The Effects of Cycling Rates and Cycling Temperature on the Battery Degradation in Vehicle-to-Grid Applications

Commercial LiFePO4/graphite batteries produced by A123 Systems were cycled by simulating urban drive modes and vehicle-to-grid (V2G) modes at the 1 C and 2 C V2G discharge rates under 25 degrees C and 45 degrees C environmental temperature. Their degeneration was explored by characterization of capacity attenuation, electrochemical impedance spectroscopy (EIS), charge/discharge test of two half cells, BET Surface Area, XRD, FE-SEM and EDS. Results show that the capacity declines obviously with the rise of V2G discharge rates and environmental temperature. With the progress of the cycle test, the ohmic resistance (R-s) doesn't change a lot before the first 900 cycles and rises a great deal during the second 900 cycles while the resistance of charge transfer (R-ct) is relatively low and the diffusion coefficient of lithium ion (D-Li(+)) climbs up and then declines. Moreover, the rise of temperature and discharge rates mainly results in the increase of R-s. The degradation of battery separator arisen from the rise of V2G discharge rates and environmental temperature is one of reasons that the battery performance was reduced. The ascend of temperature makes a more negative influence on the attenuation of anodes than that of cathodes while the rise of V2G discharge rates has similar negative influence on the degradation of cathodes and anodes.