Tracking the Influence of Thermal Expansion and Oxygen Vacancies on the Thermal Stability of Ni‐Rich Layered Cathode Materials

Abstract

The ever‐growing demand for high‐energy lithium‐ion batteries in portable electronics and electric vehicles has triggered intensive research efforts over the past decade. An efficient strategy to boost the energy and power density of lithium‐ion batteries is to increase the Ni content in the cathode materials. However, a higher Ni content in the cathode materials gives rise to safety issues. Herein, thermal expansion and oxygen vacancies are proposed as new critical factors that affect the thermal stability of charged Ni‐rich cathode materials based on a systematic synchrotron‐based X‐ray study of Li0.33Ni0.5+xCo0.2Mn0.3‐xO2 (x = 0, 0.1, 0.2) cathode materials during a heating process. Charged cathode materials with higher Ni contents show larger thermal expansion, which accelerates transition metal migration to the Li layers. Oxygen vacancies are formed and accumulate mainly around Ni ions until the layered‐to‐spinel phase transition begins. The oxygen vacancies also facilitate transition metal migration to the Li layers. Thermal expansion and the presence of oxygen vacancies decrease the energy barrier for cation migration and facilitate the phase transitions in charged cathode materials during the heating process. These results provide valuable guidance for developing new cathode materials with improved safety characteristics.

A Three in One Strategy to Achieve Zirconium Doping, Boron Doping, and Interfacial Coating for Stable LiNi0.8Co0.1Mn0.1O2 Cathode

“…Note that the discharge capacities of all doped samples at 3C and 4C are much higher than that of the pristine one, indicating that rate performance can be improved after ZrB 2 treatment. The excellent rate performance of the modified sample should be attributed to the higher lithium-ion diffusion coefficient (D Li + ) due to the enlargement of the unit cell ( Figure S10, Supporting Information), [36] indicating www.advancedsciencenews.com www.advancedscience.com that ZrB 2 doping can offer a suitable framework to facilitate ion transport kinetic.…”

Role of Residual Li and Oxygen Vacancies in Ni-rich Cathode Materials

“…Particularly, with increasing Ni content, the average bonding energy decreases due to more O−Ni bonds with lower bonding energy taking the place of Co−O or Mn−O bonds. 22,23 As a result, the Ni-rich cathode material is thermally unstable and easily decomposes during the high-temperature sintering process. 24−26 Such decomposition leaves residual Li on the surface and forms oxygen vacancies in the bulk structure, largely influencing the electrochemical properties of Ni-rich materi-als.…”

Effect of high Ni on battery thermal safety

“…Xie 16 believed that when the Li removal exceeds 70%, the H2‐H3 phase transition occurs and the c‐axis shrinks, causing the layered structure to collapse and performance to deteriorate. Lee 17 studied the effects of thermal expansion and oxygen vacancies on the thermal stability of Ni‐rich layered cathode materials. Charged cathode materials with a higher Ni content have greater thermal expansion, and oxygen vacancies promote the migration of transition metals to the Li layer.…”