High-Rate Charging Induced Intermediate Phases and Structural Changes of Layer-Structured Cathode for Lithium-Ion Batteries

Using fast time-resolved in situ X-ray diffraction, charge-rate dependent phase transition processes of layer structured cathode material LiNi_1/3Mn_1/3Co_1/3O₂ for lithium-ion batteries are studied. During first charge, intermediate phases emerge at high rates of 10C, 30C, and 60C, but not at low rates of 0.1C and 1C. These intermediate phases can be continuously observed during relaxation after the charging current is switched off. After half-way charging at high rate, sample studied by scanning transmission electron microscopy shows Li-rich and Li-poor phases' coexistence with tetrahedral occupation of Li in Li-poor phase. The high rate induced overpotential is thought to be the driving force for the formation of this intermediate Li-poor phase. The in situ quick X-ray absorption results show that the oxidation of Ni accelerates with increasing charging rate and the Ni⁴⁺ state can be reached at the end of charge with 30C rate. These results give new insights in the understanding of the layered cathodes during high-rate charging.