聚合物基固态锂金属电池相比于目前商用的液态锂离子电池可实现更高的能量密度及安全性，是下一代二次锂电池研究与发展的一个重要方向。其中核心的问题是寻找或开发一种离子电导率高、与电极材料界面相容性好尤其是对锂金属负极稳定等综合性能优异的聚合物基电解质材料。到目前为止，研究报道的聚合物基电解质虽然有很多，但各有优缺点。能制备一种可以很好地满足固态锂金属电池应用需求的聚合物基电解质材料依旧是一个大的挑战。本课题研究制备了一种新型的聚偏氟乙烯（PVDF）基固态电解质，通过对其中锂离子传导及界面化学反应机制的研究以及对其中重要组分包括锂盐、络合态溶剂以及无机填料的优化与调控，最终获得高性能电解质材料并实现了其在固态锂金属电池中的长周期稳定循环。研究了Li6.75La3Zr1.75Ta0.25O12（LLZTO）对PVDF基电解质的影响。结果表明，LLZTO可以对PVDF/LiClO4电解质进行化学及物理结构的改性，且得到的PVDF/LiClO4/LLZTO复合电解质展现了高的电导率、良好的力学性能及热稳定性。研究了LiClO4、LiN(SO2CF3)2（LiTFSI）、LiN(SO2F)2（LiFSI）三种不同锂盐对PVDF基电解质的影响。结果表明，锂盐种类会影响PVDF基电解质的电化学性能，尤其是对锂金属的界面稳定性。其中，PVDF/LiFSI电解质具有较高的电导率及最佳的对锂金属稳定性。因为该电解质与锂金属接触后可原位生成约20 nm厚的由LiF、硫的化合物、Li2CO3、LiOH、Li2O组成的致密的马赛克形界面层。研究了N,N-二甲基甲酰胺（DMF）溶剂对PVDF基电解质的影响。结果表明，DMF会与锂离子发生较强的络合，并以非自由态形式存在于电解质中。同时，络合态DMF与PVDF之间有较弱的相互作用。外加电流下，锂离子可能会借助PVDF链段并在络合态DMF与PVDF的作用位点之间进行迁移。因此，提高DMF含量可提高电解质的电导率。但较高含量的DMF会降低电解质的氧化电位、且与锂金属接触时会产生自由基进而引发界面反应导致电池出现开路失效。经过调控与优化，DMF含量为10-14 wt%的PVDF/LiFSI电解质展现了最佳的电化学性能，且由此组装的固态锂金属电池可稳定循环超过1000圈并且容量保持率大于90 %。进一步在PVDF/LiFSI中引入LLZTO，制备得到的PVDF/LiFSI/LLZTO复合电解质展现了更高的室温电导率（2.4×10-4 S cm-1）及更宽的电化学窗口（4.6 V）。
Polymer-based solid-state lithium (Li) metal batteries are attracting ever-increasing attention due to their improved energy density and safety versus commercial liquid Li-ion batteries. To achieve high-performance Li metal batteires, polymer electrolytes with high ionic conductivity and good interfacial stability with electrodes, especially the stability against a Li anode, are desired. So far, many types of polymer electrolytes have been investigated. However, challenges still remain in exploiting a suitable polymer electrolyte material with good comprehensive performance. This work presents and demonstrates a novel poly(vinylidene fluoride) (PVDF)-based electrolyte for the Li metal battery application. Based on the fundamental understanding of ionic conduction and interface chemistry, a high-performance PVDF-based electrolyte has been obtained via regulating solvent effect, Li salt and inorganic fillers in the electrolyte and the assembled solid-state Li metal batteries show an excellent long-term cycling stability.Dispersing Li6.75La3Zr1.75Ta0.25O12 (LLZTO) fillers into a PVDF/LiClO4 electrolyte can modify both the physical and chemical structures of the electrolyte and thus improve the performance of the electrolyte. The obtained PVDF/LiClO4/LLZTO composite electrolyte shows a high ionic conductivity, good mechanical properties and satisfactory thermal stability. The PVDF-based electrolytes with different Li salts show different electrochemical performance especially the stability against Li anode. Among PVDF/LiX (X= ClO4-, N(SO2CF3)2- (TFSI-), N(SO2F)2- (FSI-)) electrolytes, the PVDF/LiFSI electrolyte exhibits a high ionic conductivity and the in-situ formed nanoscale LiF-sulfur compounds-Li2CO3-LiOH-Li2O mosaic interface layer makes it very stable with Li metal. N,N-dimethylformamide (DMF) solvent plays an important role in the PVDF-based electrolyte. All of DMF molecules are in the bound state and there is no free DMF in the electrolyte. In addition, the bound DMF weakly interacts with PVDF chains. Li ions may transport, with the assistance of PVDF chains, among the interaction sites between the bound DMF and PVDF. Increasing the amount of DMF thus can increase the ionic conductivity of the electrolyte. Nevertheless, the electrolyte with a high content of DMF is easily decomposed under a high voltage. Also, DMF can induce the interfacial reaction between the electrolyte and a Li anode by generating free radicals, which finally leads to the failure of the cell. The PVDF/LiFSI electrolyte with 10-14 wt% amount of DMF shows optimal electrochemical performance and delivers excellent cycling performance in the assembled solid-state Li metal batteries (over 1000 cycles with a capacity retention larger than 90 %).In addition, the PVDF/LiFSI/LLZTO electrolyte shows the further improved ionic conductivity (2.4×10-4 S cm-1 at 25 oC) and high-voltage compatibility (4.6 V).
