Phase segregation and nanoconfined fluid O2 in a lithium-rich oxide cathode

Lithium-rich oxide cathodes lose energy density during cycling due to atomic disordering and nanoscale structural rearrangements, both of which are challenging to characterise. Here we use a combined approach of ab initio molecular dynamics and cluster-expansion-based Monte Carlo simulations to resolve the kinetics and thermodynamics of these processes in an exemplar layered Li-rich cathode, Li_1.2–xMn_0.8O₂. We identify a kinetically accessible and thermodynamically favoured mechanism to form O₂ molecules in the bulk, involving Mn migration and driven by interlayer oxygen dimerisation. At the top of charge the bulk structure locally phasesegregates into MnO₂-rich regions and Mn-deficient nanovoids, which contain O₂ molecules as a nanoconfined fluid. These nanovoids are connected in a percolating network, potentially allowing long-ranged oxygen transport, and linking bulk O₂ formation to surface O₂ loss. These insights highlight the importance of future strategies to kinetically stabilise the bulk structure of Li-rich O-redox cathodes to maintain their high energy densities.