Design And Research Of Lithium-based Electrolyte Solutions For Dual-ion Batteries

With the increased demand of large-scale energy storage devices in modern society,the new rechargeable batteries are becoming the research focus of the nextgeneration electrochemical energy storage devices,such as sodium-ion,potassium-ion and dual-ion batteries.Compared with other kinds of batteries,dual-ion batteries possess the advantages like high worrking voltage,low cost,environmental benignity and facile fabrication process mainly because graphite is usually utilized as the positive electrode material.The energy of the dual-ion battery comes from the simultaneous storage of the anion and cation of the electrolyte solutions at the positive and negative electrodes driven by the external current,so the role of electrolyte solutions in the dualion battery is very significant.Moreover,compared with both ionic liquids with the complex preparation process and concentrated aqueous solutions with rather high cost,the traditional organic electrolyte solutions employed in dual-ion battery seem more competitive in the future applications by virtues of the available preparation process and acceptable cost.However,in the traditional organic electrolyte solutions,the solvents always co-intercalate into graphite positive electrode together with anions and demonstrate different solvation manners,resulting in the varied performance of the dual-ion batteries.Up to now,1 M LIPF6-EMC(ethyl methyl carbonate)has the best compatibility with graphite electrode,however,it still faces the problems as follows:first,it is flammable;second,it is incompatible with graphite negative electrode in dual-graphite batteries;third,the performance of dual-ion batteries using this electrolyte solution isn’t very satisfactory at low temperatures;finally,the mechanism of anion storage in graphite positive electrode has not been clarified in depth.Therefore,in order to improve or optimize its functions,the following attempts in designing lithium-based electrolyte solution for dual-ion batteries have been carried out in this thesis:1.In order to solve the flammability issue of pure EMC-based electrolyte solution,the flame-retardant solvent TMP(trimethyl phosphate)was added into 1 M LiPF6EMC.It was found that TMP competes with EMC to solvate PF6-anion and then suppress the intercalation of EMC-PF6-into graphite,leading to the decrease of capacity delivered by graphite electrode in 1 M LiPF6-EMC/TMP.Then,by increasing the concentration of LiPF6,the anion storage capacity considerably rises.So the solution of 3 M LiPF6-EMC/TMP(7:3 volume ratio)could meet both needs of high capacity of graphite electrode and flame-retardant ability of electrolyte solution;At the same time,the concentrated solution of 3 M LiPF6-EMC/TMP(7:3 volume ratio)has also been found compatible with graphite negative electrode.In this case,dual-graphite battery with flame-retardant electrolyte solution was further assembled for the first time.The influences of both the mass ratio of positive to negative graphite electrode and prelithiated technology on the performance of the dual-graphite battery were investigated;On the another hand,by substituting LiPF6 with LiBF4 in EMC/TMP solutions,it was found that a small dose of TMP can promote the intercalation of EMC-BF4-into graphite.This phenomenon could be attributed to the following reason.TMP with high dielectric constant can separate more Li+-BF4-ion pairs,and the liberated Li+ is preferentially solvated by TMP with stronger electron donor ability,resulting in more BF4-solvated by EMC and then accelerating EMC-BF4-intercalation into graphite positive electrode.2.In the above works,it was observed that PF6-and BF4-could not deeply intercalate into the graphite electrode trom TMP at voltages below 5.2 V vs.Li/Li+.By increasing the concentration of electrolyte salt and elevating the cut-off voltage of dual-ion batteries,TMP-PF6-and TMP-BF4-could successfully intercalate into graphite positive electrode.Moreover,TMP-PF6-and TMP-BF4-can also intercalate into the graphite positive electrode at lower voltage through the pre-intercalation strategy of TMP-TFSI-into graphite.3.In order to improve the low-temperature performance of Li/graphite dual-ion batteries using 1 M LiPF6-EMC,MA(methyl acetate)with low melting point and low viscosity was introduced into 1 M LiPF6-EMC as a co-solvent.At the same time,the anion storage behavior of graphite electrode in 1 M LiPF6-MA was investigated.4.In the solutions of 1 M LiPF6-EC/EMC(EC(ethylene carbonate)accounted for more than 60%of the volume percentage of EMC/EC mixed solvents),the formed anion-graphite intercalation compounds(AGICs)in charged graphite electrode has an abnormal intercalated gallery height(IGH)value of 0.808 nm,which indicated that both EMC and EC co-intercalate into graphite layers together with PF6-.Furthermore,both the volume percentage of EMC in the EMC/EC mixed solvents and the concentration of LiPF6 was changed and the distribution map of the abnormal intercalated gallery height of 0.808 nm was obtained.The following fact was recognized.EC at first solvates Li+,and the remaining EC competes with EMC to solvate PF6-,then a trinity anion of EMC-PF6--EC can intercalate into the graphite layers.In addition,through the activation strategy of EMC-PF6-pre-intercalation into graphite,EC could also cointercalate into this "activated" graphite and the above abnormal intercalated gallery height could be repeated;Moreover,In the ternary-solvent solutions of 1 M LiPF6EC/EMC/MA,a novel IGH value(0.824 nm)of anion-graphite intercalation compounds was observed in the charged graphite electrodes,which could be attributed to the intercalation of PF6-anion co-solvated by EMC,EC and MA into graphite layer.By two different ways of pre-intercalation routes,this IGH vaule could be repeated,which fact further verified the above ascription.