Degradation Mechanism And Direct Regeneration Of LiNi_xCo_yMn_1-x-yO₂ Cathode Materials For Lithium-ion Batteries

Lithium-ion battery cathode materials have the dual attributes of environment and resources.Element extraction technologies(pyrometallurgy and hydrometallurgy)were mainly used to recycle metal elements from spent cathode materials.However,long path and secondary pollution are two big issues for element extraction technologies.The direct regeneration of spent cathode is aimed at a recycling process for the spent cathode materials with low impurity content,which has been paid widespread attention in recent years.In this thesis,we focus on the direct regeneration of spent cathode,and the nickel-nickel-cobalt-manganate(NCM)material is selected as the research object.The mechanism of the failure process of the material is systematically studied.Based on this,mechanochemical activation and structural restoration are used.The direct regeneration technology is a short-process method without any secondary pollution.Meanwhilie,in situ high temperature XRD was used to observe the real-time changes in the crystal structure of NCM regenerated materials during direct regeneration,and the mechanism of direct regeneration of NCM cathode waste was analyzed and proposed.The specific conclusions are listed as follows:(1)Based on SEM and XRD measurements,the evolution in the morphology and crystal structure of NCM cathode during long-term charging and discharging were studied.After the NCM cathode material has been charged and discharged for many cycles,cracks appeared on the surface of the cathode,which blocked the transport of electrons and degraded the electrochemical performance of NCM cathode material.Meanwhile,NCM cathode particles gradually cracked,the spherical morphology was very difficult to maintain effectively,and the layered structure was damaged.During the charge and discharge process,lithium ions could not be fully intercalated and deintercalated,resulting in the capacity fading of NCM cathode material.Revealing the failure mechanism of NCM cathode materials could give important insights for the direct regeneration of NCM spent cathodes.(2)Based on mechanochemical activation and structural repair,the direct regeneration of NCM cathode waste is realized.The discharge specific capacity of NCM recycled material can reach 165 m Ah g^-1 in the first cycle.After 100 cycles,the capacity retention is 80%.Without introducing any impurity,the direct regeneration process can restore the electrochemical performance of NCM spent cathode.Meanwhile,this direct regeneration process of NCM spent cathode was conduced with minimal chemical consumption and high atomic utilization,which could create environmental and economic benefits.(3)Based on the direct regeneration process of NCM cathode material,the comprehensive reuse of NCM spent cathode and vanadium-containing waste slag is realized.In combination with spray drying technology,V₂O₅ coated NCM cathode material is successfully prepared.At 0.1 C rate,100 cycles,the discharge specific capacity can reach 156.3 m Ah g^-1,and the capacity retention is 90.6%.The preparation process of NCM-V₂O₅ cathode material is simple without secondary pollution,which is consistent with the concept of green chemistry.(4)Based on in situ high temperature XRD measurements and the refinement of XRD patterns,the direct regeneration mechanisms of NCM spent cathode were investigated.It can be found that the direct regeneration process of spent NCM cathode materials is to replenish the lithium source into the NCM spent cathode that has lost lithium.When the temperature rised from room temperature to 300 ^oC,lithium carbonate gradually decomposed,and lithium started to restore the crystal structure of the spent NCM cathode material.In the temperature rising range from 300^oC to 600 ^oC,lithium began to be fully replenished into the crystal structure of the spent NCM cathode material.The NCM cell grew uniformly in different directions,the cell volume gradually increased,and the layered structure of the NCM cathode material was effectively repaired.After 600 ^oC,the lithium replenishment process basically ended,the NCM cell volume began to stabilize,indicating that the direct regeneration process of the spent NCM cathode material was finished.