Simple and Scalable Ball Milling Synthesis of MoS2/Graphite Heterostructures with High Rate Capability for Li-Ion Batteries

Abstract

With the constant rise of consumer demands for energy storage devices, improvements in current Li-ion battery (LIB) technology are paramount. At present, graphite is the most common commercial anode material, with a theoretical capacity of 372 mA h g-1. However, it is inevitable that graphite alone will no longer suffice to meet large capacity and high power requirements.Transition metal dichalcogenides (TMDs) are isostructural analogues of graphite where a transition metal layer is sandwiched between two chalcogen layers by strong intraplanar bonds while each unit is stacked together through weak van der Waal’s interplanar forces. MoS2 is a TMD which has also been considered as a potential candidate for LIB anodes as its theoretical capacity (670 mA h g-1) is almost double that of graphite. Unfortunately, the anode’s practical performance is very poor due to large volume expansion upon charging/discharging and formation of undesirable polysulphides.Combining MoS2 and graphite could generate an alternating layer heterostructure that exploits the desirable properties of each material. Most syntheses of MoS2/Graphite heterostructures in the literature either require extensive reaction conditions such as long periods of ultrasonication, or produce very low yields such as chemical vapour deposition. Additionally, ultrasonication often introduces many undesirable defects, due to the solvent used and harsh conditions. Alternatively, dry ball milling is a scalable and economically viable technique for producing MoS2/Graphite heterostructures. The process uses mechanical forces exerted by balls to strip layered materials through their weaker van der Waal’s bonding. To date, the only MoS2/Graphite composite synthesised with this method (though under Ar) for Li-ion batteries, was ball milled for 40 hrs and exhibited a practical capacity of ~700 mA h g-1 at 100 mA g-1 current density for 100 cycles.(1)This work uses simple dry ball milling under atmospheric conditions for 12 hrs to synthesise a MoS2/Graphite composite which exhibits ~350 mA h g-1 at a high current density of 1 A g-1. Additionally, this work shows that dry ball milling MoS2 alone for 12 hrs produces a phase transformation to a conductive 1T MoS2 product. Raman spectroscopy, XRD, XPS, SEM and impedance spectroscopy techniques were used to characterise the materials. By combining MoS2 and graphite with this simple and scalable method, we provide a viable solution to meet rising consumer demands of improved energy storage devices.(1) Zhao H, Zeng H, Wu Y, Zhang S, Li B, Huang Y. Facile scalable synthesis and superior lithium storage performance of ball-milled MoS2-graphite nanocomposites. J Mater Chem A. 2015;3(19):10466–70.