Advanced single-crystal layered Ni-rich cathode materials for next-generation high-energy-density and long-life Li-ion batteries

Benefiting from the high specific capacity and output voltage, Ni-rich layered oxides are one of the most promising commercial cathodes for next-generation high-energy-density Li-ion batteries (LIBs). However, typical Ni-rich cathode materials generally inherit polycrystal (PC) morphology from their precursors, which induces severe crack generations and side reactions, resulting in the rapid decay of structural and electrochemical stability upon cycling. In contrast, the Ni-rich cathodes with single-crystal (SC) morphology display remarkable structural stability and long cycle life by means of superior mechanical strength and limited side reactions, which is expected to solve the instinctive problems of PC counterparts. Herein, the synthesis strategies and the growth mechanisms of SC Ni-rich oxides are introduced and summarized in detail. Moreover, the significant differences in electrochemical behaviors between PC and SC cathodes are comprehensively compared in various voltage windows. Furthermore, the corresponding structural evolutions and morphology changes are also systematically investigated and analyzed. Additionally, the state-of-the-art characterization techniques for SC materials are elaborated. Altogether, in this review, we not only unravel the fundamental understandings of SC Ni-rich cathode materials but also provide an effective guide for realizing high-energy-density LIBs with long cycle life.