Influence of over-discharge on the lifetime and performance of LiFePO4/graphite batteries

In this study, the degradation of a LiFePO4/graphite battery under an over-discharge process and its effect on further cycling stability are investigated. Batteries are over-discharged to 1.5, 1.0, 0.5 or 0.0 V and then cycled 110 times under over-discharge conditions. The batteries over-discharged to 0.5 and 0.0 V experience serious irreversible capacity losses of 12.56% and 24.88%, respectively. The same batteries lost 7.79 and 24.46% more capacity after they were further subjected to 110 cycles between 3.65 and 2.0 V at 1C/1C, respectively. This shows that a serious loss of active lithium and loss of anode material occur at 0.0 V during both over-discharging and the normal cycling stage. Dissolution and breakdown of solid electrolyte interphase (SEI) films are suggested to be the main reason for degradation under over-discharge at low voltage and further lead to a poor cycling performance. Gas generation can be found on the cycled batteries below 1 V and the gas mainly contains H-2, CH4 and C2H6. The structures of LiFePO4 and graphite materials have almost no change according to the results of XRD tests. Half-cell study suggests that almost no irreversible capacity loss occurs at the LiFePO4 cathode, whereas a decline in the capacity is observed at the graphite anode, especially for the batteries over-discharging bellow 1.0 V. Evidence for fierce side reactions at 0.5 and 0.0 V is provided as well, as demonstrated by the developed rich surface chemistry and an significant impedance increase for the aged electrodes.