Layered materials consist of molecular layers stacked together by weak interlayer interactions. They often crystallize to form atomically smooth thin films, nanotubes, and platelet or fullerene-like nanoparticles due to the anisotropic bonding. Structures that predominately expose edges of the layers exhibit high surface energy and are often considered unstable. In this communication, we present a synthesis process to grow MoS2 and MoSe2 thin films with vertically aligned layers, thereby maximally exposing the edges on the film surface. Such edge-terminated films are metastable structures of MoS2 and MoSe2, which may find applications in diverse catalytic reactions. We have confirmed their catalytic activity in a hydrogen evolution reaction (HER), in which the exchange current density correlates directly with the density of the exposed edge sites.

Silicon is an attractive material for anodes in energy storage devices, because it has ten times the theoretical capacity of its state-of-the-art carbonaceous counterpart. Silicon anodes can be used both in traditional lithium-ion batteries and in more recent Li-O2 and Li-S batteries as a replacement for the dendrite-forming lithium metal anodes. The main challenges associated with silicon anodes are structural degradation and instability of the solid-electrolyte interphase caused by the large volume change (∼300%) during cycling, the occurrence of side reactions with the electrolyte, and the low volumetric capacity when the material size is reduced to a nanometre scale. Here, we propose a hierarchical structured silicon anode that tackles all three of these problems. Our design is inspired by the structure of a pomegranate, where single silicon nanoparticles are encapsulated by a conductive carbon layer that leaves enough room for expansion and contraction following lithiation and delithiation. An ensemble of these hybrid nanoparticles is then encapsulated by a thicker carbon layer in micrometre-size pouches to act as an electrolyte barrier. As a result of this hierarchical arrangement, the solid-electrolyte interphase remains stable and spatially confined, resulting in superior cyclability (97% capacity retention after 1,000 cycles). In addition, the microstructures lower the electrode-electrolyte contact area, resulting in high Coulombic efficiency (99.87%) and volumetric capacity (1,270 mAh cm(-3)), and the cycling remains stable even when the areal capacity is increased to the level of commercial lithium-ion batteries (3.7 mAh cm(-2)).

Silicon is regarded as one of the most promising anode materials for next generation lithium-ion batteries. For use in practical applications, a Si electrode must have high capacity, long cycle life, high efficiency, and the fabrication must be industrially scalable. Here, we design and fabricate a yolk-shell structure to meet all these needs. The fabrication is carried out without special equipment and mostly at room temperature. Commercially available Si nanoparticles are completely sealed inside conformal, thin, self-supporting carbon shells, with rationally designed void space in between the particles and the shell. The well-defined void space allows the Si particles to expand freely without breaking the outer carbon shell, therefore stabilizing the solid-electrolyte interphase on the shell surface. High capacity (∼2800 mAh/g at C/10), long cycle life (1000 cycles with 74% capacity retention), and high Coulombic efficiency (99.84%) have been realized in this yolk-shell structured Si electrode. KEYWORDS: Silicon nanoparticle, Li-ion battery, anode, yolk-shell, solid-electrolyte interphase, in situ TEM E lectrochemical energy storage has become a critical technology for a variety of applications, including grid storage, electric vehicles, and portable electronic devices. The lithium-ion battery (LIB) is an attractive energy storage device because of its relatively high energy density and good rate capability. To further increase the energy density for more demanding applications, however, new electrode materials with higher specific and volumetric capacity are required. Since the initial commercialization of the LIB two decades ago, there has been little progress in commercializing new electrode materials with significantly higher capacity. 1 To meet the increasing demand for energy storage capability, novel electrode materials with higher capacity, low cost, and the ability to be produced at large scale are of great interest.Alloy-type anodes (Si, Ge, Sn, Al, Sb, etc.) have much higher Li storage capacity than the intercalation-type graphite anode that is currently used in Li-ion batteries. 2 Among all the alloy anodes, silicon has the highest specific capacity: Experiments have demonstrated an initial specific capacity of >3500 mAh/g, which is 10 times the capacity of graphite. 3 In addition, silicon is the second most abundant element in the earth's crust (28% by mass), indicating its potential to be utilized in large quantities at low cost. 4 A further benefit is that mass production of elemental silicon is already a mature technology in the semiconductor industry. Despite these advantages, graphite anodes still dominate the marketplace due to the fact that alloy anodes have two major challenges that have prevented their widespread use. First, alloy anodes undergo significant volume expansion and contraction during Li insertion/extraction. 2 This volume change (∼300% for Si) can result in pulverization of the initial particle morphology and causes the loss of electrical contact between active mat...

