In the wake of global efforts to meet climate change goals, solid-state ionic conductors are receiving increasing interest as promising materials for electrochemical applications. One crucial feature of solid-state ionic conductors is high ionic conductivity. Enhancing ionic conductivity would allow a lower operative temperature while guaranteeing higher currents and lower overpotentials, making the material a good candidate for superior fuel cells or battery applications. Ionic conductivity follows an Arrhenius-type law, suggesting an increase in conductivity with decreasing activation energy. However, decreasing activation energy is also typically correlated with orders of magnitude decrease in the pre-exponential factor, which is known as compensation law [1]. Previous designs of ion conductors have struggled to overcome such compensation law due to a lack of fundamental understanding and descriptors (tunable design variables). Therefore, a deeper understanding of the processes governing ionic conductivity and fundamental descriptors to overcome compensation law could enable superior designs of solid-state ionic conductors.

In this presentation, we discuss designs of solid-state ion conductors with low activation energies and high pre-exponential factors. Using density functional theory simulations on perovskite oxygen ion conductors, we highlight that the entropy of migration regulates changes in the pre-exponential factor across many orders of magnitude. We uncover that such entropy of migration is the result of changes in the vibrational structure of the local environment of oxygen atoms occurring from equilibrium to the transition state during ion hops. Based on this understanding, we highlight strategies and descriptors to enable high pre-exponential factors by enabling the softening of the local environment of mobile ions during migration. On the other hand, we recently showed that the local electronic structure of lattice point defects regulates the migration barrier, which can be decreased by increasing the charge screening capability of the local host lattice [2]. Therefore, independently tuning the vibrational structure of the local environment of mobile ions and the electronic structure of lattice point defects can open a new design space to overcome compensation law. This framework is tested on perovskites and other crystal structures and mobile species. Our new findings and proposed descriptors hold great potential for accelerating the discovery of superior solid-state ion conductors leveraging emerging characterization techniques or growing approaches of computation-aided designs.