Asymmetric 2D MoS2 for Scalable and High-Performance Piezoelectric Sensors.

Piezoelectricity in two-dimensional (2D) transition-metal dichalcogenides (TMDs) has attracted significant attention due to their unique crystal structure and the lack of inversion centers when the bulk TMDs thin down to monolayers. Although the piezoelectric effect in atomic-thickness TMDs has been reported earlier, they are exfoliated 2D TMDs and are therefore not scalable. Here, we demonstrate a superior piezoelectric effect from large-scale sputtered, asymmetric 2D MoS2 using meticulous defect engineering based on the thermal-solvent annealing of the MoS2 layer. This yields an output peak current and voltage of 20 pA and 700 mV (after annealing at 450 °C), respectively, which is the highest piezoelectric strength ever reported in 2D MoS2. Indeed, the piezoelectric strength increases with the defect density (sulfur vacancies), which, in turn, increases with the annealing temperature at least up to 450 °C. Moreover, our piezoelectric MoS2 device array shows an exceptional piezoelectric sensitivity of 262 mV/kPa with a high level of uniformity and excellent performance under ambient conditions. A detailed study of the sulfur vacancy-dependent property and its resultant asymmetric structure-induced piezoelectricity is reported. The proposed approach is scalable and can produce advanced materials for flexible piezoelectric devices to be used in emerging bioinspired robotics and biomedical applications.

Nanoengineering to achieve high efficiency practical lithium-sulfur batteries.

Rapidly increasing markets for electric vehicles (EVs), energy storage for backup support systems and high-power portable electronics demand batteries with higher energy densities and longer cycle lives. Among the various electrochemical energy storage systems, lithium-sulfur (Li-S) batteries have the potential to become the next generation rechargeable batteries because of their high specific energy at low cost. However, the development of practical Li-S batteries for commercial products has been challenged by several obstacles, including unstable cycle life and low sulfur utilization. Only a few studies have considered the importance of low electrolyte and high sulfur loading to improve the overall energy densities of Li-S cells. This article reviews the recent developments of Li-S batteries that can meet the benchmarks of practical parameters and exceed the practical energy density of lithium-ion batteries (LIBs) including areal sulfur loading of at least 4 mg cm-2, electrolyte to sulfur ratio of less than 10 µL mg-1, and high cycling stability of over 300 cycles. This review presents the advancements in each component in Li-S batteries, including the enhancement of the electrochemical properties of sulfur cathodes, lithium anodes, or electrolytes. Also identified are several important strategies of nanoengineering and how they address the practical limitations of Li-S batteries to compete against LIBs. Additionally, perspectives on fundamentals, technology, and materials are provided for the development of Li-S batteries based on nanomaterials and nanoengineering so that they can enter the market of high energy density rechargeable storage systems.

Publisher Correction: 2D MoS2 as an efficient protective layer for lithium metal anodes in high-performance Li-S batteries.

In the version of this Article originally published, a technical error in typesetting led to the traces in Fig. 3a being trimmed and made to overlap. The figure has now been corrected with the traces as supplied by the authors; the original and corrected Fig. 3a are shown below. Also, in the last paragraph of the section "Mechanistic study on Li diffusion in MoS2" the authors incorrectly included the term 'high-concentration' in the text "the Li diffusion will be dominated by high-concentration Li migration on the surface of T-MoS2 with a much smaller energy barrier (0.155 eV) to overcome". This term has now been removed from all versions of the Article. Finally, the authors have added an extra figure in the Supplementary Information (Supplementary Fig. 19) to show galvanostatic tests at 1 and 3 mA cm-2 for the MoS2-coated Li symmetric cells. The caption to Fig. 3 of the Article has been amended to reflect this, with the added wording "Galvanostatic tests at 1 and 3 mA cm-2 can be found in Supplementary Fig. 19."

2D MoS2 as an efficient protective layer for lithium metal anodes in high-performance Li-S batteries.

Among the candidates to replace Li-ion batteries, Li-S cells are an attractive option as their energy density is about five times higher (~2,600 Wh kg-1). The success of Li-S cells depends in large part on the utilization of metallic Li as anode material. Metallic lithium, however, is prone to grow parasitic dendrites and is highly reactive to several electrolytes; moreover, Li-S cells with metallic Li are also susceptible to polysulfides dissolution. Here, we show that ~10-nm-thick two-dimensional (2D) MoS2 can act as a protective layer for Li-metal anodes, greatly improving the performances of Li-S batteries. In particular, we observe 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. In a Li-S full-cell configuration, using the MoS2-coated Li as anode and a 3D carbon nanotube-sulfur cathode, we obtain a specific energy density of ~589 Wh kg-1 and a Coulombic efficiency of ~98% for over 1,200 cycles at 0.5 C. Our approach could lead to the realization of high energy density and safe Li-metal-based batteries.