Study On The Interfacial Reaction Mechanism Of Fluorinated Carbon Electrode

Lithium primary batteries（LPBs）are widely used in civil and military fields.With the increasing demand of LPBs,the pursuit of high energy density and high power density has become a common goal.As one of the commercial LPBs,lithium/fluorinated Carbon primary battery（Li/CF_x）is considered to be the most promising one.Li/CF_x primary battery has been widely used in electronic equipment,medical equipment,aerospace and other fields due to its advantages of ultra-high energy density(theoretical value is 2180 Wh kg^-1),stable discharge platform,wide operating temperature range and high safety.Due to its strong C-F covalent bond and low conductivity,CF_x has serious electrode polarization,long voltage delay time,poor discharge rate ability and large heat generation during discharge,which limits its application in high power density and high energy density fields.In this thesis,the influence of graphite conductive agent content on the interface of CF₁ electrode was explained,then the electrochemical properties and interface characteristics of CF₁ with different F/C ratios were introduced.Finally,the electrochemical properties and interface characteristics of CF_1.3 modified by KOH hydrothermal were studied.The main contents and conclusions are as follows:（1）The influence factors of graphite content on fluorinated Carbon（CF₁）electrode were investigated.With the increase of graphite content,the discharge capacity of the battery was improved to some extent,but the high graphite content leaded to the deterioration of the discharge capacity of the battery.It was found that the size and morphology of Li F changed at different discharge depths,and the main component of SEI film was Li F,which may be the main factor leading to the surface passivation of CF₁ electrode.In addition,electrochemical impedance spectroscopy（EIS）and distribution of relaxation time（DRT）showed that there were four processes in the discharge process of CF₁ electrode,namely,the diffusion and migration of lithium ions by SEI,the impedance caused by Schottky contact,the ion transport process in the pores of porous electrode and the charge transfer process.Through EIS,the process of porous pore transition of CF₁/graphite electrode was provided,and the factors of porous pore transition were explained.（2）The electrochemical properties and interface characteristics of CF_x with different F/C ratios were investigated.The results showed that CF_x was a kind of micron particles with a size of about 1～30μm,and the higher the fluorination degree,the more obvious the lamellar shape,which had obvious disordered phase characteristics.With the increase of F/C ratio,the content of covalent C-F bond and C-F_x（x=2,3）increased.CV shows that Li/CF_x battery is irreversible,and the internal reaction of the battery is controlled by diffusion.In addition,normal temperature discharge and high and low temperature discharge show that the higher the fluorine content,the worse the discharge rate.The reason is that CF_x with low F/C ratio has better electronic conductivity and can accelerate the transmission of electrons during discharge,thus having higher discharge rate.CF_0.33 can discharge at a current density of 10 C,and has a power density of 7293 W kg^-1 and an energy density of 437.6 Wh kg^-1.Test of GITT shows that CF_0.33has the highest diffusion coefficient,which decreases at first and then increases,indicating that Li⁺is easier to diffuse into CF_x electrode,which makes electrochemical reaction easier to occur,so it has better kinetic properties.EIS fitting results show that the internal impedance of CF_1.2 varies with F/C ratio,and CF_1.2 has the largest R_SEI and R_con,which is one of the reasons for its poor discharge ability.（3）The surface of CF_1.3 with F/C ratio of 1.3 was modified by KOH hydrothermal method,and the modified CF_1.3 material with high capacity and high rate performance was obtained.The results show that the surface modification does not affect the core of CF_1.3 particles,and has more obvious lamellar structure and higher degree of disorder.In addition,the discharge current density of CF_1.3-160℃can reach 40 A g^-1,and the specific discharge capacity exceeds 100 m Ah g^-1.At the current density of 32 A g^-1,it has a voltage plateau of over 1.7 V and a specific discharge capacity of 586.9 m Ah g^-1.It is worth noting that the remarkable improvement of rate performance may be due to the etching of KOH,which makes the surface defluorinated and produces more structural defects and surface functional groups,greatly improves the electronic conductivity of CF_1.3 materials,provides more active sites,accelerates the transmission of lithium ions and electrons on CF_1.3 surfaces,and thus significantly improves the electrochemical performance.In this thesis,there are 75 figures,3 tables and 120 reference articles.