Optimization of superionic conductivity in batteries with solid-state electrolytes

Solid-state electrolytes (SSE) have been developed to replace the far more hazardous and flammable liquid electrolytes used in Li-ion batteries. Presently, battery researchers are struggling to find the suitable chemical elements that can form chemically-stable SSE crystal structure, which also permits large ionic conductivity comparable to the best known liquid electrolytes (10${}^{-2}$ Scm${}^{-1}$). The only way to enhance ionic conductivity in a given SSE is to select the right combinations of chemical elements, and this option is actually limited due to finite numbers of available anions and cations from the Periodic Table. In addition, the influence of crystal structure and volume (with or without defects) further complicates the tasks of evaluating and optimizing the ionic conductivity where increasing conduction dimensionality (1D to 3D) or crystal volume does not necessarily increase the ionic conductivity due to repulsive and attractive interactions between conducting cations and SSE crystal ions. Therefore, our strategy here is to first derive the ionic transport mechanism from the first principles using the ionization energy theory (IET). After which, we shall validate our ionic conductivity formula with the experimental data of well-established solid ionic conductors, which can also be used to evaluate and improve the superionic conductivity of SSE.