Superionic Conduction of Next-Generation Mobile Ions in Solids Enabled by Coordinating Ligands

Abstract

Advancements in battery technologies are a critical step towards meeting the growing demand for sustainable energy storage solutions. The development of next-generation battery technologies using "beyond-Li" ions, like Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, and Al³⁺, could potentially offer improved performance, safety, and cost-effectiveness over traditional lithium-ion systems. However, the realization of next-generation battery technology based on "beyond-Li" mobile ions is limited, in part, due to a lack of understanding of solid state conduction of next-generation ions, which governs ion transport in electrodes, interphases, and solid electrolytes. “Beyond-Li” ions tend to have relatively low mobility in solids due to: (1) the larger ionic radii (Na⁺, K⁺, Ca²⁺), which limit the accessible migration pathways, or (2) higher charge densities (Mg²⁺, Zn²⁺ Al³⁺), which results in strong electrostatic interactions within the solid.

This work discusses several structure-property relationships and structural modifications that are hypothesized to lead to facile conduction of next-generation working ions. A notable discovery is the superionic conductivity of ZnPS3 after exposure to humid environments. Water is introduced into the grain boundaries, thereby enabling Zn²⁺ ions from the material to migrate and conduct freely in the network of adsorbed water. The introduction of water leads to potential H⁺, therefore a methodology for decoupling the contributions of Zn²⁺ and H⁺ in mixed ionic conducting solids using ion-selective EIS, transference number measurements, and deposition experiments is established.

Further extending this approach, superionic conductivity of other next-generation ions in electronically-insulating inorganic solids is achieved by leveraging the established ion exchange/intercalation mechanism of MPS₃ (M = Cd, Mn) materials. The mobile cations that are introduced are coordinated with H₂O ligands which simultaneously increase the size of the bottlenecks within the migration pathway and screen the charge-dense ions resulting in high mobilities. Potential applications can be extended to water-incompatible systems by replacing the water ligands with aprotic molecules.

These insights contribute significantly to the understanding and development of next-generation battery technologies, representing an important step toward the development of more sustainable and efficient energy storage solutions.