Influence of Ion Size on Structure and Redox Chemistry in Na-Rich and Li-Rich Disordered Rocksalt Cathodes

Li-rich and Na-rich cathode materials with disordered rocksalt crystal structures represent a significant advancement in high-energy-density cathode materials for alkali-ion batteries. Li-rich disordered rocksalts, such as Li1.2Ti0.4Mn0.4O2 and Li2MnO2F, have demonstrated high reversible capacities without using Co or Ni.1,2 This notable performance has motivated the exploration of Na-rich disordered rocksalts as promising cathode materials for Na-ion batteries. Despite their potential, the mechanisms that govern their structural stability and redox chemistry remain poorly understood, especially compared to their Li-rich analogues. This study examines two isostructural disordered rocksalt oxyfluorides, Li2MnO2F and Na2MnO2F to understand how the significant difference in ion size influences their electrochemical behaviour. We reveal that Na2MnO2F can achieve an initial charging capacity of 350 mAh g⁻¹, corresponding to complete Na extraction within the practical voltage range, as shown in Figure 1. In contrast, Li2MnO2F can reach a charging capacity of 290 mAh g⁻¹ within equivalent voltage limits, raising questions about the underlying redox and structural changes that enable such a significant difference in capacity. In this study, we conduct a comprehensive characterisation of the manganese and oxygen redox couples, along with the structural changes, to elucidate the role changing from Li- to Na-ions plays in the electrochemical behaviour of disordered rocksalt oxyfluorides. Yabuuchi, N. et al. Origin of stabilization and destabilization in solid-state redox reaction of oxide ions for lithium-ion batteries. Nat Commun 7, (2016). House, R. A. et al. Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox. Energy Environ Sci 11, 926–932 (2018). Yabuuchi, N. et al. Origin of stabilization and destabilization in solid-state redox reaction of oxide ions for lithium-ion batteries. Nat Commun 7, (2016). House, R. A. et al. Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox. Energy Environ Sci 11, 926–932 (2018). Figure 1