Ultrafast saturable absorption of BiOI nanosheets prepared by chemical vapor transport.

The saturable absorption properties of BiOI nanosheets with exposed {110} facets prepared by the chemical vapor transport were investigated by Z-scan with femtosecond pulse laser. The nonlinear absorption coefficient at 400 nm is stronger and more sensitive to photoexcitation than its nonlinear response at 800 nm. The small saturation intensity could have been achieved, which is one order of magnitude smaller than that of black phosphorus nanosheets, while the Imχ(3) are determined to be -4.35×10-12esu close to theoretical prediction. According to time-resolved photoluminescence spectrum results, this strong saturated absorption at 400 nm may be attributed to the interband recombination process, whose lifetime was 230 ps.

Controllable epitaxial growth of GeSe2nanostructures and nonlinear optical properties.

Germanium diselenide (GeSe2) has emerged as a new member of anisotropic two-dimensional (2D) materials and gained increasing attention because of its excellent air stability, wide band gap and unique anisotropic properties, which exhibits promising applications in the fields of electronics, optoelectronics and polarized photodetection. However, the controllable epitaxial growth of large-scale and high-quality GeSe2nanostructures to date remains a big challenge. Herein, GeSe2nanofilms with lateral size up to centimeter scale have been successfully prepared on mica substrate by employing chemical vapor deposition technique. Experimental results demonstrated that hydrogen is the key factor for the controllable growth of GeSe2nanostructures and GeSe2-based heterostructures. Corresponding growth mechanism was proposed based on systematical characterizations. The nonlinear optical properties of as-prepared GeSe2were investigated by employing open-aperture z-scan technique exhibiting significant saturable and reverse saturable absorption behaviors at wavelengths of 400 nm and 800 nm, respectively. This study provides a new and robust route for fabricating GeSe2nanostructures and 2D heterostructures, which will benefit the development of GeSe2-based nonlinear optical and optoelectronic devices.

The Bonding Nature and Adhesion of Polyacrylic Acid Coating on Li-Metal for Li Dendrite Prevention.

The success of polyacrylic acid (PAA) to suppress Li dendrite growth suggests that the mechanical properties of polymer-based coatings, including the modulus, toughness, and interfacial adhesion are important design criteria. However, the measurement of the adhesion of thin PAA, as well as other polymer coatings to the reactive Li-metal anode surface is limited experimentally and challenging computationally. In this paper, a strategy was proposed to estimate the adhesion and delamination of the PAA(polymer)/Li interface, based on the bonding nature at the simpler PAA (oligomer)/Li interfaces using density functional theory calculations. It has been shown that the carboxylic acid groups in PAA reacted strongly with metallic Li, which significantly enhances the interfacial adhesion through the Li-O bonds formation, Li ionization and its incorporation into PAA, and -H or -OH termination of Li after decomposition of the COOH functional group. During delamination, it was found that the most likely PAA delamination route involved breaking partial Li-O bonds and lifting some ionized Li atoms from the Li-metal, especially for the Li atoms that showed a charge closer to +1 or are bonded with two O atoms from PAA. Based on the average bonding energies from PAA(oligomer)/Li interface delamination calculations, the work of separation, Wsep, of the PAA(polymer)/Li interface was estimated to be ∼1.0 (J/m2). The high Wsep of PAA (polymer)/Li was comparable with the Li2O/Li interface and higher than Li2CO3/Li and LiF/Li interfaces. This order correlated well with the areal density of Li-O bonds, which can serve as a descriptor for the interfacial adhesion. This computational approach can be applied to other interfaces with polymer-based coatings.

Hierarchical porous silicon structures with extraordinary mechanical strength as high-performance lithium-ion battery anodes.

Porous structured silicon has been regarded as a promising candidate to overcome pulverization of silicon-based anodes. However, poor mechanical strength of these porous particles has limited their volumetric energy density towards practical applications. Here we design and synthesize hierarchical carbon-nanotube@silicon@carbon microspheres with both high porosity and extraordinary mechanical strength (>200 MPa) and a low apparent particle expansion of ~40% upon full lithiation. The composite electrodes of carbon-nanotube@silicon@carbon-graphite with a practical loading (3 mAh cm-2) deliver ~750 mAh g-1 specific capacity, <20% initial swelling at 100% state-of-charge, and ~92% capacity retention over 500 cycles. Calendered electrodes achieve ~980 mAh cm-3 volumetric capacity density and <50% end-of-life swell after 120 cycles. Full cells with LiNi1/3Mn1/3Co1/3O2 cathodes demonstrate >92% capacity retention over 500 cycles. This work is a leap in silicon anode development and provides insights into the design of electrode materials for other batteries.

