*Article* **Ionic Liquid-Based Electrolyte Membranes for Medium-High Temperature Lithium Polymer Batteries**

#### **Guk-Tae Kim 1,2, Stefano Passerini 1,2, Maria Carewska 3 and Giovanni Battista Appetecchi 4,\***


Received: 11 June 2018; Accepted: 9 July 2018; Published: 10 July 2018

**Abstract:** Li+-conducting polyethylene oxide-based membranes incorporating *N*-butyl-*N*methylpyrrolidinium bis(trifluoromethanesulfonyl)imide are used as electrolyte separators for all-solid-state lithium polymer batteries operating at medium-high temperatures. The incorporation of the ionic liquid remarkably improves the thermal, ion-transport and interfacial properties of the polymer electrolyte, which, in combination with the wide electrochemical stability even at medium-high temperatures, allows high current rates without any appreciable lithium anode degradation. Battery tests carried out at 80 ◦C have shown excellent cycling performance and capacity retention, even at high rates, which are never tackled by ionic liquid-free polymer electrolytes. No dendrite growth onto the lithium metal anode was observed.

**Keywords:** ionic liquids; *N*-butyl-*N*-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide; poly(ethyleneoxide); polymer electrolytes; lithium polymer batteries
