Preparation Of Nanostructured Fluorinated Graphite And Its Application In Lithium/Sodium/Potassium Primary Batteries

Lithium/fluorinated carbon(Li/CF_x)primary battery has many advantages,such as high energy density(2180 Wh kg^-1),stable discharge platform,low self-discharge rate,wide working temperature,long storage life,and so on.It has important applications in consumer electronics,advanced medical and military equipment.The discharge specific capacity of the Li/CF_x battery increases with the increase of the F/C ratio(i.e.fluorine content)in CF_x material.When x=1,the theoretical specific capacity of CF_x can reach865 m Ah g^-1.However,the pursuit of high fluorine content will reduce the electronic conductivity and ionic conductivity of CF_x materials,resulting in large initial voltage delay,poor rate performance,and serious heating of Li/CF_x batteries.Li/CF_x batteries have poor large-rate discharge performance and low power density,which restrict their commercial application.The trade-off between high conductivity and high fluorine content is the key factor to determine the power density and energy density of CF_xmaterials.Therefore,it is a major challenge for the development of Li/CF_x battery to achieve high conductivity of CF_x material based on high fluorine content,so as to achieve high energy density and high power density of Li/CF_x battery simultaneously.In view of the above challenges,this paper aims to achieve high power density and high energy density of Li/CF_x batteries.By designing carbon sources and adjusting fluorination conditions,2D nanostructured fluorinated graphite and 3D structured fluorinated graphite assembled by nanosheets were prepared,which have high fluorine content and high conductivity.Meanwhile,the nanostructured fluorinated graphite was prepared on a kilogram scale,and the pouch-type battery was designed and assembled.On the other hand,considering that sodium and potassium have more advantages than lithium in resource reserve,cost,and ionic conductivity,new-type Na/CF_x and K/CF_xprimary batteries were developed,and their electrochemical performance and application indicators were systematically studied.Finally,to verify the influence of nanostructure on electrochemical performance and provide guidance for the optimization design of the material structure,the structural evolution,discharge mechanism,and structural stability of nanostructured fluorinated graphite during the discharge process of lithium primary battery were studied in detail.The specific research contents and conclusions are as follows:(1)2D accordion-like structure fluorinated graphite nanosheets(FGNSs)were prepared from cheap and easily available expanded graphite by combined with physical exfoliation method,freeze-drying method,and high-temperature gas fluorination method.Compared with commercial CF_1.0 with the same fluorine content,FGNSs-1.0 has a higher F/C ratio(1.0),higher conductivity(3.8*10^-8 S m^-1),more conductive sp² C=C bonds(7.14%),less electrochemical inactive C-F₂ bonds(18.17%),and larger specific surface area(216.5 m² g^-1).FGNSs-1.0 has excellent electrochemical performance as cathode material for lithium primary batteries.At 1 C,the discharge specific capacity of the Li/FGNSs-1.0 battery is 809.9 m Ah g^-1(93.6%of its theoretical specific capacity),and the maximum energy density is 1879.0 Wh kg^-1.Li/FGNSs-1.0 battery shows excellent rate discharge capability,with a maximum discharge rate of 20 C(?17.3 A g^-1)and a maximum power density of 31313.0 W kg^-1.Compared with the commercial Li/CF_1.0battery,the discharge rate is improved by 300%.(2)To further improve the energy density and reduce the cost of primary batteries,the electrochemical properties of new-type Na/FGNSs-x and K/FGNSs-x batteries were systematically studied with FGNSs-x as the cathode and Na/K as the anode with abundant resources and low cost,and their application prospects were evaluated.At the current density of 10 m A g^-1,the discharge specific capacity of Na/FGNSs-1.0 coin cell is 831.3m Ah g^-1(96.1%of its theoretical capacity),and the maximum energy density is 1960.5Wh kg^-1.The discharge specific capacity of the K/FGNSs-1.0 coin cell is 834.1 m Ah g^-1(96.4%of its theoretical capacity),and the maximum energy density is 2144.6 Wh kg^-1.At a high current density of 3000 m A g^-1,the maximum power density of sodium primary battery is 6227.4 W kg^-1.The maximum discharge current of the potassium primary battery is 4000 m A g^-1,and the maximum power density is 7076.8 W kg^-1.At the same time,FGNSs-1.0 also showed excellent electrochemical performance in the different temperature ranges(-30??100?),long-term storage(60 days),high active material loading(3.6 mg cm^-2),and pouch-type battery.These results show that the new-type Na/FGNSs-1.0 and K/FGNSs-1.0 primary batteries developed here have great application prospects in high-energy equipment requiring high power and low cost.(3)To further improve the rate performance of Li/CF_x batteries,a flower-like porous fluorinated graphite microsphere(FGS)with a 3D structure was prepared by spray drying and high-temperature gas fluorination.The homogeneous fluorination effect caused by the use of the 3D porous structure of carbon source materials and increasing fluorination temperature greatly improve the electrochemical performance of the FGS cathode.In FGS-1.0,the interconnected nanosheets and the abundant sp² C=C bonds(15%)form a fast electron conduction network,while the porous structure and a small amount of inactive C-F₂ bonds(11.74%)make Li⁺conduct rapidly.Based on this design,high power density and high energy density can be achieved simultaneously in Li/CF_x primary battery when FGS-1.03 with a high F/C ratio is used as cathode.The specific capacity of Li/FGS-1.03 coin cell at 1 C is 797.2 m Ah g^-1,corresponding to a maximum energy density of 1894.2 Wh kg^-1.Moreover,the maximum discharge rate of Li/FGS-1.03 coin cell is 50 C(?43.3 A g^-1),the discharge capacity is still 504.7 m Ah g^-1,and the maximum power density is 71180.9 W kg^-1.Compared with the literature,the discharge rate and power density are increased by 54%and 30%.The Li/FGS-1.03 pouch-type cell also showed excellent electrochemical performance with a maximum discharge rate of 20 C and a maximum power density of 12451.2 W kg^-1 based on the mass of the whole battery.Therefore,FGS-1.03 developed in this paper is a promising cathode material for commercial lithium primary batteries,and can greatly improve the discharge rate and power density of primary batteries.(4)The structure evolution,discharge mechanism,and stability of FGS-1.03 cathode at different discharge stages were studied by ex-situ XPS,EIS,SEM,and in-situ DIC techniques.The results of in situ XPS and EIS show that the excellent performance of FGS-1.03 is due to the stability of its 3D porous structure and the uniform formation of Li F crystal during the discharge process.Compared with commercial CF_1.0,the ex-situ SEM results also show that the 3D porous structure of FGS-1.03 has reserved space for the growth of Li F,which effectively alleviates the accumulation of Li F and the volume deformation of the material during the lithiation process.To quantify the deformation of the electrode during discharge and realize real-time observation,the in-situ DIC test technology is applied to the lithium battery system for the first time.The results show that the electrode deformation increases with the increase of discharge depths and the increase of discharge rate also increases the deformation.At the discharge rate of 1 C,FGS-1.03with a 3D porous structure can better relieve the volume deformation during lithiation than CF_1.0 with bulk structure,and the average principal strain decreases by 15%after full discharge.These results will provide guidance for the structural design of high-performance CF_x materials.