No abstract

Room-temperature sodium−sulfur (RT-Na/S) batteries have recently gained much attention as a low-cost candidate for application in large-scale energy storage, especially in stationary energy. For performance improvement of RT-Na/S batteries, a full understanding of the actual reaction process and discharge products is needed. In this work, we discovered the most stable structure of Na 2 S 3 and a new phase of Na 2 S 2 (γ-Na 2 S 2 ) by using first-principles unbiased structure searching calculations. Analysis of the thermodynamics and electrochemical activity indicates that Na 2 S 3 acts as a stable product like Na 2 S 2 and Na 2 S, but it can spontaneously disproportionate into Na 2 S 2 , Na 2 S, and S in an RT-Na/S battery. The structure of Na 2 S 3 not only matches the last sloping region of the experimental discharge profile but also gives a direct explanation of the experimental Raman peaks at 476, 458, and 238 cm −1 . Our work makes a contribution to a full understanding of the mechanism for the discharge progress in RT-Na/S batteries.

The promised performance of monolayer molybdenum disulfide (MoS2)-based devices is hindered by the high electrical resistance at the metal-MoS2 contact. Benefitting from the metallic phase of MoS2, 1T-MoS2 is considered a potential electrode material for the semiconducting 2H-MoS2. In this paper, we report a comparative study of the allotropic 1T/2H MoS2 contact with different contact types. The edge-contact interface has a low tunnel barrier, high charge density, and even Ohmic contact with no Schottky barrier, which implies that this contact may overcome the resistance limit for the electrode-MoS2 contact. The outstanding interface properties of the 1T/2H MoS2 edge contact originate from the excess in-plane dangling bonds in the edge position. Based on our results, the edge-contact model is ideal for the 1T/2H MoS2 contact and may solve the problem of MoS2 contact resistance.