Suppressing O-type stacking and cation migration with Mg and Si doping in P2-type Fe-Mn layered oxides for sodium-ion cathodes

Layered oxide cathodes for Na-ion batteries containing Mn or Fe are of considerable interest for sustainable energy storage applications largely due to cost issues, relative abundance and high capacities. However, these layered cathodes such as Na_xFe_0.5Mn_0.5O₂ exhibit a structural phase transformation (from P2 to O2) at high charge states accompanied by irreversible cation migration into the Na layers and formation of trapped O₂. Here we investigate whether doping into Na_xFe_0.5Mn_0.5O₂ can effectively suppress this phase transformation and cation migration, focusing on the atomic-scale effects of Mg²⁺ and Si⁴⁺ substitution, using ab initio simulation techniques. These dopants are contrasting species in terms of bonding character from divalent ionic (Mg) to tetravalent covalent (Si). Our study indicates that Mg-doping delayed Fe migration into the Na-layers in line with the experiment. In contrast, Si-doping stabilised the P2 phase over the entire charging range and suppresses Fe migration with no O-O dimer formation, suggesting that the Si-doped system should be a promising Na-ion cathode material.