Fluorinated Graphite Cathode Material For High Energy Density Batteries

It is pivotal for secondary batteries to develop electrode materials with high energy density and good rate performance.Fluorinated carbons?CF_x?have the highest energy density among reported cathodes of Li primary battery,which are widely applied in high-value electronic equipments.The discharge products of CF_x as Li primary cell after lithium insertion are LiF and C.LiF is in the micron range,and the reason why CF_x can be only used in primay batteries is that the two-phase separation of LiF and C occurs,which leads to the poor kinetics and the irreversible discharge reaction.In view of the poor kinetic performance,a new battery system is proposed with CF_x as the cathode and potassium as the anode,which utilizes the electrolyte solvation with metal ions to achieve higher rate performance and the discharge mechanism is studied.As far as the irreversibility of Li/CF_0.88,the electrochemical window is extended to explore the electrochemical behaviours of Li/CF_0.88 as the secondary battery.Started with Li primary cell,the performance of CF_0.88 demonstrates that it exhibits high specific capacities of 734 mAh?g^-1 in LiBF₄?EC/DMC and the platform of discharge as well as capacity descends quickly when the current density increases.What's more,it reveals that the surface and internal carbon structure of CF_0.88 have changed a lot after discharge and it is observed that LiF particles distribute both on the surface and in the interlayer of the material through SEM and TEM.Meanwhile the amorphous framework of CF_0.88 transforms to ordered structure,like graphite.Afterwards,owing to a smaller Stokes radius of K⁺after solvation with the electrolyte which leads to better kinetic performance,CF_0.88 as K cell cathode is studied.It reveals a better rate performance,higher operational voltage,and higher energy density than that of Li cell.The cathode in K cell offers specific capacities of 743 mAh?g^-1 and an energy density of 1869 Wh?kg^-1 at 0.05C,higher than 1824 Wh?kg^-1 of Li/CF_0.88.The reaction progress can be divided into two parts based on theoretical calculations,F-rich state and F-poor state,which represent the early stage of cathode reaction and the final stage of cathode reaction,respectively.Meanwhile,better rate performance,higher operational voltage benefit from better kinetics correlated with higher diffusion coefficient of K⁺on the basis of GITT curves.We also find crystalline KF particles with nano or micron size dispersing on the surface and framework of CF_0.88 has changed via SEM and TEM images at the discharge state of 1.5 V.Combined with the solubility of KF in the electrolyte which is measured by ICP,a dissolution-recrystallization mechanism of KF is proposed for discharge process.Moreover,the operational voltage and discharge capacity of fluorinated graphite as KIBs decrease when the salt concentration increases.It is ascribed to that higher concentration electrolyte causes bigger KF particles,which blocks diffusion paths of K⁺leading to bad performance.In addition,to broaden application range of Li/CF_0.88,the electrochemical behaviour of fluorinated graphite for Li secondary battery is also explored.Good reversibility is obtained under 0.5-4.8 V and it delivers charge and discharge specific capacities of 567mAh?g^-1 and 644 mAh?g^-1 after 10 cycles at 0.05C.It indicates that platforms are corresponding to the formation and decomposition of Li_x+yF_yC_z in terms of curves of charge-discharge and cyclic voltammetry.Besides,the reaction course of Li/CF_0.88 can be concluded through results of ex-situ XRD,SEM and Raman spectra:when discharging to 1.5 V,LiF distributing in the interlayer of carbon can react forming chemical compounds of Li_x+yF_yC_z;after charging to 4.8 V,Li_x+yF_yC_z decompose and Li ions are released from carbon layers,which leads to smaller interlayer spacing of carbon and the structure of carbon tends to be ordered.