Research On Direct Recycling Of Waste LiNi_0.5Co_0.2Mn_0.3O₂ Cathode Material

Lithium-ion batteries are widely used in Electric Vehicles due to their superior electrochemical performance.However,the nickel and cobalt resources required for Li Ni_xCo_yMn_zO₂（NCM）cathode materials mainly rely on imports.To achieve sustaina-ble development of lithium-ion batteries,comprehensive utilization of resources,and environmental protection,there is an urgent need to study the recycling and utilization of waste NCM cathode materials.In this work,the spent Li Ni_0.5Co_0.2Mn_0.3O₂（NCM523）cathode material was used as the study material.The objective is to directly regenerate and repair the NCM523 by addressing issues such as the loss of Li source within cathode materials,sodium residue after pre-processing,electrolyte decomposition after recycling and pre-processing,and mixed lithium manganate cathode materials.The crystal structure and morphology of materials were repaired through relithiation with eutectic molten salt,the doping of residual sodium,coating with Li F,Li₃PO₄,and Li₂Mn O₃,thus enabling the spent NCM523 with improved electrochemical performance.The main points of the thesis are as follows:Firstly,the effects of relithiation with different lithium sources on the reconstrcu-tion of the structure and morphology,and the improvement of electrochemical perfor-mance of spent NCM523 were investigated.The crystal structure of NCM523 cathode material comes back to the layered structure after regeneration with different lithium sources.The NCM523 regenerated by Li OH-Li₂CO₃ eutectic molten salt shows the most complete layered structure,and the lowest Li/Ni cation mixing,and the first and 200cycles are 146.3 and 130.3 m A h g^-1 at 1C,respectively,and a capacity retention rate of up to 89.06%.This should be ascribed to the fact that Li?of the eutectic molten salt diffuses into the bulk of spent NCM523 and reacts to restore the missing lithium to the original lithium sites at high temperature.The crystal structure of the regenerated NCM523 material is restored to an ordered layered structure via two stages of calcina-tion,and the Li/Ni cation mixing is reduced,thus improving the electrochemical properties of the regenerated NCM523.Secondly,Li_（1-x）Na_xNi_0.5Co_0.2Mn_0.3O₂ was prepared by using residual sodium during the direct regeneration process.The effects of different amounts of Na-doping on the phase structure,microstructure,and electrochemical properties of the recovered materials have been systematically investigated.It was shown that rock-salt structure within NCM523 has recovered into a layered structure after sodium doped.The electro-chemical properties of the regenerated material were significantly improved,with the cyclic and rate performance being the best at 3 mol%sodium.At 1 C,the discharge capacity was 136.4 and 120 m A h g^-1 for the first and 100 cycles,respectively,and the capacity retention was 87.98%.The secondary particles remain spherical and no microcracks are found after 200 cycles.This should be attributed to the support of doping sodium on crystal structure,which prevents the layered structure from transforming into the irreversible rock salt phase and provides a lithium-ion intercala-tion channel.Thirdly,the effects of coating and regenerating on the crystal structure,morphol-ogy,and electrochemical properties of NCM523 were investigated using the coating of Li F and phosphate.It is shown that temperature affect Li F coating uniformity.R-700has the best cycle stability and rate performance at 2.8-4.3 V,and the discharge specific capacities of the first and 200th cycle was 144.7 and 120.9 m A h g^-1,respectively,with a capacity retention rate of 83.55%.The average capacity retention rate was 82.95%after 200 cycles at 3 and 4.5 V.This is because the Li F coating mitigates the phase evolution and structural changes during charging process,thus improving the stability of the material.However,the inhomogeneous distribution of P is present in the coating of Li₃PO₄,and the cyclic stability and rate performance of Li₃PO₄ are also improved.The discharge specific capacity of the spent NCM is 122.5 and 0 m A h g^-1 for the first and 80 cycles.However,the first and 100 cycles for the materials coated and recycled at 650℃are 137.5 and 112.5 m A h g^-1,respectively,and the capacity retention rate is81.82%.This also indicates that the coating improves the recycling stability of the recovered materials.Finally,in order to reduce the influence of lithium manganate mixed into the NCM523 on the direct regeneration and improve the high voltage performance of the regenerated material,Li₂Mn O₃ was coated on NCM523,and the electrochemical performance of NCM523 at high voltage was improved through the coating and direct regeneration.Results shows that Li₂Mn O₃ is coated on the regenerated cathode material in form of fine particles and the secondary particles of the regenerated material are not dense.The capacity retention of 3 mol%Li₂Mn O₃ after 100 cycles at 3-4.7 V is 78.83%.In addition,it has better rate performance with a capacity retention rate of 72.06%at5 C,indicating that the Li₂Mn O₃ coating reduces the polarization and thus improves the electrochemical properties at high voltages of NCM523.