Rechargeable Li/S batteries have attracted significant attention lately due to their high specific energy and low cost. They are promising candidates for applications, including portable electronics, electric vehicles and grid-level energy storage. However, poor cycle life and low power capability are major technical obstacles. Various nanostructured sulfur cathodes have been developed to address these issues, as they provide greater resistance to pulverization, faster reaction kinetics and better trapping of soluble polysulfides. In this review, recent developments on nanostructured sulfur cathodes and mechanisms behind their operation are presented and discussed. Moreover, progress on novel characterization of sulfur cathodes is also summarized, as it has deepened the understanding of sulfur cathodes and will guide further rational design of sulfur electrodes.

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Due to their high energy density and low material cost, lithium-sulfur batteries represent a promising energy storage system for a multitude of emerging applications, ranging from stationary grid storage to mobile electric vehicles. This review aims to summarize major developments in the field of lithium-sulfur batteries, starting from an overview of their electrochemistry, technical challenges and potential solutions, along with some theoretical calculation results to advance our understanding of the material interactions involved. Next, we examine the most extensively-used design strategy: encapsulation of sulfur cathodes in carbon host materials. Other emerging host materials, such as polymeric and inorganic materials, are discussed as well. This is followed by a survey of novel battery configurations, including the use of lithium sulfide cathodes and lithium polysulfide catholytes, as well as recent burgeoning efforts in the modification of separators and protection of lithium metal anodes. Finally, we conclude with an outlook section to offer some insight on the future directions and prospects of lithium-sulfur batteries.

We developed two-step solution-phase reactions to form hybrid materials of Mn(3)O(4) nanoparticles on reduced graphene oxide (RGO) sheets for lithium ion battery applications. Selective growth of Mn(3)O(4) nanoparticles on RGO sheets, in contrast to free particle growth in solution, allowed for the electrically insulating Mn(3)O(4) nanoparticles to be wired up to a current collector through the underlying conducting graphene network. The Mn(3)O(4) nanoparticles formed on RGO show a high specific capacity up to ∼900 mAh/g, near their theoretical capacity, with good rate capability and cycling stability, owing to the intimate interactions between the graphene substrates and the Mn(3)O(4) nanoparticles grown atop. The Mn(3)O(4)/RGO hybrid could be a promising candidate material for a high-capacity, low-cost, and environmentally friendly anode for lithium ion batteries. Our growth-on-graphene approach should offer a new technique for the design and synthesis of battery electrodes based on highly insulating materials.

Lithium metal has re-emerged as an exciting anode for high energy lithium-ion batteries due to its high specific capacity of 3860 mAh g and lowest electrochemical potential of all known materials. However, lithium has been plagued by the issues of dendrite formation, high chemical reactivity with electrolyte, and infinite relative volume expansion during plating and stripping, which present safety hazards and low cycling efficiency in batteries with lithium metal electrodes. There have been a lot of recent studies on Li metal although little work has focused on the initial nucleation and growth behavior of Li metal, neglecting a critical fundamental scientific foundation of Li plating. Here, we study experimentally the morphology of lithium in the early stages of nucleation and growth on planar copper electrodes in liquid organic electrolyte. We elucidate the dependence of lithium nuclei size, shape, and areal density on current rate, consistent with classical nucleation and growth theory. We found that the nuclei size is proportional to the inverse of overpotential and the number density of nuclei is proportional to the cubic power of overpotential. Based on this understanding, we propose a strategy to increase the uniformity of electrodeposited lithium on the electrode surface.