Solid state lithium ion conductors: Design considerations by thermodynamic approach

The most common previously employed methods of designing useful solid state lithium ion conductors (SSLICs) are reviewed and a new approach for the rational design of advanced SSLICs is described, which makes use of thermodynamic considerations. The described method is based on the Gibbs energy of formation of binary compounds of substitutional or additional cations (including dopants) and is demonstrated by the improvement of the lithium ion conductivity of SSLICs having perovskite-, NASICON- and Li₄SiO ₄-type structures. Dopant metal oxides with higher negative Gibbs energies of formation than that of the parent metal oxide increase commonly the lithium ion conduction. The stronger binding forces of the oxide ion with the dopant cation result in an electrostatic shielding of the attractive forces between the lithium ions and the anions which facilitates the ionic motion. Irrespective of the crystal structure, it is expected that this thermodynamic rule holds also for other mobile ionic species.