Fluorinated Carbon Materials:Synthesis Mechanism,Structural Regulation,and Their Application In Batteries

The lithium/fluorinated carbon（Li/CF_x）battery uses fluorinated carbon（CF_x）as the cathode material and lithium as the anode,which has extremely high theoretical specific energy(2180Wh kg^-1)and theoretical specific capacity(865 m Ah g^-1).Moreover,the Li/CF_x battery possesses many other advantages,including excellent safety,long storage life,wide operating temperatures,and stable discharge voltage platform.Therefore,it has important applications in high-technology fields such as military industry,defense,aerospace and implantable medical device.However,there are several critical issues to be solved in Li/CF_x system:first,poor conductivity of CF_x materials and slow diffusion of Li⁺lead to low discharge voltage and poor rate performance of Li/CF_x batteries.Second,the synthesis mechanism of CF_x materials is still unclear,especially the influence mechanism of structural defects and pore structure of carbon sources on the fluorination reaction and electrochemical performance of CF_x materials.Third,the lack of lithium resources hinders the large-scale application of Li/CF_x primary batteries.In view of the above problems,the modification,fluorination mechanism and discharge mechanism of CF_x materials are studied in this paper by using fluorinated graphene as the model material.The detailed studies are as follows:（1）Preparation and lithium storage properties of carbon coated multilayer fluorinated graphene.Fluorinated graphene is considered to be the most promising CF_x cathode to achieve the theoretical electrochemical performance,due to its adjustable F/C ratio and C-F bonding properties,good diffusion kinetics of Li⁺and large specific surface area.However,the commercialized multilayer fluorinated graphene has some issues to overcome,such as poor conductivity and low discharge voltage.Polyvinyl pyrrolidone（PVP）was used as carbon source for coating fluorinated graphene（CF_x）to improve the electrochemical performance of Li/CF_xbatteries.The effects of CF_x/PVP ratio and carbonization temperature on structure and electrochemical performance of the CF_x@PVP materials were studied systematically.Electrochemical results show that the carbon coating significantly improves the discharge voltage and rate performance of CF_x.For example,at 10 m A g^-1,the discharge specific capacity and voltage of CF_x@PVP-10-300 are 747.4 m Ah g^-1 and 2.73 V,respectively;while those of CF_xare 844.1 m Ah g^-1 and 2.65 V.Meanwhile,the CF_x@PVP-10-300 can discharge smoothly at3000 m A g^-1,while the maximum operating current density of CF_x is 1000 m A g^-1.Subsequently,ball milling was used to reduce the particle size of the CF_x@PVP-10-300,and new F-containing surfaces appeared.This method not only promotes the diffusion of Li⁺but also improves the utilization rate of F atoms in the discharge process.As a result,the rate performance of the material is further improved.It can discharge smoothly at 5000 m A g^-1,showing a maximum power density of 9215.5 W kg^-1and a maximum discharge specific capacity of 830.3 m Ah g^-1.This study confirmed that ball milling can alleviate the reduction of specific capacity caused by modification of carbon coating.Meanwhile,the rate performance of the material is further improved,making it exhibits an ultra-high power density.（2）Preparation,structural regulation and electrochemical performance of porous fluorinated graphene.The pore structure and defects of graphene are important factors affecting the structure of fluorinated graphene,and ultimately affect the electrochemical performance of Li/fluorinated graphene batteries.Reduced graphene oxide aerogel（Ar GO）with different pore structures and defect structure was prepared by chemical activation using zinc chloride.Porous fluorinated graphene（FAr GO-x-y-z）materials were prepared by using Ar GO as raw material and F₂ as fluorine source.The effect mechanism of pore structure and defects on fluorination reaction and electrochemical performance of CF_x material was studied by using FAr GO-x-y-z as model material.Firstly,experimental and characterization results show that the oxygen functional groups and porous structure of carbon precursors can promote the fluorination reaction,and theπ-πconjugate structure will resist the attack of high temperature and fluorine free radicals.Theoretical calculations confirm that the hierarchical porous structure promotes the fluorination reaction by providing more reaction sites（dangling bonds）for the insertion of F atoms.Secondly,the average binding energy of C-F bonds is enhanced by high fluorination temperature,which is accompanied by an increase in the content of electrochemical inert functional groups,thus significantly reducing the electrochemical performance of the prepared porous fluorinated graphene materials.Moreover,prolonging the fluorination time also causes the same result.And lastly,the FAr GO-200-4-0.5 sample achieves the best rate performance,which may benefit from its relatively large specific surface area,well-developed pore structure,wide layer spacing,and less content of electrochemically inert groups（-CF₂ and-CF₃）.It can discharge smoothly at a high current density of 3 A g^-1 and has a maximum voltage of 3.0 V.（3）Controllable synthesis of three-dimensional（3D）fluorinated graphene and high power potassium primary batteries.The K/CF_x battery,as an alternative to Li/CF_x battery system with ultra-high energy density,has gradually attracted the attention of researchers.However,the large radius of K⁺makes its diffusion slow in the cathode material,which seriously restricts the reaction kinetics of the potassium-based battery system.To solve this problem,we prepared 3D porous fluorinated graphene materials（PFG-x）and investigated their electrochemical performance as a cathode material for lithium,sodium and potassium primary batteries.Electrochemical results show that the specific capacity and energy density of K/PFG-0.79 are similar to those in Li/CF_x system.However,the K/PFG-0.79 battery performs significantly better than the Li/PFG-0.79 battery in terms of rate performance.At 10 m A g^-1,the K/PFG-0.79achieves the highest voltage of 2.9 V,the specific capacity is 752 m Ah g^-1,and the corresponding energy density is 1985.2 Wh kg^-1.Besides,it possess a maximum power density of 11700 W kg^-1 at 6000 m A g^-1.Moreover,the K/PFG-0.79 battery shows satisfactory electrochemical performance under various extreme conditions（-40～50oC）.In contrast,with fluorinated graphite as cathode material,the rate performance of lithium primary batteries is superior to that of potassium primary batteries.Combined with the ex-situ XPS,in-situ XRD,galvanostatic intermittent titration technique（GITT）,and first-principles（DFT）calculations,we revealed that the 3D porous fluorinated graphene architecture and low desolvation energy of K⁺boost the high-power-density of K/CF_x primary batteries.