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, Li1.2–xMn0.8O2. 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 phase-segregates 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.