Recycling
of spent lithium-ion batteries is extremely urgent with
their increasing decommission. In this work, eutectic molten salts
of LiOH–Li2CO3 used as lithium sources
for direct regeneration of LiNi0.5Co0.2Mn0.3O2 were developed. Based on the phase diagram
of LiOH and Li2CO3, the effects of different
lithium sources on material regeneration have been investigated. The
cathode materials regenerated with eutectic molten salts have high
capacity, good cycling performance, and rate performance. The discharge
capacities during the 1st and 200th cycles at 1 C are 146.3 and 130.3
mA h g–1, respectively, and the capacity retention
rate reaches 89.06%. Using the combined X-ray diffraction (XRD), high-resolution
transmission electron microscopy (HRTEM), and X-ray photoelectron
spectroscopy (XPS) analysis, the original layered structure of spent
cathode materials was restored. Therefore, the eutectic molten salt
of LiOH–Li2CO3 is feasible for direct
regeneration of spent 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.
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