Researchers at The University of Texas at Dallas have used two-dimensional (2D) MoS₂ (molybdenum disulfide) as a protective layer for Li-metal anodes, greatly improving the performances of Li–S batteries.

In a paper in the journal Nature Nanotechnology, the researchers report that such a use of MoS₂ results in stable Li electrodeposition and the suppression of dendrite nucleation sites. The deposition and dissolution process of a symmetric MoS₂-coated Li-metal cell operates at a current density of 10 mA cm⁻² with low voltage hysteresis and a threefold improvement in cycle life compared with using bare Li-metal.

For a Li–S full-cell configuration, using the MoS₂-coated Li as anode and a 3D carbon nanotube–sulfur cathode, they reported obtain a specific energy density of ~589 Wh kg⁻¹ and a Coulombic efficiency of ~98% for more than 1,200 cycles at 0.5 C.

This approach, they suggested, could lead to the realization of high energy density and safe Li-metal-based batteries.

Long-term cycling performance of Li–S battery with the 3D CNT–S cathode (~33 wt% S content) and MoS2-coated Li anode measured at 0.5 C for 1,200 cycles. The average specific capacity decay is 0.013% per cycle and the Coulombic efficiency at 1,200th cycle is around 98%. The schematic inset illustrates the basic kinetics of Li+ between two electrodes during cycling. Chae et al. Click to enlarge.

Despite many advantages, Li–S batteries are plagued with practical issues that limit their applications: (1) the poor electronic conductivity of sulfur that retards electron transfer during the charge/ discharge processes; (2) the formation of intermediate polysulfides generated during cycling, which leads to the shuttle effect and increases the impedance of both electrodes; (3) the intrinsic issues of Li-metal anodes, which are often associated with uncontrollable dendrite formation during repeated Li deposition and dissolution processes; and (4) the formation of an unstable solid electrolyte interphase (SEI) layer between the electrolyte and Li metal due to inhomogeneous deposition of Li. These issues lead to the reduction of Coulombic efficiency and the subsequent fast termination of battery life.

To tackle the issues regarding the sulfur cathode (1 and 2), various carbon-based materials, for example, porous carbon, carbon nanotubes (CNTs), graphene and fibrous carbon networks, have been shown to enhance the electrochemical properties of sulfur in Li–S batteries. Recently, we reported that three-dimensional (3D) CNT composite structures can improve the electrical conductivity of the sulfur cathode while preventing polysulfides from shuttling during cycling. Despite the progress made on the cathode of Li–S batteries, the issues associated with Li metal and its interactions with electrolytes (3 and 4) still remain a major concern, including battery safety. Therefore, it is crucial to resolve the issues with Li-metal anodes to realize the full potential of Li–S batteries.

… Here, we propose to passivate Li metal with atomic layers of two-dimensional molybdenum disulfide (2D MoS₂) and create a protective barrier between the Li metal and electrolyte. Large amounts of Li atoms can intercalate into the atomically layered MoS₂ structure to reduce the interfacial resistance and facilitate a consistent flow of Li⁺ into and out of bulk Li metal. Unlike other carbon-, composite-, or ceramic-based protective layers, the unique atomically layered structure and its phase-change characteristics (semiconductor to metallic trait) circumvent the issues of high impedance and/ or poor interfacial-contact. Also, the enhanced conductivity of the MoS₂ interlayer eliminates the preferential sites for Li dendrite nucleation. With the use of a MoS₂ passivated Li-metal anode and a CNT–S composite cathode to encapsulate soluble polysulfides, it is possible to realize high-performing Li–S batteries.

—Cha et al.

Dr. Kyeongjae “K.J.” Cho, professor of materials science and engineering, along with research associate Dr. Jeongwoon Hwang, both of the Erik Jonsson School of Engineering and Computer Science, worked with other regional scientists to improve lithium-sulfur batteries.

Lithium-sulfur batteries have important advantages over lithium-ion batteries; they are less expensive to make, weigh less, store almost twice the energy of lithium-ion batteries and are better for the environment. However, sulfur is a poor electrical conductor and can become unstable over just several charge-and-recharge cycles. Electrodes breaking down is another reason lithium-sulfur batteries aren’t yet mainstream.

Molybdenum, a metallic element often used to strengthen and harden steel, creates a material that adjusts the thickness of the coating when combined with two atoms of sulfur. Cho and his colleagues found it improved stability and compensated for poor conductivity of sulfur, thus allowing for greater power density and making lithium-sulfur batteries more commercially viable.

The full-cell assembly of Li–S batteries with a Li–MoS₂ anode and a CNT–S cathode demonstrates high specific energy and power densities of 589 Wh kg⁻¹ and 295 W kg⁻¹, respectively; it also exhibits a high capacity retention of 84% for up to 1,200 cycles. These results indicate that the major challenge encountered in the development of Li–S batteries can be effectively resolved. We believe that this strategy can be applied with other 2D materials to provide a route for the development of high-performance Li-metal based rechargeable batteries.Cha et al.