Cation disorder dominates the defect chemistry of high-voltage LiMn_1.5Ni_0.5O₄ (LMNO) spinel cathodes

High-voltage spinel LiMn_1.5Ni_0.5O₄ (LMNO) can exist in a Mn/Ni ordered P4₃32 or disordered Fd3̄m arrangement with a majority of literature studies reporting improved electrochemical performance for the disordered phase. Through modifying synthesis conditions, the Mn/Ni ordering can be tuned, however oxygen and Mn³⁺ stoichiometries are also affected, making it difficult to decouple these responses and optimise performance. Here, we investigate all intrinsic defects in P4₃32 LMNO under various growth conditions, using density functional theory (DFT) calculations. We find that the majority of defects are deep and associated with small polarons (Mn³⁺, Mn²⁺ and Ni³⁺) formation. The tendency for cation disorder can be explained by the low formation energy of the antisite defects and their stoichiometric complexes. The intrinsic Fermi level of LMNO varies from moderately n-type under oxygen-poor conditions to weakly p-type under oxygen-rich conditions. Our work explains experimental observations (e.g. the Mn/Ni disorder) and provides guidelines for defect-controlled synthesis.