【作者】 张磊；

【导师】 王宏宇；

【作者基本信息】 中国科学技术大学 ， 物理化学， 2022， 博士

【摘要】 随着现代社会对大规模储能器件的需求日益增强,钠离子电池、钾离子电池、双离子电池等新型电池逐渐成为下一代电化学储能器件的重点研究对象。由于正极材料常使用的是石墨,双离子电池相比于其它两种电池优势明显,其工作电压更高,成本更低、制作流程更简便,对环境也更加友好。双离子电池的能量直接来源于外部电流驱使电解液中阴、阳离子的在正、负极的定向存储,因此电解液在双离子电池中扮演的角色非同一般。相比于离子液体的复杂制备和超浓水系电解液的昂贵价格,传统有机电解液制备工艺更成熟,价格更低廉,在双离子电池的应用中具有较明显的优势。然而,在传统有机电解液中,溶剂随阴离子一同插嵌石墨正极时会表现出不同的行为,导致这些双离子电池的性能各不相同。即使是现阶段与石墨正极兼容性最好的电解液1 M LiPF6-EMC(碳酸甲乙酯),也面临着易燃、与石墨负极相容性较差、低温性能不佳和储能机理不明晰等问题。因此,在本论文中,围绕着改良优化锂基双离子电池电解液的设计尝试展开了以下几方面工作:1、我们尝试通过加入阻燃剂TMP(磷酸三甲酯)来解决以EMC为基础溶剂的电解液易燃的问题。结果发现TMP会与EMC竞争络合PF6-阴离子,抑制阴离子插嵌石墨,导致在较低浓度的LiPF6-EMC/TMP电解液中石墨正极阴离子存储容量减小。但是随着LiPF6浓度的增大,在LiPF6-EMC/TMP电解液中石墨的阴离子存储容量逐步得以提升。浓度达到3 M时,该系电解液可以同时兼顾石墨正极的高容量和电解液阻燃性;另外又发现电解液3 MLiPF6-EMC/TMP(7:3体积比)与石墨负极相容。于是利用该种电解液组装了双石墨全电池,并探讨了正、负极石墨质量比以及石墨负极预锂化处理对全电池性能的影响;此外,将1 M LiPF6-EMC/TMP中的LiPF6换成LiBF4,我们发现少量TMP的引入会促进EMC-BF4-插嵌石墨正极,并将这一现象的主要原因归结为:拥有高介电常数的TMP能够分开更多的Li+-BF4-离子对,从离子对释放出的Li+优先与TMP络合,而BF4-与EMC络合,从而促进了EMC-BF4-插嵌石墨。2、在上述工作中,我们发现PF6-和BF4-在纯TMP中都无法在5.2 V(vs.Li/Li+)以下深层次插嵌石墨正极。通过增大电解质浓度、提高电压,TMP-PF6-和TMP-BF4-均成功插嵌石墨正极;另一方面,通过TMP-TFSI-的预插嵌,TMP-PF6-和TMP-BF4-在较低电压处也能插嵌至石墨正极。3、使用1 M LiPF6-EMC电解液的Li/石墨双离子电池的低温性能尚不够理想,通过在电解液中加入低熔点、低粘度的共溶剂MA(乙酸甲酯),提高了低温(-25℃)下Li/石墨双离子电池的电化学性能。另外,也研究了 1 M LiPF6-MA电解液中石墨正极的电化学行为。4、在1 M LiPF6-EMC电解液中加入EC(碳酸乙烯酯)(EC占EMC/EC混合溶剂体积比超过60%),石墨正极充电后所形成的阴离子-石墨嵌层化合物(AGICs)具有异常的插嵌层间距数值0.808 nm,表明EMC与EC同时溶剂化PF6-并共插嵌石墨层间。我们进一步改变混合溶剂中EMC、EC的体积占比和LiPF6浓度,绘制了可以得到此插嵌层间距数值的电解液组分分布图,从中发现电解液中的EC溶剂分子会抢先与Li+络合,然后剩余的EC分子再与EMC争夺络合PF6-并形成三位一体的溶剂化阴离子(EMC-PF6--EC),并共同插嵌至石墨正极。同时,利用EMC-PF6-预插嵌石墨之后,EC-PF6--随后也能插嵌此“活化”石墨,再现上述插嵌层间距数值;此外,在三元混合溶剂组成的1 M LiPF6-EMC/EC/MA电解液体系中,石墨正极充电后可以得到特殊的阴离子-石墨嵌层化合物层间距数值0.824 nm,将之归因于三种溶剂EMC/EC/MA同时溶剂化PF6-阴离子并插嵌石墨层间。并且通过两种不同的预插嵌活化途径,成功地获得了此插嵌层间距数值,进一步证实了其归属。更多还原

【Abstract】 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.更多还原