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 Li 1.2 Ti 0.4 Mn 0.4 O 2 and Li 2 MnO 2 F, 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, Li 2 MnO 2 F and Na 2 MnO 2 F to understand how the significant difference in ion size influences their electrochemical behaviour. We reveal that Na 2 MnO 2 F 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, Li 2 MnO 2 F 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). Figure 1