Solid-State Divalent Ion Conduction in ZnPS₃

Next-generation batteries based on divalent working ions have the potential to both reduce the cost of energy storage devices and increase performance. Examples of promising divalent systems include those based on Mg²⁺, Ca²⁺, and Zn²⁺ working ions. Development of such technologies is slow, however, in part due to the difficulty associated with divalent cation conduction in the solid state. Divalent ion conduction is especially challenging in insulating materials that would be useful as solid-state electrolytes or protecting layers on the surfaces of metal anodes. Furthermore, there are no reports of divalent cation conduction in insulating, inorganic materials at reasonable temperatures, prohibiting the development of structure–property relationships. Here, we report Zn²⁺ conduction in insulating ZnPS₃, demonstrating divalent ionic conductivity in an ordered, inorganic lattice near room temperature. Importantly, the activation energy associated with the bulk conductivity is low, 351 ± 99 meV, comparable to some Li⁺ conductors such as LTTO, although not as low as the superionic Li⁺ conductors. First-principles calculations suggest that the barrier corresponds to vacancy-mediated diffusion. Assessment of the structural distortions observed along the ion diffusion pathways suggests that an increase in the P–P–S bond angle in the [P₂S₆]^4– moiety accommodates the Zn²⁺ as it passes through the high-energy intermediate coordination environments. ZnPS₃ now represents a baseline material family to begin developing the structure–property relationships that control divalent ion diffusion and conduction in insulating solid-state hosts.