Degradation analysis of a Ni-based layered positive-electrode active material cycled at elevated temperatures studied by scanning transmission electron microscopy and electron energy-loss spectroscopy

We investigate the local structural changes in a positive electrode of a lithium ion secondary battery (LiNi0.8Co0.15Al0.05O2 (NCA) as the active material) associated with charge–discharge cycling at elevated temperatures by scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS). STEM–EELS spectral imaging reveals the evolution of a NiO-like phase localized near the surface and grain boundary regions after many cycles. The amounts of capacity fading and resistance increase are discussed based on the results of the semiquantitative estimation of NiO-like and other product phases. We also identify the chemical state of lithium in the NiO-like phase substituting for Ni.

► We reports quantitative analyses of the NCA-type cathode material in a Li-ion secondary battery. ► The spatial distributions of the degraded phase at nanometer resolution were imaged. ► The amount of the degraded phase successfully explained the macroscopic capacity fading.