Abstract:

A gel electrolyte was prepared by using lithium hexafluorophosphate (LiPF₆) as the polymerization initiator of tetrahydrofuran, and at the same time as a fluorine source, a LiF-rich solid electrolyte interface (SEI) was constructed in-situ on the surface of the lithium metal anode to suppress the growth of lithium dendrites and side reactions between metallic lithium/electrolyte. The as-prepared gel polymer electrolyte presents an ionic conductivity of 1.33 mS·cm^-1 at room temperature and shows a high electrochemical stability up to 4.5 V. Compared with linear sweep voltammetry in 335C electrolyte, lower reduction current is observed at the range of 0—1.5 V (vs Li/Li⁺) in the cell with gel polymer electrolyte, which indicates that gel polymer electrolyte can mitigate the side reaction of metallic lithium with electrolyte. The lithium metal anode in the symmetric-cell with in-situ polymerization gel polymer electrolyte exhibits no obvious lithium dendrite and damaged morphology. X-Ray photoelectron spectroscopy results further reveal that the lithium metal anode in gel polymer electrolyte is formed a stable SEI with LiF-rich compound, which is much enhanced than that of in 335C electrolyte. Consequently, the Li|LiFePO₄ cells with gel polymer electrolyte exhibits a long-term cycling stability, which can release a reversible capacity 118.7 mAh·g^-1 after 400 cycles at a current density of 1 C, as well as coulombic efficiency of 99.5%. Benefit from the ring opening polymerization process of tetrahydrofuran, PF₅ participates the formation reaction of intermediate product [(THF)⁺(PF₅)^-], which caused the equilibrium of the decomposition reaction shift to the right and therefore increase the formation of LiF on the surface of lithium metal anode. This process results in a great enhancement of the cyclability due to LiF-rich formed in the SEI. Therefore, the growth of lithium dendrite can be prevented by the stable SEI, as well as the side reaction between lithium metal and electrolyte.

Key words: LiF, SEI, in-situ polymerization, gel polymer electrolyte, lithium mental batteries

摘要：

以六氟磷酸锂（LiPF₆）为四氢呋喃的聚合引发剂制备凝胶电解质，同时作为氟源在金属锂负极表面原位构建富含LiF的固态电解质界面层（solid electrolyte interface，SEI）来抑制锂枝晶的生长以及金属锂/电解液之间的副反应。所制备的凝胶电解质具有较高的室温离子电导率（1.33 mS·cm^-1）和较宽的电化学稳定窗口（4.5 V）。原位聚合方式组装金属锂对称电池循环后，锂负极表面没有明显的锂枝晶和被损毁的形貌出现；XPS结果表明锂负极表面生成了富含LiF的SEI。组装的LiFePO₄全电池在1 C的电流密度下，稳定循环400周后仍保持118.7 mAh·g^-1的放电比容量。得益于四氢呋喃在开环聚合反应过程中，促进了LiPF₆分解反应平衡的正向移动，在锂负极表面形成稳定的富含LiF的SEI，能够抑制锂枝晶的生长并防止其被持续性的腐蚀破坏。

关键词: 氟化锂, 固态电解质界面层, 原位聚合, 凝胶聚合物电解质, 金属锂电池