安全性高温高压锂离子电池特种电解质的合成及其化学反应机理研究

High-voltage, high specific-energy and specific-capacity are necessary for lithium-ion batteries in electric vehicles and hybrid electric vehicles. Although improvement of their performance has been achieved, there exists potential safety hazard and cycling stability. The fundamental way out is to develop safe electrolyte with non-flammability and superior comprehensive property at elevated temperature. In this project, by changing th structures of ionic liquids (IL) and lithium salts, we try to obtain new IL-based electrolytes which can keep stable storage and operating performance at elevated temperature 60-150 ℃. Based on the different oxdativeand reductive reaction mechanisms of IL, lithium salt and additives at charge and discharge cycle, stable SEI membranes from on the cathode surface. With scanning probe microscope, the changing process and the developing mechanism of the SEI membranes will be explored, an on the basis, the intrinsic structure - activity relationship will be discussed between SEI membrane and electrolytes. In combination with physical property analysis, the effect of electrolyte structure and composition on intermolecular forces will be studied. The migration ability of Li+ at different concentraiton will be evaluated. The structure and composition of the IL-based electrolyes will be optimized to meet the demand of special elevated-temperature electrolytes for high-voltage Li-Ion battery with safety. In addition, the compatibility of the IL-based electrolytes with cathode LiCoxNiyMn1-x-yO2 or LiNixMn1-xO2 and aluminum substrate will be studied at 60-150 ℃.The action and coordinating mechanism of the electrolytes will be explored.

动力锂离子电池需要具有高电压、高比能量、高比容量和安全性能。尽管锂离子电池的综合性能有较大改善，但还不能完全满足动力锂离子电池的要求,尤其是其安全性和高温循环稳定性。本项目通过分子设计，采用烷基化反应和磺酰化反应等方法合成在60-120 ℃高温储存和运行稳定的新型离子液体特种电解质；根据商用高温高压特种电解质发展要求，优化安全性离子液体特种电解质结构与组成，研究电解质在充放电时的氧化还原反应机理，探讨离子液体特种电解质与高压正极、铝集流体等关键部件的相容性和高温循环稳定性，构建具备良好Li+传输性的耐高温正极表面钝化膜（SEI）；运用扫描探针显微镜和X射线能谱仪等现代分析技术，研究正极SEI膜的表面结构演化过程，阐明SEI膜在电极表面的生长机理，探索各组分间的协同机制，揭示电解质结构、组成与耐高温SEI膜之间的内在构效关系。

锂离子电池具备高能量密度、无记忆效应、无污染等突出优势，是目前综合性能最好的化学电源之一，但有机碳酸酯/LiPF6体系，与高压正极材料相容性差；高温稳定性差，高于55 ℃时电池容量快速衰减；有机溶剂易燃烧，过充等极端条件下发生安全风险。因此，合成安全性耐高温高压锂离子电池特种电解质、研究并提高锂离子电池耐高压高温循环的稳定性及其机制，具有重要的现实和科学意义。本项目制备了6类新型耐高温高压的安全型离子液体电解质，与高压正极材料LiNi0.8Co0.1Mn0.1O2、LiCo1/3Ni1/3Mn1/3O2、LiNi0.5Mn1.5O4、富锂正极材料Li1.15(Ni0.36Mn0.64)0.85O2、Li1.1Ni0.25Mn0.65O2、负极石墨和锂金属相容性好。2035扣式电池在70℃、1 C最高放电比容量倍率达220.1 mAh g-1，循环300圈后容量保持率为81.9%；85 ℃，1 C、50圈后放电比容量达到218.7 mAh g-1；Li1.1Ni0.25Mn0.65O2/Graphite全电池在0.5 C、40 ℃循环100次，充放电比容量为181.05、180 mAh g-1，库伦效率99.42%，容量保持率达87.54%。DFOB-在正极表面有助于生成稳定的SEI膜，其在正极表面优先氧化生成的B-O/B-F类化合物、LiF抑制了正极表面金属离子的溶解，保护了正极结构在高压高温条件下的稳定；当进一步引入氮-甲基-氮-烯丙基丙基哌啶双三氟甲磺酰亚胺（PP13*TFSI），与 LiDFOB协同在正极表面形成双层SEI膜，更有效地保护正极结构稳定。内层膜主要由LiDFOB分解产物构成，多孔且不均匀，厚度为18.63-43.18 nm；外层主要由PP13*TFSI分解产物构成，颜色深且密度较大，厚度约11.15 nm；该实验结果与理论计算证明了双层结构SEI膜形成机制。本项目研究安全型离子液体电解质可完全抑制TFSI-对铝集流体腐蚀，提出了DFOB-阴离子抑制铝腐蚀的机理：DFOB-在铝箔表面优先氧化生成的B-O/B-F类化合物，在铝表面形成一层致密的钝化膜，抑制LiTFSI引起的铝腐蚀；以上实验结果和研究数据对指导制备耐高压高温的安全型离子液体电解质提供了科学依据，为制备满足动力电池的电解质提供了新的选择和解决方案，为优化现有电解质提供数据支撑与指导。