Metallic 1T Na_<i>x</i>MoS₂ as Sulfur Host for Room Temperature Na-S Batteries.

Abstract

Sodium intercalation in layered materials is important for beyond lithium ion-based energy storage technologies. We show that the intercalation of sodium ions using a solid-state reaction between layered molybdenum disulfide (MoS₂) and sodium borohydride (NaBH₄) at 300 °C in an argon atmosphere leads to a transformation from the semiconducting to metallic phase and the formation of sodium-intercalated MoS₂ (Na_<i>x</i>MoS₂). However, the removal of borohydride by washing with water also dissolves the intercalated Na ions, resulting in nonsodiated 1T phase MoS₂. Therefore, 1T phase Na_<i>x</i>MoS₂ has been difficult to isolate and thus has remained largely unexplored. Here, we show that sequential washing with dimethylformamide (DMF) leads to the effective removal of borohydride while retaining intercalated sodium ions. This process yields stabilized Na_<i>x</i>MoS₂ (<i>x</i> ≈ 0.6 as determined by electrochemical deposition) with a 1T phase concentration of ∼60% measured by X-ray photoelectron spectroscopy (XPS). High resolution transmission electron microscopy (HR-TEM) results show that the 1T phase is uniformly distributed within the flake. ²³Na magic-angle-spinning solid-state nuclear magnetic resonance (²³Na ssNMR) confirms the interlayer intercalation of sodium ions. Finally, we employ 1T Na_<i>x</i>MoS₂ as a sulfur host in Na-S batteries. Cathodes based on 1T Na_<i>x</i>MoS₂ deliver higher specific capacity and improved cycling stability compared to conventional carbon/sulfur cathodes. These results suggest that 1T Na_<i>x</i>MoS₂ is promising for room temperature Na-S batteries.