Showing papers by "Yi Cui published in 2015"

The synergetic effect of lithium polysulfide and lithium nitrate to prevent lithium dendrite growth

Abstract: Lithium dendrite growth is a serious hazard in battery operations. Here, the authors show that when using lithium polysulfide and lithium nitrate as additives in ether-based electrolyte, a stable and uniform solid electrolyte interphase forms on the lithium surface, which prevents dendrite growth.

A phosphorene–graphene hybrid material as a high-capacity anode for sodium-ion batteries

Abstract: Sodium-ion batteries have recently attracted significant attention as an alternative to lithium-ion batteries because sodium sources do not present the geopolitical issues that lithium sources might. Although recent reports on cathode materials for sodium-ion batteries have demonstrated performances comparable to their lithium-ion counterparts, the major scientific challenge for a competitive sodium-ion battery technology is to develop viable anode materials. Here we show that a hybrid material made out of a few phosphorene layers sandwiched between graphene layers shows a specific capacity of 2,440 mA h g−1 (calculated using the mass of phosphorus only) at a current density of 0.05 A g−1 and an 83% capacity retention after 100 cycles while operating between 0 and 1.5 V. Using in situ transmission electron microscopy and ex situ X-ray diffraction techniques, we explain the large capacity of our anode through a dual mechanism of intercalation of sodium ions along the x axis of the phosphorene layers followed by the formation of a Na3P alloy. The presence of graphene layers in the hybrid material works as a mechanical backbone and an electrical highway, ensuring that a suitable elastic buffer space accommodates the anisotropic expansion of phosphorene layers along the y and z axial directions for stable cycling operation. The sodiation–desodiation properties of few-layer phosphorene are mostly preserved by sandwiching the material between graphene layers, a behaviour that makes phosphorene–graphene hybrids a potentially suitable anode material for sodium-ion batteries.

Polarization-sensitive broadband photodetector using a black phosphorus vertical p–n junction

Abstract: The ability to detect light over a broad spectral range is central to practical optoelectronic applications and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here, we demonstrate a broadband photodetector using a layered black phosphorus transistor that is polarization-sensitive over a bandwidth from ∼400 nm to 3,750 nm. The polarization sensitivity is due to the strong intrinsic linear dichroism, which arises from the in-plane optical anisotropy of this material. In this transistor geometry, a perpendicular built-in electric field induced by gating can spatially separate the photogenerated electrons and holes in the channel, effectively reducing their recombination rate and thus enhancing the performance for linear dichroism photodetection. The use of anisotropic layered black phosphorus in polarization-sensitive photodetection might provide new functionalities in novel optical and optoelectronic device applications. The anisotropic optical properties of black phosphorus can be exploited to fabricate photodetectors with linear dichroism operating over a broad spectral range.

Transparent air filter for high-efficiency PM2.5 capture

Abstract: Particulate matter pollution is a public health concern in industrialized and urban areas. Here, the authors control the surface chemistry and microstructure of filtration materials to fabricate effective and transparent air filters for the capture of PM2.5 pollutants.

Understanding the Anchoring Effect of Two-Dimensional Layered Materials for Lithium–Sulfur Batteries

Abstract: Although the rechargeable lithium–sulfur battery system has attracted significant attention due to its high theoretical specific energy, its implementation has been impeded by multiple challenges, especially the dissolution of intermediate lithium polysulfide (Li2Sn) species into the electrolyte. Introducing anchoring materials, which can induce strong binding interaction with Li2Sn species, has been demonstrated as an effective way to overcome this problem and achieve long-term cycling stability and high-rate performance. The interaction between Li2Sn species and anchoring materials should be studied at the atomic level in order to understand the mechanism behind the anchoring effect and to identify ideal anchoring materials to further improve the performance of Li–S batteries. Using first-principles approach with van der Waals interaction included, we systematically investigate the adsorption of Li2Sn species on various two-dimensional layered materials (oxides, sulfides, and chlorides) and study the de...

