Water Vapor Induced Superionic Conductivity in ZnPS₃

Next-generation batteries based on sustainable multivalent working ions, such as Mg²⁺, Ca²⁺, or Zn²⁺, have the potential to improve the performance, safety, and capacity of current battery systems. Development of such multivalent ion batteries is hindered by a lack of understanding of multivalent ionics in solids, which is crucial for many aspects of battery operation. For instance, multivalent ionic transport was assumed to be correlated with electronic transport; however, we have previously shown that Zn²⁺ can conduct in electronically insulating ZnPS₃ with a low activation energy of 350 meV, albeit with low ionic conductivity. Here, we show that exposure of ZnPS₃ to environments with water vapor at different relative humidities results in room-temperature conductivity increases of several orders of magnitude, reaching as high as 1.44 mS cm⁻¹ without decomposition or structural changes. We utilize impedance spectroscopy with ion selective electrodes, ionic transference number measurements, and deposition and stripping of Zn metal, to confirm that both Zn²⁺ and H⁺ act as mobile ions. The contribution from Zn²⁺ to the ionic conductivity in water vapor exposed ZnPS₃ is high, representing superionic Zn²⁺ conduction. The present study demonstrates that it is possible to enhance multivalent ion conduction of electronically insulating solids as a result of water adsorption and highlights the importance of ensuring that increased conductivity in water vapor exposed multivalent ion systems is in fact due to mobile multivalent ions and not solely H⁺.