Ion gel

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An Ion gel (or Ionogel) is a composite material consisting of an ionic liquid immobilized by an inorganic or a polymer matrix. [1] [2] [3] 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 ionic liquid. A common practice is to utilize a block copolymer which is polymerized in solution with an ionic liquid 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 or refractory materials such as boron nitride.

Contents

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. [1] 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: [1]

Polymeric Classes: [3]

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. [4]

Applications

Ion gels have been utilized in many electrical device systems such as in capacitors as dielectrics, [5] as insulators for field effect transistors, [6] and as electrolytes for lithium-ion batteries. [1] 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. [7] 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. [1] 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. [8]

Thermal Stability

Ion gels have been known to be able to sustain upwards of 300 °C before onset of degradation. [9] The high temperature capability is typically limited by the underlying ionic liquid, which can have a wide range of thermal stability, but are typically stable to at least 250 °C. [10] 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. [11]

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. [7] [9] These types of elastomeric materials offer high degree of elastic 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. [12] 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 applications, which seek to deter the growth of lithium dendrites in the cell that can result in an internal short-circuit. A relationship has been established in lithium-ion batteries between high modulus, strong, solid electrolytes and a reduction in lithium dendrite growth. [13] 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. 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). [14] The magnitude of the storage modulus and its ratio to the loss modulus dictate the strength and the toughness of composite material. [9] Storage modulus values for ion gels can vary from approximately 1.0 kPa for typical polymeric-based matrices [15] up to approximately 1.0 MPa for refractory-based matrices. [11]

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. [11] [13] 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 ionic liquid. [11] The higher the interaction forces between the components in the ion gel composite results in a higher force required for plastic deformation 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. [16] 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. [17] 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. [16]

Related Research Articles

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 dissolving, the substance separates into cations and anions, which disperse uniformly throughout the solvent. Solid-state electrolytes also exist. In medicine and sometimes in chemistry, the term electrolyte refers to the substance that is dissolved.

<span class="mw-page-title-main">Lithium polymer battery</span> Lithium-ion battery using a polymer electrolyte

A lithium polymer battery, or more correctly lithium-ion polymer battery, is a rechargeable battery of lithium-ion technology using a polymer electrolyte instead of a liquid electrolyte. High conductivity semisolid (gel) polymers form this electrolyte. These batteries provide higher specific energy than other lithium battery types and are used in applications where weight is a critical feature, such as mobile devices, radio-controlled aircraft and some electric vehicles.

<span class="mw-page-title-main">Nanofiber</span>

Nanofibers are fibers with diameters in the nanometer range. Nanofibers can be generated from different polymers and hence have different physical properties and application potentials. Examples of natural polymers include collagen, cellulose, silk fibroin, keratin, gelatin and polysaccharides such as chitosan and alginate. Examples of synthetic polymers include poly(lactic acid) (PLA), polycaprolactone (PCL), polyurethane (PU), poly(lactic-co-glycolic acid) (PLGA), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and poly(ethylene-co-vinylacetate) (PEVA). Polymer chains are connected via covalent bonds. The diameters of nanofibers depend on the type of polymer used and the method of production. All polymer nanofibers are unique for their large surface area-to-volume ratio, high porosity, appreciable mechanical strength, and flexibility in functionalization compared to their microfiber counterparts.

<span class="mw-page-title-main">Fast-ion conductor</span>

In materials science, fast ion conductors are solid conductors with highly mobile ions. These materials are important in the area of solid state ionics, and are also known as solid electrolytes and superionic conductors. These materials are useful in batteries and various sensors. Fast ion conductors are used primarily in solid oxide fuel cells. As solid electrolytes they allow the movement of ions without the need for a liquid or soft membrane separating the electrodes. The phenomenon relies on the hopping of ions through an otherwise rigid crystal structure.

<span class="mw-page-title-main">Lithium tetrafluoroborate</span> Chemical compound

Lithium tetrafluoroborate is an inorganic compound with the formula LiBF4. It is a white crystalline powder. It has been extensively tested for use in commercial secondary batteries, an application that exploits its high solubility in nonpolar solvents.

<span class="mw-page-title-main">Nanobatteries</span> Type of battery

Nanobatteries are fabricated batteries employing technology at the nanoscale, particles that measure less than 100 nanometers or 10−7 meters. These batteries may be nano in size or may use nanotechnology in a macro scale battery. Nanoscale batteries can be combined to function as a macrobattery such as within a nanopore battery.

Rechargeable lithium metal batteries are secondary lithium metal batteries. They have metallic lithium as a negative electrode, sometimes referred to as the battery anode. The high specific capacity of lithium metal, very low redox potential and low density make it the ideal anode material for high energy density battery technologies. Rechargeable lithium metal batteries can have a long run time due to the high charge density of lithium. Several companies and many academic research groups are currently researching and developing rechargeable lithium metal batteries as they are considered a leading pathway for development beyond lithium-ion batteries. Some rechargeable lithium metal batteries employ a liquid electrolyte and some employ a solid-state electrolyte.

<span class="mw-page-title-main">Solid state ionics</span>

Solid-state ionics is the study of ionic-electronic mixed conductor and fully ionic conductors and their uses. Some materials that fall into this category include inorganic crystalline and polycrystalline solids, ceramics, glasses, polymers, and composites. Solid-state ionic devices, such as solid oxide fuel cells, can be much more reliable and long-lasting, especially under harsh conditions, than comparable devices with fluid electrolytes.

A solid-state battery uses solid electrodes and a solid electrolyte, instead of the liquid or polymer gel electrolytes found in lithium-ion or lithium polymer batteries.

The lithium–air battery (Li–air) is a metal–air electrochemical cell or battery chemistry that uses oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current flow.

