A self-healable and highly stretchable supercapacitor based on a dual crosslinked polyelectrolyte

Superior self-healability and stretchability are critical elements for the practical wide-scale adoption of personalized electronics such as portable and wearable energy storage devices. However, the low healing efficiency of self-healable supercapacitors and the small strain of stretchable supercapacitors are fundamentally limited by conventional polyvinyl alcohol-based acidic electrolytes, which are intrinsically neither self-healable nor highly stretchable. Here we report an electrolyte comprising polyacrylic acid dual crosslinked by hydrogen bonding and vinyl hybrid silica nanoparticles, which displays all superior functions and provides a solution to the intrinsic self-healability and high stretchability problems of a supercapacitor. Supercapacitors with this electrolyte are non-autonomic self-healable, retaining the capacitance completely even after 20 cycles of breaking/healing. These supercapacitors are stretched up to 600% strain with enhanced performance using a designed facile electrode fabrication procedure.

Compared with pure PAA, a low content of VSNPs (0.1 wt.%) significantly increases the strain to over 3700%. The fact that strain decreases remarkably with the increase of VSNPs is attributed to a decrease in the average chain length of the cross-linked PAA chains by VSNPs.
Tensile strengths of VSNPs-PAA at various VSNPs contents are all higher than that of pure PAA.
With an initial increase in VSNPs content, the tensile strength increases. This is explained by the remarkable increase in the density of cross-linking points and by a decrease in the average chain length over a small range. With a further increase in VSNPs content, the density of cross-linking points becomes too high and the average chain length decreases remarkably. This makes the polyelectrolyte brittle and thus, decreasing the tensile strength at higher VSNPs contents. with breaking (red) of hydrogen bonds by urea. To reveal the self-healing mechanism of the VSNPs-PAA and verify the proposed hydrogen bonding mechanism, we used urea 1,2 to prevent the formation of hydrogen bonds. The VSNPs-PAA healed following the urea treatment possessed much lower strain, tensile strength, and tensile modulus in comparison with that healed without the treatment. This clearly reveals that hydrogen bonding is the dominant mechanism for the self-healing of VSNPs-PAA materials. Experimental details are as follows: The cut surfaces were coated with several drops of 1 M aqueous solution of urea (an efficient hydrogen-bond-breaking reagent) for 5 minutes. Both the urea-treated and non-urea-treated VSNP-PAAs were healed using identical procedures. Then they were mounted on a Zwick-Roell Z005 machine for tensile tests. Under all scan rates and charging/discharging currents, the capacitance increases with water content in the range of 1.7 to 96.1 wt.%, varying up to four orders of magnitude ( Supplementary   Fig. 8c and d). The enhanced capacitances can be attributed to high ion mobility at high water contents and convenient ion transfer in the moisturized electrolyte/electrode interface.
Electrochemical impedance spectroscopy measurements ( Supplementary Fig. 8e) also reflect these results. The supercapacitors exhibit small systematic resistance (the intercept at the Z`axis) and small overall impedance (the end point in the Nyquist plot) at high water contents. The sufficiently extended polymer chains favor ion transportation in the electrolyte and at the electrolyte/electrode interface, thereby reducing the resistance and increasing the capacitance. In addition to water content, the protons that penetrate the VSNPs-PAA polyelectrolyte also contribute to capacitance enhancement. As observed in Supplementary Fig. 8a-e, the performance of VSNPs-PAA polyelectrolyte without H 3 PO 4 penetration, though at higher water contents, is inferior. The difference in electrolytes with and without H 3 PO 4 is more obvious at faster scan rates and higher charging/discharging currents, revealing the important role that transportation of available ions plays during the fast electrochemical dynamic process. Obviously, there is a scar on the PPy@CNT electrode after cutting, which suggests a poor electronic connection between the two electrodes. This will greatly increase internal resistance of the supercapacitor. However, after the CNT patch is applied on the electrode scar, the electric connection is rebuilt between the two broken pieces of electrode. This is the key to achieving high self-healing efficiency for supercapacitor devices.  Fig. 17a-c). The pressure-improved interfacial contact between the electrolyte and the electrodes is the main reason for the improved performance under compression. As seen in Supplementary Fig. 17d, the supercapacitor has a smaller systematic resistance (the intercept at the Z`-axis) at a higher compressive strain. This result is further confirmed by the decreased IR drop shown in Supplementary Fig. 17b. With increased compressive strain, IR drop decreases.

Supplementary
This suggests smaller systematic resistance, and thus, improved contact at the interface of the electrolyte/electrode under compression. Such compress-induced capacitance improvement is consistent with many studies on supercapacitors under pressure 5,6 . To summarize, the 20 compressive stress benefits the interfacial contact and, therefore, the ion transfer from the electrolyte to the surface of electrode, which contributes to the higher capacitance. various VSNPs-PAA electrolyte thicknesses. In the range of electrolyte thicknesses we studied, the performance of supercapacitors was not affected. Thus, the effect of electrolyte thickness on the capacitance can be ignored in our devices during compression.

The very facile fabrication of self-healable supercapacitors is demonstrated in Supplementary
Movie 1: Self-heal the supercapacitor. The entire repairing process simply brings the cut interfaces together, paving two small patches of CNT paper on the wounds at room temperature.
The repaired supercapacitor successfully powers an LED bulb. Supplementary Movie 2 (Stretch the supercapacitor) shows the process of stretching the supercapacitor. The supercapacitor is easily stretched to high degree, meanwhile still lighting an LED bulb.

Supplementary Methods
Tensile strength of VSNPs-PAA polyelectrolyte samples (cylindrical with diameter 5 mm) was measured by a mechanical testing system (Zwick-Roell Z005, Ulm, Germany) at a strain rate of 100 mm min −1 . The microstructure and morphology of electrodes were characterized by scanning electron microscope (SEM) (JEOL JSM-6335F) with an acceleration voltage of 5 kV.
The particle size of VSNPs was observed by transmission electron microscope (TEM) (Hitachi H7700) at an acceleration voltage of 100 kV. FTIR (AVATAR 380) was used to visualize vinyl groups on the as-synthesized VSNPs. Raman spectra were obtained by RENISHAW Raman microscope (RA100) with an excitation wavelength of 633 nm. For the compressible supercapacitor evaluation, different weights (100 g, 280 g, and 500 g) were placed on the supercapacitor to impose compression during electrochemical measurement. To confirm the hydrogen bonding effect on the self-healing of VSNPs-PAA polyelectrolyte, several droplets of urea solution (1 M) were dropped on the surface of the wound in order to break the hydrogen bonding at the interface.