Unraveling the Effect of Cu ²⁺ on the Environmental Stability of Sodium-Ion Layered Cathode Materials

Poor environmental stability is one of the main reasons to limit the cost advantage of sodium-ion layered cathode materials. Among various improvement strategies, Cu substitution has been recognized as a simple and effective method to overcome air sensitivity, but its micromechanism remains unclear. Combining density-functional theory calculations, crystal field theory, d-band center theory and impurity formation mechanism, we compared the surface properties of O3-type NaNi1/3Fe1/3Mn1/3O2 (NFM) and NaCu1/3Fe1/3Mn1/3O2 (CFM) and demonstrate that the specific contribution of Cu2+ to the environmental stability of sodium-ion layered oxides includes the following: (i) Cu2+ with a stable Jahn–Teller distortion can itself induce strong electrostatic repulsion of eg-orbital electrons from air molecules in highly coordinated surface environments rather than that it inhibits surface active oxygens or significantly increases the proportion of inert surfaces; (ii) the adsorption of air molecules on CFM surfaces is generally weaker than on NFM surfaces because Cu2+ is more prone to surface segregation and has a lower d-band center than Ni2+; (iii) the surface Na thermal stability of CFMs is generally better than that of NFMs, which contributes to the inhibition of surface Na detachment from the lattice sites and thus reaction sustainability.