Controllable Preparation Of Three-dimensional Hollow Fluorinated Carbon Materials For Energy Storage

Lithium/Fluorocarbon（Li/CF_x）batteries are primary batteries with the highest theoretical specific energy（～2180 Wh/kg）and are popular in high-end fields,such as military weapons,aerospace,and medical equipment.However,the intrinsic conductivity of carbon fluoride（CF_x）materials is relatively low by virtue of the strong covalent nature of the C-F bond,and CF_x has poor wettability with electrolyte because the surface energy of CF_x is low.Therefore,it is easy to generate serious polarization problem that results in incomplete utilization of the capacity and poor rate performance of the Li/CF_x batteries when CF_x is used as the electrode material,which not only causes an enormous waste of energy,but also fails to meet the requirement for high-power discharge of the battery in a specific environment.Herein,CF_x cathode materials are systematically studied from two aspects:the structure designing of carbon source and carbon-coated modification.Firstly,a new type of carbon source,three-dimensional（3D）hollow carbon capsule（CC）,was prepared by catalytic chemical vapor deposition（CCVD）and then the fluorinated carbon capsule（FCC）electrode material was synthesized via the direct gas fluorination method.It was shown that FCC maintained the three-dimensional（3D）hollow capsule-like structure when fluorination temperature was between 325°C and360°C.Nevertheless,the fluorination temperature higher than 370°C triggered the disintegration of the hollow capsule-like structure of the FCC to convert the sheet-like structure.Moreover,the F/C ratio and the content of semi-ionic C-F bonds followed the law of first increasing and then decreasing with the increase of fluorination temperature.Secondly,the performance of lithium primary battery of FCC material was systematically studied through constant current discharge test and AC impedance test,and the association between structure and achievement was clarified.It was found that the synthesized FCC-360 electrode material delivered a discharge specific capacity of 1005m Ah/g（107%of the theoretical specific capacity）and an energy density of 2323 Wh/kg at the cut-off voltage of 1.5 V.Also,it delivered a discharge specific capacity of 1445m Ah/g（154.5%of the theoretical specific capacity）and an energy density of 2953 Wh/kg at the cut-off voltage of 1 V,which was better than most of the reported literature.Its ultra-high electrochemical performance was mainly because the FCC-360 material contained more semi-ionic C-F bonds and the high curvature of the capsule-like structure weakened the covalent nature of the C-F bonds,which was contributory to the participation of the covalent C-F bonds in the discharge.Moreover,the extra capacity was mainly due to the FCC-360’s edge defect structure adsorbing extra Li⁺during the discharge process.Finally,the atomic layer deposition-chemical vapor deposition（ALD-CVD）joint technology was innovatively developed to carbon-coated modification of FCC and the enhanced lithium primary battery performance of FCC@C was scrutinized.The amorphous Al₂O₃ layers were deposited on the FCC surface through ALD technology,and then the carbon-coated FCC electrode material（FCC@C）was synthesized by CVD method.It was shown that amorphous carbon was evenly stacked on the surface of FCC by the form of nanosheets.More importantly,the specific capacity of FCC@C exhibited massive advancement.At a small current density of 0.01 A/g,the specific capacity of FCC@C enhanced from 723.4 m Ah/g（81.6%of theoretical specific capacity）to 852.6m Ah/g（96.1%of theoretical specific capacity）,indicating an improvement of 17.9%.Moreover,at a high current density of 0.5 A/g,the specific capacity escalated from 417.5m Ah/g（47.1%of the theoretical specific capacity）to 630.7 m Ah/g（71.1%of the theoretical specific capacity）,denoting an enhancement of 51.1%.The gigantic improvement of performance was attributed to the amorphous carbon layer with good conductivity that could form the conductive network in the electrode,which was contributory to the conduction of electrons.In addition,the amorphous carbon possessed good compatibility with the electrolyte,which enhanced the infiltration between the electrode material and the electrolyte.Furthermore,the 3D hollow capsule-like structure and the small voids left by the stacking and coating of amorphous carbon nanosheets benefited the transport of electrolyte and promoted the diffusion of Li⁺.Briefly,this work designed a 3D hollow FCC cathode material with ultra-high energy density and then developed an ALD-CVD joint technology for the carbon-coated modification of FCC with enhanced rate performance,laying a significant research foundation for the development of next generation of Li/CF_x batteries with high energy density and high power density.