Abstract
To evaluate the degradation characteristics of Li-ion cells at low temperatures, commercially available 18650 LiCoO2–graphite cells are charged/discharged at low temperature via two different charge/discharge cycling protocols. Cell degradation during these protocols is investigated using cell capacity measurements and differential capacity (dQ/dV vs. V) curves. Subsequently, the cell temperature is restored to 25 °C, and the cells are charged/discharged to confirm the capacity fading. Under the first charge/discharge procedure (reversible degradation), the cell capacity significantly fades during charge/discharge cycling at 5 °C, and the apparent capacity loss is recovered to the initial value after the temperature is restored to 25 °C. However, under the second charge/discharge procedure (irreversible degradation), although the capacity is recovered to a certain extent when the temperature is restored to 25 °C, it is considerably lower than the initial value. Most importantly, the mechanism of cell degradation reversibility was investigated by peak attribution of the dQ/dV vs. V curves. The dQ/dV vs. V analysis reveals that the low-temperature degradation is low relative to the cathode. Reversible degradation is predominantly related to the reversible overvoltage and reduction of the lithium inventory for the phase transition of the graphite anode, whereas irreversible degradation is associated with the irreversible reduction of the lithium inventory for the same process.
