Correlations of concentration changes of electrolyte salt with resistance and capacitance at the surface of a graphite electrode in a lithium ion battery studied by in situ microprobe Raman spectroscopy

Concentration changes of electrolyte salt in practical lithium ion batteries occur due to various factors during operation, and the changes causes serious degradation of battery performance. It is important to identify elementary factors and how each of the factors induces the concentration changes in batteries by using a simplified system. The concentration of ions in the electrolyte solution between a highly oriented pyrolytic graphite (HOPG) electrode and a lithium foil electrode in a model battery was studied during charge/discharge cycles by in situ microprobe Raman spectroscopy. The concentration of ions decreased during de-intercalation of Li+ from HOPG. The decreased concentration recovered during subsequent rest time with a time constant of several hours. The results of impedance spectroscopy showed that the cycle dependence of the resistance of the surface film on the graphite was similar to that of the time constant. On the other hand, the cycle dependence of the capacitance at the graphite surface and the cycle dependence of the charge transfer resistance at the graphite surface were similar to that of the degree of concentration change. The results suggest that the resistance of the surface film can be evaluated from the rate of the recovery.