Study On Regulation And Electrochemical Performance Of Fluorine-based Electrolyte/Lithium Metal Interface

Lithium(Li)metal,with the highest theoretical specific capacity(3860 mAh g-1)and the most negative electrochemical potential(-3.04 V vs.standard hydrogen electrode),has been considered the most promising next-generation anode candidate.However,the highly reactive nature of Li metal can trigger severe side reactions with the electrolyte and form an electrically insulating but ionically conducting solid electrolyte interphase(SEI)on its surface.In general,SEI film is principally composed of organic species ROCO2Li,which continuously fracture and regenerate during cycling,thereby affecting the Coulombic efficiency(CE)and cycling stability.In addition,the organic ROCO2Li has high ion transfer resistance and sluggish Li+transport,which gives rise to nonhomogeneous Li deposition accompanied by dendrite formation.During cycling,the dendrite can pierce the separator and result in the short-circuiting of battery,even causing fire and explosion.Therefore,the formation and composition of SEI play critical roles in the CE and stability of Li batteries.In this thesis,on account of the large interface resistance,unstable structure and low anode CE in conventional electrolytes,"Interfacial interaction regulated by the fluorine-based anions and solvents in the electrochemical process" is considered the starting point.The emphasis is placed on the center of Li+solvation sheath,including the regulation of anion concentration,the Li+-dipole interaction with fluorinated anions,and the Li+-dipole interaction with fluorinated solvent.As a result,low-impedance and electrochemically stable SEI film with abundance of LiF is produced,which inhibits the dendrite formation,achieves high anode CE and maintains stable cycling stability.The main results are summarized as follows:(1)A copper substrate covered by anion-derived SEI,which is produced by electroreduction of highly concentrated water-in-salt electrolyte with 21 M lithium bis(trifluoromethane sulfonyl)imide(LiTFSI),is used as the working electrode in organic solvent system.In this high-concentration electrolyte,TFSI" principally participates in the Li+solvation sheath and produces SEI dominated by inorganic species.Compared with the SEI that has high ion resistance in conventional electrolyte,this stable SEI reduces the Li+diffusion resistance and achieves uniform Li deposition.Scanning electron microscopy images show that smooth and spherical Li forms under the protection of anion-derived SEI.Such SEI facilitates fast Li+transport and produces uniform spatial distribution for suppressing dendrite growth at local positions.At 0.5 mA cm-2,Li?Cu half cells enable average CE of 98.2%over 140 cycles.When combined with sulfur cathode,stable cycling stability over 400 cycles with high CE of 99.1%is achieved.(2)Lithium trifluoroacetate(LiTFA)with carbonyl and fluorine functional groups is used to regulate the Li+solvation sheath.With its strong coordination between the carbonyl groups(C=O)and Li+,TFA-modulates the environment of the Li+solvation sheath and facilitates fast desolvation kinetics.In addition,due to relatively smaller lowest unoccupied molecular orbital energy than solvents,TFA-has preferential reduction to produce stable SEI with uniform distribution of LiF and Li2O.Such stable SEI effectively reduces the energy barrier for Li+diffusion,contributing to low nucleation overpotential,fast ion transfer kinetics,and uniform Li+deposition with high cycling stability.Coupled with lithium iron phosphate(LiFePO4)and LiNi0.6Co.2Mn0.2O2 cathodes,the full cells with limited Li as the anode enable long-term cycling life than the control samples.(3)Di-fluoro ethylene carbonate(DFEC)with two substituted fluorine atoms is used to reduce the intrinsic solvating power of solvent and regulate the Li+solvation sheath.Fluorinated functional groups have strong electron-withdrawing ability,which can reduce the density of lone electron cloud of carbonyl groups and weaken the dipolar force between Li+and dipole moment of DFEC.Such DFEC-based electrolyte displays 6 times faster Li+desolvation rate over that of non-fluorinated EC-based electrolyte at-20?,indicating great potential to improve the low-temperature charge transfer kinetics.In addition,the fluorinated functional groups can greatly reduce the lowest unoccupied molecular orbital energy and the highest occupied molecular orbital energy of solvent molecules.During SEI formation,the expelled DFEC from the solvation sheath is preferentially reduced for its high reduction potential to form Li+-conducting SEI layer with an abundance of LiF.Such SEI induces low Li plating/stripping overpotential and uniform Li deposition with high anode CE.In addition,the highest occupied molecular orbital(HOMO)energy of DFEC pushes the oxidation stability beyond 5.2 V,in contrast to 4.2 V for EC-based electrolyte.When combined with the LiNi0.8Co0.1Mn0.1O2(NCM811)at room temperature,NCM811?Li cells in DFEC-based electrolyte retain 91%original capacity after 300 cycles with an average CE of 99.6%.Benefited from the fast interfacial transfer kinetics,about 89%(160 mAh g-1)and 51%(93 mAh g-1)of room-temperature capacities are delivered even at 0? and-30?,respectively.