An Ion gel (or Ionogel) is a composite material consisting of an

ionic liquid An ionic liquid (IL) is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below a specific temperature, such as . While ordinary liquids such as water and gasoline are predominantly made of ...

immobilized by an inorganic or a polymer matrix. The material has the quality of maintaining high ionic conductivity while in the solid state. To create an ion gel, the solid matrix is mixed or synthesized in-situ with an

. A common practice is to utilize a

block copolymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...

which is polymerized in solution with an

so that a self-assembled nanostructure is generated where the ions are selectively soluble. Ion gels can also be made using non-copolymer polymers such as cellulose, oxides such as

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

or refractory materials such as

boron nitride Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal ...

Types of Ion Gels

Ion gels can be divided in two broad classes based on the major component of the matrix in the composite: polymeric and inorganic. These broad classes can be further subdivided based on the chemical class of the matrix. Across typical ion gel applications, it is desired that the matrix components be electrically insulating to separate contacts within a device and supply ionic conductivity alone. The matrix selection of a material has ramifications on the ionic conductivity as well as the mechanical properties of the final composite material. Inorganic Classes: * Non-metal

oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...

(e.g. SiO₂) * Functionalized non-metal oxide *

Metal Oxide An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...

* Ionic liquid tethered nanoparticles *

Metal Organic Framework A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

* Refractory Materials (e.g. boron nitride) Polymeric Classes: *

Poly(ethylene oxide) Polyethylene glycol (PEG; ) is a polyether compound derived from petroleum with many applications, from industrial manufacturing to medicine. PEG is also known as polyethylene oxide (PEO) or polyoxyethylene (POE), depending on its molecular we ...

Poly(methyl methacrylate) Poly(methyl methacrylate) (PMMA) belongs to a group of materials called engineering plastics. It is a transparent thermoplastic. PMMA is also known as acrylic, acrylic glass, as well as by the trade names and brands Crylux, Plexiglas, Acrylite, ...

* Poly(vinylidene fluoride) * Poly(ethylene glycol) diacrylate *

Poly(acrylonitrile) Poly, from the Greek πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong incorporated Chinese property developer ** Poly Real ...

Although these subtypes of ion gels can classify many materials in this broad class, there are yet still hybrid materials that fall outside these categorizations. Examples have been demonstrated of ion gels with both polymeric and inorganic materials to provide both flexibility and strength in the final composite.

Applications

Ion gels have been utilized in many electrical device systems such as in

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

s as

dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...

s, as

insulators Insulator may refer to: * Insulator (electricity), a substance that resists electricity ** Pin insulator, a device that isolates a wire from a physical support such as a pin on a utility pole ** Strain insulator, a device that is designed to work ...

for

field effect transistors The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs (JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs control ...

, and as

electrolyte An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon dis ...

s for

lithium-ion batteries A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...

. The solid state and yet flexible form of ion gels are attractive for modern mobile devices such as formable screens, health monitoring systems, and solid state batteries. Especially for solid state battery applications, the high viscosity of ion gels provides sufficient strength to serve as both an electrolyte and separator between the anode and cathode. In addition, ion gels are sought after in battery applications as the viscoelastic flow of the gel under stress creates a high quality electrode/electrolyte contact compared to other solid state electrolytes.

Thermal Stability

Ion gels have been known to be able to sustain upwards of 300 °C before onset of degradation. The high temperature capability is typically limited by the underlying

, which can have a wide range of thermal stability, but are typically stable to at least 250 °C. This high temperature stability has been exploited to operate lithium ion battery cells at lab scale up to 175 °C, which is well beyond the capabilities of current commercial electrolytes.

Mechanical Properties

Given the variety of ion gels, the mechanical properties of this broad class of materials spans a wide range. Often mechanical properties are tailored towards the desired application. Applications that require high flexibility target a highly elastic matrix material such as a cross-linked polymer. These types of

elastomer An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus and high failure strain compared with other materials. The term, a portmanteau of ''elastic p ...

ic materials offer high degree of

elastic Elastic is a word often used to describe or identify certain types of elastomer, elastic used in garments or stretchable fabrics. Elastic may also refer to: Alternative name * Rubber band, ring-shaped band of rubber used to hold objects togeth ...

strain with full recovery, which is desirable in wearable devices that will undergo many stress cycles during their lifetime. Additionally, these types of materials can achieve up to 135% strain at failure indicating a degree of

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

. Applications that require higher strength ion gel will often use a refractory matrix to generate composite strengthening. This is particularly desirable in

lithium-ion battery A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...

applications, which seek to deter the growth of lithium

dendrites Dendrites (from Greek δένδρον ''déndron'', "tree"), also dendrons, are branched protoplasmic extensions of a nerve cell that propagate the electrochemical stimulation received from other neural cells to the cell body, or soma, of the n ...

in the cell that can result in an internal

short-circuit A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit. ...

. A relationship has been established in

between high modulus, strong, solid electrolytes and a reduction in lithium dendrite growth. Thereby, a strong ion gel composite can improve the longevity of lithium-ion batteries through reduced internal short circuit failures. The elastic resistance to flow of ion gels is often measure via

Dynamic Mechanical Spectroscopy Dynamic mechanical analysis (abbreviated DMA) is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymer, polymers. A sinusoidal stress is applied and the strain in the material is ...

. This method reveals the storage modulus as well as the loss modulus, which define the stress-strain response of the gel. All ion gels are in the quasi-solid to solid state regime indicating that the storage modulus is higher than the loss modulus (i.e. elastic behavior prevails over the energy dissipating liquid-like behavior). The magnitude of the storage modulus and its ratio to the loss modulus dictate the strength and the

toughness In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing. Storage modulus values for ion gels can vary from approximately 1.0 kPa for typical polymeric-based matrices up to approximately 1.0 MPa for refractory-based matrices. The structure of the composite matrix can play a large role in the outcome of the final bulk mechanical properties. This is especially true for inorganic based matrix materials. Several lab-scale examples have demonstrated a general trend that smaller matrix particle sizes can result in orders of magnitude increase in storage modulus. This has been attributed to higher surface area to volume ratio of the matrix particles and the higher concentration of nanoscale interactions between the particle and the immobilized

. The higher the interaction forces between the components in the ion gel composite results in a higher force required for

plastic deformation In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strain ...

and an overall stiffer material. Another degree of freedom in ion gel design lies in the ratio of matrix to ionic liquid in the final composite. As the concentration of ionic liquid in the matrix increases, the material will become more liquid-like in general corresponding to a decrease in storage modulus. Conversely, a decrease in concentration will generally strengthen the material and depending on the matrix material can generate a more elastomeric or brittle stress-strain response. The general tradeoff in a reduced concentration in ionic liquid is a subsequent decrease in ionic conductivity of the overall composite making optimization necessary for the specific application.

References

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