Phase Segregation and Nanoconfined Fluid O₂ in an Oxygen Redox Battery Cathode

Next-generation, sustainable high energy density batteries require high-capacity cathodes composed of Earth-abundant elements. Lithium and manganese rich oxide cathodes are leading candidates because they achieve high capacity from a combination of transition metal and oxygen redox.[1] O-redox, however, is accompanied by structural changes that cause a large loss of energy density.[2] To apply O-redox cathodes in practical devices, these structural changes must be understood and prevented. These structural changes are exceedingly difficult to characterise for both experimental and computational modelling,[3] which has led to uncertainties and ambiguities in the mechanisms of O-redox in battery cathodes.<br/><br/>The characterisation of O-redox behaviour is particularly challenging for computational modelling. Because O-redox cathodes undergo atomic rearrangements during cycling, cathode structures after the early stages of the first charge are not known <i>a priori</i> and must be solved <i>in silico</i>. For computational modelling studies to make credible predictions of O-redox behaviour, both the kinetics and thermodynamics of structural rearrangements must be considered. Modelling schemes that predict O-redox behaviour from structures that i) form via kinetically inaccessible pathways, ii) are kinetically unstable or iii) are far from the thermodynamic ground state, can produce unrealistic descriptions of O-redox. Furthermore, cycled O-redox cathodes exhibit crystallographic site disorder and nanoscale structural changes, such as the formation of nanovoids. Computational modelling using DFT, however, cannot directly investigate disorder and nanoscale structures, meaning that additional modelling methods are required to provide a complete picture of O-redox behaviour.<br/><br/>Here, we use a computational strategy that directly addresses these kinetic and thermodynamic factors. To identify kinetically viable atomic-scale rearrangements during the first charge, we have used long-timescale ab initio molecular dynamics (AIMD). In parallel, to account for disorder and nanoscale structural changes produced after many cycles, we have developed a DFT-derived cluster-expansion model of oxygen-redox, which we have used to perform large-scale Monte Carlo simulations. This approach allows us to efficiently search the vast configurational space for thermodynamically low-energy structures at the top of charge, and to conduct this search in structures containing ~50,000 atoms, so that nanoscale structural rearrangements can be examined.<br/><br/>We apply this strategy to high-capacity O2-layered Li_1.2–xMn_0.8O₂, which is an exemplar system for understanding Li-rich oxide cathodes. We identify a kinetically viable O-redox mechanism, in which the formation of interlayer superoxide and peroxide intermediates drives out-of-plane Mn migration, resulting in O₂ molecules forming within the bulk structure. The thermodynamic ground-state structure at the top of charge exhibits phase segregation into a two-phase mixture of MnO₂ and O₂. Bulk O₂ molecules are confined within nanometre-sized Mn-deficient voids that form a connected, percolating network. These O₂ molecules have a nanoconfined supercritical fluid character and can potentially diffuse through the network of voids, providing a mechanistic link between bulk O₂ formation and surface O₂ loss.[4]<br/><br/>[1] P. Rozier & J.M. Tarascon Review — Li-Rich Layered Oxide Cathodes for Next-Generation Li-Ion Batteries: Chances and Challenges. J. Electrochem. Soc. 162, A2490–A2499 (2015)<br/><br/>[2] R.A. House, G.J. Rees, K. McColl et al., Delocalized electron holes on oxygen in a battery cathode, Nat Energy 8, 351–360 (2023)<br/><br/>[3] K. McColl et al., Transition metal migration and O₂ formation underpin voltage hysteresis in oxygen-redox disordered rocksalt cathodes. Nat. Commun. 13, 5275 (2022).<br/><br/>[4] K. McColl et al., Phase segregation and nanoconfined fluid O₂ in a lithium-rich oxide cathode. ChemRxiv (https://doi.org/10.26434/chemrxiv-2023-9v2dw (2023).