低温电解液的构筑及Li+迁移强化新方法

Low temperature operated lithium ion battery (LIB) was call “the pearl on the crown” in electrochemical energy storage area. The low transport speed of lithium ions in both electrolyte and electrode/electrolyte interface under low temperature is the main bottleneck which hindering the research of low temperature operated LIB. Polyether with the structure of CH3O(CH2CH2O)nCH3 (short as Gn) often shows lower melting point, lower vapour pressure as well as higher dielectric constant compared to common esters or carbonates which usually be used in electrolytes for LIB, while it was rarely used for its poor compatibility with graphite. We intend to introduce the concept of “solvent in salt”, which means to mix polyether with equal molar of lithium salt with proper ionize capability to form a special ionic liquid with the structure of [Li(Gn)][X], moreover, we will try to combine theoretical calculation and experimental research to improve the electrochemical stability of polyether and settle the compatibility issue. Meantime, we will try to mix [Li(Gn)][X] with different n, which means the length of ether chain, to reduce the viscosity and melting point, as well as to improve the conductivity. And by introduce organic solvents with low melting point, such as DEC or EMC, to further improve the low temperature performances of the electrolyte. Functional additives will also be used to enhance the control of solid state interface between electrode and electrolyte. The final goal is to obtain electrolyte with wide liquid range, good electrochemical stability, low vapour pressure, and capable of work between the temperature range of -60~60℃. On which we will try to issue a new method to reinforce the low temperature Li+ transportation, and breakthrough the bottle neck in low temperature lithium ion battery development.

低温锂离子电池被誉为电化学储能领域“皇冠上的明珠”，低温下Li+在电解液和电极/电解液界面的传输是其发展的主要瓶颈。与电解液常用的羧酸酯、碳酸酯类相比，CH3O(CH2CH2O)nCH3（Gn）结构多元醚具有熔点低、蒸汽压低、介电常数高等特点，但由于与石墨相容性差而很少被用于锂离子电池电解液。本项目拟引入“solvent in salt”的概念，将多元醚与特殊结构的锂盐复合，从而制备出具有[Li(Gn)][X]结构的离子液体，提高多元醚的电化学稳定性、解决相容性差的问题，同时将具有不同长度醚链的离子液体复配，引入具有较低熔点的有机溶剂如DEC、EMC，以降低电解液粘度和熔点、提升电导率，加入具有特定功能的添加剂，从而得到具有液程范围宽、电化学稳定性好、饱和蒸汽压低、可在-60~60℃的温度范围内工作的安全型低温离子液体电解液，建立低温下强化Li+迁移新方法，破除低温锂离子电池研究的瓶颈。

Li+在电极/电解液中扩散（迁移）变慢、SEI膜增厚导致的极化增大是电池在低温下难以稳定充放电的主要原因，本项目从电解液的角度出发，设计和制备新型电解液，通过调制其中Li+的迁移机制，提高了电导率和锂离子迁移数(t_(〖Li〗^+ ))，稳定了Li+在负极的沉积、抑制了枝晶产生，同时保护了正极，从而提升了电池的低温性能。取得的创新性成果如下：（1）制备了纳米结构大分子锂盐添加剂，其结构可在一定程度上调控Li+的迁移，从而提高电解液锂离子迁移数、有效保护负极、抑制低温下锂枝晶和死锂的产生；同时该添加剂可很好的消耗电解液中的HF，从而达到保护正极材料的目的，电池的低温性能有显著提升；（2）纳米结构离子液体添加剂具有制备工艺简单、电化学稳定性好等优势，其可在低温下抑制电解液的凝固、并形成离子迁移活性较高的微区，从而提高电解液的低温电导率；同时该添加剂有助于负极表面电导率较高的致密SEI的生成，用于NCM622||LTO和LCO||LTO全电池均表现了良好的低温性能；（3）制备了基于溶剂化离子液体的局部高浓电解液，基于其制备的局部高浓电解液表现了良好的电极相容性，且其凝固温度低至-70℃，电池在-40~60℃间均表现了稳定的循环性能。.通过以上研究我们发现，与室温工作的电池电荷转移阻抗（与Li+去溶剂化过程直接相关）是主要的限制因素不同，低温下由于负极表面界面膜的不断累计，其界面阻抗往往可以达到与电荷转移阻抗相似的量级，成为限制电池性能的又一主要因素。因此，要从电解液的角度改善电池的低温性能，一方面需要关注Li+的溶剂化形态，此外负极表面副反应产生的SEI膜成分也是需要重点关注的因素。设计具有低去溶剂化能、可在负极表面形成薄且致密、稳定SEI膜的电解液是开发低温电解液的核心。