Effects of Lithium Over-Stoichiometry in Li1+xCoO2−δ and Li1+xCo0.95Ni0.05O2−δ on High-Voltage Float Durability and Cyclability

Finding a balance between high energy density and long life is a fundamental challenge in the development of lithium-ion batteries. The energy density of lithium-ion batteries can be enhanced by increasing the upper limit charging voltage of the LiCoO 2 positive electrode, but fatal deterioration can occur, especially during high-voltage float charging. In this study, we investigate the 4.5 V float charging durability and cyclability on lithium over-stoichiometric Li 1+x CoO 2− δ and Li 1+x Co 0.95 Ni 0.05 O 2− δ (x = 0–0.14). The leakage current of accompanied by the structural collapse of positive electrode materials under float charging at 4.5 V could be suppressed by increasing the lithium over-doping. On the other hand, increases in the lithium over-doping resulted in a larger float current at the early stage, which was found by a side reaction accompanied cobalt/nickel dissolution directly linked to the deterioration of the positive electrode materials. The 4.5 V cyclability and float durability of Li 1+x CoO 2− δ and Li 1+x Co 0.95 Ni 0.05 O 2− δ showed a clear positive correlation, thus indicating that a higher 4.5 V float durability leads to higher cyclability. Our systematic evaluation of both positive electrode materials revealed that x ≈ 0.04 in Li 1+x CoO 2− δ and x ≈ 0.03 in Li 1+x Co 0.95 Ni 0.05 O 2− δ demonstrated high float durability and cyclability.