Restoring the Crystal Structure of Single-Crystalline High Ni Ncm with Synthetic Heat Treatment Profile Control

Abstract

Single-crystalline high Ni NCM is regarded as a next generation cathode candidate for high-energy lithium ion batteries with great potential due to its advanced crystal structural and chemical stability. The synthesis of single-crystalline high Ni NCM was studied using in-operando heating XRD. The change of the crystal structure of single-crystalline NCM was observed while raising the synthesis temperature. The synthesis was carried out in two steps, with the first step at 860 degrees to form the phase and grow the particle size, and the second step around 740 degrees to recover the collapsed structure by the first heat treatment. XRD data and Rietveld refinement results showed that the half width of (104) peak decreased over time in each step, but tended to increase as it moved from 1st step to 2nd step. In addition, it was confirmed that Li/Ni cation mixing maintained a high value at the 1st step but then steadily decreased at the 2nd step, which means relatively low temperature heat treatment is effective to recover crystal structural anti-site defects. To evaluate the electrochemical properties of structural restoration, coin half cell characteristics were evaluated. It was showed that the longer time of 2nd step of the heat treatment, the higher efficiency and capacity due to structural recovery.Overall, this study provides insights into the synthesis of single-crystalline high Ni NCM and their crystal structure changes under real-time heating. This findings highlight the importance of the two-step synthesis process and the role of relatively low temperature of 2nd step heat treatment that can reduce Li/Ni cation mixing through structural recovery.