Fast-ion conductor

A proton conductor, specifically, superionic ice, in a static electric field.

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.

Mechanism

Fast ion conductors are intermediate in nature between crystalline solids which possess a regular structure with immobile ions, and liquid electrolytes which have no regular structure and fully mobile ions. Solid electrolytes find use in all solid-state supercapacitors, batteries, and fuel cells, and in various kinds of chemical sensors.

Classification

In solid electrolytes (glasses or crystals), the ionic conductivity σ_i can be any value, but it should be much larger than the electronic one. Usually, solids where σ_i is on the order of 0.0001 to 0.1 Ω⁻¹ cm⁻¹ (300 K) are called superionic conductors.

Proton conductors

Proton conductors are a special class of solid electrolytes, where hydrogen ions act as charge carriers. One notable example is superionic water.

Superionic conductors

Superionic conductors where σ_i is more than 0.1 Ω⁻¹ cm⁻¹ (300 K) and the activation energy for ion transport E_i is small (about 0.1 eV), are called advanced superionic conductors. The most famous example of advanced superionic conductor-solid electrolyte is RbAg₄I₅ where σ_i > 0.25 Ω⁻¹ cm⁻¹ and σ_e ~10⁻⁹ Ω⁻¹ cm⁻¹ at 300 K.<ref>Akin, Mert; Wang, Yuchen; Qiao, Xiaoyao; Yan, Zhiwei; Zhou, Xiangyang (September 2020). "Effect of relative humidity on the reaction kinetics in rubidium silver iodide based all-solid-state battery". Electrochimica Acta. 355: 136779. doi:10.1016/j.electacta.2020.136779. S2CID 225553692.</ref><ref>Wang, Yuchen; Akin, Mert; Qiao, Xiaoyao; Yan, Zhiwei; Zhou, Xiangyang (September 2021). "Greatly enhanced energy density of all‐solid‐state rechargeable battery operating in high humidity environments". International Journal of Energy Research. 45 (11): 16794–16805. doi:10.1002/er.6928.</ref> The Hall (drift) ionic mobility in RbAg₄I₅ is about 2×10⁻⁴ cm²/(V•s) at room temperatures.<ref>Stuhrmann C.H.J.; Kreiterling H.; Funke K. (2002). "Ionic Hall effect measured in rubidium silver iodide". Solid State Ionics. 154–155: 109–112. doi:10.1016/S0167-2738(02)00470-8.</ref> The σ_e – σ_i systematic diagram distinguishing the different types of solid-state ionic conductors is given in the figure.<ref>Александр Деспотули; Александра Андреева (2007). Высокоёмкие конденсаторы для 0,5 вольтовой наноэлектроники будущего. Современная Электроника (in русский) (7): 24–29. Alexander Despotuli; Alexandra Andreeva (2007). "High-capacity capacitors for 0.5 voltage nanoelectronics of the future". Modern Electronics (7): 24–29.</ref><ref name=Despotuli>Despotuli, A.L.; Andreeva, A.V. (January 2009). "A Short Review on Deep-Sub-Voltage Nanoelectronics and Related Technologies". International Journal of Nanoscience. 8 (4&5): 389–402. Bibcode:2009IJN.....8..389D. doi:10.1142/S0219581X09006328.</ref>

Classification of solid-state ionic conductors by the lg (electronic conductivity, σ_e) – lg (ionic conductivity, σ_i) diagram. Regions 2, 4, 6 and 8 are solid electrolytes (SEs), materials with σ_i ≫ σ_e; regions 1, 3, 5 and 7 are mixed ion-electron conductors (MIECs). 3 and 4 are superionic conductors (SICs), i.e. materials with σ_i > 0.001 Ω⁻¹cm⁻¹. 5 and 6 are advanced superionic conductors (AdSICs), where σ_i > 10⁻¹ Ω⁻¹cm⁻¹ (300 K), energy activation E_i about 0.1 eV. 7 and 8 are hypothetical AdSIC with Ei ≈ k_BT ≈0.03 eV (300 К).

No clear examples have been described as yet, of fast ion conductors in the hypothetical advanced superionic conductors class (areas 7 and 8 in the classification plot). However, in crystal structure of several superionic conductors, e.g. in the minerals of the pearceite-polybasite group, the large structural fragments with activation energy of ion transport E_i < k_BT (300 К) had been discovered in 2006.<ref>Bindi, L.; Evain M. (2006). "Fast ion conduction character and ionic phase-transitions in disordered crystals: the complex case of the minerals of the pearceite– polybasite group". Phys Chem Miner. 33 (10): 677–690. Bibcode:2006PCM....33..677B. doi:10.1007/s00269-006-0117-7. S2CID 95315848.</ref>

