Electrochemical impedance study on the low temperature of Li-ion batteries

Cycling performance of Li-ion cells was studied by using electrochemical impedance spectroscopy (EIS). Results showed that total resistance (Rcell) of the Li-ion cells is mainly composed of bulk resistance (Rb), solid-state interface resistance (Rsei) and charge-transfer resistance (Rct). During cycling, the Rb and Rsei remain unchanged while the Rct displays two minima in the same voltage regions where the major peaks of differential capacities are present. The Rct can be linked to kinetics of the cell electrochemical reaction. In response to the temperature change, the Rb and Rsei vary in a very similar manner, while the Rct shows significant difference. In the fully charged and discharged states as well as at the low temperatures (≤20 °C), the Rcell of the Li-ion cells is predominated by the Rct. Using the term of the Rct, we explained two low temperature phenomena of the Li-ion battery: (1) charging of a fully discharged cell is much more difficult than discharging of a fully charged cell, and (2) both the power (operating voltage) and energy (delivered capacity) are substantially reduced.