards genuine high-energy cathode materials.