Research And Application Of Zwitterionic Ionic Liquid In Lithium Ion Conduction And Flexible Sensor

Ionic liquids,completely composed of cation and anion,are molten salts having melting points lower than 100?.They have many unique physical and chemical properties such as negligible vapor pressure,good thermal stability,nonflammability,high ionic conductivity and wide electrochemical window,which endow ionic liquids with great prospect in energy-related fields.The properties of ionic liquids are usually affected by the types of cations and anions,and the interactions between cations and anions,which inspired us to design ionic liquids with specific functions.The applicability of ionic liquids and their derivatives can be broaden by modulating the structures of cation and anion.The functional diversity of ionic liquids has attracted much attention in many fields,especially in modern electrochemical devices such as energy storage/power generation devices,actuators,strain sensors and soft robots.In recent years,the design and synthesis of ionic liquid polymers with specific functions has become a hot topic in materials science,especially poly(ionic liquid)s.As an important branch of ionic liquids,poly(ionic liquid)s are obtained by polymerization of ionic liquid monomer which combine the unique physical and chemical properties of ionic liquid as well as the desired mechanical properties and processing properties of polymer.Based on the excellent ionic conductivity and electrochemical stability,poly(ionic liquid)s can be applied in electrolytes and sensors,and the excellent mechanical processing properties are conducive to the development of electrochemical devices towards ultra-thinness,miniaturization and flexibility.In addition,the development of functional zwitterionic structures originated from the improvement of the drawbacks of ionic liquids.In ordinary ionic liquids,cations and anions migrate relatively independently under the electrostatic interactions and hydrogen bonds or dispersion forces of other ions.Thus,their constituent ions can migrate with potential gradients in ionic liquids-based electrolytes.When only target ions should be transported such as lithium ions for lithium ion batteries,ionic liquids-based electrolytes exhibit the low target ion migration number.Unlike ionic liquids,zwitterions,an ion pair where cation and anion are covalently tethered,keep their own ion pair form despite many interactions with other ions.The net charge of zwitterions is neutral,so they do not migrate even under potential gradients.Besides,zwitterions can interact with other salts to form ionic liquids with low melting points based on ion-dipole and dipole-dipole interactions.Therefore,zwitterions can be considered to be new electrolyte materials.High dipole moment provides strong solvation capacity for various polarity or ion species through the electrostatic interaction.Zwitterions without carrier ions provide a unique ionic environment to transport target ions,thus effectively inhibiting the migration of non-target ions.In addition,as functional derivatives of zwitterions,polymerizable zwitterions retain the original properties of zwitterions and have high shape plasticity,which can meet the requirements for the preparation of functional devices with special structures,such as flexible batteries and flexible strain sensors.In this dissertation,based on the prominent designability of ionic liquids,ionic liquids and poly(ionic liquid)s with specific functions were designed and synthesized through zwitterions,and their applications in electrochemical fields such as electrolytes in lithium ion batteries and strain sensor were studied.The research work was mainly divided into the following four parts:1.We designed and synthesized two kinds of zwitterions,3-(1-methyl-3-imidazolic)propanesulfonate(MIPS)and polymerizable 3-(1-vinyl-3-imidazolic)propanesulfonate(VIPS).Zwitterions can be used as effective "ion dissociator" to prepare room temperature ionic liquid and poly(ionic liquid)s with the equimolar mixture of lithium bis(trifluoromethanesulfonyl)imide(LiTFSI).The copolymer poly(vinylidene fluoride-cohexafluoropropylene)PVdF-Co-HFP was used as matrix to prepare the self-supporting electrolytes.Two kinds of polymer electrolytes,namely MIPS-based gel-type polymer electrolyte and VIPS-based all-solid polymer electrolyte,were designed and synthesized based on zwitterions with different structures by solution casting technique.The thermal stability,mechanical and electrochemical properties of the tw o kinds of polymer electrolytes were investigated in detail:(1)The IR and 7Li NMR spectra indicated that both zwitterions MIPS and VIPS have strong ion-dipole interactions with lithium salts(LiTFSI),and effectively dissociated the existing anion/cation pairs or clusters in lithium salts and released more mobile lithium ions,which facilitated the increase of lithium ion migration number(tLi+).The XRD profile proved that the addition of zwitterions can reduce the crystallinity of polymer.Zwitterion,as a lithium salt solvent,interacts with lithium salt to form ionic liquid which acts as plasticizer to soften the polymer backbone.Linear sweep voltammetry(LSV)results showed that the electrochemical stability of the electrolytes can be significantly improved after the addition of zwitterions.