Water Vapor Induced Superionic Conductivity in ZnPS.

Next-generation batteries based on sustainable multivalent working ions, such as Mg, Ca, or Zn, 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 Zn can conduct in electronically insulating ZnPS with a low activation energy of 350 meV, albeit with low ionic conductivity. Here, we show that exposure of ZnPS 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 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 Zn and H act as mobile ions. The contribution from Zn to the ionic conductivity in water vapor exposed ZnPS is high, representing superionic Zn 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.