| In recent years,in the context of achieving the national "double carbon" goals,lithium-ion batteries for energy storage in the field of electronic consumer electronics and new-energy vehicles is growing rapidly.However,after 3-8 years of service life,we are facing the climax of the retirement of large numbers of lithium-ion batteries.Forced by the dual pressure of environmental protection and resource conservation,we need to recycle the components of spent lithium-ion batteries.All the time,the recovery of valuable metals from retired lithium-ion batteries has been critical,while relatively little research has been done on the lower value-added of graphite.The recycling of anode materials can not only avoid polluting environment,but also alleviate the shortage of plumbago.Hence,in this paper,two types of anode materials(graphite leaching residues and electrode stripping material)from typical retired lithium-ion batteries are taken as the research objectives,and systematic research is conducted on the key aspects of sulfuric acid curing-acid leaching,high-temperature treatment,carbon coating and silicon-carbon material synthesis.Firstly,spent graphite deriving from retired ternary lithium-ion batteries was recovered by sulfuric acid curing-acid leaching combined with heat treatment process.A series of characterizations including XRD,Raman,SEM-EDS,SAEM,physical property and electrochemical properties are carried out on spent graphite(SG)and recycled graphite(RG)as well as commercial graphite(CG),and the result shows that:the vast majority of impurities in the SG are effectively removed.the fixed carbon content of graphite is increased from 96.8%to 99.6%;besides,the degree of crystallization and graphitization of graphite are enhanced as well;moreover,the morphology is greatly improved,and accompanied by a smooth and flat surface;Electrochemical performance shows RG displays a initial specific capacity of 349 mAh/g and capacity retention of 98.8%after 50 cycles at 0.1 C,respectively,which are both close to those of CG(352 mAh/g and 99.7%),which is superior to that of SG(304.4 mAh/g and 59.2%).Subsequent electrochemical analysis(CV,EIS)also explains the differences in electrochemical properties between RG and CG.Subsequently,the compositional and structural analysis of spent graphite from the spent LFP battery shows the presence of iron phosphate(FePO4),iron phosphate dihydrate(FePO4·2H2O)and Al.Based on the thermodynamic calculations,and SEM-EDS analysis was performed on the materials before and after curing process,the phase transformation mechanism of impurities is further proposed:namely,these impurities containing Fe and P.as well as elemental Al can be converted into corresponding water-soluble sulfate or phosphate.The fixed carbon content of graphite was increased from 88.1%to 99.1%after optimization of the curing and acid leaching process conditions.The initial coulomb efficiency and capacity retention of the regenerated graphite were 86.1%and 99.4%at 0.1 C,respectively,which were much higher than those of the spent graphite(72.5%,80.9%)and lower than those of the commercial graphite(99.4%,99.6%).In particular,for regenerated graphite,the capacity contribution of lithium ions was 10.0%in liquid phase Ⅲ and 47.4%in solid phase Ⅲ and Ⅱ;which is better than that of spent graphite(40.0%,40.6%)and close to that of commercial graphite(8.1%,49.2%),further demonstrating the improved initial coulomb efficiency.Secondly,for the spent anode materials stripped from the negative electrode.sulfuric acid leaching was preformed to obtain purified graphite.The effects of different temperatures on the graphite structure and electrochemical properties were subsequently investigated,and the result showed that an optimum heat treatment is 900℃ for 2 h,wherein recovered graphite has the best crystallinity.In addition,the content of C=C(C-sp2)in heat-treated graphite at 900℃ increases from 57.2%to 65.8%and the content of C-C(C-sp3)decreases from 18.5%to 12.6%,besides,the overall O content decreases from 9.06%to 2.34%.This shows a significant increase in the degree of carbon atom ordering and a significant reduction in oxygen defects.Lastly,heat treated graphite at 900℃ exhibits the best cycling performance and rate capability,and subsequent EIS tests and calculation of lithium-ion diffusion coefficient in CV tests also proved that HTG-900 has a better wetting effect between the electrode and electrolyte,and a more stable crystal structure.Thirdly,in order to solve the problem of poor electrochemical performance of spent graphite of some failed lithium-ion batteries,a surface modification method(asphalt coating)was proposed to improve.The results show that the asphalt has been successfully coated onto the graphite material and the amorphous carbon formed can effectively repair the surface pores and cracks,making the graphite surface smooth and flat,which further facilitates the electrolyte infiltration.In addition,the particle size of the carbon-coated graphite has been increased and the specific surface area has been significantly reduced,which helps to reduce the formation of SEI film due to the consumption of Li+.The electrochemical properties showed that the carbon-coated graphite exhibited increased electrochemical performance,in particular,the initial coulomb efficiency and reversible capacity of carbon-coated graphite with 10%asphalt content were 80.5%and 334.5 mAh/g at 0.1 C,which were significantly better than those of the purified graphite before coating(74.7%,313.8 mAh/g)at 0.1 C.Finally,in order to realize the high value-added utilization of spent graphite,graphite was doped with silicon nanoparticles to prepare silicon-carbon compoise.The results of morphology showed that spherical silicon carbon composite(SSCC)with different silicon content(5%～20%),the particle size range of 20～50 μm and flaky silicon carbon composite(FSCC)with 10%silicon content are successfully prepared.Besides,the silicon element is more evenly dispersed in the SSCC with 10%silicon content compared with the flaky silicon carbon material(FSCC).In addition,the specific surface area and average pore volume of the spherical particles(8.115 m2/g and 0.040 cm3/g)are lower than those of the flaky particles(9.302 m2/g and 0.048 cm3/g).Moreover,the electrochemical performance showed that the initial coulomb efficiencies of 5%,10%,15%and 20%silicon content SSCC are 77.9%,77.8%,83.4%and 82.6%respectively,and the initial specific capacities were 388.0 mAh/g,431.8 mAh/g,664.7 mAh/g and 752.5 mAh/g at 0.1 C,respectively.Among them,the electrochemical performance of the SSCC is slightly lower than that of flaky material(78.4%,440.36 mAh/g).Furthermore,when the current density increases to 1.0 C,the capacity retention of SSCC with 10%Si is 91.6%after 200 cycles,which is higher than that of the FSCC(88.4%).More importantly,the morphological characterization of the electrodes shows:after 100 cycles the electrode expansion rate of the spherical material(208.6%)is much lower than that of the flake material(296.6%).Meanwhile,the surface damage of the spherical material electrodes after cycling is superior to that of the flake material. |
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