与高电压锂离子电池碳负极兼容的含氟溶剂的设计及性能研究

The demand for a high-voltage electrolyte has become a high priority for the integration of high-capacity lithium ion batteries (LIBs) in transportation and grid storage applications. With the advantages of high anodic stability, fluoroethylene carbonate (FEC) based electrolytes appear to be a suitable electrolyte candidate for high voltage electrolyte solvents. However, the chemical, physical, and structural properties of the solid electrolyte interphase (SEI) film at carbon negative using FEC-based electrolyte solutions are still poorly known. In this project, we study the formation mechanism of SEI films on the carbon negative with FEC-based high-voltage electrolytes. Typically, because fluorinated carbonates and ethers participate in SEI formation, several specific fluorinated solvents are used as co-solvents or SEI formation additives in the electrolytes to improve the compatibility of graphite anode with FEC-based electrolytes..In this project, we selected a completely different molecular structure from the fluorinated C4H10-xFxO (0≤x≤10) derivatives, and extensively investigated their physical and electrochemical properties and their applications to lithium-ion batteries. Moreover, the density functional theory (DFT) with the B3PW91 functional using the Gaussian09w program package is used for calculating molecular properties for different molecular structures of fluorinated ethers in this project. Subsequently, we will design several fluorinated ethers with high anodic stability and good SEI-filming property as co-solvents or additives for high-voltage electrolytes, and also explore their electrochemical performances in the battery environment and the presence, structure and formation mechanisms of interfacial surface films on the carbon negative. It can be expected that the research results from this project would provide scientific basis and theoretical supports for building high-voltage electrolytes in the future.

开发高电压电解液是大容量锂离子电池在电动汽车、储能电站等新能源技术领域商业化应用的迫切需求。氟代碳酸乙烯酯（FEC）基电解液因具有高的抗氧化能力已成为当前高电压电解液的研究热点。然而，FEC基高电压电解液与碳负极的兼容性差且缺乏深入的认识。本项目拟弄清楚FEC基电解液在碳负极表面的成膜机制，并利用具有成膜作用的含氟溶剂作共溶剂或添加剂来改善其与碳负极的兼容性，保证高电压锂离子电池的应用。.在含氟溶剂的选择中，我们拟C4H10-xFxO（0≤x≤10）类含氟醚为研究对象，通过Gaussian09w程序理论计算分子结构对其氧化还原行为的影响规律，揭示含氟醚分子结构与其相关性能的规律。在此基础上，设计出具有氧化电位高且还原电位合适的含氟溶剂作为高电压电解液共溶剂或成膜剂，并考察它们在碳负极表面的成膜机理及其在电池环境中的实际性能。本项目的完成为高电压电解液的组分设计和预测提供科学依据和理论指导。

氟代碳酸乙烯酯FEC基电解液因具有高的抗氧化能力已成为当前高电压电解液的研究热点。本项目通过Gaussian09w程序理论计算和实验方法研究了FEC的氧化分解机理及其在高电压正极、石墨负极表面的成膜机制，计算70多种含氟溶剂的HOMO/LUMO能级，并归纳了其分子结构与其氧化还原性能的规律，筛选出合适的氟代溶剂作为FEC基电解液的共溶剂，考察了其在电池环境中的实际性能。研究发现，FEC溶剂分子在正极表面失去一个电子形成FEC•+，其最可能的分解产物是CO2和2-氟乙醛阳离子自由基，这些阳离子自由基易受到FEC分子的攻击，最终生成醛和烷基碳酸酯的低聚物等；FEC基电解液与高电压正极兼容性好，而MCMB负极在其中的循环稳定性较差，究其原因在于FEC易还原分解，导致FEC基电解液不能在MCMB负极表面形成稳定的SEI膜；为了改善FEC基电解液与石墨类碳负极的兼容性问题，一方面，通过在电解液中加入0.5 wt%成膜添加剂LiDFOB不仅明显改善FEC基电解液与碳负极的兼容性的循环稳定性，而且有效捕获电解液中的少量水和HF，提高高电压正极LiNi0.5Mn1.5O4的电化学行为。另一方面，通过理论设计计算含氟溶剂的HOMO和LUMO能级，发现相比于相应的非氟代溶剂，氟代溶剂具有更好的抗氧化性，且其LUMO能级较低，表明容易得到电子，可参与形成稳定的SEI膜；氟取代个数越多，含氟溶剂的氧化稳定性越好，但还原稳定性与氟取代位置关系复杂；对于相同分子式但不同分子构型的含氟溶剂来说，其HOMO和LUMO能级差异较大，且直链比支链含氟溶剂的氧化稳定性更高。筛选了具有成膜作用的氟代醚-六氟异丙基甲基醚HFPM作为共溶剂，构建的氟代电解液不仅浸润性好、完全不燃，而且该氟代电解液在高电压锂离子电池显示了优异的循环稳定性。发表SCI论文7篇，申请发明专利5项，FEC基电压电解液的工作得到了国内外同行的认可和评述。本项目的顺利实施，阐明了FEC基电解液在石墨类碳负极表面的成膜机理，归纳了氟代溶剂分子结构与其氧化还原行为及电化学性能之间的内在关联，为锂离子电池电解液的组分设计和预测提供科学依据和理论指导。