The genesis and control of microcracks in nickel-rich cathode materials for lithium-ion batteries

Due to the advantages of high reversible capacity and low cost, Ni-rich layered metal oxides (NROs) are considered among the most competitive cathode materials for the next generation of lithium-ion batteries (LIBs). Despite the obvious contribution to energy density from increasing Ni content, the development of NROs is inevitably challenged by the severe chemical and structural instability, especially for Ni contents higher than 80%, which were manifested by notorious chemo-mechanical problems including parasitic reactions with organic electrolytes and continuous structural failure during extended cycles, thereby leading to serious problems related to the reliability and safety of LIBs. Particularly, the formation of microcracks inside the NRO particles attributed to the uneven stress field is considered a characteristic feature, whose evolution inside the particles continues to expose new electrode-electrolyte interface, accordingly aggravating the cycle stability and jeopardizing their practical application. Herein, we update the knowledge on NRO microcracks starting with a detailed discussion on those essential factors trigging their formation, and then the crack-related failure mechanism of NRO particles was introduced to elucidate the structure-performance relationship of NRO microcracks. Different control strategies focusing on modulating the physicochemical properties both on the surface and in the bulk of NRO particles are analyzed to clarify their contribution to alleviating the adverse impact of the microcracks. We also envision future research directions toward crack-free NRO materials so that robust cathode materials with high energy density and high cycling stability could be simultaneously ensured for next-generation LIBs. A schematic diagram of the formation, detrimental impacts of microcracks, and the corresponding modification strategies.