Synthesis of intrinsically sodium intercalated ultra-thin layered MnO₂ and its ionic charge transport

Ionic motion at the interlamellar space in layered materials can provide superior properties in electrochemical energy storage, neuromorphic computing, and ion-mediated charge transport. Herein, we demonstrate the synthesis of ultra-thin, single-crystalline MnO₂ nanosheets intercalated with Na⁺ ions during molten-salt-assisted chemical vapor deposition. Unlike the post-intercalation of alkali ions into a host material, this method facilitates spontaneous Na⁺ intercalation during crystal formation, yielding highly ordered layered δ-MnO₂ (birnessite-type phase) with structural stability. Our in-depth studies, including X-ray diffraction, Raman spectroscopy, and transmission electron microscopy, on the as-synthesized Na-MnO₂ crystals elucidate the crystal properties and highlight that ambient moisture significantly affects the movement of ions and improves their electrochemical stability. Furthermore, we used two-terminal devices of Na-MnO₂ crystals to show hysteretic behavior due to the ionic charge transport mechanisms, making them highly suitable for neuromorphic applications.