A layered and highly porous poly-ionic liquid-based membrane as a gel polymer electrolyte for rechargeable lithium-ion batteries

Abstract

Gel electrolytes have great significance in solid-state batteries; in this regard, lithium-ion conducting gel polymer electrolyte (GPE) membranes were developed using an optimized blend of the polymeric ionic liquid poly(diallyldimethylammonium)bis(trifluoromethanesulfonyl)imide (PDADMATFSI), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. The resulting GPEs were systematically characterized for their morphological and electrochemical properties. The morphological study of the GPE membrane clearly shows a unique layered structure with distinct layers. The optimized membrane exhibited a high ionic conductivity of 1.48 × 10⁻³ S cm⁻¹ at 25 °C, which increased to 3.17 × 10⁻³ S cm⁻¹ at 65 °C, indicating efficient lithium-ion transport. It also demonstrated excellent mechanical flexibility, a broad electrochemical stability window of 4.8 V, and a lithium-ion transference number of 0.75. When employed in a Li|GPIL-3|LiFePO₄ half-cell configuration, the membrane enabled a high initial discharge capacity of 127.09 mAh g⁻¹ at 0.1C. which increased to 155.75 mAh g⁻¹ by the fifth cycle. Moreover, it delivered 151 mAh g⁻¹ at 1C with a capacity retention of 72.91% over 600 cycles, highlighting its robust rate capability and cycling stability. Ionic transport in the GPE membrane occurs primarily within the amorphous regions, where mobile Li⁺ cations coordinate with electronegative atoms along the polymer backbone, enabling stepwise migration between transient coordination sites and facilitating efficient conduction. Additionally, the membrane's layered structure is responsible for ion transport. Notably, the ionic conductivity achieved at room temperature surpasses previously reported values for PDADMA-based GPE systems. These findings underscore the potential of the optimized GPE composition for high-performance lithium-ion batteries.