(2)[VIPS][LiTFSI]-containing all-solid polymer electrolytes have slightly low conductivities due to the polymerization of VIPS which impedes ionic motion.However,the all-solid electrolyte showed better mechanical properties and higher lithium ion migration number and can avoid leakage,electrode corrosion and other safety hazards.The above results indicated that poly(ionic liquid)s combine the advantages of ionic liquids and polymers,and can achieve safer batteries and energy storage.2.We designed and synthesized a room temperature zwitterionic poly(ionic liquid)by equimolar mixture of zwitterion 3-(1-vinyl-3-imidazolic)propanesulfonate(VIPS)and lithium salt 4-styrenesulfony(trifluoromethylsulfony)imde(LiSTFSI)for single lithium-ion conduction.The flexible comonomer poly(ethylene glycol)methyl ether methacrylate(PEGM)can improve the rigidity of poly(ionic liquid)and contribute to achieving lithium ion hopping transport transmission.PC was used as plasticizer due to its high dielectric constant to further facilitate lithium salt dissociation.The polymer electrolyte films were prepared by in situ photopolymerization.We investigated the effects of zwitterion VIPS on the transport properties of lithium ions and the electrochemical stability of the electrolytes.The IR spectra confirmed the electrostatic interactions between lithium ions and sulfonate groups of zwitterions which facilitates the conduction of lithium ions and provides active site for lithium ion conduction.The 7Li NMR spectra showed that the zwitterionic effect significantly promotes the dissociation of lithium salt,and the lithium ion migration number can reach 0.94.TGA and LSV analysis showed that the polymer electrolytes had good thermal stability and electrochemical stability.During 100 charge-discharge cycles,the discharge capacity is maintained at a steady state of 120 mA h g-1,and the Coulombic efficiency is maintained above 93%,which indicates the tremendous potential for single Li-ion conductors.3.We prepared the double-network(DN)hydrogels with orthogonal property combining natural and synthetic polymers.In order to match with the ?-carrageenan primary skeleton,a polymerizable K+-containing ionic liquid(IL)is proposed as the chemically cross-linked skeleton.The equimolar mixture of zwitterion VIPS and 3-sulfopropyl methacrylate potassium salt KSM monomers produces PIL with a low melting point.The DN hydrogels were obtained after UV-induced polymerization.This unique DN network structure significantly improved the mechanical strength of hydrogels and facilitates multifunctional integration.Based on the thermal sensitivity of ?-carrageenan,3D printing can be realized.The addition of PIL significantly improves the mechanical strength and fatigue resistance of hydrogels,and endows the system with good ionic conductivity,adhesion and self-healing ability.In addition,the K+ions in PIL could act as junction sites can regulate charge-driven association between the PIL and ?-carrageenan and induce phase separation.Coulombic cohesion between ?-carrageenan and PIL induced octopus suckers-like bionic surface morphology.The flexible strain sensors based on multifunctional double-network hydrogels showed high sensitivity,information quality and wide monitoring range.4.We constructed an organohydrogel based on glycerol/water cosolvent.Biocompatible dopamine-functionalized hyaluronic acid(HA-CA)and polyacrylamide served as "soft" polymer skeleton,and Laponite served as ionic conductor and "hard"nanoreinforcing domains where polymer chains attached to the nanosheet surfaces through physical interfacial interactions to form a dynamic crosslinking network.Such an "soft and hard" synergetic architecture endowed organohydrogels suitable mechanical strength(below the epidermal modulus of skin),super tensile property(>8000%),excellent self-adhesion,good self-healing ability.This unique network structure provided high-quality monitoring of daily movements and specific recognition of important physiological activities such as swallowing,frowning,pulse,walking state and handwriting.Furthermore,the organohydrogel strain sensors can perceive the mass of the object during grasping process and detect large-scale bending angles from 0° to 180° and have temperature-discrimination capacity.In addition,due to the advantages of glycerol/water cosolvent,this organohydrogel sensors have excellent temperature tolerance and long-term stability.At the same time,in vitro cell toxicity evaluation demonstrated that organohydrogel could be used for wearable sensors.