Long-Term Cycling and Mechanisms of Cell Degradation of Single Crystal LiNi_0.95Mn_0.04Co_0.01O₂/Graph ite Cells

Extremely high nickel content positive electrode materials have high specific capacity leading to high energy density Li-ion cells. The long-term cycling stability of pouch cells with a single crystal LiNi0.95Mn0.04Co0.01O2 positive electrode material was studied here. Cells with such high nickel content demonstrated excellent cycling when only charged to 4.04 V (about 75% state of charge (SOC)), while they showed more capacity loss when charged to 4.18 V or 100% SOC. Lowering the upper cut-off voltage is in favor of the cycling stability however decreases the cell energy density. The main reason for the capacity loss at 40 degrees C is due to positive electrode impedance growth, which originated from parasitic reactions between the positive electrode material and the electrolyte, especially when the cells are operated to 4.18 V. There was no noticeable positive electrode particle cracking by scanning electron microscopy and no significant active mass loss even for cells operated to 4.18 V. XRD of cycled positive electrodes indicated no appreciable amount of nickel migrating into the lithium layer, so the impedance growth mainly comes from the positive electrode particle surfaces. Using 1.2 M LiPF6 fluoroethylene carbonate: ethyl methyl carbonate 20:80 electrolyte with 1 wt% lithium difluorophosphate allows cycle life to be extended by reducing impedance growth of the cells.