Design And Performance Of Fluorine-based Electrolyte For Li Metal Batteries

With the development of microelectronic technology,hybrid electric vehicles and miniaturized electronic equipment are growing with each passing day,leading to large-scale production of lithium-ion batteries.However,developed lithium-ion batteries increasingly close to the limits of their inherent capabilities cannot meet the current dramatic demand for high energy density.As a typical representative of post-lithium ion batteries,lithium metal batteries with low electrode potential（-3.04 V）and high theoretical specific capacity(3860 m Ah g^-1)are promising to greatly improve the energy density.Although many efforts have been paid,some thorny issues still restrict the practical application of lithium metal battery.On one hand,the excessive growth of lithium dendrite caused by the uneven deposition of lithium ion during the cycle of lithium metal battery is prone to puncture the separator associated with short circuit of the battery.On the other hand,the solid electrolyte interface spontaneously formed between lithium metal and electrolyte on the electrode surface is generally unstable to trigger continuous exposure of fresh lithium,in which a series of side reactions between fresh lithium and electrolyte inevitably cause a large amount of lithium metal and electrolyte consumption and severely deteriorate battery cycle life.In recent years,the electrolyte engineering has been proposed by researchers to address the above issues.Therefore,this thesis mainly focused on the design and preparation of fluorinated electrolytes,and conducted electrochemical tests on batteries with various fluorinated electrolytes followed by the systematic analysis on the composition and morphology of interface phases,aiming at obtaining high-performance lithium metal batteries.The main research contents of this thesis are as follows:（1）Different density functional theory（DFT）methods were compared and the optimal functional method was screened to calculate the electrochemical properties of solvents in lithium metal batteries.Firstly,a small redox potential database named as Redox20 base database was established using the Wuhan-Minnesota scaling（WMS）method developed by our group.The database included the Ox10 subset and Red10subset,both of which were consist of 10 oxidation potentials and 10 reduction potentials,respectively.The Redox20 database was used to evaluate the performance of 35reported DFT methods.The results showed that M05-2X,M08-HX,M08-SO,M06-2X and rev M06 functions ranked in the top five to predict the oxidation potential,N12,M06-HF,M08-HX,HSE06 and PW6B95-D3 ranked in the top five to predict the reduction potential.M08-HX had the smallest mean unsigned error,which was considered as the best DFT method for calculating the redox potential.Therefore,suitable solvents can be effectively screened by precisely calculating the redox potential,the highest occupied molecular orbital（HOMO）and the lowest unoccupied molecular orbital（LUMO）energy levels of many solvents for high-performance lithium metal batteries.（2）An electrolyte strategy to directly regulate the solvation structure of lithium ion was proposed to construct the ideal interfacial chemistry for high-performance lithium metal batteries.Based on the optimal DFT method of the first part,HOMO/LUMO energy levels of fluorinated solvents were evaluated,and thus some fluorinated carbonate solvents with high HOMO/LUMO stood out:ethyl（2,2,2-trifluoroethyl）carbonate（ETFEC）and fluoroethylene carbonate（FEC）.The electrolytes with different fluoride degrees were prepared by selective combination of conventional solvents（EC and DEC）,fluorinated solvents（FEC and ETFEC）,and Li PF₆ as Li salt.Significantly,the fluorinated carbonate electrolyte can effectively induce high-quality passivation interface phase on the electrode surface,and it was compatible with high-voltage cathode.Through the classical molecular dynamics（MD）simulation,it was found that all-fluorinated carbonate electrolyte directly optimized the solvation sheath of lithium ion,and the coordination number of lithium ion with solvents decreased from 3.75 in non-fluorinated electrolyte to 3.02 in all-fluorinated electrolyte.The experimental results further proved that the rapid dissolution of lithium ion in all-fluorinated electrolyte and the formation of a stable Li F-rich interface phase on the electrode surface jointly restrained the growth of lithium dendrites as well as side reaction between lithium metal and electrolyte.The average coulomb efficiency（CE）of the assembled Li||Cu cell reached 98.3%.Even at high voltage（4.6 V）,the Li||NMC811 cell retained 72.3%of the initial capacity with an average CE of 99.8%after 225 cycles.（3）A fluorinated carbonate electrolyte with nonflammable and high-voltage resistant characteristics was prepared for lithium metal battery,in which the solvation structure of the electrolyte was analyzed,and different types of cells were assembled to test their electrochemical performances.On the basis of the ETFEC proposed in the second part in this thesis,further fluorination yielded the solvent with two–CF₃ groups,namely,di-2,2,2-trifluoroethyl carbonate（TFEC）.According to DFT calculation,TFEC had superior antioxidant and reducing activity to those of DEC.Hence,this thesis combined TFEC with FEC,and Li PF₆ as FT-type electrolyte.The nonflammability of the electrolyte favored the safety of lithium metal battery.The MD simulation results further disclosed that the FT-type electrolyte can not only effectively reduce the lithium ion coordination number,but also enhance the rapid dissolution ability of lithium ion.The cells assembled with the FT-type electrolyte delivered excellent cycling stability（Li||Li cell:>900 h）and high CE（Li||Cu cell:98.4%）thanks to the formation of a stable SEI on the fluorinated electrolyte-treated Li anode surface.Li||NCM811 cells retained58%of initial capacity combined with an average CE of 99.8%after 300 cycles at 4.6V,suggesting the good compatibility of the electrolyte with high-voltage cathode.（4）An all-fluorinated electrolyte composed of fluorinated ether and fluorinated carbonates was designed for highly-fluorinated interphases.In the second and third part of this thesis,the combined solvents belonged to fluorinated carbonates.In this part,a new component,fluorinated ether,namely1,1,2,3,3,3-hexafluoropropyl-2,2,2-trifluoroethyl ether（HFTFE）was introduced to electrolyte system.Remarkably,HFTFE with high HOMO energy level possessed strong antioxidant activity.It is promising to combine HFTFE with fluorinated carbonate solvents（FEC and FEMC）and Li PF₆ as a novel all-fluorinated electrolyte.The electrochemical performance of cells assembled with all-fluorinated electrolyte was systematically evaluated and the interface characteristics were analyzed.It can be concluded that that the cells with all-fluorinated electrolyte suggested high cycling stability（Li||Li:>1100 h）and Li plating/stripping CE（97.1%after 800 cycles）.In addition,Li||Ni92 cell retained 68%of the initial capacity with an average CE of 99.8%after 500 cycles at 5 V.The combination of fluorinated ether and fluorinated carbonate provides an effective strategy for the safe operation of high-performance lithium metal battery under high voltage.