Preparation Of Polyacrylonitrile Carbonized Coating And Tin Dioxide Composite Natural Graphite Anode Material

Graphite,as the main anode material for lithium ion batteries,has a high energy density and its theoretical lithium storage capacity is up to 372 mAh/g.Secondly,the conductivity of natural graphite is good,and its electronic conductivity can reach 10⁴S/cm.Moreover,the structural stability of natural graphite is better,and there is a stable charge-discharge voltage platform between 0.1V and 0.2V voltage.The lamellar structure of graphite is less than 10%of the strain in the process of deintercalation of lithium,and the structural strain of the lamellar structure in graphite is less than 10%in the process of deembedding lithium,so graphite has good cycling stability.More importantly,graphite reserves are abundant and processing costs are low.With the popularization of automobile electrification and the explosive growth of various mobile energy-consuming devices,the performance of lithium-ion batteries is required higher in the market.The lithium-ion batteries should meet the requirements of high energy storage density and long cycle durability.Therefore,the use of innovative technical means to improve the existing negative materials or the development of new negative materials are suitable ways,but graphite as a traditional anode materials still have a lot of room for improvement.As an anode material for lithium-ion batteries,natural graphite has many advantages,but it also has some disadvantages,such as low capacity,poor performance and poor safety due to co-embedding of solvents.The main reasons for the poor electrochemical performance of natural graphite are the sp³ hybrid carbon atoms,the imperfect structure of carbon chains and marginal carbon atoms.These defects are the main active sites leading to electrolyte decomposition,which causing a large irreversible capacity.At the same time,the natural graphite is difficult to exist stably in the electrolyte containing propylene carbonate.Because of the co-embedding of electrolyte solvent,the graphite lamellar falls off and the cycle stability is reduced.Therefore,it is necessary to study the modification of natural graphite.The method of air oxidation and carbon coating can eliminate the structural defects in natural graphite,improve the cycle stability and increase the capacity.Metal composite graphite is used to improve the capacity and the ratio of the graphite.The main contents of this thesis include the following aspects:?1?the surface oxidation-modified surface-oxidized spherical graphite?SGO?was prepared by air oxidation,and a natural spherical graphite was adopted as a raw material,the surface oxidized spherical graphite materials were prepared at air oxidation temperature of 300?,350?,400?,450?,500?,550?for 6h.The structure,composition,micropore distribution and apparent morphology of the surface oxidized spherical graphite were characterized by XRD,Raman,XPS,BET,SEM and so on.The semi-cell test of the prepared material shows the SGO-350?prepared at 350?has a initial efficiency of 92.60%,and the initial charge specific capacity was up to 371.8 mAh/g,and the capacity remained 99.46%after 100 cycles under 0.1C current.The results showed that the electrochemical properties of graphite materials can be improved at this oxidation temperature.?2?The surface oxidized spherical graphite powder was used as raw material,liquid polyacrylonitrile?LPAN?was used as carbon source to coat surface oxidized spherical graphite.Polyacrylic carbonation coated natural graphite?CG?negative electrode materials were prepared after high temperature carbonation.The effects of different content of LPAN and different carbonation temperature on the structure and properties of the material were investigated.The results showed that when the dosage of LPAN is 10%and the carbonation temperature is 1000?for 6 h,the initial efficiency of 10%CG-1000?was 94.29%and the initial charge specific capacity reached 391.0 mAh/g,which charge specific capacity of 0.1C at 100th cycle is up to386.1 mAh/g.The coating of LPAN improved the initial efficiency and cycle stability of the material.The 18650 battery was fabricated by matching the 10%CG-1000?with the lithium iron phosphate positive electrode,and the capacity retention rate was80.45%at 3C rate current for 400 cycles.?3?On the basis of the above mentioned,the graphite was further modified with10%nano-tin dioxide,and LPAN was used as the coating agent and binder,the addition of LPAN was 5%,10%,15%of the total mass.The results showed that the energy density of the prepared CG-10%SnO₂ is 400 mAh/g after 100 cycles.The experimental results showed that the electrochemical performance of10%CG-10%SnO₂ prepared by ball milling was the best when the dosage of LPAN was 10%,and its first efficiency was 88.04%corresponding to the first cycle specific capacity of 438.8 mAh/g,the retention rate of 100-turn cycle capacity was 93.94%at100 mA/g current,which indicates that the material has good cycle stability.