Application Of Fluorinated Electrolyte Additives In Lithium Metal Batteries

Lithium is one of the most attractive anode materials for electrochemical energy storage.However,the growth of lithium dendrite in the electrochemical deposition process leads to low coulomb efficiency and safety problems,which has hindered the application of rechargeable lithium metal batteries for a long time.Therefore,adding additives to the electrolyte to inhibit the growth of lithium dendrite and stabilize lithium metal anode is an effective strategy.In this paper,the protective effect of electrolyte additives on lithium metal anode was studied,and the mechanism was explained by a series of characterization methods.In addition,Li Ni_xMn_yCo_zO₂（NCM）was used as cathode to verify the practicability of the electrolyte containing additives.It mainly includes the following contents:（1）1,1,1,3,3,3-Hexafluoroisopropyl Trifluoromethanesulfonate（HFPTf）was developed as an electrolyte additive.By formin g a stable solid electrolyte interface layer（SEI）,the growth of lithium dendrite was greatly inhibited and the Li metal anode was protected.The SEI has Li F bottom layer which can reduce Li⁺diffusion barrier and Li₂SO_x（x=0,3,4）which is highly conductive distributed in the upper layer.The special structure of SEI can conduct Li⁺rapidly and avoid the growth of Li dendrite.HFPTf additive can also change the structure of the first-order sheath of Li⁺solvation,making Li⁺easier to desolvent.The results show that the Li‖Li symmetrical cell with 1 wt%HFPTf-contained electrolyte achieves cycle stability of more than 1000 h at 0.5 m A cm^-2 current density and 0.25 m Ah cm^-2 deposition capacity.In addition,a thin and uniform cathode electrolyte interface layer（CEI）was formed,which inhibits the dissolution of metal ions from the cathode material.The capacity retention rate of Li‖NCM811 cell is 70%after 150 cycles with current density of 100 m A g^-1.（2）1-（Heptafluorobutyryl）imidazole（HFBMZ）was developed as an electrolyte additive to stabilize the anode of lithium metal batteries.First,HFBMZ can remove the trace H₂O and HF in electrolyte,and can reduce by-products on SEI surface and inhibit the dissolution of metal ions of NCM622.Secondly,HFBMZ can make the electrolyte diaphragm interface have good wettability,which is conducive to increase the flow rate of Li⁺and inhibit the growth of dendrites.In addition,HFBMZ is reduced to stable SEI and even CEI on the surface of lithium metal,which improves the cycle performance and the rate performance of the cells.The experimental results show that the Li‖Li symmetrical cells has achieved stable cycle for more than 1200 h under the condition of current density of 0.5 m A cm^-2 and capacity of 0.25 m Ah cm^-2in electrolyte containing 1 wt%HFBMZ.The capacity retention rate of Li‖NCM622cell is 92%after 150 cycles with current density of 100 m A g^-1.