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 the properties and performance of QSPEs supported on polyolefin microporous separators, composed of poly(vinylidene-fluoride-co-hexafluoropropylene) (PVdF-HFP) as polymer matrix, ethylene carbonate as plasticizer, and with different lithium salt mixtures, encompassing lithium bis(fluorosulfonylimide) (LiFSI), lithium bis(oxalate)borate (LiBOB), and LiNO₃ as SEI-forming additive. These QSPEs have ionic conductivity of ca. 1 mS cm⁻¹ at room temperature and excellent resistance against Li dendrites, thanks to the presence of the tough polyolefin separator. The effect of the Li salts mixture composition on the Li plating/stripping performance and on the electrooxidation stability was studied in detail, showing that LiNO₃, while having a clear positive effect on the plating/stripping performance, possibly has also a deleterious effect on the electrolyte oxidative stability, accelerating the degradation of the cathode/electrolyte interface. QSPEs with binary LiFSI/LiBOB salt mixtures were tested, at room temperature, in a LiNi_0.8Mn_0.1Co_0.1O₂||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 PVdF-HFP, EC, LiFSI and LiBOB, for application in high-voltage quasi solid state lithium metal batteries.