Abstract:

Mechanical failure is a major problem faced by ternary oxide cathode materials in high-capacity applications. In this paper, single crystal and polycrystalline NMC811 materials were prepared using the molten salt and coprecipitation methods, respectively. The evolution and mechanical properties of single crystal and polycrystalline NMC811 materials during the electrochemical process were studied using XRD, FIB-SEM, and stress analysis. The relationship between crack initiation and propagation, as well as the stress and strain, of single crystal and polycrystalline NMC811 materials during the charging and discharging processes was established, and the cause of the structural stability degradation of the material during lithium intercalation was revealed. The experimental results show that after 100 weeks of charging and discharging at 0.5 ℃, almost no cracks appear, and the residual stress of the single crystal NMC811 material is small. However, after 100 cycles of charging and discharging at 0.5 ℃, a large number of cracks appeared along the grain boundary in the polycrystalline NMC811 material, and the maximum residual stress was three times that of the single crystal material. Thus, the cycle and rate performances of the single crystal NMC811 material are better than that of the polycrystalline NMC811. The preparation and development of single crystal NMC811 materials will be an important method of restraining crack propagation during charging and discharging and improving the life cycle of high nickel ternary materials.

Key words: lithium-ion battery, NMC811, polycrystalline and single crystal, stress and strain