Modulating Phase Separation via Multiple Hydrogen Bonding in Polyurethane-Based Gel Polymer Electrolytes for All-Solid-State Supercapacitors

Abstract

Hybrid gel polymer electrolytes (GPEs) based on thermoplastic polyurethane (PU) and ionic liquid (IL) are successfully synthesized by incorporating silica nanoparticles through a one-pot in situ sol-gel process. The ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [(BMIM)⁺(TFSI)^-], acts as a miscible solvent for both the hard and soft segments of PU, simultaneously serving as an ionic charge carrier and plasticizer. Fourier transform infrared (FTIR) Spectroscopy and differential scanning calorimetry (DSC) analyses reveal that increasing the IL concentration weakens the hydrogen bonding interactions between hard-hard and hard-soft segments, leading to a reduction in glass transition temperature and suppression of phase separation. Furthermore, incorporating silica nanoparticles as phase-modulators by creating hydrogen bonding between -Si-OH with TFSI^- anion, inducing phase-separation, thus enhancing the room temperature ionic conductivity of the GPE, and at the same time, the mechanical strength of the GPE also improves upon the addition of 0.02 wt.% SN. The all-solid-state supercapacitors using hybrid GPE delivered a high energy density (E_D = 183 Wh kg^-1) and power density (P_D = 7 kW kg^-1) within an extended voltage window of up to 3.5 V, along with excellent cycling stability, retaining ≈98% of their initial capacitance after 12, 000 cycles. These results indicate that the GPEs can be promising candidates for future all-solid-state energy storage devices.

Keywords: all‐solid‐state supercapacitors; hydrogen bonding; ionic liquids; silica nanoparticles; thermoplastic polyurethane.