Surface Growth and Intergranular Separation of Polycrystalline Particles for Regeneration of Stable Single-Crystal Cathode Materials.

The direct regeneration technology has been developed because of its short-range, high efficiency, and green characteristics. However, the existing direct regeneration method is hardly applied in collaborative reconstruction of the damaged crystal and particle of spent polycrystalline layered materials. The single-crystal regeneration with restructuring the morphology and crystal structure was herein achieved for the first time by low-temperature lithium supplementation followed with high-temperature molten salt conversion, which could effectively solve the structural defects of spent polycrystalline layered materials. We found that the realization of single-crystal regeneration with the molten salt process is attributable to that the original crystal growth of primary particles in the polycrystal transfer to the subsequent division along the grain boundary. At the test conditions of 25 °C and 2.8-4.3 V, the capacity retention capacity of the regenerated single-crystal materials reach 83.3% after 200 cycles at 1 C, which is much higher than 20.0% for conventional direct lithiation regeneration and 61.6% for low-temperature molten salt regeneration. Interestingly, the regenerated single-crystal NCM622 in the graphite full-cell test displays a capacity retention rate of 85.24% after 800 cycles at a rate of 1 C at 2.5-4.35 V. This work opens up a new way for the direct regeneration of spent polycrystalline layered cathode materials.