Microstructural visualization of structural and compositional changes induced by transition metal dissolution in Ni-rich layered cathode materials by high-resolution particle analysis
Journal
Nano Energy
Date
2019.01.30
Abstract
The dissolution of transition metals in LiMO2 (M = Ni, Co and Mn) cathodes and their subsequent side reactions on the anode and electrolyte in Li-ion battery are well known phenomena resulting the capacity and power degradation. However, it is not yet accurately revealed the microstructural origins and its mechanism of transition metals dissolution, because it is difficult to find adequate analysis method for keeping the tracking of subtle changes in the chemical compositions corresponding specific location. In this study, we probe the dissolution of transition metals from Ni-rich layered cathode, deposition on graphite anode and reactions with electrolyte due to capacity degradation in aspect of microstructure. The changes of transition metals and oxygen in Ni-rich NCM (LiNi0.87Co0.09Mn0.04O2) cathode materials are visualized on two-dimensional microstructure using by high-resolution chemical particle analysis throughout transmission electron microscopy and X-ray energy dispersive spectroscopy. As a result, there is not much chemical difference between pristine and degraded Ni-rich NCM (capacity retention rate is less than 80 ％) particles only with grain-boundary cracking due to repeating the expansion and contraction of volume by charge/discharge. But, in case of broken and pulverized particles, which is very heavily fracture and broken so that the surface of particles are exposed directly to the electrolyte, in degraded Ni-rich NCM, it showed that the chemical compositions of all transition metals was decreased throughout sectionalized chemical maps. Therefore, the dissolution of transition metals, which is one of origins for the material degradations, would have occurred actively at the interface of cathode materials and electrolyte.