The low temperature performance of Li-ion batteries

A symmetric cell was adopted to analyze low temperature performance of Li-ion battery. Results showed that impedances of both Li-ion and symmetric cells are mainly composed of bulk resistance (R-b), surface layer resistance (R-sl) and charge-transfer resistance (R-ct). Among these three components, the R-ct is most significantly increased and becomes predominant as the temperature falls to below -10 degreesC. Therefore, we may ascribe the poor low temperature performance of Li-ion battery to the substantially high R-ct of the graphite and cathode. Comparing impedance spectra of the symmetric cells, we found that at -30 degreesC the delithiated graphite and lithiated cathode, both of which correspond to a discharged state in a Li-ion battery, have a much higher R-ct than when charged. This means that the Li-ion battery in the discharged state suffers a higher polarization. This result explains the phenomenon that at low, temperatures, charging of a discharged Li-ion battery is more difficult than discharging of a charged battery. (C) 2002 Elsevier Science B.V. All rights reserved.