UV-radiation Inducing Strategy To Modified Fluorinated Carbon Materials For Energy Storage

With the development and upgrading of special equipment and facilities,aerospace devices and medical devices,lithium/carbon fluoride（Li/CF_x）battery has become a research hotspot in these fields because of its ultra-high theoretical energy density（～2180 Wh/kg）.However,due to the strong electronegativity of F element,the electron cloud density is concentrated around the F element,resulting in a highly polarization of the C-F bond,which leads to the low intrinsic conductivity of the fluorinated carbon material.Correspondingly,the low conductivity limits wide application of Li/CF_xbattery.Aiming at these problems,this work systematically studies from three aspects:the graphitization of soft carbon,the design and preparation of hard/soft carbon,and the enhancement of rate performance by UV irradiation modification.Firstly,the degree of graphitization for carbon materials is controlled by selecting soft carbon as the carbon source and using a high-temperature annealing method.The effects of different annealing and fluorination temperatures on the performance of Li/CF_x battery are investigated.In order to obtain a large volume/mass energy density,soft carbon with a density of 0.88 g/cm~3 is preferred as the carbon source.In addition,the abundant soft carbon edges at the interface of the layered structure are beneficial to forming C-F₂ bonds,providing more activity.The as-synthesized fluorinated soft carbon delivers an ultra-high specific capacity of 925 m Ah/g at 0.01 C.Secondly,the hard/soft carbon is developed by mixing the low-cost starch and coal tar as the hard carbon and soft carbon precursors.The influence of different fluorination temperatures on the electrochemical performance for lithium primary batteries is researched by pre-carbonization,emulsification and later carbonization at a high temperature of 900°C,and further high-energy ball milling and refinement.Fluorinated hard/soft carbon reaches a specific capacity of 847.82 m Ah/g at a relatively low fluorination temperature of 350°C.Finally,a technique to modify CF_x by UV irradiation was developed to enhance the rate capability,and the mechanism was investigated.UV irradiation modification technology slices the bulk CF_x materials into fragments and form more abundant transmission and diffusion channels.UV irradiation also increases the specific surface area and pore size of the material,providing more reaction activation sites and larger ion transport channels for electrochemical reactions.UV irradiation modification also controls the bond type to decrease content of inactive C-F₃ and increase of semi-ionic C-F bonds,which contributes to improve the conductivity of CF_x.FSC-5W-72h has a specific capacity of 762.1 m Ah/g at 1 C,showing good high-rate performance.In conclusion,this work systematically studies the influencing factors such as carbon source optimization interface design,graphitization control and precise fluorination,and develops a kind of low-cost fluorinated hard/soft carbon material,which further improves performance of Li/CF_x primary batteries.Meanwhile,UV irradiation modifying method is developed to effectively improve the high-rate discharge capability of CF_x materials.It provides an important strategy for the design,preparation and modification of cathode material CF_x.