Aligned Ionogel Electrolytes for High-Temperature Supercapacitors

Abstract

Ionogels are a new class of promising materials for use in all-solid-state energy storage devices in which they can function as an integrated separator and electrolyte. However, their performance is limited by the presence of a crosslinking polymer, which is needed to improve the mechanical properties, but compromises their ionic conductivity. Here, directional freezing is used followed by a solvent replacement method to prepare aligned nanocomposite ionogels which exhibit enhanced ionic conductivity, good mechanical strength, and thermal stability simultaneously. The aligned ionogel based supercapacitor achieves a 29% higher specific capacitance (176 F g^-1 at 25 °C and 1 A g^-1) than an equivalent nonaligned form. Notably, this thermally stable aligned ionogel has a high ionic conductivity of 22.1 mS cm^-1 and achieves a high specific capacitance of 167 F g^-1 at 10 A g^-1 and 200 °C. Furthermore, the diffusion simulations conducted on 3D reconstructed tomography images are employed to explain the improved conductivity in the relevant direction of the aligned structure compared to the nonaligned. This work demonstrates the synthesis, analysis, and use of aligned ionogels as supercapacitor separators and electrolytes, representing a promising direction for the development of wearable electronics coupled with image based process and simulations.

Keywords: X‐ray tomography; aligned ionogels; supercapacitors; thermal tolerance; tortuosity factors.

Figure 1

Proposed schematic illustration of preparation of aligned hydrogel applying a directional freezing process. a) The precursor aqueous solution of clay‐Ns, DMAA, and TiO₂‐NP in water. b,c) The monomer‐nanoparticle composite is excluded from the directionally freezing of the orientated ice crystals. d) The monomer is cryopolymerized to produce hydrogel with an aligned structure. e) SEM image of the freeze‐dried polymer matrix with aligned structure. f) The aligned ionogel can be obtained by adding ionic liquid to freeze‐dried aligned polymer matrix.

Figure 2

SEM images of the 3D structures of the gels after vacuum freeze‐drying treatment. a) Nonaligned porous polymer structure of the gel matrix without directional freezing treatment; b) SEM images from longitudinal and cross sections of the aligned porous polymer structures (inset image) of the gel matrix applying directional freezing process. c) Galvanostatic charge–discharge curves of the nonaligned and aligned ionogel electrolyte based supercapacitors measured at 1 A g⁻¹. d) The specific capacitance of the two ionogels at various current densities for the supercapacitors.

Figure 3

The imaging and simulation pipeline for the two samples, showing the reconstructed grayscale images from the XCT; the same images, overlaid with the trained Weka segmentation results; the same images, now converted to binary format; the full segmented 3D volume (showing scale and directional labeling); and, finally, sample simulation results highlighting the projected flux density resulting from the diffusion simulation, where the bright regions represent higher flux.

Figure 4

a) CV curves of the aligned ionogel electrolyte based supercapacitor applying various temperatures (25, 80, 100, and 200 °C). b) The correlation of specific capacitance with current density applying various temperatures for aligned ionogel based supercapacitor. c) The effect of temperature on the ionic conductivity and viscosity of the aligned ionogel electrolyte. d) The correlation of specific capacitance with current density applying various temperatures for nonaligned ionogel based supercapacitor. e) Nyquist plots and f) Bode plots of the phase angle versus frequency of the aligned and nonaligned ionogel based supercapacitors applying various temperatures (25 and 200 °C).