Capacity fading of a LiFePO₄/graphite cell during shallow cycling due to planar inhomogeneity in LiFePO₄

The capacity fading mechanism of a LiFePO4/graphite cell (nominal capacity 3 Ah) was investigated. We report that capacity fading resulted from the centimeter-scale planar inhomogeneous reaction on the LiFePO4 cathode after operation at a shallow state of charge (SOC), operated between SOC 40% and 60%. It was confirmed by (i) wide variations of electrode capacities of the disassembled electrode and by (ii) the two-dimensional X-ray diffraction (XRD) analysis. Li-ion content could be estimated by (i) comparing the residual capacities of several electrode regions of the LiFePO4 sheet after disassembly of the cell and (ii) the intensity ratio variations of the typical XRD peaks of LiFePO4 and FePO4 at 5 min x 5 mm grids. The inhomogeneity was induced by using only the two-phase region in the LiFePO4. Electrode reversible capacities are not a dominant factor for capacity fading, and we found the inhomogeneity has a possibility for restoration. Therefore, we should consider a suitable operating procedure of the storage system for electricity grid use with long-term service life.