Reinforced Composite Film on Lithium Metal Electrodes through Aryl Chlorosilane Treatment.

Lithium metal has great potential to become the anode for the next generation of high-energy-density batteries because of high capacity (3860 mAh g-1), lowest negative electrochemical potential, and low density. Low cycle efficiency and dendrite growth are two critical barriers for rechargeable batteries using Li metal as the anode, mainly due to the coupled mechanical/chemical degradation of the solid electrolyte interphase (SEI) layer formed on the Li metal surface. In this work, we found that a composite film of lithium aryl silanolate with uniformly distributed submicron LiCl-dominant particles can in situ form on the Li metal surface by treating Li with a single phenyl substituted chlorosilane. The synergistic effect of the high modulus of the composite film resulted from both well-dispersed LiCl particles and phenyl ring structures, and the extra reinforcement by the π-π interaction of aryl silanolate molecules that coated on LiCl particles and Li electrode surface endows the artificial surface coating film with high modulus and stability, and, thus, suitable as an artificial SEI layer. The coin cells using the lithium metal electrodes with this Lithium silanolate/LiCl particle composite coating layer showed an improved cycle efficiency and the extended life in a relatively harsh cycling condition.

Tuning Solid Electrolyte Interphase Layer Properties through the Integration of Conversion Reaction.

The solid electrolyte interphase (SEI) layer plays an important role in altering the ion transport and modifying the structural evolution of the Li metal anode during repeated cycling. While the fundamental understanding of the SEI properties has been continuously advanced in recent years, effectively tuning the SEI components, especially the inorganic constituents, is still challenging. In this work, tungsten trioxide, WO3, is found to promote the formation of inorganic salts, for example, LiF/Li2CO3 in SEI layers, thereby enhancing the SEI properties such as mechanical and chemical stabilities. Additionally, WO3 is simultaneously reduced to electronic W nanoparticles during the electrochemical process, mitigating the formation of "dead" Li, which otherwise is completely wrapped by the accumulated insulating SEI layers. The possibility of WO3 in catalyzing electrolyte decomposition, through favored reaction pathway, to produce robust SEI layers is discussed. This work provides new insights into the control of the SEI properties on Li metal surfaces.

A Bottom-Up Formation Mechanism of Solid Electrolyte Interphase Revealed by Isotope-Assisted Time-of-Flight Secondary Ion Mass Spectrometry.

Understanding the solid electrolyte interphase (SEI) formation mechanism is critically important for the performance and durability of lithium-ion batteries. However, the details of how SEI builds up into a nanometer-thick layer from molecular level reduction reactions on negative electrodes are missing. Here, isotope-assisted time-of-flight secondary ion mass spectrometry analyses were designed to answer this fundamental question. By investigating the isotope ratio profile in SEI during the initial SEI formation cycle, it is discovered that the topmost SEI near the electrolyte formed first and the SEI near the electrode formed later. This new "bottom-up" SEI growth mechanism was then correlated to the electrolyte one-electron and two-electron reduction reaction dynamics, which in turn explains the formation of the two-layered organic-inorganic SEI composite structure.

Silicon-Based Anodes for Lithium-Ion Batteries: From Fundamentals to Practical Applications.

Silicon has been intensively studied as an anode material for lithium-ion batteries (LIB) because of its exceptionally high specific capacity. However, silicon-based anode materials usually suffer from large volume change during the charge and discharge process, leading to subsequent pulverization of silicon, loss of electric contact, and continuous side reactions. These transformations cause poor cycle life and hinder the wide commercialization of silicon for LIBs. The lithiation and delithiation behaviors, and the interphase reaction mechanisms, are progressively studied and understood. Various nanostructured silicon anodes are reported to exhibit both superior specific capacity and cycle life compared to commercial carbon-based anodes. However, some practical issues with nanostructured silicon cannot be ignored, and must be addressed if it is to be widely used in commercial LIBs. This Review outlines major impactful work on silicon-based anodes, and the most recent research directions in this field, specifically, the engineering of silicon architectures, the construction of silicon-based composites, and other performance-enhancement studies including electrolytes and binders. The burgeoning research efforts in the development of practical silicon electrodes, and full-cell silicon-based LIBs are specially stressed, which are key to the successful commercialization of silicon anodes, and large-scale deployment of next-generation high energy density LIBs.

Strain-Induced Lithium Losses in the Solid Electrolyte Interphase on Silicon Electrodes.

