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 phenomenaduringhigh-temperature calcination of Ni-based layered cathode materialsfor Li-ion batteries, leading to the formation of a cation-disorderedrock salt phase on the surface of cathode particles. The rock saltphase, which is made of divalent transition metal oxides, is vulnerableto reaction with the liquid electrolyte and requires a surface coatingto suppress transition metal dissolution and to enhance cycling stability.In this study, a layered LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode material with a cation-disorderedrock salt phase on its surface is surface-modified via H3PO4 treatment. A uniform coating layer is formed on thesurface of NMC particles after calcination at 375 or 475 & DEG;C.However, impedance measurements along with the analysis of distributionof relaxation times (DRT) depicted that the higher calcination temperatureled to a more stable and less reactive coating layer in contact withthe electrolyte, as it exhibited lower cathode-electrolyteinterphase (CEI) layer resistance. Additionally, the coating layerformed at 475 & DEG;C increases the capacity retention of the pristineNMC by 10.3 and 19.4% after 100 cycles at room temperature and 50 & DEG;C, respectively. Besides, the coating layer successfully enhancedthe rate capability of the NMC electrode, which is attributed to thesuppression of transition metal dissolution, providing a lower chargetransfer resistance compared to the pristine NMC.