Ionic Conductivity Enhancement of Polymer Electrolytes with Ceramic Nanowire Fillers

Abstract: Solid-state electrolytes provide substantial improvements to safety and electrochemical stability in lithium-ion batteries when compared with conventional liquid electrolytes, which makes them a promising alternative technology for next-generation high-energy batteries. Currently, the low mobility of lithium ions in solid electrolytes limits their practical application. The ongoing research over the past few decades on dispersing of ceramic nanoparticles into polymer matrix has been proved effective to enhance ionic conductivity although it is challenging to form the efficiency networks of ionic conduction with nanoparticles. In this work, we first report that ceramic nanowire fillers can facilitate formation of such ionic conduction networks in polymer-based solid electrolyte to enhance its ionic conductivity by three orders of magnitude. Polyacrylonitrile-LiClO4 incorporated with 15 wt % Li0.33La0.557TiO3 nanowire composite electrolyte exhibits an unprecedented ionic conductivity of 2.4 × 10–4 S cm–1 at...

A Highly Reversible Room-Temperature Sodium Metal Anode.

Abstract: Owing to its low cost and high natural abundance, sodium metal is among the most promising anode materials for energy storage technologies beyond lithium ion batteries. However, room-temperature sodium metal anodes suffer from poor reversibility during long-term plating and stripping, mainly due to formation of nonuniform solid electrolyte interphase as well as dendritic growth of sodium metal. Herein we report for the first time that a simple liquid electrolyte, sodium hexafluorophosphate in glymes (mono-, di-, and tetraglyme), can enable highly reversible and nondendritic plating–stripping of sodium metal anodes at room temperature. High average Coulombic efficiencies of 99.9% were achieved over 300 plating–stripping cycles at 0.5 mA cm–2. The long-term reversibility was found to arise from the formation of a uniform, inorganic solid electrolyte interphase made of sodium oxide and sodium fluoride, which is highly impermeable to electrolyte solvent and conducive to nondendritic growth. As a proof of conc...

Physical and chemical tuning of two-dimensional transition metal dichalcogenides

Abstract: The development of two-dimensional (2D) materials has been experiencing a renaissance since the adventure of graphene. Layered transition metal dichalcogenides (TMDs) are now playing increasingly important roles in both fundamental studies and technological applications due to their wide range of material properties from semiconductors, metals to superconductors. However, a material with fixed properties may not exhibit versatile applications. Due to the unique crystal structures, the physical and chemical properties of 2D TMDs can be effectively tuned through different strategies such as reducing dimensions, intercalation, heterostructure, alloying, and gating. With the flexible tuning of properties 2D TMDs become attractive candidates for a variety of applications including electronics, optoelectronics, catalysis, and energy.

Integrated digital inverters based on two-dimensional anisotropic ReS2 field-effect transistors.

Abstract: Many two-dimensional materials exhibit isotropic properties, but anisotropy can extend the functionality of future devices. Here, the authors fabricate field-effect transistors from single and few-layer rhenium disulfide and observe an anisotropic ratio of three to one along the two principle axes

Transition-metal Doped Edge Sites in Vertically Aligned MoS2 Catalysts for Enhanced Hydrogen Evolution

Abstract: Highly active and low-cost catalysts for electrochemical reactions such as the hydrogen evolution reaction (HER) are crucial for the development of efficient energy conversion and storage technologies. Theoretical simulations have been instrumental in revealing the correlations between the electronic structure of materials and their catalytic activity, and guide the prediction and development of improved catalysts. However, difficulties in accurately engineering the desired atomic sites lead to challenges in making direct comparisons between experimental and theoretical results. In MoS2, the Mo-edge has been demonstrated to be active for HER whereas the S-edge is inert. Using a computational descriptor-based approach, we predict that by incorporating transition metal atoms (Fe, Co, Ni, or Cu) the S-edge site should also become HER active. Vertically standing, edge-terminated MoS2 nanofilms provide a well-defined model system for verifying these predictions. The transition metal doped MoS2 nanofilms show an increase in exchange current densities by at least two-fold, in agreement with the theoretical calculations. This work opens up further opportunities for improving electrochemical catalysts by incorporating promoters into particular atomic sites, and for using well-defined systems in order to understand the origin of the promotion effects.

