A solid composite material has been made that conducts electricity through the rapid transport of silver ions, which diffuse faster than in some liquids. The material holds promise for applications in charge-storage devices. See Article p.159
What happens when two compounds that have contrary properties are mixed together? Do they work against each other, or do they combine constructively to produce unexpected effects? On page 159, Chen et al.1 report an impressive example of the second outcome. They have combined a material that conducts electricity purely through negatively charged electrons with one that conducts through the fast movement of positively charged ions, to create a composite that they call an artificial mixed conductor. The composite exhibits impressively fast ion diffusion, and has the potential to be of use in batteries.
The electron conductor in the composite is graphite2, a carbon allotrope composed of layers made up of six-membered carbon rings. This layered structure means that conduction in graphite varies with the direction of the current. Graphite can conduct negatively charged electrons but can also host various highly mobile ions, and is a widely used electrode in energy-storage devices. Atomically thin layers of graphite are called graphene, and can act as a membrane that conducts protons3.
The other component in the authors' composite is rubidium silver iodide (RbAg4I5), the best known solid conductor of silver ions at room temperature4. This compound can itself be thought of as a composite of silver iodide (AgI) and rubidium iodide (RbI). Silver ions are positively charged, and are large and heavy compared with most other charge carriers. But the rubidium ions in the iodide framework of RbAg4I5 are perfectly arranged to provide vacant sites for the silver ions to 'jump' into. This allows the silver ions to move almost freely in all directions through the solid.
If graphite and RbAg4I5 are combined, then a solid, mixed conductor system might be generated that enables charge transport or conduction by two different charge carriers at the interfaces between the two compounds, potentially offering extremely high conductivity. Conductors that naturally allow conduction through both electron and ion transport have been widely studied and are used in processes that benefit from such optimized conductivity. For example, they have applications in energy-storage devices such as batteries and supercapacitors, and in sensor devices5,6,7.
Duality of charge transport has another advantage: it can be used to change the stoichiometry (the ratio of atom types described by a chemical formula) of a conductor, because the addition or removal of a given charge carrier can be compensated for by adding or removing the oppositely charged carriers. This enables fast transport, storage and redistribution of mass, which is also useful for charge-storage devices. For example, an extra positively charged ion such as Ag+ can be compensated for by the addition of a negatively charged electron, whereas a vacant Ag+ ion can be balanced by adding an electron hole (a quasiparticle corresponding to the absence of an electron; Fig. 1). In pure electron- or ion-conducting systems, such compensation processes are disfavoured or almost impossible, in most cases because of the lack of oppositely charged carriers. A solution to this problem that allows certain stoichiometry changes could be to combine both types of system to form a hybrid.
Chen and colleagues' graphite–RbAg4I5 composite is just such a clever combination. The authors prepared the material by grinding the components together in a mortar and then melting the mixture, to bring the two types of conductor into intimate contact with each other. In a series of experiments, the authors observed impressively fast (occurring within seconds) and pronounced stoichiometry changes (from approximately –10−5 to about 4 × 10−6 for silver) at the interfaces between the two compounds. This behaviour was combined with an extraordinarily high diffusion of silver ions at room temperature.
But how do the components of the composite enable stoichiometric changes? In the case of silver being added to the composite, the extra silver is stored within the ion conductor (RbAg4I5), whereas the compensatory electrons are hosted by the electron conductor (graphite). Both compounds contribute their best talents to this joint effort — RbAg4I5 effectively transports silver ions and stores them at vacant surface sites, and graphite acts as an electron sponge. The situation is different in the case of silver being removed from the composite: RbAg4I5 releases silver and forms vacancies that are compensated for by electron holes in the graphite. Once again, the job is shared by the two compounds.
Chen and co-workers also provide a detailed analysis of the physics behind the observed fast diffusion process, and show that the classical theory of chemical diffusion must be reconsidered in the case of interface-driven job-sharing processes. In particular, there should be a reassessment of the roles of chemical capacitance (a material's ability to take up or release chemical components such as silver ions) and of electrostatic energy in changing the charge-carrier concentration at the interface.
Finally, the authors built two all-solid-state prototype energy-storage devices — a battery and a supercapacitor — to demonstrate potential practical applications of their composite. The battery can be reversibly charged and discharged using extremely high currents (and therefore within 0.05 seconds), whereas the supercapacitor provides ultrafast charge release, which is needed for various applications of these devices. Both effects are a direct result of the high mass transport in the system and its compositional flexibility.
It will be exciting to see whether the concept of an artificial mixed conductor system can be transferred to other solid ion conductors, such as the promising 'argyrodite-type' solids, in which lithium ions have unusually high mobility8,9. Another question is whether graphite is the optimal electron conductor in these mixed conductors. Perhaps graphene sheets, or graphite consisting of just a few stacked graphene sheets, could be used instead, to optimize the number of interfaces per unit volume between the two types of conductor. Materials scientists, chemists and physicists will no doubt be keen to adopt this concept to create materials that have hybrid functionality, potentially opening up fresh applications.