NOVELTY - The method involves setting (S3) the physical parameters such as reaction rate coefficient. The calculation data of preset number of charging/discharging cycles is computed (S4) with respect to capacity retention ratio by using types of diffusion coefficients with respect to the solvent molecules and physical parameter of physical model. The mean square error of measurement data in preset number of charging/discharging cycles, and calculated data is computed (S5). The value of diffusion coefficient from which the mean-square-error is at minimum is selected (S8).    USE - Method for predicting life of lithium ion battery of smart phone, electric vehicle, and industrial battery.    ADVANTAGE - The calculation data of preset number of charging/discharging cycles is computed with respect to capacity retention ratio by using types of diffusion coefficients with respect to the solvent molecules and physical parameter of physical model and the value of diffusion coefficient from which the computed mean square error of measurement data in preset number of charging/discharging cycles, and calculated data is at minimum is selected so as to generate the precursor of passive film and to give physical interpretation. Thus, the lifetime of lithium ion battery with the charging/discharging cycling characteristics and reserve time characteristic of capacity retention ratio, elucidation of physical mechanism of capacity degradation is improved effectively.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a life expectancy predicting system of Li ion battery.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart illustrating the process of predicting life of lithium ion battery. (Drawing includes non-English language text)    Step for defining physical model of lithium ion cell (S1)    Step for setting physical parameters such as reaction rate coefficient with which precursor of passive film is formed (S3)    Step for computing calculation data of preset number of charging/discharging cycles with respect to capacity retention ratio by using types of diffusion coefficients with respect to solvent molecules and physical parameter of physical mode (S4)    Step for computing mean square error of measurement data in preset number of charging/discharging cycles and calculated data (S5)    Step for selecting diffusion coefficient value from which mean-square-error is minimum (S6)