Quasi-solid polymer electrolytes with binary and ternary salt mixtures for high-voltage lithium metal batteries
Nicola Boaretto, Oihane García‐Calvo, Mónica Cobos, Asier Fernández de Añastro, Marta Diez Viera, Mohammed Shakir, Simon Lindberg, Rosalía Cid, Gérôme Godillot, Leif Olav Jøsang, Andriy Kvasha, María Martínez‐Ibáñez
Abstract
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.