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 to 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 probes), proxies for transition-metal migration, and tracking of microstructural change (voids, reconstruction, impedance growth). Finally, we translate diagnostics into actionable design rules spanning (i) bulk/composition (Mn-valence control, Li/TM ordering, concentration gradients, high-entropy chemistries), (ii) architecture and interfaces (primary-particle coatings; thin, Li⁺-conductive, acid-scavenging layers; oxygen-tolerant CEIs), and (iii) electrolytes (fluorinated and localized-high-concentration systems with targeted additives). Emerging concepts—dynamic oxygen buffers, self-regenerating interphases, and solid/gel interlayers—are assessed against application-relevant benchmarks (areal loading, temperature, gas evolution, N/P balancing, scalable synthesis). We conclude with prioritized experiments and go/no-go criteria to accelerate durable, high-voltage LMR commercialization.