Environmental and Thermal Stability of Chemically Exfoliated Li<inf>x</inf>MoS<inf>2</inf> for Lithium-Sulfur Batteries

Abstract

Molybdenum disulfide (MoS2) can exist in semiconducting (2H) or metallic (1T) phase. The metallic 1T phase of MoS2 is achieved by lithium intercalation using n-butyllithium. The resulting 1T MoS2 can be in lithiated form (LixMoS2) or as pure MoS2. The 1T phase of MoS2 is metastable and relaxes to the stable 2H phase upon heating. Here we study the thermal and environmental stability of metallic phase LixMoS2 and pure 1T phase MoS2 for comparison. We find that the thermal stability of 1T MoS2 is enhanced by lithiation so that LixMoS2 is phase stable up to 400°C in argon, while non-lithiated MoS2 relaxes to the 2H phase above 150°C. The stability of the LixMoS2 was monitored under ambient conditions (23°C, 50% relative humidity). We find that the LixMoS2 phase progressively oxidizes up to 48 hours of exposure in ambient after which further oxidation ceases. X-ray diffraction shows that the oxidation occurs via loss of sulfur and the formation of lithium molybdate (Li2MoO4). Our analysis reveals that moisture is the primary cause of oxidation and that the 1T phase and chemistry of LixMoS2 can be preserved in a dry oxygen rich environment at up to 250°C. This enhanced stability allows the use of conventional molten diffusion method for cathode preparation for lithium-sulfur batteries.