Role of Superexchange Interaction on Tuning of Ni/Li Disordering in Layered Li(NixMnyCoz)O2

Ni/Li exchange (disordering) usually happens in layered Li(Ni_xMn_yCo_z)O₂ (NMC) materials and affects the performance of the material in lithium-ion batteries. Most of previous studies attributed this phenomenon to the similar size of Ni²⁺ and Li⁺, which implies that Ni²⁺ should be more favorable than Ni³⁺ to be located at Li 3b sites in the Li slab. However, this theory cannot explain why in Ni-rich NMC materials where most Ni cations are Ni³⁺, Ni/Li exchange happens even more frequently. Using extensive ab initio calculations combined with experiments, here we report that a superexchange interaction between transition metals plays a dominating role in tuning the Ni/Li disordering in NMC materials. Under this scheme, we also propose a new charge compensation mechanism that describes that after Ni³⁺/Li exchange the nearest Co³⁺ transforms to Co⁴⁺ in Ni-rich NMC materials. On the basis of this theory, the existence of Co⁴⁺ in the initial Ni-rich NMC samples was predicted for the first time, which was further confirmed by our synchrotron-based soft X-ray absorption spectroscopy.