Application Of High Voltage Hydrogel Electrolytes In Zinc-ion Hybrid Supercapacitors/Batteries

Facing the shortage of nonrenewable energy,energy storage devices with low cost,long cycle life,high capacity,and safety have come into people’s vision.Because of its ultra-high theoretical specific capacity(820 Ah kg^-1)and environmental friendliness,zinc,as one of the negative electrodes with great potential,has attracted extensive attention in the application of energy storage devices.With the development of rechargeable zinc ion hybrid supercapacitors（ZHSs）/batteries（ZIBs）,flexible ZHSs and ZIBs are widely used in wearable electronic devices.The application of hydrogel electrolytes in ZHSs and ZIBs can ensure safety,bear external force,and prolong the life of energy storage devices by inhibiting the growth and side reaction of zinc dendrites.However,it is not yet mature that the research on the application of hydrogel electrolytes in energy storage devices,and there are still many problems:（1）due to its inherent characteristics,the voltage window of energy storage devices is generally narrow（0-1 V）,resulting in low energy density.（2）the mechanical properties of hydrogel electrolytes are not enough to inhibit the growth of zinc dendrites,leading to zinc dendrites piercing the electrolyte and short-circuiting energy storage devices.（3）Under extreme conditions,such as ultra-low temperature and splitting under external force,it cannot continue to work.Therefore,it is an effective way to improve the electrochemical performance of ZHSs and ZIBs by reasonably designing hydrogel electrolyte materials to increase the energy density and prolong the service life of energy storage devices.Based on this,we designed and synthesized three kinds of hydrogel electrolytes that can work at high voltage.They exhibited good electrochemical properties in ZHSs and ZIBs.These three hydrogel electrolytes enhance the mechanical strength of hydrogels by using the natural polymer network structure.By increasing the voltage window of electrolytes by using high salt/zwitterionic gel,the energy density is increased,and the problems existing in ZHSs and ZIBs are effectively solved.The main contents are as follows:1.PAM-SA-Ca²⁺hydrogel electrolyte was prepared by chemical crosslinking to form polyacrylamide（PAM）matrix and crosslinking with sodium alginate（SA）and Ca²⁺.A high concentration of Zn Cl₂,low temperature,and high voltage hydrogel electrolyte(PAM-SA-Ca²⁺)was prepared.Because Ca²⁺is bonded with SA with the abundant group,the network structure formed has good mechanical strength,and the elongation of PAM-SA-Ca²⁺hydrogel is 2013.5%.At the same time,because the PAM-SA-Ca²⁺hydrogel contains a high concentration of Zn Cl₂,the prepared electrolyte has a wide working voltage window（0-2.2 V）and its working temperature can be as low as-75^oC.When PAM-SA-Ca²⁺hydrogel is applied to ZHSs,it can still work stably at high voltage（2.2V）and ultra-low temperature（-75^oC）.The energy density at 1 A g^-1 is 202.3 Wh kg^-1,especially at 3 A g^-1,and the capacity retention rate is 85.82%after 11000 cycles.In addition,whether at 25 ^oC or-75^oC,the flexible ZHSs can still drive the timer and has good environmental adaptability.2.Synthesis of zwitterionic hydrogel PMAZn:the natural polymer SA and zwitterionic monomer[2-（Methacryloyloxy）ethyl]dimethyl-（3-sulfopropyl）（MS）are heated to polymerize to form hydrogels,and by immersing Zn²⁺into hydrogels to form PMAZn hydrogels.PMAZn hydrogel has good mechanical strength（elongation up to705.06%）,self-healing,high working voltage（2.4 V）and safety.Because MS and SA chains have abundant groups,the authors form an intertwined network structure through the simple process of SA and MS polymerized segments（PMS）,while Zn²⁺and SA cross-link to form a 3D network structure,resulting in good mechanical properties and self-healing properties of hydrogels.The PMS chain has unique zwitterionic characteristics,which can inhibit the decomposition of water.This makes PMAZn hydrogel possess a high working voltage and energy density.Especially in the zinc-based energy storage devices assembled by PMAZn,the hydrogel electrolyte not only acts as the electrolyte for transport ion and flexible and deformable diaphragm but also reduces the impact of the external mechanical shock on the electrode due to its good mechanical strength.The PMAZn hydrogel prepared in this work has good electrochemical performance for ZHSs and ZIBs.3.Stable zwitterionic hydrogel（PMCS）was prepared by introducing polymer carboxylated chitosan（CS）into the zwitterionic gel matrix and obtaining ionic conductivity by immersion in Zn SO₄ salt solution,thus constructing zwitterionic hydrogel electrolyte（PMCS）.The prepared PMCS hydrogel electrolyte has a wide voltage window（0-2.4 V）,good ionic conductivity and mechanical properties.The assembled ZHSs has high energy density,power density and good cyclic life(the capacity retention rate after 10000 cycles at 5 A g^-1 is 84.34%).In addition,a VN cathode with high performance was prepared by hydrothermal and calcination methods for ZIBs.The battery showed good electrochemical properties such as specific capacity and cycle.Therefore,the prepared PMCS hydrogel electrolyte has good electrochemical performance in ZHSs and ZIBs.In conclusion,we have designed and synthesized three kinds of hydrogel electrolytes with high voltage applied to ZHSs and ZIBs.Both of them exhibit good electrochemical properties,which provide a new idea for the design and preparation of high voltage hydrogel electrolytes.