Evolutions of Crystal Structure, Stoichiometry and Electrochemical Behavior with Co Substitution in LiNi_1-yCo_yO₂ Positive Electrodes

LiNi_1-yCo_yO₂(0.1^_≤ y^_≤0.4) positive electrode materials are synthesized by a chemical method with stoichiometric acetates of related cations. Their crystal structure, stoichiometry and electrochemical behaviors versus Co concentration are investigated by x-ray diffraction, synchrotron-based x-ray absorption fine structure and galvanostatic cycling measurements. The results reveal that the non-stoichiometric Ni²⁺, Li/Ni cation mixing and polarization are reduced as the amount of Co substitution increases, clearly indicating that the Co element is a medium for easily oxidizing Ni²⁺ to Ni³⁺ during the synthesis process.