Thermal Expansion Neutralization Enhancing the Cycling Stability of Ni-Rich LiNi_0.6Co_0.2Mn_0.2O₂ Cathode Material

As promising cathode candidates with advantageous capacity and price superiority for lithium-ion batteries, Ni-rich materials are severely impeded in the practical application due to their poor microstructural stability induced by the intrinsic Li⁺/Ni²⁺ cation mixing and mechanical stress accumulation upon cycling. In this work, a synergetic approach is demonstrated to improve the microstructural and thermal stabilities of Ni-rich LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) cathode material through taking advantage of the thermal expansion offset effect of the LiZr₂(PO₄)₃ (LZPO) modification layer. The optimized NCM622@LZPO cathode exhibits a significantly enhanced cyclability with a capacity retention of 67.7% after 500 cycles at 0.2 C and delivers a specific capacity of 115 mAh g^-1 with a capacity retention of 64.2% after 300 cycles under 55 °C. Exploiting the chemical environment analysis of the Ni element detected by the synchrotron radiation technique, it is found that the mixing degree of Li⁺/Ni²⁺ cations in the bulk Ni-rich material can be effectively depressed through interfacial Zr⁴⁺ doping during the preparation of the LZPO-modified material. Additionally, time- and temperature-dependent powder diffraction spectra were collected to monitor the structure evolutions of pristine NCM622 and NCM622@LZPO cathodes in the initial cycles and under various temperatures, revealing the contribution of negative thermal expansion LZPO coating in promoting microstructural stability of the bulk NCM622 cathode. The introduction of NTE functional compounds might provide a universal strategy to address the stress accumulation and volume expansion issues of various cathode materials for advanced secondary-ion batteries.