Combination of float charging and occasional discharging to cause serious LIB degradation analyzed by operando neutron diffraction

Abstract

With the widespread use of lithium-ion batteries (LIBs) for multiple applications, understanding the degradation factors of LIBs is essential for their life extension. In this work, durability tests of commercially available 18650 LIB cells were performed for about 400 days with float charging (floating), continuous cycling (cycling), and float charging with occasional discharging (floating–cycling) with the intention to mitigate the film formation during the float charging. The results indicated that severe capacity degradation was observed in floating–cycling, which was much worse than the sum of the degradation by floating and cycling. The dV/dQ curve and impedance analyses suggested that this serious cell degradation is due to the deterioration of the positive electrodes. Operando neutron diffraction analysis of the cells degraded by the floating–cycling mode revealed considerable reaction inhomogeneity in the positive electrode, and the Rietveld refinement of the lattice parameter transitions suggested that the origin of the reaction inhomogeneity is the internal resistance increase in the positive electrode to limit the available capacity ranges of the cell. Post-mortem analysis of the positive electrode degraded after the floating–cycling test indicated that the cracks caused by repeated charging–discharging cycles were covered with thick resistive films by the floating operation, leading to the non-uniform increase in impedance and the reaction inhomogeneity in the positive electrode. The deterioration factors and mechanisms found in this study will give insights on the capacity loss in the LIBs and promote the development of the strategy to reduce the LIB degradation.