Basic Research On Wet Chemical Recovery And Regeneration Of Typical Cathode Materials Of Lithium-ion Batteries

Lithium-ion batteries(LIBs)have been widely and largely exploited as the energy carrier in modern civilization.In LIBs,especially the cathode material,there are many kinds of valuable metals.During the life cycle of LIBs,large amount of cathode materials have been derived for now and needed to be recycled urgently,such as the scraps and leftovers in the manufacture and the spent cathode material in retired LIBs.In this thesis,the wet chemical recovery and re-synthesis of typical cathode materials LiFePO4 and LiNixCoyMnzO2 were taken as the research objects.Basic researches were carried out to focus on the key links of the wet chemical recovery and re-synthesis of cathode materials.Firstly,E?pH diagrams of the Li-Fe-P-H2O system from 298 K to 473 K were plotted.The dominant areas of LiFePO4 and relevant species in the E?pH diagrams were analyzed.These dominant areas will change at different conditions of temperature,pH,and oxidation-reduction potential(ORP).And these changes have been utilized to conduct thermodynamics analysis on:(1)the hydrothermal synthesis of LiFePO4 and its suitable conditions,(2)the mechanism of hydrothermal synthesis,and(3)the hydrometallurgical route of recycling spent LiFePO4.It was found that the conversion between LiFePO4 and FePO4 is thermodynamically feasible in aqueous solution.The conversion can be realized by changing the aqueous conditions of temperature and ORP.Based on this acknowledgement,the selectively leaching of lithium from LiFePO4,and the hydrothermal regeneration of spent LiFePO4 were proposed and investigated in the present thesis.Secondly,the experimental research of selectively leaching of Li was carried out starting from the thermodynamics analysis.The results indicated that common oxidants such as O2,O3,NaClO,H2O2,and Na2S2O8 were effective in selectively leaching Li.Among them,the leaching system of H2SO4 and H2O2 are low-cost and efficient.Over 98%of Li,and less than 0.1%of Fe and P were extracted from LiFePO4 at the leaching conditions of room temperature,pH 2.5,and L/S ratio of 3?5 L/kg.The characterizations of XRD,SEM,FT-IR,and XPS were performed on the solid phases of before and after leaching,and the results indicated that the olivine structure of LiFePO4 was not destroyed during the leaching.The leaching process is equivalent to the charging of LiFePO4 batteries.Li in the leachate can be precipitated as Li2CO3,and Li2CO3 can be furtherly refined to the purity of battery level.Thirdly,a one-step hydrothermal system for regenerating spent LiFePO4 was set up according to the feature of Li-loss in spent LiFePO4 and the prediction of thermodynamics analysis.The research results showed that spent LiFePO4 can be regenerated in the hydrothermal system which composed by Li salt aqueous and reductant.Characterizations by XRD,XPS,SEM,TEM indicated that the hydrothermal regeneration process of spent LFP is equivalent to the discharging of LiFePO4 batteries.After optimizing the hydrothermal conditions,the regenerated LiFePO4 sample with excellent electrochemical performance was obtained.At the current density of 0.2,0.5,1,2,and 5 C,the specific discharge capacity of the regenerated LiFePO4 can reach up to 146.2,145.0,141.9,137.6,and 128.2 mAh/g,respectively.The capacity retention can reach up to 99.5%even after 200 charge-discharge cycles.The cost analysis of reagents implied that the proposed one-step hydrothermal method may have good economic benefits.At last,relevant researches were carried out to focus on the hydrothermal synthesis of LiNixCoyMnzO2,which is the key segment during the wet chemical recycle of LiNixCoyMnzO2.On the basis of analyzing previous research,a series of factors which may effect the hydrothermal synthesis were studied.It was found that the enhancement of hydrothermal conditions can promote the ratio of hydrothermal lithiation,but full lithiation is still hardly to achieve.Meanwhile,the low-Ni type cathode material is more suitable to achieve hydrothermal lithiation than the high-Ni type cathode material.After that,a novel hydrothermal route for synthesizing LiNi1/3CO1/3Mn1/3O2 was proposed,which had the advantages of no need of extra Li-usage and no solid-liquid separation.The electrochemical performance of the hydrothermally prepared cathode material turned out to be obviously better than that of the traditional roasting method,and the specific discharge capacity at 0.2,0.5,1,2,and 5 C can reach up to 161.6,154.0,151.1,145.9,and 135.6 mAh/g,respectively.The hydrothermal lithiation is the key to achieve excellent electrochemical performance.Research results showed that the hydrothermal prepared cathode material had better dispersion,higher Li+diffusion coefficient,and more stable crystal structure.Besides,compared to traditional hydrothermal methods,the proposed method has the advantages of low-Li-usage,and no discharging of Li-contained waste water.