Multi-Step Oxygen Redox Mechanism in the Polyanionic Lithium-Rich Cathode Li2FeSiO4.

Lithium-rich cathode materials offer significantly higher energy densities than conventional cathode materials, due to their ability to store charge beyond the formal transition-metal redox limit, via so-called "oxygen redox". In lithium-rich oxides, oxygen redox is accompanied by molecular O2 formation and transition-metal migration, associated with irreversible structural change and poor cycling stability. Lithium-rich silicates provide an alternative lithium-rich chemistry, and have been proposed to be resilient to O2 formation at the top of charge, due to their covalently bonded [SiO4]4- framework. We report a computational study of the top-of-charge behavior of the prototypical silicate cathode Li2FeSiO4. Using density functional theory, we predict that fully delithiated FeSiO4 is thermodynamically unstable with respect to oxidation of oxide ions to form molecular O2 within the bulk material. Ab initio molecular dynamics shows this decomposition occurs spontaneously, via a multistep mechanism involving FeIV migration and disproportionation to FeIII and FeV, and subsequent FeV reduction alongside molecular O2 formation. Our findings demonstrate that polyanionic frameworks do not inherently prevent O2 formation in lithium-rich cathodes, and illustrate how different previously proposed oxygen redox mechanisms can function as steps within a compound process.