Improved Electrochemical Performance of the Cation-Disordered NMC Cathode of Lithium-Ion Batteries by Lithium Phosphate Coating

Li loss and cation mixing are two highly probable phenomena during high-temperature calcination of Ni-based layered cathode materials for Li-ion batteries, leading to the formation of a cation-disordered rock salt phase on the surface of cathode particles. The rock salt phase, which is made of divalent transition metal oxides, is vulnerable to reaction with the liquid electrolyte and requires a surface coating to suppress transition metal dissolution and to enhance cycling stability. In this study, a layered LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode material with a cation-disordered rock salt phase on its surface is surface-modified via H3PO4 treatment. A uniform coating layer is formed on the surface of NMC particles after calcination at 375 or 475 °C. However, impedance measurements along with the analysis of distribution of relaxation times (DRT) depicted that the higher calcination temperature led to a more stable and less reactive coating layer in contact with the electrolyte, as it exhibited lower cathode–electrolyte interphase (CEI) layer resistance. Additionally, the coating layer formed at 475 °C increases the capacity retention of the pristine NMC by 10.3 and 19.4% after 100 cycles at room temperature and 50 °C, respectively. Besides, the coating layer successfully enhanced the rate capability of the NMC electrode, which is attributed to the suppression of transition metal dissolution, providing a lower charge transfer resistance compared to the pristine NMC.