Rechargeable Thin‐Film Lithium Microbattery Using a Quasi‐Solid‐State Polymer Electrolyte

Abstract A thin‐film microbattery was designed after synthesizing a unique gel polymer electrolyte (GPE), using polyvinylidene fluoride‐co‐hexafluoropropylene (PVdF‐HFP) and cross‐linked poly(ethylene oxide) (PEO), with an ionic liquid and salt (LiTFSI) mixture. The polymers resulted in a semi‐interpenetrated polymer network (semi‐IPN), hosting an ionic liquid (IL)/salt mixture, and thus exhibited high ionic conductivity and excellent mechanical properties. Nuclear magnetic resonance (NMR) diffusion measurements and relaxation rates analysis highlighted the existence of interactions between Li⁺ ions and oxygen within the polymers, preventing electrolyte leakage and ensuring excellent mechanical strength which enabled a unique quasi‐solid electrolyte. Such mechanical strength and chemical stability made this electrolyte to be first ever reported GPE to withstand thermal evaporation deposition and hence direct deposition of lithium metal. The electrolyte could be shaped as self‐standing thin films, and thus worked as both separator and electrolyte in a thin‐film lithium microbattery. The thin‐film microbattery exhibited excellent performances showing no short‐circuit current, an open circuit voltage of ∼3.0 V, higher nominal voltage plateau, lower equivalent series resistance by comparison to a thin‐film microbattery designed in conventional way with a popular ceramic electrolyte LiPON.