Composite solid electrolytes: recent advances and design strategies

Abstract

The physical properties of composite solid electrolytes are briefly reviewed. The surface potential formation and the point defects equilibrium at free surfaces and interfaces are considered in frames of the unified Stern model. Special attention is drawn to true size effects due to the change of the bulk characteristics of ionic salts in the nanocomposites. The main thermodynamic reason of the nanocomposite formation relates to the adhesion energy, γ a. At sufficiently high γ a values, the ionic salt tends to spread along the oxide surface and to form a nanocomposite if the oxide is nanocrystalline or nanoporous. Analysis of the experimental data shows that non-autonomous interface phases, crystalline or amorphous, exist in some composites. The reason for the stabilization of non-equilibrium states is the complex influence of several factors, including the interfacial interaction between components, particle size effect, and elastic strains in the lattice of the ionic salt. The results of molecular dynamic simulations show that the main origins of the conductivity enhancement are the adsorption of ions to oxide surface with the formation of a space charge layer, the lattice deformation near interface, and an appearance of interdomain boundaries generated by misfit dislocations. The equations proposed earlier for the conductivity description are analyzed. Among them, the general mixing rule has a rather simple analytical form and provides appropriate description of the experimental data on both ordinary insulator–conductor composites and composite solid electrolytes in a broad concentration range. Main approaches for the improvement or creating new composite systems are analyzed.