The degradation of LiCoO₂/graphite batteries at different rates

The LiCoO₂/graphite full cells (CP523450A) are cycled at different rates (0.6C, 1.2C, 1.8C and 3.0C), and the degradation mechanisms are evaluated in this paper. The lithium metal is deposited on the anode when the cycling rate is higher than or equal to 1.8C, which is confirmed by the tests of the incremental capacity (IC) curves, XPS, DSC and SEM. The root cause of the lithium deposition is analyzed by the available capacities of the single electrodes as well as the actual capacity ratio of anode to cathode (actual A/C ratio). The rate capability of the anodes recovered from the fresh and the aged full cells is quite poor in comparison with the cathodes. Therefore, the capacity of full cell is limited by the anode when the test rate is relatively high. In such situation, the coulombic efficiency is less than 1, and its value is slightly larger than the actual A/C ratio which is the root cause of the deposition of lithium metal. Compared with the generation of SEI film, the lithium deposition leads to an accelerated loss rate of active lithium which results in the rapid capacity fade of full cell. The research about the actual A/C ratio provides a guidance for the design of the initial reversible capacity ratio of anode to cathode (initial A/C ratio) of lithium ion batteries.