Nano-Fluorocarbon Materials:Preparation And Their Application In Lithium/Sodium Primary Batteries

The lithium/fluorocarbon?Li/CF_x?battery is known to be the highest specific energy solid-state lithium primary battery,and have been widely used.Fluorinated graphite,the most common CF_x material,still has many problems to be solved as the cathode material of LPBs,including low discharge voltage platform,poor rate performance,voltage hysteresis,and intense fever phenomenon in discharge process.In addition,we are facing the problem of lithium shortage.Sodium batteries are considered tobe the most likely candidate because of the low cost of Na and its similar chemical properties to lithium.This thesis is mainly to prepare nano-fluorocarbon materials and study their application in lithium primary batteries?LPBs?and sodium primary batteries?SPBs?.1.Preparation of fluorinated carbon nanotubes and its application in lithium/sodium primary batteries:In this work,two series of fluorinated carbon nanotubes?F-CNTs?are prepared by from the ordinary carbon nanotubes?CNTs?and graphitized carbon nanotubes.The effects of graphitization of the carbon nanotubes on the structure of F-CNTs and C–F bonding are systematically studied.FGCNT-x prepared from the graphitized CNTs exhibit a lower ratio of F/C and weaker C–F bonding than those of the FCNT-x prepared from the ordinary CNTs at the same fluorination temperature.The FGCNT-0.81 sample exhibits very high specific capacities of 798.8 mAh g^-1 and 751 mAh g^-1 in LPBs and SPBs,repectively,nearly the theoretical capacity calculated by F/C ratio.The FGCNT-0.81 sample possesses the maximum energy density of2006.6 Wh kg^-1 in Li/CF_x batteries and 1733.4 Wh kg^-11 in Na/CF_x batteries.The maximum power density of 3861.1 W kg^-1 can be achieved for FCNT-0.81 in Li/CF_x batteries,but a lower value of 1863.9 W kg^-1 for FGCNT-0.81 in Na/CF_x batteries.It is proved that the binding energy of C–F bond,the electronegativity and the polarization force of the metal ions collaboratively determine the electrochemical performance of F-CNTs materials.The new Na/CF_x primary batteries developed here are promising in low power applications as high-energy density and low-cost batteries.2.Preparation of fluorinated graphene and its application in lithium/sodium primary batteries:In this work,fluorinated graphene?FG?materials with different fluorine contents are prepared through a direct fluorination method at different temperatures.In order to study the effect of fluorination temperature on electrochemical performances of the FG materials,we tested the FG samples as the cathode materials of LPBs and SPBs.It is found that the C-F bonding strength is the main effect factor of electrochemical performances at low discharge current densities.Thus,the FG-0.75 with weaker C–F bonds achieves the maximum specific capacity(856.9 mAh g^-1)and energy density(2239.8 Wh kg^-1)at 10 mA g^-1 in LPBs.However,at high discharge current densities,interlayer spacing and the particle size of the FG samples significantly influence their electrochemical performance.Consequently,the FG-0.95 with the smallest particle size and maximum layer spacing obtains the best rate capability.In LPBs,FG-0.95 shows a remarkable specific capacity of 851.1 mAh g^-1 and a discharge voltage of 2.65 V at10 mA g^-1,and 725.5 mAh g^-1 and 2.05 V at 2000 mA g^-1.In SPBs,FG-0.95 exhibits a discharge voltage of 2.35 V and a high specific capacity of 824.4 mAh g^-11 at 10 mA g^-1,and 1.9 V and757.6 mAh g^-11 at 500 mA g^-1.It is proved that the C–F bonds?including the binding energy and the content of the C-F bonds?,interlayer spacing,material particle size,the electronegativity and the polarization force of the metal ions collaboratively determine the electrochemical performance of FG materials.Moreover,the obtained FG materials show much better electrochemical performances than that of the commercially fluorinated graphite material?Daikin?,such as better rate performance,higher average discharge voltage and larger discharge specific capacity.Finally,the discharge reaction mechanism of LPBs and SPBs are explained by ex-situ characterization of XRD and XPS.