Transition metal migration and O2 formation underpin voltage hysteresis in oxygen-redox disordered rocksalt cathodes

Abstract

Lithium-rich disordered rocksalt cathodes display high capacities arising from redox chemistry on both transition-metal ions (TM-redox) and oxygen ions (O-redox), making them promising candidates for next-generation lithium-ion batteries. However, the atomic-scale mechanisms governing O-redox behaviour in disordered structures are not fully understood. Here we show that, at high states of charge in the disordered rocksalt Li2MnO2F, transition metal migration is necessary for the formation of molecular O2 trapped in the bulk. Density functional theory calculations reveal that O2 is thermodynamically favoured over other oxidised O species, which is confirmed by resonant inelastic X-ray scattering data showing only O2 forms. When O-redox involves irreversible Mn migration, this mechanism results in a path-dependent voltage hysteresis between charge and discharge, commensurate with the hysteresis observed electrochemically. The implications are that irreversible transition metal migration should be suppressed to reduce the voltage hysteresis that afflicts O-redox disordered rocksalt cathodes.

Unraveling the Dynamic Correlations between Transition Metal Migrations and the Oxygen Dimer Formation in the Layered LiCoO2 Cathode

“…Furthermore, the effect of the Ti dopant on the kinetics of the cathode material's structural change is discussed based on insights provided by the DeePMD simulations. Here, we note that although the generation of oxygen molecules from the cathode surfaces is easier in batteries, the TM cation migrations in the bulk were reported by experiments, [46][47][48][49][50][51] which also could lead to the oxygen release. For instance, the layered-to-rocksalt-phase transition, usually induced by TM cation migrations, was found in the bulk LixCoO2 phase by Li et al 47 , which draws attention to the dynamic behaviors of the TM and oxygen atoms in a highly delithiated bulk-phase cathode material.…”

Grand-canonical Monte-Carlo simulation methods for charge-decorated cluster expansions

“…We performed CE-MC simulations in a disordered rocksalt system. A rocksalt crystal structure is a basic prototype of ionic materials consisting of an FCC cation and an FCC anion sub-lattice, mimicking the basic chemistry of some novel Li-ion cathode systems which have been modeled with CE-MC in recent studies [53][54][55]. In our system, Li + , Mn 3+ , Zr 4+ are distributed on the cation sub-lattice, and O 2− , F − are present on the anion sublattice.…”

Understanding the limits to Short-range order Suppression in Many-Component Disordered Rock Salt Lithium-ion Cathode Materials

“…Each Monte Carlo simulation was run at 2000 K. 2000 K falls within the range of empirical simulation temperatures commonly used for Monte Carlo simulations of disordered rock salt structures [27,34,61] and we expect it to capture experimentally relevant trends in the short range order formation. Each simulation ran for at least 5,000,000 trial steps.…”

“…It is found the connectivity of the lithium diffusion network is poorer than expected in a totally random system in almost all cases, and substantial lithium excesses are required to give good long-range connectivity of the 0-TM network [25]. Minimising the lithium excess is desirable, as the additional lithium substitutes for the redox-active transition metal, reducing the capacity or increasing the anionredox activity which can cause voltage hysteresis and structural degradation in disordered rock salt cathodes [27].…”

“…To extend the current understanding of short range order suppression in many-component disordered rock salt transition metal oxyflouride cathode materials, we examine the lithium-transition metal ordering in rocksalt-structured 3d-transition-metal lithium oxyfluorides using cluster-expansion-driven Monte Carlo simulations. Such simulations have proved to be an excellent tool in unravelling short range ordering preferences in lithium-ion cathode materials [25,27,34,38]. We examine how the lithium percolation network evolves as a function of the number of transition metals on the cation sublattice and varying lithium excess and fluorine content.…”