In-situ Construction And Applications Of Hydrogel Electrolyte Towards Zn-ion Batteries

With the fast development of the flexible and wearable electronic devices,the flexible energy storage batteries have drawn more and more attentions.Aqueous Zn-ion batteries have been one of the most promising candidates for flexible batteries due to its high capacity,low cost,high safety and environment friendly.Nevertheless,the Zn dendrite and side reaction issues still impede the further practical applications for Zn-ion batteries,which not only reduce the ion transport efficiency but also lead to rapid decrease of the cycling life span or even cause battery failure.Besides,the mechanical property of the hydrogel electrolyte in Zn-ion batteries should also be carefully designed to withstand the bending,hammering during operating.In addition,the freezing point of the aqueous electrolyte should also be optimized to enable the battery to operate at wide temperature range.As one of the most important components in batteries,hydrogel electrolyte is composed with polymer chain and liquid electrolyte.The hydrophilic segment of polymer chain could constrain the motion of liquid electrolyte to reduce the risk of leakage and short circuit,thus improving the safety performance of the flexible and wearable batteries.In addition,the abundant functional groups such as hydroxyl,carboxyl,sulfonic acid and amino groups on polymer chain can interact with Zn²⁺to reduce the amounts of water on Zn solvation structure to effectively inhibit dendrite growth,and improve the electrochemical performance of the battery.Moreover,gel electrolytes with high mechanical strength,toughness,self-healing ability,anti-freezing and other special functions can be prepared via the characteristics of different functional groups on the polymer chain.Therefore,the application of polymer electrolyte to high-performance zinc ion battery will help to promote its development.Hofmeister effect has been demonstrated to be effective on regulating the hydrogel property with salting-in effect（which can reduce the amount of bound water to decrease freezing point and improve the solubility of macromolecules）and salting-out effect（which can improve the hydrophobic effect of macromolecules to improve the mechanical properties of gel electrolytes）.Thereby,in this paper,gel electrolytes with anti-freezing and high mechanical strength were in situ designed according to the Hofmeister effect.The details are as follows:（1）A hydrogel electrolyte with flexible and anti-freezing performance regulated by Hofmeister effect was in-situ designed via 2 M Zn（Cl O₄）₂ salt.The Zn（Cl O₄）₂can break the hydrogen bond between water molecules to achieve anti-freezing performance of hydrogel electrolyte at-30 ^oC.The chaotropic Cl O₄^-can form weak HB with hydrogel and water molecules,thus rendering the hydrogel with good mechanical flexibility.The results not only improve the interface contact stability between electrode and electrolyte but also conform to the electrolyte flexibility of the zinc-ion battery.In addition,Zn（Cl O₄）₂ salts can improve the polymer chain hydrophilicity and lead to a good water retention,which ensure that the battery cycling stable under long time operation.The gel electrolyte effectively alleviates zinc dendrites and side reactions by regulating the free water amounts in electrolytes.As a consequence,the symmetrical Zn/Zn battery can keep dendrite free for more than 1200 hours,and the flexible Zn-PANI battery can deliver excellent cycling performance at different bending angles at-30 ^oC.This work not only demonstrates a facile and universe strategy to develop a low temperature tolerated electrolyte with chaotropic salts,but also provides a new path way to rationally regulate the hydrogel mechanical flexibility with chaotropes via the Hofmeister effect.（2）A hydrogel electrolyte with high mechanical properties was designed based on Hofmeister effect by introducing 2 M Zn SO₄ into the gel electrolyte.The salting-out effect of SO₄^2-can improve the hydrophobicity of the polymer chain and regulate the mechanical properties of the gel.In addition,the interaction between the carboxyl group of carboxymethyl chitosan and Zn²⁺can adjust the Zn²⁺solvation sheath to guide the zinc metal growth along the（002）hexagons during the stripping-plating process,which effectively alleviates side reactions and dendrites formation.The Zn/Zn asymmetric cell exhibits high reversibility after 1000 h.This rational design strategy provides a simple way on regulating the hydrogel mechanical properties and engineering the interfacial contact of with the electrode for high performance aqueous zinc-ion batteries.