The chemical and mechanical stability of SEI layers are particularly important for high capacity anode materials such as silicon, which undergoes large volume changes (∼300%) during cycling. In this work, we present a novel approach for applying controlled strains to SEI films with patterned Si electrodes to systematically investigate the impact of large volume changes on SEI formation and evolution. Comparisons between patterned silicon islands and continuous silicon thin films make it possible to correlate the irreversible capacity losses due to expansion and contraction of underlying silicon. The current work demonstrates that strain in the SEI layer leads to more lithium consumption. The combination of in situ AFM and electrochemical lithium loss measurements provides further information on SEI layer growth. These experiments indicate that in-plane strains in the SEI layer lead to substantial increases in the amount of inorganic phase formation, without significantly affecting the overall SEI thickness. These observations are further supported with EIS and TOF-SIMS results. A map of irreversible capacity evolution with strain in the SEI is obtained from the experimental results.

Hot-Chemistry Structural Phase Transformation in Single-Crystal Chalcogenides for Long-Life Lithium Ion Batteries.

Tuned chalcogenide single crystals rooted in sulfur-doped graphene were prepared by high-temperature solution chemistry. We present a facile route to synthesize a rod-on-sheet-like nanohybrid as an active anode material and demonstrate its superior performance in lithium ion batteries (LIBs). This nanohybrid contains a nanoassembly of one-dimensional (1D) single-crystalline, orthorhombic SnS onto two-dimensional (2D) sulfur-doped graphene. The 1D nanoscaled SnS with the rodlike single-crystalline structure possesses improved transport properties compared to its 2D hexagonal platelike SnS2. Furthermore, we blend this hybrid chalcogenide with biodegradable polymer composite using water as a solvent. Upon drying, the electrodes were subjected to heating in vacuum at 150 °C to induce polymer condensation via formation of carboxylate groups to produce a mechanically robust anode. The LIB using the as-developed anode material can deliver a high volumetric capacity of ∼2350 mA h cm-3 and exhibit superior cycle stability over 1500 cycles as well as a high capacity retention of 85% at a 1 C rate. The excellent battery performance combined with the simplistic, scalable, and green chemistry approach renders this anode material as a very promising candidate for LIB applications.

Carbon-Coated Silicon Nanowires on Carbon Fabric as Self-Supported Electrodes for Flexible Lithium-Ion Batteries.

A novel self-supported electrode with long cycling life and high mass loading was developed based on carbon-coated Si nanowires grown in situ on highly conductive and flexible carbon fabric substrates through a nickel-catalyzed one-pot atmospheric pressure chemical vapor deposition. The high-quality carbon coated Si nanowires resulted in high reversible specific capacity (∼3500 mA h g-1 at 100 mA g-1), while the three-dimensional electrode's unique architecture leads to a significantly improved robustness and a high degree of electrode stability. An exceptionally long cyclability with a capacity retention of ∼66% over 500 cycles at 1.0 A g-1 was achieved. The controllable high mass loading enables an electrode with extremely high areal capacity of ∼5.0 mA h cm-2. Such a scalable electrode fabrication technology and the high-performance electrodes hold great promise in future practical applications in high energy density lithium-ion batteries.

Design of Nanostructured Heterogeneous Solid Ionic Coatings through a Multiscale Defect Model.

Understanding of the electrical conduction, that is, ionic and electronic conduction, through the solid electrolyte interphase (SEI) is critical to the design of durable lithium-ion batteries (LIBs) with high rate capability and long life. It is believed that an ideal SEI should not only be an ionic conductor, but also an electronic insulator. In this study, we present a theoretical design of an artificial SEI consisting of lithium fluoride (LiF) and lithium carbonate (Li2CO3) on a LIB anode based on a newly developed density functional theory (DFT) informed space charge model. We demonstrate that the migration of lattice Li ions from LiF phase to form Li interstitials in Li2CO3 is energetically favorable near the LiF/Li2CO3 interface. At equilibrium, this interfacial defect reaction establishes a space charge potential across the interface, which causes the accumulation of ionic carriers but the depletion of electronic carriers near the LiF/Li2CO3 interface. To utilize this space charge effect, we propose a computationally designed, nanostructured artificial SEI structure with high density of interfaces of LiF and Li2CO3 perpendicular to the electrode. On the basis of this structure, the influences of grain size and volume ratio of the two phases were studied. Our results reveal that reducing the grain size of Li2CO3 in the nanostructured composite can promote ionic carriers and increase the ionic conductivity through the composite SEI by orders of magnitude. At the same time, the electronic conductivity is reduced due to electron depletion near the LiF/Li2CO3 interface. Furthermore, an optimal volume fraction that ensures high ionic and low electronic conduction was predicted.

