Lithium-sulfur batteries benefit from MoS₂ encapsulation

A simple procedure for wrapping sulfur particles in thin sheets of molybdenum disulfide may offer a way to capitalize on the promise of high-performance lithium-sulfur batteries, according to a study (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b05371). Lithium batteries made with sulfur, a low-cost and abundant element, have the potential to store five times as much energy by weight as conventional Li-ion batteries. But long-lasting Li-S batteries have remained elusive because of several materials and chemistry problems. For example, electrochemical reactions in these batteries form troublesome lithium polysulfides, which dissolve in the electrolyte solution, reducing the availability of energy-rich lithium. In addition, lithiation reactions lead to substantial swelling of the cathode, which can trigger cracking and failure of that electrode. In an attempt to bypass those problems, Wei Tang and Kian Ping Loh of National University of Singapore and coworkers reacted a polyvinylpyrrolidone suspension of hollow sulfur particles with ultrathin MoS2 flakes, causing the flakes to tightly encapsulate the sulfur particles. The team made batteries fitted with cathodes prepared from the MoS2-S hybrid material and found that they retained much of their high initial charge capacity even after 1,000 charging cycles. The researchers attribute the good performance to strong van der Waals forces between the MoS2 layers, which help prevent escape of lithium polysulfides. They also found that wrinkles in the MoS2 wrap provide ample space to accommodate swelling of the sulfur particles upon lithiation.