Effect of crystal morphology of ultrahigh-nickel cathode materials on high temperature electrochemical stability of lithium ion batteries

Higher nickel content endows Ni-rich cathode materials LiNi_xCo_yMn_1−x−yO₂ (x > 0.6) with higher specific capacity and high energy density, which is regarded as the most promising cathode materials for Li-ion batteries. However, the deterioration of structural stability hinders its practical application, especially under harsh working conditions such as high-temperature cycling. Given these circumstances, it becomes particularly critical to clarify the impact of the crystal morphology on the structure and high-temperature performance as for the ultrahigh-nickel cathodes. Herein, we conducted a comprehensive comparison in terms of microstructure, high-temperature long-cycle phase evolution, and high-temperature electrochemical stability, revealing the differences and the working mechanisms among polycrystalline (PC), single-crystalline (SC) and Al doped SC ultrahigh-nickel materials. The results show that the PC sample suffers a severe irreversible phase transition along with the appearance of microcracks, resulting a serious decay of both average voltage and the energy density. While the Al doped SC sample exhibits superior cycling stability with intact layered structure. In-situ XRD and intraparticle structural evolution characterization reveal that Al doping can significantly alleviate the irreversible phase transition, thus inhibiting microcracks generation and enabling enhanced structure. Specifically, it exhibits excellent cycling performance in pouch-type full-cell with a high capacity retention of 91.8% after 500 cycles at 55 °C. This work promotes the fundamental understanding on the correlation between the crystalline morphology and high-temperature electrochemical stability and provides a guide for optimization the Ni-rich cathode materials.