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.

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.

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.

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.

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.

Polydopamine-coated, nitrogen-doped, hollow carbon-sulfur double-layered core-shell structure for improving lithium-sulfur batteries.

To better confine the sulfur/polysulfides in the electrode of lithium-sulfur (Li/S) batteries and improve the cycling stability, we developed a double-layered core-shell structure of polymer-coated carbon-sulfur. Carbon-sulfur was first prepared through the impregnation of sulfur into hollow carbon spheres under heat treatment, followed by a coating polymerization to give a double-layered core-shell structure. From the study of scanning transmission electron microscopy (STEM) images, we demonstrated that the sulfur not only successfully penetrated through the porous carbon shell but also aggregated along the inner wall of the carbon shell, which, for the first time, provided visible and convincing evidence that sulfur preferred diffusing into the hollow carbon rather than aggregating in/on the porous wall of the carbon. Taking advantage of this structure, a stable capacity of 900 mA h g(-1) at 0.2 C after 150 cycles and 630 mA h g(-1) at 0.6 C after 600 cycles could be obtained in Li/S batteries. We also demonstrated the feasibility of full cells using the sulfur electrodes to couple with the silicon film electrodes, which exhibited significantly improved cycling stability and efficiency. The remarkable electrochemical performance could be attributed to the desirable confinement of sulfur through the unique double-layered core-shell architectures.

Engineered Si electrode nanoarchitecture: a scalable postfabrication treatment for the production of next-generation Li-ion batteries.

A novel, economical flash heat treatment of the fabricated silicon based electrodes is introduced to boost the performance and cycle capability of Li-ion batteries. The treatment reveals a high mass fraction of Si, improved interfacial contact, synergistic SiO2/C coating, and a conductive cellular network for improved conductivity, as well as flexibility for stress compensation. The enhanced electrodes achieve a first cycle efficiency of ∼84% and a maximum charge capacity of 3525 mA h g(-1), almost 84% of silicon's theoretical maximum. Further, a stable reversible charge capacity of 1150 mA h g(-1) at 1.2 A g(-1) can be achieved over 500 cycles. Thus, the flash heat treatment method introduces a promising avenue for the production of industrially viable, next-generation Li-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.

Unraveling manganese dissolution/deposition mechanisms on the negative electrode in lithium ion batteries.

The structure, chemistry, and spatial distribution of Mn-bearing nanoparticles dissolved from the Li1.05Mn2O4 cathode during accelerated electrochemical cycling tests at 55 °C and deposited within the solid electrolyte interphase (SEI) are directly characterized through HRTEM imaging and XPS. Here we use air protection and vacuum transfer systems to transport cycled electrodes for imaging and analytical characterization. From HRTEM imaging, we find that a band of individual metallic Mn nanoparticles forms locally at the SEI/graphite interface while the internal and outermost layer of the SEI contains a mixture of LiF and MnF2 nanoparticles, which is confirmed with XPS. Based on our experimental findings we propose a new interpretation of how Mn is reduced from the cathode and how metallic Mn and Mn-bearing nanoparticles form within the SEI during electrochemical cycling.

Multifunctional TiO2-C/MnO2 core-double-shell nanowire arrays as high-performance 3D electrodes for lithium ion batteries.

The unique TiO2-C/MnO2 core-double-shell nanowires are synthesized for the first time using as anode materials for lithium ion batteries (LIBs). They combine both advantages from TiO2 such as excellent cycle stability and MnO2 with high capacity (1230 mA h g(-1)). The additional C interlayer intends to improve the electrical conductivity. The self-supported nanowire arrays grown directly on current-collecting substrates greatly simplify the fabrication processing of electrodes without applying binder and conductive additives. Each nanowire is anchored to the current collector, leading to fast charge transfer. The unique one-dimensional core-double-shell nanowires exhibit enhanced electrochemical performance with a higher discharge/charge capacity, superior rate capability, and longer cycling lifetime.

Li segregation induces structure and strength changes at the amorphous Si/Cu interface.

The study of interfacial properties, especially of their change upon lithiation, is a fundamentally significant and challenging topic in designing heterogeneous nanostructured electrodes for lithium ion batteries. This issue becomes more intriguing for Si electrodes, whose ultrahigh capacity is accompanied by large volume expansion and mechanical stress, threatening with delamination of silicon from the metal current collector and failure of the electrode. Instead of inferring interfacial properties from experiments, in this work, we have combined density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations with time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements of the lithium depth profile, to study the effect of lithiation on the a-Si/Cu interface. Our results clearly demonstrate Li segregation at the lithiated a-Si/Cu interface (more than 20% compared to the bulk concentration). The segregation of Li is responsible for a small decrease (up to 16%) of the adhesion strength and a dramatic reduction (by one order of magnitude) of the sliding resistance of the fully lithiated a-Si/Cu interface. Our results suggest that this almost frictionless sliding stems from the change of the bonding nature at the interface with increasing lithium content, from directional covalent bonding to uniform metallic. These findings are an essential first step toward an in-depth understanding of the role of lithiation on the a-Si/Cu interface, which may contribute in the development of quantitative electrochemical mechanical models and the design of nonfracture-and-always-connected heterogeneous nanostructured Si electrodes.

