Unveiling the effect of cycle time and ambient temperature on the failure mechanisms of commercial LiCoO2/artificial graphite pouch cells

Abstract

Many efforts have been devoted to investigate failure mechanisms of LiCoO₂-based lithium-ion batteries at ≥4.55 vs. Li⁺/Li. However, most of these works are conducted on coin-type half-cells and rarely consider the effects of cycle time and ambient temperature on mechanisms. So, it calls an alarming demand for making clear the cyclabilities of cells at different temperatures, then differentiating the root causes for their capacity degradations. Herein, the cyclabilities of an ∼3.1 Ah commercial LiCoO₂/artificial graphite pouch cell at 3–4.45 V are compared at 25 and 60 °C. It is found that the cycle lives at 25 and 60 °C are 2944 and 122 cycles, respectively. The structural variations of the cycled electrodes are systematically studied. Results reveal that the main cause for the capacity beginning to decline after 3000 cycles at 25 °C is the damage of LiCoO₂ structure, while its impedance growth and other side reactions are not severe. Differently, the capacity dropping upon cycling at 60 °C is caused by the parasitic reactions including successive electrolyte decomposition, severe cobalt dissolution and Li deposition. Obviously, these findings can provide a theoretical basis for further optimizing of LiCoO₂ cells.