Rechargeable lithium ion batteries (LIBs) have been applied in portable electronics and electricity powered transportation due to the high energy and power density. Unlike the traditional batteries packed in a cylindrical type, currently, most LIBs used in the consumer electronics (like smart phones and pads) are prismatic batteries. To meet the requirement of the electronics with particular shape such as wearable devices the flexible energy storage devices have been developed. For the safety concern, the solid electrolytes, such as polymer electrolyte, are developed to avoid leakage of liquid electrolyte. Nevertheless, the larger interfacial resistance between the solid electrolyte and electrodes and less flexibility reduced its application. Therefore, the gel-type electrolytes are favored because lithium ion is more easily to transport in gel electrolyte than solid electrolyte. Yet, as reported, certain amount of the organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) have been usually used with polymers in the traditional gel electrolytes, causing the suspicion of solvent leakage. In this work, the solvent-free polymer-based ionogel electrolyte (IGE) composed of nonflammable and thermally stable N-butyl-N-methylpyrrolidinum bis(trifluorome­thanesulfonyl) imide (PYR₁₄TFSI) room temperature ionic liquid and polyvinylidenefluoride-co-hexafluoropropylene (P(VdF-HFP)) are prepared by the solution casting method. The thermal and electrochemical properties are determined by the DSC and DMA measurements. The electrical properties are investigated by the linear sweep voltammetry, cyclic voltammetry and impedance spectroscopy. It is found that the as-prepared ionogel electrolytes are flexible self-standing films with wide electrochemical windows. The best conductivity obtained at room temperature is 6.0 × 10⁻⁴ S cm⁻¹. The highest capacity of the half-cell (Li/IGE/LiFePO₄) achieves 143 mA h g^-1 at 0.2C rate. And even after 50 cycles, the capacity retains 135 mA h g^-1at 0.2C. This study reveals that the ionogel electrolyte is a promising safe electrolyte for application in the flexible LIBs.

Acknowledge:

The authors gratefully acknowledge the Ministry of Science and Technology and Chung Yuan Christian University, Taiwan, R. O. C. for supporting this research work.