Application Of Sulfone-based Electrolytic Solutions In Dual-ion Batteries

Recently,dual-ion batteries(DIBs)has drawn extensive attention mainly because the high working voltages and cheap positive materials.However,anions are involved in electrochemical reactions on positive electrodes,which makes DIBs different from traditional secondary batteries like lithium-ion batteries.Meanwhile,intercalation of anions into graphite electrodes occurs at high potentials and the tolerance towards caustic oxidation environment is required.Moreover,the solvation of anions in organic solutions complicates the intercalation behavior.At present,the following problems need further explorations.1.Establish a reliable methodology to calibrate the structural parameters of anion-graphite intercalation compounds(AGICs);2.Unambiguously recognize the species inside AGICs formed in complex solutions(mixed solvents or salts);3.Clarify the impacts of concentrated solutions on the electrochemical performance of graphite positive electrodes and the structure of AGICs.In this PhD thesis,the solutions based on LiBF4 salt and sulfone solvents coupled with natural graphite positive electrode are applied for dual-ion batteries.A systematic study regarding the above theme includes the following attempts:1.In the solution of 1M LiBF4-sulfolane(SL),graphite positive electrode is subjected to several electrochemical in situ characterizations during charge-discharge cycles in the potential range of 3-5.5 V vs.Li/Li+.The characteristic structures of AGICs are calibrated and the structural changes are traced.It is confirmed that Stage 2 AGIC is formed at the super-high potential of 5.5V,which corresponds to the anion storage capacity of about 90 mAh g-1.2.LiBF4 solutions based on the binary solvent mixtures of SL/ethyl methyl carbonate(EMC)and 3-methyl sulfolane(MSL)/EMC are applied for DIBs.The maximum capacity values delivered by graphite positive electrode are in the solutions composed of mixed solvents rather than those composed of pure solvents.X-ray diffraction(XRD)measurements show that EMC dominates the co-intercalation with BF4-in most solutions.Nevertheless,sulfone solvents have higher DN number and permittivity,which promote the dissociation of LiBF4.The synergistic effect of both solvation and ion-pairing profoundly influnces the electrochemical performance of graphite positive electrode.3.In LiBF4 solutions based on the mixed solvents of ethyl methyl sulfone(EMS)and SL,the storage behavior of anions in graphite electrodes are tested.While the capacities delivered by graphite positive electrodes in LiBF4-pure sulfones are close,the ratio of SL/EMS imposes little influence on capacities.However,the potential plateaus in charge-discharge curves have changed,which means that the electrochemical intercalations in EMS and SL are quite distinct.In situ XRD measurements display that in a large range of solvents ratios,IGH of GICs could transfer from 0.80 to 1.07 nm with potentials increasing,which indicates anions intercalate accompanied by EMS at first and then subsequently SL are involved in the co-intercalation with the anion.The impacts of cut-off potential in the charge process,salt concentration and pre-intercalation of anion have been addressed.4.Highly concentrated LiBF4-SL or LiClO4-SL solutions are applied for dual-ion batteries.It’s found that a novel type of AGICs with the characteristic IGH(Intercalation gallery height)value of 0.80 nm are formed in concentrated solutions with capacities decreasing.Infrared spectra of solutions show that more and more SL molecules are strongly bond with Li+as concentration increasing.Therefore,the lack of free solvents could result in the shrink of solvation shell around each anion inside GICs change and then the in-plane density of anions decreases,which could account for the lessening electrochemical activity.The influences of partial substitutions of SBPBF4 or LiPF6 for LiBF4 prove the above deduction.5.LiDFOB+LiBF4-SL or LiClO4-SL solutions are applied for DIBs.Lower discharge capacities are gotten when small amounts of LiDFOB are added.In situ XRD tests show that DFOB-intercalates into graphite at relatively lower potentials and SL-BF4-@GIC or SL-ClO4-@GIC phases appears later.Moreover,the influence of DFOB-addition on ClO4--SL intercalation is weaker than that on BF4--SL intercalation,which fact may result from the lower intercalation potential of ClO4than that of BFa-from SL.