Polymer Nanofiber-Guided Uniform Lithium Deposition for Battery Electrodes

Abstract: Lithium metal is one of the most promising candidates as an anode material for next-generation energy storage systems due to its highest specific capacity (3860 mAh/g) and lowest redox potential of all. The uncontrolled lithium dendrite growth that causes a poor cycling performance and serious safety hazards, however, presents a significant challenge for the realization of lithium metal-based batteries. Here, we demonstrate a novel electrode design by placing a three-dimensional (3D) oxidized polyacrylonitrile nanofiber network on top of the current collector. The polymer fiber with polar surface functional groups could guide the lithium ions to form uniform lithium metal deposits confined on the polymer fiber surface and in the 3D polymer layer. We showed stable cycling of lithium metal anode with an average Coulombic efficiency of 97.4% over 120 cycles in ether-based electrolyte at a current density of 3 mA/cm2 for a total of 1 mAh/cm2 of lithium.

Nonfilling Carbon Coating of Porous Silicon Micrometer-Sized Particles for High-Performance Lithium Battery Anodes

Abstract: Silicon is widely recognized as one of the most promising anode materials for lithium-ion batteries due to its 10 times higher specific capacity than graphite. Unfortunately, the large volume change of Si materials during their lithiation/delithiation process results in severe pulverization, loss of electrical contact, unstable solid–electrolyte interphase (SEI), and eventual capacity fading. Although there has been tremendous progress to overcome these issues through nanoscale materials design, improved volumetric capacity and reduced cost are still needed for practical application. To address these issues, we design a nonfilling carbon-coated porous silicon microparticle (nC-pSiMP). In this structure, porous silicon microparticles (pSiMPs) consist of many interconnected primary silicon nanoparticles; only the outer surface of the pSiMPs was coated with carbon, leaving the interior pore structures unfilled. Nonfilling carbon coating hinders electrolyte penetration into the nC-pSiMPs, minimizes the electr...

Roll-to-Roll Encapsulation of Metal Nanowires between Graphene and Plastic Substrate for High-Performance Flexible Transparent Electrodes

Abstract: Transparent conductive film on plastic substrate is a critical component in low-cost, flexible, and lightweight optoelectronics. Industrial-scale manufacturing of high-performance transparent conductive flexible plastic is needed to enable wide-ranging applications. Here, we demonstrate a continuous roll-to-roll (R2R) production of transparent conductive flexible plastic based on a metal nanowire network fully encapsulated between graphene monolayer and plastic substrate. Large-area graphene film grown on Cu foil via a R2R chemical vapor deposition process was hot-laminated onto nanowires precoated EVA/PET film, followed by a R2R electrochemical delamination that preserves the Cu foil for reuse. The encapsulated structure minimized the resistance of both wire-to-wire junctions and graphene grain boundaries and strengthened adhesion of nanowires and graphene to plastic substrate, resulting in superior optoelectronic properties (sheet resistance of ∼8 Ω sq–1 at 94% transmittance), remarkable corrosion resis...

A high tap density secondary silicon particle anode fabricated by scalable mechanical pressing for lithium-ion batteries

Abstract: Much progress has been made in developing high capacity lithium ion battery electrode materials such as silicon anodes. With the powerful nanomaterial design approach, cycle life of silicon anodes has been increased significantly. However, nanomaterials have three major issues to be addressed, including severe side reactions due to a large surface area, low tap density and poor scalability. Nanostructured Si secondary clusters (nano-Si SC) are promising for reducing side reactions and increasing tap density, yet the scalability and tap density could still be further improved. Here, we propose a mechanical approach for SC fabrication to address all the problems. With the mechanical approach, >20 g of nano-Si SC per batch was produced even at our university lab scale, with >95% yield. Moreover, much denser packing of nanostructures can be achieved (1.38 g cm−3, pellet form), which gives much higher tap density (0.91 g cm−3, powder form) and better electrical contact. Accordingly, over 95% of initial capacity is retained after 1400 cycles at 1C, with an average specific capacity of ∼1250 mA h g−1. Stable cycling with >2 mg cm−2 of areal mass loading (∼3.5 mA h cm−2) is obtained. After uniformly integrating carbon nanotubes (CNTs) into SCs, intracluster electrical conductivity is further improved. As a result, notably enhanced rate capability is attained, with a high reversible specific capacity of ∼1140 mA h g−1 and ∼880 mA h g−1 at 2C and 4C, respectively.

