Abstract: Ni-rich layered oxide cathode materials hold great promise for enhancing the energy density of lithium-ion batteries (LIBs) due to their impressive specific capacity. However, the chemical and structural stability issues associated with the materials containing a high Ni content have emerged as a primary safety concern, particularly in the context of traction batteries for electric vehicles. Typically, when these materials are in a highly charged state, their metastable layered structure and highly oxidized transition metal ions can trigger detrimental phase transitions. This leads to the generation of oxygen gas and the degradation of the material’s microstructure, including the formation of cracks, which can promote the interactions between Ni-rich materials and electrolytes, further generating flammable gases. Consequently, various strategies have been devised at the material level to mitigate potential safety hazards. This review begins by providing an in-depth exploration of the sources of instability in Ni-rich layered oxides, drawing from their crystal and electronic structures, and subsequently outlines the safety issues that arise as a result. Subsequently, it delves into recent advancements and approaches aiming at modifying Ni-rich cathode materials and electrolytes to enhance safety. The primary objective of this review is to offer a concise and comprehensive understanding of why Ni-rich cathode materials are susceptible to safety incidents and to present potential methods for improving the safety of Ni-rich cathode materials in high-density LIBs.

Key words: Lithium-ion battery, Ni-rich layered oxide, Cathode material, Battery safety, Thermal runaway