Hydrogel Electrolytes Based On Slidable Polymer Networks And Their Application In Flexible Supercapacitor

Hydrogel electrolytes are composed of hydrophilic polymers,water,and conductive ions.They are soft,deformable,safe,and environmental friendly.Hydrogel electrolytes have significant advantages in constructing flexible supercapacitors that can withstand deformation and resist physical damage.However,hydrogel electrolytes formed by the direct combination of hydrophilic polymers and electrolytes often have poor stretchability and self-healing ability,and low ionic conductivity.The introduction of dynamic bond in the hydrogel polymer network is an important direction to solve these key scientific and technical bottlenecks.In this study,based on the cross-linking sites of dynamic imine bond and boronate bond,a slidable polymer network was designed to fabricate hydrogels with highly stretchable and rapid self-healing properties.The hydrogels have high accommodating capability for different electrolytes.The as formed hydrogel electrolytes not only have good mechanical properties and rapid self-healability but also exhibit excellent ionic conductivity.Based on the study of the structure and performance regulation of the slidable network polymer hydrogels and the corresponding hydrogel electrolytes,we further applied them in flexible supercapacitors,which provides a reference for the design of high-performance wearable devices.The detail contents of this research are shown as follows:(1)Using polyethyleneimine(PEI),polyvinyl alcohol(PVA)and 4-formylbenzeneboronic acid(Bn)as starting materials,PEI-PVA-Bn hydrogels with slidable network were prepared through the formation of dynamic imine bonds and boronate bonds.Fourier transform infrared(FT-IR)and X-ray photoelectron spectroscopy(XPS)methods were used to verify the formation of dynamic bonds(imine and boronate)in hydrogels and the structural characteristics of slidable networks;the hydrogen bonding between the polymer network and water molecules was analyzed by Nuclear Magnetic Resonance spectroscopy(1H NMR).The mechanical properties and self-healing properties of PEI-PVA-Bn hydrogels can be adjusted by the content of cross-linking agent Bn,the molecular weight of PEI and the mass ratio of the two polymers.The PEI-PVA-Bn hydrogel constructed with a Bn content of 0.5 wt%,a PEI molecular weight of 1800 g mol-1,and a polymer mass ratio of 1:1 has the best performance with a mechanical strength of～59.5 kPa,1429%tensile unbroken,100%strain self-healing efficiency within 2 min.(2)The hydrogel electrolytes were further constructed by adding different types of electrolytes(LiCl.NaCl,KC1 and ionic liquid 1-allyl-3-methylimidazolium chloride(AMC))and different contents of LiCl into PEI-PVA-Bn hydrogel.The mechanical properties,self-healing properties and ionic conductivity of hydrogel electrolytes with different compositions were studied.The results showed that the PEI-PVA-Bn hydrogel exhibited excellent accommodation capability to different electrolytes.Among them,PEI-PVA-Bn-LiCl hydrogel electrolyte showed excellent mechanical properties(mechanical tensile strength of 34.6 kPa.elongation at break of 1223%),self-healing performance(94.3%strain self-healing efficiency within 2 min)and high conductivity(21.49 mS cm-1).The interaction mechanism between electrolytes and hydrogel polymer cross-linked network was confirmed by 1H NMR.The results showed that the electrolyte could form a strong binding force between the nitrogen atom of the amino group and the imine bond.(3)A sandwich-type flexible supercapacitor was assembled using a PEI-PVA-Bn-LiCl hydrogel electrolyte with excellent comprehensive performance and multiwalled carbon nanotube(MWCNT)electrodes,and the electrochemical and mechanical properties of flexible supercapacitor based on this hydrogel electrolyte were discussed.The results showed that the device demonstrated a broaden operating potential window of 1.4 V.specific capacitance of 16.7 mF cm-2,high cycling stability up to 10.000 charge/discharge cycles and good resistance to bending deformation.