Degradation Behavior in Lithium Iron Phosphate Secondary Cells Under High Rate Pulsed Discharge

Three 26650 LiFePO4 (LFP) cells cycled at 15C (40 A) under a pulsed discharge (PD) profile showed an abrupt drop in their 15C recharge capacity to 0 Ahr within a 60 cycle span. EIS at 100% state of charge (SOC) shows separation in the mid-frequency semi-circle indicating rapid increase of one electrochemical component’s impedance. XAS analyses on cycled cathodes show 85% retention of active material at the bulk level and almost 100% in the surface region. XPS on PD anodes show surface films that are rich in LiF which have high impedance compared to Li2CO3 rich films. LiF rich films on the anode cause the real impedance of the anodic half-cell to increase faster than the other, slowing the charge transfer kinetics of the battery dramatically. Results indicate the bipolar PD profile increases the rate of LiF deposition on the surface of the anode compared to high rate continuous cycling.