Personal Thermal Management by Metallic Nanowire-Coated Textile

Abstract: Heating consumes large amount of energy and is a primary source of greenhouse gas emission. Although energy-efficient buildings are developing quickly based on improving insulation and design, a large portion of energy continues to be wasted on heating empty space and nonhuman objects. Here, we demonstrate a system of personal thermal management using metallic nanowire-embedded cloth that can reduce this waste. The metallic nanowires form a conductive network that not only is highly thermal insulating because it reflects human body infrared radiation but also allows Joule heating to complement the passive insulation. The breathability and durability of the original cloth is not sacrificed because of the nanowires’ porous structure. This nanowire cloth can efficiently warm human bodies and save hundreds of watts per person as compared to traditional indoor heaters.

In Situ Electrochemical Oxidation Tuning of Transition Metal Disulfides to Oxides for Enhanced Water Oxidation

Abstract: The development of catalysts with earth-abundant elements for efficient oxygen evolution reactions is of paramount significance for clean and sustainable energy storage and conversion devices. Our group demonstrated recently that the electrochemical tuning of catalysts via lithium insertion and extraction has emerged as a powerful approach to improve catalytic activity. Here we report a novel in situ electrochemical oxidation tuning approach to develop a series of binary, ternary, and quaternary transition metal (e.g., Co, Ni, Fe) oxides from their corresponding sulfides as highly active catalysts for much enhanced water oxidation. The electrochemically tuned cobalt–nickel–iron oxides grown directly on the three-dimensional carbon fiber electrodes exhibit a low overpotential of 232 mV at current density of 10 mA cm–2, small Tafel slope of 37.6 mV dec–1, and exceptional long-term stability of electrolysis for over 100 h in 1 M KOH alkaline medium, superior to most non-noble oxygen evolution catalysts repor...

Artificial Solid Electrolyte Interphase-Protected LixSi Nanoparticles: An Efficient and Stable Prelithiation Reagent for Lithium-Ion Batteries.

Abstract: Prelithiation is an important strategy to compensate for lithium loss in lithium-ion batteries, particularly during the formation of the solid electrolyte interphase (SEI) from reduced electrolytes in the first charging cycle. We recently demonstrated that LixSi nanoparticles (NPs) synthesized by thermal alloying can serve as a high-capacity prelithiation reagent, although their chemical stability in the battery processing environment remained to be improved. Here we successfully developed a surface modification method to enhance the stability of LixSi NPs by exploiting the reduction of 1-fluorodecane on the LixSi surface to form a continuous and dense coating through a reaction process similar to SEI formation. The coating, consisting of LiF and lithium alkyl carbonate with long hydrophobic carbon chains, serves as an effective passivation layer in the ambient environment. Remarkably, artificial-SEI-protected LixSi NPs show a high prelithiation capacity of 2100 mA h g–1 with negligible capacity decay in ...

Li Intercalation in MoS2: In Situ Observation of Its Dynamics and Tuning Optical and Electrical Properties

Abstract: Two-dimensional layered materials like MoS2 have shown promise for nanoelectronics and energy storage, both as monolayers and as bulk van der Waals crystals with tunable properties. Here we present a platform to tune the physical and chemical properties of nanoscale MoS2 by electrochemically inserting a foreign species (Li+ ions) into their interlayer spacing. We discover substantial enhancement of light transmission (up to 90% in 4 nm thick lithiated MoS2) and electrical conductivity (more than 200×) in ultrathin (∼2–50 nm) MoS2 nanosheets after Li intercalation due to changes in band structure that reduce absorption upon intercalation and the injection of large amounts of free carriers. We also capture the first in situ optical observations of Li intercalation in MoS2 nanosheets, shedding light on the dynamics of the intercalation process and the associated spatial inhomogeneity and cycling-induced structural defects.

