Failure Mechanism And Direct Regeneration Strategy Of Retired Power Battery Ternary Cathode Materials

Lithium-ion batteries（LIBs）play an important role in the field of energy storage and electrification due to the long cycle life,high power density and low maintenance costs.Especially in the context of carbon neutrality,the demand for power batteries in the electric vehicle industry is growing exponentially.However,fluctuating material costs and limited resources for key materials make power batteries a serious material supply problem,especially for the expensive LIB cathode materials.On the other hand,the introduction of a large number of LIBs in electric vehicles will result in a huge amount of retired power batteries within eight to ten years（about 11 million tonnes of discarded power batteries are expected to be generated by 2030）,which will cause significant environmental and ecological pressure if the batteries without properly disposed of.The recycling of batteries is an effective way to address both the ecological and resource pressures.However,the failure mechanism of retired batteries is not yet clear,and there is no research on how to effectively recover retired electrode materials and regenerate them directly for different levels of failure.At the same time,how to elucidate the mechanism of direct regeneration and rational design of the regeneration process are key issues that need to be addressed.This paper investigates the recycling and direct regeneration of LiNi_0.5Co_0.2Mn_0.3O₂（NCM523）cathode electrodes from waste batteries,including the pre-treatment of electrode materials,the characterization of the failure of NCM523 cathode materials with different degrees of failure,the investigation of the mechanism,the repair and regeneration technology of NCM523 with different degrees of failure,the expansion and application of the repair and regeneration solution,and the economic evaluation of the regeneration solution.The main conclusions of the paper are as follows.（1）This work systematically investigates the discharge and disassembly of NCM523-based LIBs and the pretreatment process of the positive electrode.A mechanical crushing-stripping scheme was added before the pretreatment,followed by NMP immersion treatment of the stripped NCM523 powder combined with high-temperature sintering to obtain pure cathode powder.The pretreated cathode material was assembled into a 10 Ah pouch cell and the capacity was maintained at over 90%after 500 cycles,with favorable cycling stability.（2）This work presents an in-depth analysis of the failure mechanism of NCM523single crystal cathode materials with shallow failure due to long-time storage.It shows that when NCM523 cathode materials are exposed to humid air,they react easily with H₂O and CO₂ to form LiOH,LiHCO₃ and Li₂CO₃ on the surface of the particle,and that an indefinite phase and a small amount of rock-salt phase appear on the near-surface of the material.The shallowly degraded NCM523 can be regenerated by direct high-temperature annealing of the cathode material.（3）The failure mechanism of the deeply degraded NCM523 was analyzed by spherical differential electron microscopy,in-situ X-ray diffraction analysis,in-situ electrochemical impedance testing and density flooding theory.The formation of a cathodic electrolyte interface on the material surface,cation mixing,gas production,formation of oxygen atom vacancies,internal crystal cracks and the formation of rock-salt phases ultimately led to the deeply degraded NCM523 cathode material.The capacity of the deeply degraded NCM523cathode material is less than 10%of that of commercial NCM523.The deeply degraded NCM523 cathode material has a capacity of only 3.8 m Ah g^-1 at 1 C.The CV test shows no redox peaks and the in-situ EIS does not respond to voltage,indicating that the deeply degraded NCM523 cathode material has poor electrochemical performance.（4）This work adopts a hydrothermal synergistic molten salt solution for the regeneration of the deeply degraded NCM523 cathode powder.The hydrothermal material was directly centrifuged and dried,so that the surface of the degraded material was coated with a LiOH shell layer,which helped the material to come into uniform contact with the lithium salt so that the rock salt phase within the bulk phase of the material could react to form a layered material.The surface of the regenerated cathode material is coated with a layer of LiF,which effectively prevents the oxidation of the electrolyte on the active surface of the material.Which in turn leads to a better symmetry of the charge/discharge curve and a higher Coulomb efficiency of the material.Finally,the electrode material was prepared into a 1.7 Ah pouch cell,and the capacity could still be maintained at 90.8%after 500 cycles.（5）In this work,the regeneration technology of hydrothermal synergistic molten salt was extended to the regeneration of NCM111 as well as NCM523 of retired power batteries,and the capacity was all at commercial levels at 1 C.At the same time,the economic valuation of the production line equipment as well as raw materials was evaluated.If the LiOH solution is reused in the production process,the economic benefit of RMB 100000per ton of NCM-based cathode material can be achieved,which has a large profit margin.Finally,an in-depth discussion on the possibility of forming an integrated model of industry-academia-research cooperation and industrialization application was further analyzed.In summary,this work presents a reliable pretreatment solution for the cathode material of used batteries and provides a detailed characterization and in-depth discussion of the failure mechanisms of NCM523 with different failure levels.On the basis of this,a hydrothermal synergistic molten salt method is used for the repair and regeneration of NCM523 with different failure levels,and the repair and regeneration mechanism is elucidated.Finally,this repair and regeneration scheme can be extended to the recovery of NCM111 and NCM523 cathode materials from retired power batteries,and has shown great economic and environmental benefits,thus providing a new feasible idea for the direct regeneration of used batteries,especially the expensive cathode materials.