Synergetic Effects of Inorganic Components in Solid Electrolyte Interphase on High Cycle Efficiency of Lithium Ion Batteries.

The solid electrolyte interphase (SEI), a passivation layer formed on electrodes, is critical to battery performance and durability. The inorganic components in SEI, including lithium carbonate (Li2CO3) and lithium fluoride (LiF), provide both mechanical and chemical protection, meanwhile control lithium ion transport. Although both Li2CO3 and LiF have relatively low ionic conductivity, we found, surprisingly, that the contact between Li2CO3 and LiF can promote space charge accumulation along their interfaces, which generates a higher ionic carrier concentration and significantly improves lithium ion transport and reduces electron leakage. The synergetic effect of the two inorganic components leads to high current efficiency and long cycle stability.

Self-generated concentration and modulus gradient coating design to protect Si nano-wire electrodes during lithiation.

Surface coatings as artificial solid electrolyte interphases have been actively pursued as an effective way to improve the cycle efficiency of nanostructured Si electrodes for high energy density lithium ion batteries, where the mechanical stability of the surface coatings on Si is as critical as Si itself. However, the chemical composition and mechanical property change of coating materials during the lithiation and delithiation process imposed a grand challenge to design coating/Si nanostructure as an integrated electrode system. In our work, we first developed reactive force field (ReaxFF) parameters for Li-Si-Al-O materials to simulate the lithiation process of Si-core/Al2O3-shell and Si-core/SiO2-shell nanostructures. With reactive dynamics simulations, we were able to simultaneously track and correlate the lithiation rate, compositional change, mechanical property evolution, stress distributions, and fracture. A new mechanics model based on these varying properties was developed to determine how to stabilize the coating with a critical size ratio. Furthermore, we discovered that the self-accelerating Li diffusion in Al2O3 coating forms a well-defined Li concentration gradient, leading to an elastic modulus gradient, which effectively avoids local stress concentration and mitigates crack propagation. Based on these results, we propose a modulus gradient coating, softer outside, harder inside, as the most efficient coating to protect the Si electrode surface and improve its current efficiency.

Inward lithium-ion breathing of hierarchically porous silicon anodes.

Silicon has been identified as a highly promising anode for next-generation lithium-ion batteries (LIBs). The key challenge for Si anodes is large volume change during the lithiation/delithiation cycle that results in chemomechanical degradation and subsequent rapid capacity fading. Here we report a novel fabrication method for hierarchically porous Si nanospheres (hp-SiNSs), which consist of a porous shell and a hollow core. On charge/discharge cycling, the hp-SiNSs accommodate the volume change through reversible inward Li breathing with negligible particle-level outward expansion. Our mechanics analysis revealed that such inward expansion is enabled by the much stiffer lithiated layer than the unlithiated porous layer. LIBs assembled with the hp-SiNSs exhibit high capacity, high power and long cycle life, which is superior to the current commercial Si-based anode materials. The low-cost synthesis approach provides a new avenue for the rational design of hierarchically porous structures with unique materials properties.

Evidence of covalent synergy in silicon-sulfur-graphene yielding highly efficient and long-life lithium-ion batteries.

Silicon has the potential to revolutionize the energy storage capacities of lithium-ion batteries to meet the ever increasing power demands of next generation technologies. To avoid the operational stability problems of silicon-based anodes, we propose synergistic physicochemical alteration of electrode structures during their design. This capitalizes on covalent interaction of Si nanoparticles with sulfur-doped graphene and with cyclized polyacrylonitrile to provide a robust nanoarchitecture. This hierarchical structure stabilized the solid electrolyte interphase leading to superior reversible capacity of over 1,000 mAh g(-1) for 2,275 cycles at 2 A g(-1). Furthermore, the nanoarchitectured design lowered the contact of the electrolyte to the electrode leading to not only high coulombic efficiency of 99.9% but also maintaining high stability even with high electrode loading associated with 3.4 mAh cm(-2). The excellent performance combined with the simplistic, scalable and non-hazardous approach render the process as a very promising candidate for Li-ion battery technology.

Unravelling the Impact of Reaction Paths on Mechanical Degradation of Intercalation Cathodes for Lithium-Ion Batteries.