Examples

Zirconia-based materials

A common solid electrolyte is yttria-stabilized zirconia, YSZ. This material is prepared by doping Y₂O₃ into ZrO₂. Oxide ions typically migrate only slowly in solid Y₂O₃ and in ZrO₂, but in YSZ, the conductivity of oxide increases dramatically. These materials are used to allow oxygen to move through the solid in certain kinds of fuel cells. Zirconium dioxide can also be doped with calcium oxide to give an oxide conductor that is used in oxygen sensors in automobile controls. Upon doping only a few percent, the diffusion constant of oxide increases by a factor of ~1000.<ref>Shriver, D. F.; Atkins, P. W.; Overton, T. L.; Rourke, J. P.; Weller, M. T.; Armstrong, F. A. “Inorganic Chemistry” W. H. Freeman, New York, 2006. ISBN 0-7167-4878-9.</ref>

Other conductive ceramics function as ion conductors. One example is NASICON, (Na₃Zr₂Si₂PO₁₂), a sodium super-ionic conductor

beta-Alumina

Another example of a popular fast ion conductor is beta-alumina solid electrolyte.<ref>Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.</ref> Unlike the usual forms of alumina, this modification has a layered structure with open galleries separated by pillars. Sodium ions (Na⁺) migrate through this material readily since the oxide framework provides an ionophilic, non-reducible medium. This material is considered as the sodium ion conductor for the sodium–sulfur battery.

Fluoride ion conductors

Lanthanum trifluoride (LaF₃) is conductive for F⁻ ions, used in some ion selective electrodes. Beta-lead fluoride exhibits a continuous growth of conductivity on heating. This property was first discovered by Michael Faraday.

Iodides

A textbook example of a fast ion conductor is silver iodide (AgI). Upon heating the solid to 146 °C, this material adopts the alpha-polymorph. In this form, the iodide ions form a rigid cubic framework, and the Ag+ centers are molten. The electrical conductivity of the solid increases by 4000x. Similar behavior is observed for copper(I) iodide (CuI), rubidium silver iodide (RbAg₄I₅),<ref>Akin, Mert; Wang, Yuchen; Qiao, Xiaoyao; Yan, Zhiwei; Zhou, Xiangyang (20 September 2020). "Effect of relative humidity on the reaction kinetics in rubidium silver iodide based all-solid-state battery". Electrochimica Acta. 355: 136779. doi:10.1016/j.electacta.2020.136779. S2CID 225553692.</ref> and Ag₂HgI₄.

Other Inorganic materials

• Silver sulfide, conductive for Ag⁺ ions, used in some ion selective electrodes
• Lead(II) chloride, conductive at higher temperatures
• Some perovskite ceramics – strontium titanate, strontium stannate – conductive for O²⁻ ions
• <chem>Zr(HPO4)2.\mathit{n}H2O</chem> – conductive for H⁺ ions
• <chem>UO2HPO4.4H2O</chem> (hydrogen uranyl phosphate tetrahydrate) – conductive for H⁺ ions
• Cerium(IV) oxide – conductive for O²⁻ ions

Organic materials

• Many gels, such polyacrylamides, agar, etc. are fast ion conductors<ref name="dry">"The Roll-to-Roll Battery Revolution". Ev World. Archived from the original on 2011-07-10. Retrieved 2010-08-20.</ref><ref>Perzyna, K.; Borkowska, R.; Syzdek, J. A.; Zalewska, A.; Wieczorek, W. A. A. (2011). "The effect of additive of Lewis acid type on lithium–gel electrolyte characteristics". Electrochimica Acta. 57: 58–65. doi:10.1016/j.electacta.2011.06.014.</ref>
• A salt dissolved in a polymer – e.g. lithium perchlorate in polyethylene oxide<ref>Syzdek, J. A.; Armand, M.; Marcinek, M.; Zalewska, A.; Żukowska, G. Y.; Wieczorek, W. A. A. (2010). "Detailed studies on the fillers modification and their influence on composite, poly(oxyethylene)-based polymeric electrolytes". Electrochimica Acta. 55 (4): 1314. doi:10.1016/j.electacta.2009.04.025.</ref>
• Polyelectrolytes and Ionomers – e.g. Nafion, a H⁺ conductor

History

The important case of fast ionic conduction is one in a surface space-charge layer of ionic crystals. Such conduction was first predicted by Kurt Lehovec.<ref>Lehovec, Kurt (1953). "Space-charge layer and distribution of lattice defects at the surface of ionic crystals". Journal of Chemical Physics. 21 (7): 1123–1128. Bibcode:1953JChPh..21.1123L. doi:10.1063/1.1699148.</ref> As a space-charge layer has nanometer thickness, the effect is directly related to nanoionics (nanoionics-I). Lehovec's effect is used as a basis for developing nanomaterials for portable lithium batteries and fuel cells.

See also

• Mixed conductor