| Graphite has become one of the most widely used anode materials in the commercial lithium-ion battery market because of its high crystallinity,good electrical conductivity,low potential plateau close to the lithium metal potential,high reversible capacity,and excellent laminar structure.However,in the LIBs recycling market,the spent graphite anode is often abandoned due to its low added value and strict separation process.As anode material,graphite accounts for large proportion in LIBs.If the graphite contained in the spent LIBs(about 10%)cannot be treated effectively,it will cause resource waste and environmental pollution.Graphite is 5 to 10 times more abundant than lithium in lithium-ion batteries.There exists a large amount of graphite anodes that are not effectively recycled every year,which leads to a continuous increase in graphite prices.It is found that spent graphite has great potential to be reused as anode material of LIBs.The effective recycling and regeneration of spent graphite has great social benefits.This paper focused on the recovery and regeneration of spent graphite and is committed to improving the microstructure and electrochemical properties of spent graphite.The main conclusions are as follows:(1)The effect of graphite separation process on battery performance was studied.The spent graphite samples were obtained by deionized water immersion separation,ethanol separation,sulfuric acid separation and mechanical grinding separation.The recovery rate,element content,XRD and electrochemical properties of the samples were analyzed.Analysis showed that the spent graphite obtained from deionized water immersion separation has high recovery rate,low impurity content and high crystallinity.The corresponding initial discharge/charge capacity of the battery were 335.7 m Ah/g and 310.4 m Ah/g,showing the initial coulomb efficiency of 92%.The capacity retention rate was 93.3%after 230 cycles at 0.1C current density.(2)In order to improve the cycling stability of spent graphite and its compatibility with electrolyte,the spent graphite was modified by carbon coating with asphalt.The influence of asphalt coating process on the structure and electrochemical properties of modified graphite was further studied.The modified graphite samples were prepared at 900 ~oC by toluene wet mixing,ethanol wet mixing and solid phase mixing.The XRD,Raman,SEM and electrochemical properties of the samples were analyzed.The results showed that the liquid phase impregnation method could obtain more uniform carbon coating than the solid phase mixing method.The modified graphite obtained by toluene wet blending was the most uniform,which had a highest capacity retention rate of 95.5%after 230 cycles.Compared with the spent graphite,the initial embedded lithium capacity of the modified graphite obtained by toluene wet blending increased from335.7 m Ah/g to 393 m Ah/g.(3)The influences of repair temperature and asphalt coating amount on the performance improvement of spent graphite was explored.The optimal process parameters were determined:asphalt doping coating amount was 1%,repair temperature was 1200 ~oC and heat preservation was1h.it was found that the temperature could effectively enhance the reversible capacity of spent graphite,and asphalt pyrolysis carbon coating improved the cycle stability of spent graphite while providing capacity.The structure of graphite particles after coating modification was effectively improved:the average particle size became larger,the morphology became smooth,and the specific surface area was reduced.The electrochemical properties were effectively improved:the reversible capacity,rate performance and cycle stability had better performance.The reversible capacity was up to 378m Ah/g and the irreversible capacity was only 17.9m Ah/g,with a first coulomb efficiency of 96%.The capacity attenuation was small under the different current density and the cycle capacity retention rate was 98%after 250 cycles at 0.1C current density. |
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