Modular <i>M</i>PS<sub>3</sub>‑Based\nFrameworks for Superionic Conduction of Monovalent and Multivalent\nIons

Next-generation batteries based on more sustainable working\nions\ncould offer improved performance, safety, and capacity over lithium-ion\nbatteries while also decreasing the cost. Development of next-generation\nbattery technology using “beyond-Li” mobile ions, especially\nmultivalent ions, is limited due to a lack of understanding of solid\nstate conduction of these ions. Here, we introduce ligand-coordinated\nions in <i>M</i>PS<sub>3</sub>-based (M = Mn, Cd) solid\nhost crystals to simultaneously increase the size of the interlayer\nspacing, through which the ions can migrate, and screen the charge-dense\nions. The ligand-assisted conduction mechanism enables ambient temperature\nsuperionic conductivity of various next-generation mobile ions in\nthe electronically insulating <i>M</i>PS<sub>3</sub>-based\nsolid. Without the coordinating ligands, all of the compounds show\nlittle to no ionic conductivity. Pulsed-field gradient nuclear magnetic\nresonance spectroscopy suggests that the ionic conduction occurs through\na hopping mechanism, where the cations are moving between H<sub>2</sub>O molecules, instead of a vehicular mechanism which has been observed\nin other hydrated layered solids. This modular system not only facilitates\ntailoring to different potential applications but also enables us\nto probe the effect of different host structures, mobile ions, and\ncoordinating ligands on the ionic conductivity. This research highlights\nthe influence of cation charge density, diffusion channel size, and\neffective charge screening on ligand-assisted solid state ionic conductivity.\nThe insights gained can be applied in the design of other ligand-assisted\nsolid state ionic conductors, which will be especially impactful in\nrealizing solid state multivalent ionic conductors. Additionally,\nthe ion-intercalated <i>M</i>PS<sub>3</sub>-based frameworks\ncould potentially serve as a universal solid state electrolyte for\nvarious next-generation battery chemistries.