Xuejie Huang

Abstract:

With the increasing demand for advanced energy storage devices in portable electronics and electrical vehicles, nikel-rich LiNixCoyMn1-x-yO2 (with x ≥ 0.8) layered cathode materials, which are low cost, have low toxicity, high practical specific capacity (> 200 mAh/g), and a relatively high operating voltage, are attracting increasing attention for their promising applications in next-generation cathode materials.[1] However, Ni-rich cathode materials have poor electrochemical performance, particularly during cycling at elevated temperatures, which significantly limits their application.
X-ray nano-computed tomography (nano-CT) and deep learning combined with Cs- corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy were employed to investigate the atomic to microscopic structural evolution of LiNi0.8Co0.1Mn0.1O2 (NCM) upon cycling at 55 ◦C.[2] Two types of intergranular cracks were clearly distinguished by nano-CT for cycled cathode particles; denoted open and closed cracks depending on whether or not the cracks reach the surface of the NCM secondary particles. The volume of high-temperature cycling-induced cracks quantified by deep learning increased drastically, particularly for the open cracks, and this phenomenon was accompanied by rapid degradation of capacity retention. Further precise STEM analysis of the crack regions revealed that
migration of transition metal (TM) ions to the Li layer forms a rocksalt-like structure, and the associated reduction of TM ions, e.g., Ni3+ to Ni2+, predominately occurred in the open crack regions in the presence of penetrated electrolyte, even for regions extending to the center of the secondary particle. In contrast, in the closed crack regions, no significant atomic-scale structure distortion and limited reduction of TM
ions was observed.