Self-assembled three-dimensional and compressible interdigitated thin-film supercapacitors and batteries

Abstract: Traditional thin-film energy-storage devices consist of stacked layers of active films on two-dimensional substrates and do not exploit the third dimension. Fully three-dimensional thin-film device ...

Design and fabrication of bioelectrodes for microbial bioelectrochemical systems

Abstract: Microbial bioelectrochemical systems (BESs) interconvert electrical and chemical energy, enabling electricity generation, hydrogen production, chemical synthesis, wastewater treatment, desalination, and remediation. The focus of this review is design of bioelectrodes for BESs. Desirable features are high conductivity, stability, and biocompatibility. We trace the history of bioelectrode design from nonporous designs to modern porous designs that are particle-based, fiber-based, or monolithic, and compare performance characteristics. The most promising strategies use porous structures conducive to microbial colonization and surface materials that promote efficient electron transfer.

High‐Areal‐Capacity Silicon Electrodes with Low‐Cost Silicon Particles Based on Spatial Control of Self‐Healing Binder

Abstract: Dr. Z. Chen, Dr. C. Wang, J. Lopez, Prof. Z. Bao Department of Chemical Engineering Stanford University Stanford , CA 94305 , USA E-mail: zbao@stanford.edu Dr. Z. Lu, Prof. Y. Cui Department of Materials Science and Engineering Stanford University Stanford , CA 94305 , USA E-mail: yicui@stanford.edu Prof. Y. Cui Stanford Institute for Materials and Energy Sciences SLAC National Accelerator Laboratory Menlo Park , CA 94205 , USA

Ultrahigh surface area three-dimensional porous graphitic carbon from conjugated polymeric molecular framework

Abstract: Porous graphitic carbon is essential for many applications such as energy storage devices, catalysts, and sorbents. However, current graphitic carbons are limited by low conductivity, low surface area, and ineffective pore structure. Here we report a scalable synthesis of porous graphitic carbons using a conjugated polymeric molecular framework as precursor. The multivalent cross-linker and rigid conjugated framework help to maintain micro- and mesoporous structures, while promoting graphitization during carbonization and chemical activation. The above unique design results in a class of highly graphitic carbons at temperature as low as 800 °C with record-high surface area (4073 m2 g–1), large pore volume (2.26 cm–3), and hierarchical pore architecture. Such carbons simultaneously exhibit electrical conductivity >3 times more than activated carbons, very high electrochemical activity at high mass loading, and high stability, as demonstrated by supercapacitors and lithium–sulfur batteries with excellent pe...

Pressure induced metallization with absence of structural transition in layered molybdenum diselenide

Abstract: Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ∼ 60 GPa using multiple experimental techniques and ab-initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.

Vertical Heterostructure of Two-Dimensional MoS2 and WSe2 with Vertically Aligned Layers

Abstract: Two-dimensional (2D) layered materials consist of covalently bonded 2D atomic layers stacked by van der Waals interactions. Such anisotropic bonding nature gives rise to the orientation-dependent functionalities of the 2D layered materials. Different from most studies of 2D materials with their atomic layers parallel to substrate, we have recently developed layer vertically aligned 2D material nanofilms. Built on these developments, here, we demonstrate the synthesis of vertical heterostructure of n-type MoS2 and p-type WSe2 with vertically aligned atomic layers. Thin film of MoS2/WSe2 vertical structure was successfully synthesized without significant alloy formation. The heterostructure synthesis is scalable to a large area over 1 cm2. We demonstrated the pn junction diode behavior of the heterostructure device. This novel device geometry opens up exciting opportunities for a variety of electronic and optoelectronic devices, complementary to the recent interesting vertical heterostructures with horizont...

