Decoupling oxygen redox from O2 release in Li- and Mn-rich layered cathodes: mechanisms, metrics, and design rules

Abstract

Lithium- and manganese-rich (LMR) layered oxides can deliver >250 mAh g⁻¹ by engaging anionic (oxygen) redox, yet their promise is undermined when oxygen redox couples with O₂ formation, triggering transition-metal migration, layered → spinel/rock-salt reconstruction, interfacial breakdown, and voltage fade. This review reframes LMR development around a single objective—decouple reversible oxygen redox from O₂ release—and organizes the field into mechanisms, metrics, and design rules. We first clarify the mechanistic pathways that produce oxidized-oxygen species versus molecular O₂ and map how these pathways propagate stress, porosity/voids, and interfacial reactivity. We then define a decision-grade metric set to distinguish O-redox from O₂ evolution under practical conditions, including gas quantification at realistic cutoffs (≥4.5 V), operando O-species fingerprints (e.g., RIXS/¹⁷O-NMR probes), proxies for transition-metal migration, and tracking of microstructural change. Finally, we translate diagnostics into actionable design rules spanning (i) bulk/composition, (ii) architecture and interfaces, and (iii) electrolytes. We conclude with prioritized experiments and go/no-go criteria to accelerate durable, high-voltage LMR commercialization. This review summarizes the key degradation mechanisms of high-voltage lithium-rich cathodes and highlights material and interface strategies to stabilize them for long-term use.