Quasi-solid polymer electrolytes with binary and ternary salt mixtures for high-voltage lithium metal batteries

Quasi-solid polymer electrolytes (QSPEs) are considered a promising alternative to liquid electrolytes for high-voltage lithium metal batteries. Herein, we present their properties and performance supported on polyolefin microporous separators. These QSPEs consist of a poly(vinylidene-fluoride-co-hexafluoropropylene) polymer matrix, ethylene carbonate as a plasticizer, and various lithium salt mixtures, including lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalate)borate (LiBOB), and LiNO3 as a solid electrolyte interface-forming additive. They exhibit an ionic conductivity of ca. 1 mS cm-1 at room temperature and excellent resistance against lithium dendrites, attributed to the presence of the tough polyolefin separator. The effect of the lithium salt mixture composition on lithium plating/stripping performance and electrooxidation stability was studied in detail, showing that LiNO3, while having a clear positive effect on the plating/stripping performance, may also adversely affect the oxidative stability of the electrolyte, accelerating the degradation of the cathode/electrolyte interface. QSPEs with binary LiFSI/LiBOB salt mixtures were tested at room temperature in a LiNi0.8Mn0.1Co0.1O2||Li monolayer pouch cell with a cathode area capacity of ca. 2.5 mAh cm-2. This cell delivered an initial capacity close to 200 mAh g-1 at C/20, 150 mAh g-1 at C/1, and 80% capacity retention after 100 cycles at 25 °C. The results demonstrate the viability of supported QSPEs, based on poly(vinylidene-fluoride-co-hexafluoropropylene), ethylene carbonate, LiFSI and LiBOB, for application in high-voltage quasi solid-state lithium metal batteries.