A potassium-ion battery or K-ion battery is a type of battery and analogue to lithium-ion batteries, using potassium ions for charge transfer instead of lithium ions. It was invented by the Iranian/American chemist Ali Eftekhari in 2004.

<span class="mw-page-title-main">Separator (electricity)</span>

A separator is a permeable membrane placed between a battery's anode and cathode. The main function of a separator is to keep the two electrodes apart to prevent electrical short circuits while also allowing the transport of ionic charge carriers that are needed to close the circuit during the passage of current in an electrochemical cell.

Research in lithium-ion batteries has produced many proposed refinements of lithium-ion batteries. Areas of research interest have focused on improving energy density, safety, rate capability, cycle durability, flexibility, and cost.

<span class="mw-page-title-main">NASICON</span>

NASICON is an acronym for sodium (Na) Super Ionic CONductor, which usually refers to a family of solids with the chemical formula Na1+xZr2SixP3−xO12, 0 < x < 3. In a broader sense, it is also used for similar compounds where Na, Zr and/or Si are replaced by isovalent elements. NASICON compounds have high ionic conductivities, on the order of 10−3 S/cm, which rival those of liquid electrolytes. They are caused by hopping of Na ions among interstitial sites of the NASICON crystal lattice.

Lithium hybrid organic batteries are an energy storage device that combines lithium with an organic polymer. For example, polyaniline vanadium (V) oxide (PAni/V2O5) can be incorporated into the nitroxide-polymer lithium iron phosphate battery, PTMA/LiFePO4. Together, they improve the lithium ion intercalation capacity, cycle life, electrochemical performances, and conductivity of batteries.

Oxycarbide glass, also referred to as silicon oxycarbide, is a type of glass that contains oxygen and carbon in addition to silicon dioxide. It is created by substituting some oxygen atoms with carbon atoms. This glass may contain particles of amorphous carbon, and silicon carbide. SiOC materials of varying stoichiometery are attractive owing to their generally high density, hardness and high service temperatures. Through diverse forming techniques high performance parts in complex shapes can be achieved. Unlike pure SiC, the versatile stoichiometry of SiOC offers further avenues to tune physical properties through appropriate selection of processing parameters.

Calcium (ion) batteries are energy storage and delivery technologies (i.e., electro–chemical energy storage) that employ calcium ions (cations), Ca2+, as the active charge carrier in the electrolytes as well as in the electrodes (anode and cathode). Calcium (ion) batteries remain an active area of research, with studies and work persisting in the discovery and development of electrodes and electrolytes that enable stable, long-term battery operation.

<span class="mw-page-title-main">Solid-state electrolyte</span>

A solid-state electrolyte (SSE) is a solid ionic conductor and electron-insulating material and it is the characteristic component of the solid-state battery. It is useful for applications in electrical energy storage (EES) in substitution of the liquid electrolytes found in particular in lithium-ion battery. The main advantages are the absolute safety, no issues of leakages of toxic organic solvents, low flammability, non-volatility, mechanical and thermal stability, easy processability, low self-discharge, higher achievable power density and cyclability. This makes possible, for example, the use of a lithium metal anode in a practical device, without the intrinsic limitations of a liquid electrolyte thanks to the property of lithium dendrite suppression in the presence of a solid-state electrolyte membrane. The utilization of a high capacity anode and low reduction potential, like lithium with a specific capacity of 3860 mAh g−1 and a reduction potential of -3.04 V vs SHE, in substitution of the traditional low capacity graphite, which exhibits a theoretical capacity of 372 mAh g−1 in its fully lithiated state of LiC6, is the first step in the realization of a lighter, thinner and cheaper rechargeable battery. Moreover, this allows the reach of gravimetric and volumetric energy densities, high enough to achieve 500 miles per single charge in an electric vehicle. Despite the promising advantages, there are still many limitations that are hindering the transition of SSEs from academia research to large-scale production, depending mainly on the poor ionic conductivity compared to that of liquid counterparts. However, many car OEMs (Toyota, BMW, Honda, Hyundai) expect to integrate these systems into viable devices and to commercialize solid-state battery-based electric vehicles by 2025.

A polymer electrolyte is a polymer matrix capable of ion conduction. Much like other types of electrolyte—liquid and solid-state—polymer electrolytes aid in movement of charge between the anode and cathode of a cell. The use of polymers as an electrolyte was first demonstrated using dye-sensitized solar cells. The field has expanded since and is now primarily focused on the development of polymer electrolytes with applications in batteries, fuel cells, and membranes.

<span class="mw-page-title-main">Lithium aluminium germanium phosphate</span> Chemical compound

Lithium aluminium germanium phosphate, typically known with the acronyms LAGP or LAGPO, is an inorganic ceramic solid material whose general formula is Li
1+xAl
xGe
2-x(PO
4)
3. LAGP belongs to the NASICON family of solid conductors and has been applied as a solid electrolyte in all-solid-state lithium-ion batteries. Typical values of ionic conductivity in LAGP at room temperature are in the range of 10–5 - 10–4 S/cm, even if the actual value of conductivity is strongly affected by stoichiometry, microstructure, and synthesis conditions. Compared to lithium aluminium titanium phosphate (LATP), which is another phosphate-based lithium solid conductor, the absence of titanium in LAGP improves its stability towards lithium metal. In addition, phosphate-based solid electrolytes have superior stability against moisture and oxygen compared to sulfide-based electrolytes like Li
10GeP
2S
12 (LGPS) and can be handled safely in air, thus simplifying the manufacture process. Since the best performances are encountered when the stoichiometric value of x is 0.5, the acronym LAGP usually indicates the particular composition of Li
1.5Al
0.5Ge
1.5(PO
4)
3, which is also the typically used material in battery applications.

References

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