Prof. Won-Sub Yoon’s research team successfully derives the key factors on the thermal instability of Ni-rich cathode material for lithium-ion batteries. This research work was published in the Advanced Science (IF: 15.804) on April 24 as a back cover, with the title "Tracking the Influence of Thermal Expansion and Oxygen Vacancies on the Thermal Stability of Ni‐Rich Layered Cathode Materials."

The extensive demand for high energy storage in electric devices, electric vehicles and smart grid systems has led to the commercialization of rechargeable lithium ion batteries using Ni-rich layered cathode material. Simultaneously, the safety issue from the thermal instability of Ni-rich cathode materials has been introduced. Until now, the strategy to enhance its thermal stability has been approached based on fragmentary facts that the thermal instability is merely attributed to the thermally unstable Ni element. However, these approaches have limitations in providing a custom solution to the fundamental cause of thermal instability involving Ni element.

Prof. Won-Sub Yoon's research team showed that the thermal expansion of Li slab become larger as the increase of Ni content in the cathode material before the onset temperature of layered to disordered spinel phase transition is reached. This thermal expansion of Li slab induces the elongation of intermediate tetrahedron in pathway of cation migration, resulting in an expanded pathway with a low energy barrier for cation migration. In addition, the oxygen vacancies are formed and accumulate around only Ni ions before the onset temperature of phase transition is reached. In presence of oxygen vacancies, the cation migration pathway is energetically mitigated as Ni ion migrates through the face-shared oxygen plane that does not contain oxygen vacancies. Herein, the team proposed the thermal expansion and oxygen vacancies as new critical factors to affect thermal stability of charged Ni-rich cathode materials as shown in the figure below.

[Figure 1] Occurrence of thermal expansion and oxygen vacancies in Ni-rich layered cathode materials due to increase of temperature and the process of change in the crystal structure and phase transition

※ Title: Tracking the Influence of Thermal Expansion and Oxygen Vacancies on the Thermal Stability of Ni‐Rich Layered Cathode Materials
※ Original Article URL: https://onlinelibrary.wiley.com/doi/full/10.1002/advs.201902413