Effects of overdischarge on performance and thermal stability of a Li-ion cell

Overdischarge effects on cycle-life and thermal stability of a commercially available Li-ion cell (rated at 780 mA h) were investigated. Cells were overdischarged and kept at 2.0, 1.5, 1.0, 0.5 or 0.0 V for 72 h (3 days) and then cycled five times (discharge to 3.0 V at 0.4 A and charged to 4.2 V at 0.8 A). This process was repeated five times. The cells overdischarged between 2.0 and 0.5 V experienced irreversible capacity losses of 2–16%. The same cells lost between 8 and 26% more capacity after they were cycled 100 times between 4.2 and 3.0 V at 0.8 A. Behavior of the cells overdischarged to 0.0 V was unpredictable. Some cells lost nearly 65% of their initial capacities after 15 days of being kept at 0.0 V, and others failed in different stages of overdischarging to 0.0 V. Overdischarging to 0.5 V had minimal effects on thermal stability, overcharge performance and a.c. impedance, but led to considerable swelling of the cells. Overdischarge to 0.0 V caused cell thickness and a.c. impedance to increase by ∼70 and 250% of their initial values, respectively. This article addresses concerns that overdischarging of Li-ion cells below 1.5 V may cause capacity losses and/or thermal stability changes which could impact tolerance to abuse conditions.