The intercalation compounds are generally considered as ideal electrode materials for lithium-ion batteries thanks to their minimum volume expansion and fast lithium ion diffusion. However, cracking still occurs in those compounds and has been identified as one of the critical issues responsible for their capacity decay and short cycle life, although the diffusion-induced stress and volume expansion are much smaller than those in alloying-type electrodes. Here, we designed a thin-film model system that enables us to tailor the cation ordering in LiNi(0.5)Mn(1.5)O4 spinels and correlate the stress patterns, phase evolution, and cycle performances. Surprisingly, we found that distinct reaction paths cause negligible difference in the overall stress patterns but significantly different cracking behaviors and cycling performances: 95% capacity retention for disordered LiNi(0.5)Mn(1.5)O4 and 48% capacity retention for ordered LiNi(0.5)Mn(1.5)O4 after 2000 cycles. We were able to pinpoint that the extended solid-solution region with suppressed phase transformation attributed to the superior electrochemical performance of disordered spinel. This work envisions a strategy for rationally designing stable cathodes for lithium-ion batteries through engineering the atomic structure that extends the solid-solution region and suppresses phase transformation.

Reduced Graphene Oxide/Tin-Antimony Nanocomposites as Anode Materials for Advanced Sodium-Ion Batteries.

Reduced graphene oxides loaded with tin-antimony alloy (RGO-SnSb) nanocomposites were synthesized through a hydrothermal reaction and the subsequent thermal reduction treatments. Transmission electron microscope images confirm that SnSb nanoparticles with an average size of about 20-30 nm are uniformly dispersed on the RGO surfaces. When they were used as anodes for rechargeable sodium (Na)-ion batteries, these as-synthesized RGO-SnSb nanocomposite anodes delivered a high initial reversible capacity of 407 mAh g(-1), stable cyclic retention for more than 80 cycles and excellent cycle stability at ultra high charge/discharge rates up to 30C. The significantly improved performance of the synthesized RGO-SnSb nanocomposites as Na-ion battery anodes can be attributed to the synergetic effects of RGO-based flexible framework and the nanoscale dimension of the SnSb alloy particles (<30 nm). Nanosized intermetallic SnSb compounds can exhibit improved structural stability and conductivity during charge and discharge reactions compared to the corresponding individuals (Sn and Sb particles). In the meantime, RGO sheets can tightly anchor SnSb alloy particles on the surfaces, which can not only effectively suppress the agglomeration of SnSb particles but also maintain excellent electronic conduction. Furthermore, the mechanical flexibility of the RGO phase can accommodate the volume expansion and contraction of SnSb particles during the prolonged cycling, therefore, improve the electrode integrity mechanically and electronically. All of these contribute to the electrochemical performance improvements of the RGO-SnSb nanocomposite-based electrodes in rechargeable Na-ion batteries.

Toward practical application of functional conductive polymer binder for a high-energy lithium-ion battery design.

Silicon alloys have the highest specific capacity when used as anode material for lithium-ion batteries; however, the drastic volume change inherent in their use causes formidable challenges toward achieving stable cycling performance. Large quantities of binders and conductive additives are typically necessary to maintain good cell performance. In this report, only 2% (by weight) functional conductive polymer binder without any conductive additives was successfully used with a micron-size silicon monoxide (SiO) anode material, demonstrating stable and high gravimetric capacity (>1000 mAh/g) for ∼500 cycles and more than 90% capacity retention. Prelithiation of this anode using stabilized lithium metal powder (SLMP) improves the first cycle Coulombic efficiency of a SiO/NMC full cell from ∼48% to ∼90%. The combination enables good capacity retention of more than 80% after 100 cycles at C/3 in a lithium-ion full cell.

A systematic investigation of polymer binder flexibility on the electrode performance of lithium-ion batteries.

The mechanical failure at the electrode interfaces (laminate/current collector and binder/particle interfaces) leads to particle isolation and delamination, which has been regarded as one of the main reasons for the capacity decay and cell failure of lithium-ion batteries (LIBs). Polymer binder provides the key function for a good interface property and for maintaining the electrode integrity of LIBs. Triethylene glycol monomethyl ether (TEG) moieties were incorporated into polymethacrylic acid (PMAA) to different extents at the molecular level. Microscratch tests of the graphite electrodes based on these binders indicate that the electrode is more flexible with 5 or 10% TEG in the polymer binders. Crack generation is inhibited by the flexible TEG-containing binder, compared to that of the unmodified PMAA-based electrode, leading to the better cycling performance of the flexible electrode. With a 10% TEG moiety in the binder, the graphite half-cell reaches a reversible capacity of >270 mAh/g at the 1C rate, compared to a value of ∼190 mAh/g for the unmodified PMAA binder.