Green Recycle And Functional Optimization Of Graphite Anode In Spent Lithium-ion Batteries

The demand for lithium-ion batteries（LIBs）is growing due to the increase in the purchasing power of the public and the demand for new products,such as cell phones and electric cars.However,LIBs with a limited life span are discarded in large quantities after the retirement of electronic devices and electric cars,which not only causes a huge loss of mineral resources,but also brings serious harm to the ecological environment.Therefore,the rational disposal of waste LIBs is of great importance for solving the problems of resource shortage and environmental pollution.In recent years,a lot of work has focused on exploring the recovery of metal elements in the cathode of waste LIBs,while less research has been done on the reclamation of anode graphite.Considering the high mass proportion of graphite in LIBs（12-21 wt%）and the expensive production cost of battery-grade graphite,the spent graphite（SG）must be properly treated in order to realize the resource utilization of LIBs.Thus the regeneration of SG is a significant issue on the waste LIBs recycling.The objective of this thesis is to efficiently recover anode graphite from waste LIBs by a green approach without employing strong acids or bases in the regeneration process.The main research contents and conclusions of this thesis are shown in the following:（1）Nitrogen-doped defect-enriched graphite（NG）was prepared from spent anode graphite（Spent graphite,SG）by a one-step recovery method using ammonium chloride-assisted roasting strategy.The effects of the mass ratio of NH₄Cl to graphite and roasting temperature on the electrochemical properties of NG were investigated.It was found that the prepared as-made NG-1-650 had richer pores(BET specific surface area of 23.1 m² g^-1 and pore volume of 0.14 cm³ g^-1)and higher surface nitrogen content（2.14 at%）.NH₄Cl roasting not only removed a large amount of impurities such as fluorine-bearing electrolyte and binder in SG,but also introduce nitrogen into the graphite matrix by NH₃ derived from NH₄Cl decomposition.The NG-1-650 delivers a remarkably improved reversible specific charge capacity of 455.9 m Ah g^-1 for the 1st cycle and 452.1 m Ah g^-1 after 50 cycles at 0.1 A g^-1 with nearly no attenuation,and also exhibits a desirable rate performance(a capacity of 175.3 m Ah g^-1is achieved at 2 A g^-1),in comparison to the SG and the commercial natural flake graphite（CG）.After 50charge/discharge cycles,it still maintains a specific capacity of 452.1 m Ah g^-1,which shows excellent cycle stability.（2）Mechanochemical reconstructed graphite（MG）was successfully prepared by,pyrolysis,followed with mechanochemical ball milling of SG using urea as a grinding aid.The effects of the mass ratio of urea to thermal treated graphite（TG）,ball milling time and speed on the structure,pore feature,morphology,surface chemistry and electrochemical properties of MG were investigated.It was revealed that the MG-10-400-10 obtained under the conditions of urea to graphite mass ratio of 10:1,ball milling time of 10 h and ball milling speed of 400 r min^-1 has the characteristics of high graphitization,less impurities and abundant defects,which provide more active sites and storage space for lithium ion de-embedding and make the cell have better electrochemical performance with excellent lithium ion charge storage capacity as well as rate behavior.the initial reversible capacity of MG-10-400-10 reached a reversible specific capacity of 521.3 m Ah g^-1 at a current density of 0.1 A g^-1,and the specific capacity only decreased from 569.9 m Ah g^-1 to 538 m Ah g^-1 after 50 charge/discharge cycles,revealing a better cycling stability.