Fluorinated Carbons As Rechargeable Battery Cathodes

Fluorinated carbon has high energy density(2189 Wh·kg^-1)and high theoretical specific capacity(865 m Ah·g^-1)in lithium primary batteries.It stands out from commercial primary batteries due to the advantages of stable discharge platform,long storage life,wide operating temperature,and wide application range.The research on fluorinated carbon primary batteries has attracted attention in recent years,mainly focusing on the improvement of the preparation method of fluorinated carbon cathodes,screening out more suitable high-voltage and high-capacity fluorinated carbon materials;preparing a suitable electrolyte system to improve the interface stability,so that it can obtain the optimal performance at high and low temperatures;modify the lithium metal negative electrode to improve the performance of fluorinated carbon batteries as a whole to improve the energy density,power density,discharge capacity,and rate performance of fluorinated carbon under harsh temperature conditions.However,less attention focus on the secondary performance of fluorinated carbons,so it is necessary to systematically study to adequately utilize the advantages of high specific energy and high capacity of fluorinated carbon materials in secondary batteries.This dissertation mainly starts from the lithium-ion/sodium-ion/potassium-ion battery system,which is a hot spot and has a enormous development prospect,to explore the secondary performance and reversible mechanism of fluorinated carbon in the three systems.The concrete researchs are as follows:First of all,the secondary performances of different types of fluorinated carbon in lithium-ion liquid batteries are initially explored.After comparative screening of electrochemical performance,fluorinated graphite CF_0.88 can rechargeable after expanding the voltage window to 0.5-4.8 V.After the first discharge of 1.5 V to 0.5 V,the ternary phase Li_1+xFC will be generated,and it can transformes into LiF+C after charging,and reversible in the subsequent cycles.By expanding the voltage window,the fresh electrochemical behavior of fluorinated graphite CF_0.88 appears,which paves the way for the further exploration of fluorinated carbon in lithium-ion secondary batteries.Considering that the ternary phase Li_1+xFC generated after the expansion of the window is reversible,it does not veritable the reversibility of fluorinated carbon in lithium-ion batteries.In the previous study,we found that fluorinated carbon can be reversible in solid-state electrolytes.On this basis,the ionic conductivity of solid-state electrolytes is improved by adding inorganic Si O₂ and polymer PPC fillers,and the electrochemical performance of fluorinated carbon in solid-state electrolytes are further improved.The reason for the reversibility is that amorphous LiF is generated in the solid-state electrolyte.Compared with the crystalline LiF generated in the liquid electrolyte,the energy required for amorphous LiF to be reversible into fluorinated carbons is weak.Inspired for this,it is formidable to convert crystallized LiF into covalent C-F bond in a liquid electrolyte.By adjusting the type of C-F bond and the F/C ratio,the fluorinated graphene CF_1.12 generates the semi-ionic C-F bond upon initial charging,and the voltage and energy required for LiF to be reversible into a semi-ionic fluorinated carbon are weak.Besides,high-voltage sulfolane electrolyte system is selected,and the synergistic effect of the semi-ionic C-F bond and the electrolyte is realized,and the synergistic effect of the semi-ionic C-F bond and the electrolyte realizes the secondary reversibility of fluorinated carbon in lithium-ion batteries.The research on the secondary performance of fluorinated carbons in sodium/potassium-ion batteries were studies.Different electrolyte PC and FEC solvent systems is selected,in the voltage range of 1.0-4.8 V.The secondary behavior of fluorinated graphite CF_0.88 appeared in the sodium-ion battery,fluorinated graphite CF_0.88exhibits secondary behavior in sodium-ion batteries.An additional discharge platform appeared in the voltage range of 1.0-1.5 V,which is characterized by solvent co-embedding.The reason is that the solvation energy of FEC is larger and it is easier to co-embed.In addition,the F/C bond length is regulated to form the ternary phase K_xFC between the carbon layers,avoiding the formation of the two-phase KF+C.At the same time,the fluorinated soft carbon CF_1.01 with larger carbon layer spacing and more defects,which is easier to store potassium and has the optimal electrochemical performance.This study realizes the rechargeability of fluorinated carbons in potassium-ion batteries,and reveals that fluorinated carbon electrode materials can reflect different electrochemical behaviors in potassium-ion batteries.