Tracking the Influence of Thermal Expansion and Oxygen Vacancies on the Thermal Stability of Ni-Rich Layered Cathode Materials.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.  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+  Co0.2Mn0.3-  O2 (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.  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. 
THERM