Yuguang C. Li

Fast and Efficient Preparation of Exfoliated 2H MoS2 Nanosheets by Sonication-Assisted Lithium Intercalation and Infrared Laser-Induced 1T to 2H Phase Reversion.

Exfoliated 2H molybdenum disulfide (MoS2) has unique properties and potential applications in a wide range of fields, but corresponding studies have been hampered by the lack of effective routes to it in bulk quantities. This study presents a rapid and efficient route to obtain exfoliated 2H MoS2, which combines fast sonication-assisted lithium intercalation and infrared (IR) laser-induced phase reversion. We found that the complete lithium intercalation of MoS2 with butyllithium could be effected within 1.5 h with the aid of sonication. The 2H to 1T phase transition that occurs during the lithium intercalation could be also reversed by IR laser irradiation with a DVD optical drive.

Controlled Exfoliation of MoS2 Crystals into Trilayer Nanosheets

The controlled exfoliation of transition metal dichalcogenides (TMDs) into pristine single- or few-layer nanosheets remains a significant barrier to fundamental studies and device applications of TMDs. Here we report a novel strategy for exfoliating crystalline MoS2 into suspensions of nanosheets with retention of the semiconducting 2H phase. The controlled reaction of MoS2 with substoichiometric amounts n-butyllithium results in intercalation of the edges of the crystals, which are then readily exfoliated in a 45 vol % ethanol-water solution. Surprisingly, the resulting colloidal suspension of nanosheets was found (by electron microscopy and atomic force microscopy) to consist mostly of trilayers. The efficiency of exfoliation of the pre-intercalated sample is increased by at least 1 order of magnitude relative to the starting MoS2 microcrystals, with a mass yield of the dispersed nanosheets of 11-15%.

Interfacial Chemistry Regulation via a Skin-Grafting Strategy Enables High-Performance Lithium-Metal Batteries

The lithium (Li) metal anode suffers severe interfacial instability from its high reactivity toward liquid electrolytes, especially carbonate-based electrolytes, resulting in poor electrochemical performance of batteries that use 4 V high-capacity cathodes. We report a new skin-grafting strategy that stabilizes the Li metal-liquid electrolyte interface by coating the Li metal surface with poly((N-2,2-dimethyl-1,3-dioxolane-4-methyl)-5-norbornene-exo-2,3-dicarboximide), a chemically and electrochemically active polymer layer. This layer, composed of cyclic ether groups with a stiff polycyclic main chain, serves as a grafted polymer skin on the Li metal anode not only to incorporate ether-based polymeric components into the solid-electrolyte interphase (SEI) but also to accommodate Li deposition/dissolution under the skin in a dendrite/moss-free manner. Consequently, a Li-metal battery employing a Li metal anode with the grafted skin paired with LiNi0.5Co0.2Mn0.3O2 cathode has a 90.0% capacity retention after 400 charge/discharge cycles and a capacity of 1.2 mAh/cm2 in a carbonate-based electrolyte. This proof-of-concept study provides a new direction for regulating the interfacial chemistry of Li metal anodes and for enabling high-performance Li-metal batteries.

General Method of Manipulating Formation, Composition, and Morphology of Solid-Electrolyte Interphases for Stable Li-Alloy Anodes

Li-alloy-based anode materials are very promising for breaking current energy limits of lithium-ion battery technologies. Unfortunately, these materials still suffer from poor solid-electrolyte interphase (SEI) stability, resulting in unsatisfied electrochemical performances. The typical SEI formation method, electrochemical decomposition of electrolytes onto the active material surface, lacks a deliberate control of the SEI functions and structures. Here we propose a general method of manipulating the formation process, chemical composition, and morphology of the SEI for Li-alloy anodes, using Si and Ge nanoparticle anodes as the platform. The SEI was fabricated through a covalent anchoring of multiple functional components onto the active material surface, followed by electrochemical decomposition of the functional components and conventional electrolyte. Click reaction, serving as the covalent anchoring approach, allows an accurate control of the SEI composition and structure at the molecular level through tuning the chemical structure and amount of variety of functional components and provides an intimate contact between the SEI and the Li-alloy material surface contributed by the covalent bonding. The optimized Si nanoparticle SEI, functionalized by a unique combination of diverse components and containing a high concentration of organic components attributed to the preanchored functional components, presented a stable composition and durable morphology during cycling and led to an improved first cycle efficiency of Si nanoparticle anodes and its long cycle life in a full cell. This general method displays potential benefits to construct stable SEIs for other Li-alloy anodes.

Light-Ultrasound Driven Collective “Firework” Behavior of Nanomotors

Abstract It is of great interest and big challenge to control the collective behaviors of nanomotors to mimic the aggregation/separation behavior of biological systems. Here, a light‐acoustic combined method is proposed to control the aggregation/separation of artificial nanomotors. It is shown that nanomotors aggregate at the pressure node in acoustic field and afterward present a collective “firework” separation behavior induced by light irradiation. The collective behavior is found to be applicable for metallic materials and polymers even different light wavelengths are used. Physical insights on the collective firework behavior resulting from the change of acoustic streaming caused by optical force are provided. It is found that diffusion velocity and diffusion region of cluster can be controlled by adjusting light intensity and acoustic excitation voltage, and irradiation direction, respectively. This harmless, controllable, and widely applicable method provides new possibilities for groups of nanomachines, drug release, and cargo transport in nanomedicine and nanosensors.

The balance of electric field and interfacial catalysis in promoting water dissociation in bipolar membranes

The lamination of a cation exchange layer (CEL) and an anion exchange layer (AEL) to form a hybrid bipolar membrane (BPM) can have several unique advantages over conventional monopolar ion exchange membranes in (photo-)electrolysis. Upon application of a reverse bias, the ordinarily slow water dissociation reaction at the AEL/CEL junction of the BPM is dramatically accelerated by the large electric field at the interface and by the presence of catalyst in the junction. Using electrochemical impedance spectroscopy (EIS), we have found a counterbalanced role of the electric field and the junction catalyst in accelerating water dissociation in a BPM. Experimental BPMs were prepared from a crosslinked AEL and a Nafion CEL, with a graphite oxide (GO) catalyst deposited at the junction using layer-by-layer (LBL) assembly. BPMs with an interfacial catalyst layer were found to have smaller electric fields at the interface compared to samples with no added catalyst. A comprehensive numerical simulation model showed that the damping of the electric field in BPMs with a catalyst layer is a result of a higher water dissociation product (H+/OH−) flux, which neutralizes the net charge density of the CEL and AEL. This conclusion is further substantiated by EIS studies of a high-performance 3D junction BPM that shows a low electric field due to the facile catalytic generation and transport of H+ and OH−. Numerical modeling of these effects in the BPM provides a prescription for designing membranes that function at lower overpotential. The potential drop across the synthetic BPM was lower than that of a commercial BPM by more than 200 mV at >100 mA cm−2 reverse bias current density, with the two membranes having similar long-term stability.