Large-Area Nanosphere Self-Assembly by a Micro-Propulsive Injection Method for High Throughput Periodic Surface Nanotexturing

Abstract: A high throughput surface texturing process for optical and optoelectric devices based on a large-area self-assembly of nanospheres via a low-cost micropropulsive injection (MPI) method is presented. The novel MPI process enables the formation of a well-organized monolayer of hexagonally arranged nanosphere arrays (NAs) with tunable periodicity directly on the water surface, which is then transferred onto the preset substrates. This process can readily reach a throughput of 3000 wafers/h, which is compatible with the high volume photovoltaic manufacturing, thereby presenting a highly versatile platform for the fabrication of periodic nanotexturing on device surfaces. Specifically, a double-sided grating texturing with top-sided nanopencils and bottom-sided inverted-nanopyramids is realized in a thin film of crystalline silicon (28 μm in thickness) using chemical etching on the mask of NAs to significantly enhance antireflection and light trapping, resulting in absorptions nearly approaching the Lambertian...

Reversible Multivalent (Monovalent, Divalent, Trivalent) Ion Insertion in Open Framework Materials

Abstract: The reversible electrochemical insertion of multivalent ions into materials has promising applications in many fields, including batteries, seawater desalination, element purification, and wastewater treatment. However, finding materials that allow for the insertion of multivalent ions with fast kinetics and stable cycling has proven difficult because of strong electrostatic interactions between the highly charged insertion ions and atoms in the host framework. Here, an open framework nanomaterial, copper hexacyanoferrate, in the Prussian Blue family is presented that allows for the reversible insertion of a wide variety of monovalent, divalent, and trivalent ions (such as Rb+, Pb2+, Al3+, and Y3+) in aqueous solution beyond that achieved in previous studies. Electrochemical measurements demonstrate the unprecedented kinetics of multivalent ion insertion associated with this material. Synchrotron X-ray diffraction experiments point toward a novel vacancy-mediated ion insertion mechanism that reduces electrostatic repulsion and helps to facilitate the observed rapid ion insertion. The results suggest a new approach to multi­valent ion insertion that may help to advance the understanding of this complex phenomenon.

Electrochemical tuning of olivine-type lithium transition-metal phosphates as efficient water oxidation catalysts

Abstract: The oxygen evolution reaction is of paramount importance in clean energy generation and storage. While the common approach in search of active, durable and cost-effective oxygen evolution catalysts involves the development of novel materials, it is equally important to tune the properties of existing materials so as to improve their catalytic performance. Here, we demonstrate the general efficacy of electrochemical lithium tuning in organic electrolyte on enhancing the oxygen evolution catalytic activity of olivine-type lithium transition metal phosphates, a widely-researched family of cathode materials in lithium ion batteries. By continuously extracting lithium ions out of lithium transition metal phosphates, the materials exhibited significantly enhanced water oxidation catalytic activity. Particularly, the electrochemically delithiated Li(Ni,Fe)PO4 nanoparticles anchored on reduced graphene oxide sheets afforded outstanding performance, generating a current density of 10 mA cm−2 at an overpotential of only 0.27 V for over 24 h without degradation in 0.1 M KOH, outperforming the commercial precious metal Ir catalysts.

Vertical nanopillars for in situ probing of nuclear mechanics in adherent cells

Abstract: Arrays of vertical nanopillars are used to study the deformability of the cell nucleus and its mechanical coupling with the cell membrane.

A Sulfur Cathode with Pomegranate-Like Cluster Structure

Abstract: Dr. W. Li, Z. Liang, Dr. Z. Lu, Dr. H. Yao, Z. W. Seh, Dr. K. Yan Department of Materials Science and Engineering Stanford University Stanford , CA 94305 , USA E-mail: yicui@stanford.edu Dr. G. Zheng Department of Chemical Engineering Stanford University Stanford , CA 94305 , USA Dr. Y. Cui Stanford Institute for Materials and Energy Sciences SLAC National Accelerator Laboratory 2575 Sand Hill Road Menlo Park , CA 94025 , USA