Unexpected capacity fade and recovery mechanism of LiFePO4/graphite cells for grid operation

The capacity fade mechanism of large-scale LiFePO4/graphite cells (>100 Wh) with shallow state of charge (SOC) cycling was investigated. In the case of shallow SOC operation, for ΔSOC = 20% between SOCs of 40% and 60%, the cells showed capacity fade after 250 days of operation. However, the tentatively lost capacity was recovered by subsequent full SOC cycling operation, and the highest capacity recovery of 20% was recorded at C/2 after the recovery cycling. The phenomena cannot be explained by the previously reported negative electrode overhung model or the partial plating of lithium. In this study, we have proposed a model where an inhomogeneity of the LiFePO4 planar surface induces the decrease in capacity because of the flat potential operating region of LiFePO4, and this decrease in the cell capacity can be restored by full SOC operation. Large capacity energy storage systems connected to the electric grid require the use LiFePO4/graphite cells for medial SOC operation because of the low installation cost and the requirement for rapid response of the battery system. In such cases, we should consider the properties of the electrodes in the cell.