Toward an ideal polymer binder design for high-capacity battery anodes.

The dilemma of employing high-capacity battery materials and maintaining the electronic and mechanical integrity of electrodes demands novel designs of binder systems. Here, we developed a binder polymer with multifunctionality to maintain high electronic conductivity, mechanical adhesion, ductility, and electrolyte uptake. These critical properties are achieved by designing polymers with proper functional groups. Through synthesis, spectroscopy, and simulation, electronic conductivity is optimized by tailoring the key electronic state, which is not disturbed by further modifications of side chains. This fundamental allows separated optimization of the mechanical and swelling properties without detrimental effect on electronic property. Remaining electronically conductive, the enhanced polarity of the polymer greatly improves the adhesion, ductility, and more importantly, the electrolyte uptake to the levels of those available only in nonconductive binders before. We also demonstrate directly the performance of the developed conductive binder by achieving full-capacity cycling of silicon particles without using any conductive additive.

In situ TEM investigation of congruent phase transition and structural evolution of nanostructured silicon/carbon anode for lithium ion batteries.

It is well-known that upon lithiation, both crystalline and amorphous Si transform to an armorphous Li(x)Si phase, which subsequently crystallizes to a (Li, Si) crystalline compound, either Li(15)Si(4) or Li(22)Si(5). Presently, the detailed atomistic mechanism of this phase transformation and the degradation process in nanostructured Si are not fully understood. Here, we report the phase transformation characteristic and microstructural evolution of a specially designed amorphous silicon (a-Si) coated carbon nanofiber (CNF) composite during the charge/discharge process using in situ transmission electron microscopy and density function theory molecular dynamic calculation. We found the crystallization of Li(15)Si(4) from amorphous Li(x)Si is a spontaneous, congruent phase transition process without phase separation or large-scale atomic motion, which is drastically different from what is expected from a classic nucleation and growth process. The a-Si layer is strongly bonded to the CNF and no spallation or cracking is observed during the early stages of cyclic charge/discharge. Reversible volume expansion/contraction upon charge/discharge is fully accommodated along the radial direction. However, with progressive cycling, damage in the form of surface roughness was gradually accumulated on the coating layer, which is believed to be the mechanism for the eventual capacity fade of the composite anode during long-term charge/discharge cycling.

Decoration of graphitic surfaces with Sn nanoparticles through surface functionalization using diazonium chemistry.

Composites of tin nanoparticles (Sn NP) and graphene are candidate materials for high capacity and mechanically stable negative electrodes in rechargeable Li ion batteries. A uniform dispersion of Sn NP with controlled size is necessary to obtain high electrochemical performance. We show that the nucleation of Sn particles on highly ordered pyrolitic graphite (HOPG) from solution can be controlled by functionalizing the HOPG surface by aryl groups prior to Sn deposition. On the contrary, we observe heterogeneous deposition of micrometer sized Sn islands on HOPG subjected to oxidation prior to deposition in the same conditions. We demonstrate that functional groups act as nucleation sites for Sn NP nucleation, and that homogeneous nucleation of small particles can be achieved by combining surface functionalization with diazonium chemistry and appropriate stabilizers in solution.

Viscoelastic behavior and force nature of thermo-reversible epoxy dry adhesives.

The gecko adhesion phenomenon has stimulated efforts to produce synthetic patterned dry adhesives. Besides introducing surface patterns on dry adhesives, it is also highly desirable to understand their intrinsic material properties. This communication reports the viscoelastic behavior of non-patterned epoxy elastomers exhibiting intrinsic adhesion that is much higher than that of elastomers typically used for structure patterning. The diverse molecular origin of the adhesion is revealed through the study of adhesion against various substrates.

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.

Revealing triple-shape memory effect by polymer bilayers.

Bilayer polymers that consist of two epoxy dual-shape memory polymers of well-separated glass transition temperatures have been synthesized. These bilayer epoxy samples exhibit a triple-shape memory effect (TSME) with shape fixities tailorable by changing the ratio between the two layers. The triple-shape fixities of the bilayer epoxy polymers can be explained by the balance of stress between the two layers. Based on this work, it is believed that the following three molecular design criterions should be considered in designing triple-shape memory polymers with optimum TSME: 1) well-separated thermal transitions, 2) a strong interface, and 3) an appropriate balance of moduli and relative ratios between the layers (or microphases).