Preparation,Modification And Electrochemical Properties Of Fluorinated Graphite And Fluorinated Graphene

Lithium primary batteries have been widely used due to their large energy density,high operating voltage,long service life,strong adaptability,light weight,and environmental friendliness.Lithium carbon fluoride（Li-CF_x）batteries possess the highest theoretical energy density among the solid lithium primary batteries,and have many other advantages,like large specific capacity,stable discharge platform,and long shelf life.The key material of this device is fluorinated carbon cathode material.However,the fluorinated carbon material has the disadvantage of poor conductivity,leading to significantly voltage lag at the initial stage of discharge,and the discharge platform far below the theoretical value.In addition,the fluorinated carbon materials are generally prepared by direct fluorination of carbon materials.The fluorination mechanism,such as the effect of the chemical structure of carbon materials on the fluorination process is not yet clear.This thesis carried out the following three aspects of research in response to the above issues.（1）Preparation and fluorination mechanism of N,F-codoped fluorinated graphene:Fluorinated graphene（FG）has been a star material as a new derivative of graphene.In this section,a series of fluorinated graphene materials are prepared by using N,O-doped graphene aerogel as precursor via a direct fluorination method,and the effect of fluorination temperature on the FG structure is investigated.The prepared FG samples are systematically characterized by scanning and transmission electron microscopy,X-ray photoelectron spectroscopy,X-ray diffraction,Fourier transform infrared spectroscopy,and Raman spectroscopy.It is found that the structure of FG,such as layer size,chemical composition,chemical bond state of the component elements,et al.is significantly relative to the fluorination temperature.With the change of the fluorination temperature,the fluorine atom enters the graphene framework by the substitution process of the N,O-containing groups such as residual phenol,ether,carbonyl group,or C-N groups,and the addition to C=C bonds,then formed a fluoride with different fluorine contents.The fluorine content increases as the fluorination temperature increases from 200°C to 300°C,but decreases at a fluorination temperature of 350°C due to the decomposition of the fluorinated graphene.The prepared FG samples are used as cathode material of lithium primary batteries.The FG sample prepared at 300°C gives a high specific capacity of 632 m Ah g^-1 and discharge plateau of 2.35 V at a current density of 10 m A g^-1,corresponding to an high energy density of 1485 Wh kg^-1.（2）Preparation and electrochemical performance of carbon-coated fluorinated graphite:According to its work mechanism,the specific capacity of Li-CF_x batteries is proportional to the ratio of fluorine to carbon.Generally,in order to obtain high gravimetric capacity,fluorocarbon materials require a higher F/C ratio.However,the C-F bond in the fluorocarbon material with a high fluorocarbon ratio is typical covalent,resulting poor electrical conductivity,thus greatly limits its discharge characteristics.In this section,we prepared carbon-coated fluorinated graphite in which the carbon coating is believed to increase its conductivity.The effects of carbon precursor,coating ratio,carbonization temperature and other factors on the carbon coating modification are systematical studied.It was found that the when using phenolic resin as a carbon precursor,a uniform conductive carbon film was formed on the surface of the fluorinated graphite,which significantly improved the conductivity of the fluorinated graphite.As a result,the discharge platform and rate performance are obvious improved.With an optimized carbon coating ratio（carbon/fluorinated graphite=1:14）,the discharge platform of the carbon-coated material increased from2.5 V to 2.7 V at a 0.1 C rate,and the specific capacity increased from 734 m Ah·g^-1 to836 m Ah·g^-1.（3）Preparation and electrochemical performance of fluorinated graphene:In this section,reduced graphene oxide（RGO）was prepared by using hydrazine hydrate as a reducing agent,and was further reduced by a hydrogen-argon gas mixture（H₂ content of 7%）at different temperatures.After this two-step reduction,the content of oxygen-containing functional groups in the graphene material reduced.Using F₂ as a fluorine source,graphene was transferred into fluorinated graphene.It is found that as the fluorination temperature increases the fluorine content increases.At the same fluorination temperature,the fluorine content of fluorinated graphene produced without hydrogen reduction or with hydrogen reduction of graphene is similar,but the latter shows higher discharge capacity and discharge platform.The main reason is that hydrogen reduction eliminates most of the oxygen-containing functional groups and reduces the electrochemically inert groups of-CF₂ and-CF₃.We tested the electrochemical performance of fluorinated graphene as a cathode material for lithium batteries and compared their performance.The RFG-300 sample exhibited a very high specific gravimetric capacity of 707 m Ah·g^-1 with a discharge platform of 2.6 V at 0.1C.