Stretchable electronic devices, such as p-n diodes, [1] photovoltaic devices, [2,3] transistors, [4,5] and functional electronic eyes, [6] have been fabricated using buckled single-crystal (e.g., Si, GaAs) thin films supported by elastomeric substrates. Recently, carbon nanotube (CNT)-based highly conducting elastic composites [7,8] and stretchable graphene films [9] have been reported, which are suitable as interconnects in stretchable electronic devices. As an indispensable component of stretchable electronics, a stretchable power-source device should be able to accommodate large strains while retaining intact function. Of various power-source devices, supercapacitors have attracted great interest in recent years due to their high power and energy densities compared with lithium-ion batteries and conventional dielectric capacitors, respectively. The most active research in supercapacitors is the development of new electrode materials. Recently, CNTs have been studied as good candidates for electrode materials [10][11][12][13][14][15][16] because of several advantages, including a high surface area, nanoscale dimensions, and excellent electrical conductivity.Here, we report stretchable supercapacitors based on periodically sinusoidal single-walled carbon nanotube (SWNT) macrofilms (a 2D network of randomly oriented SWNTs). The stretchable supercapacitors comprise two sinusoidal SWNT macrofilms as stretchable electrodes, an organic electrolyte, and a polymeric separator. Electrochemical tests were performed and the fabricated stretchable supercapacitors are found to possess energy and power densities comparable with those of supercapacitors using pristine SWNT macrofilms as electrodes. Remarkably, the electrochemical performance of the stretchable supercapacitors remains unchanged even under 30% applied tensile strain.The preparation of the periodically sinusoidal SWNT macrofilms is of primary importance for stretchable supercapacitors. The synthesis of high-quality, purified, and functionalized SWNT macrofilms is, thus, an important preprocess, which has been presented elsewhere.[17] The purified SWNT macrofilm was then shaped to a sinusoidal form by following the steps shown in Figure 1a. The procedure introduced here (step i in Fig. 1a) involves the uniaxial prestretching (e pre ) of an elastomeric substrate of a poly(dimethylsiloxane) (PDMS) slab (e pre ¼ DL/L for length changed from L to L þ DL), followed by a chemical surface treatment to form a hydrophilic surface (see Experimental Section). The exposure of UV light introduces atomic oxygen, an activated species that reacts with PDMS and, thus, changes the Figure 1. Fabrication steps of a buckled SWNT macrofilm on an elastomeric PDMS substrate. a) Illustration of the fabrication flow comprising surface treatment, transfer, and relaxation of the prestrained PDMS substrate. b) Optical microscopy image of a 50-nm-thick, buckled SWNT macrofilm on a PDMS substrate with 30% prestrain, where the well-defined periodic buckling structure is shown. c) SEM image of ...

Porous silicon nanowires have been well studied for various applications; however, there are only very limited reports on porous silicon nanowires used for energy storage. Here, we report both experimental and theoretical studies of porous doped silicon nanowires synthesized by direct etching of boron-doped silicon wafers. When using alginate as a binder, porous silicon nanowires exhibited superior electrochemical performance and long cycle life as anode material in a lithium ion battery. Even after 250 cycles, the capacity remains stable above 2000, 1600, and 1100 mAh/g at current rates of 2, 4, and 18 A/g, respectively, demonstrating high structure stability due to the high porosity and electron conductivity of the porous silicon nanowires. A mathematic model coupling the lithium ion diffusion and the strain induced by lithium intercalation was employed to study the effect of porosity and pore size on the structure stability. Simulation shows silicon with high porosity and large pore size help to stabilize the structure during charge/discharge cycles.

The effect of compressive stress on the electrochemical behavior of flexible supercapacitors assembled with single-walled carbon nanotube (SWNT) film electrodes and 1 M aqueous electrolytes with different anions and cations were thoroughly investigated. The under-pressed capacitive and resistive features of the supercapacitors were studied by means of cyclic voltammetry measurements and electrochemical impedance analysis. The results demonstrated that the specific capacitance increased first and saturated in corresponding decreases of the series resistance, the charge-transfer resistance, and the Warburg diffusion resistance under an increased pressure from 0 to 1723.96 kPa. Wettability as well as ion-size effect of different aqueous electrolytes played important roles to determine the pressure dependence behavior of the suerpcapacitors under an applied pressure. An improved high-frequency capacitive response with 1172 Hz knee frequency, which is significantly higher compared to reported values, was observed under the compressive pressure of 1723.96 kPa, indicating an improving and excellent high-power capability of the supercapacitors under the pressure. The experimental results and the thorough analysis described in this work not only provide fundamental insight of pressure effects on supercapacitors but also give an important guideline for future design of next generation flexible/stretchable supercapacitors for industrial and consumer applications.

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Recently, silicon-based lithium-ion battery anodes have shown encouraging results, as they can offer high capacities and long cyclic lifetimes. The applications of this technology are largely impeded by the complicated and expensive approaches in producing Si with desired nanostructures. We report a cost-efficient method to produce nanoporous Si particles from metallurgical Si through ball-milling and inexpensive stain-etching. The porosity of porous Si is derived from particle's three-dimensional reconstructions by scanning transmission electron microscopy (STEM) tomography, which shows the particles' highly porous structure when etched under proper conditions. Nanoporous Si anodes with a reversible capacity of 2900 mAh/g was attained at a charging rate of 400 mA/g, and a stable capacity above 1100 mAh/g was retained for extended 600 cycles tested at 2000 mA/g. The synthetic route is low-cost and scalable for mass production, promising Si as a potential anode material for the next-generation lithium-ion batteries with enhanced capacity and energy density.

Silicon is of great interest for use as the anode material in lithium-ion batteries due to its high capacity. However, certain properties of silicon, such as a large volume expansion during the lithiation process and the low diffusion rate of lithium in silicon, result in fast capacity degradation in limited charge/discharge cycles, especially at high current rate. Therefore, the use of silicon in real battery applications is limited. The idea of using porous silicon, to a large extent, addresses the above-mentioned issues simultaneously. In this review, we discuss the merits of using porous silicon for anodes through both theoretical and experimental study. Recent progress in the preparation of porous silicon through the template-assisted approach and the non-template approach have been highlighted. The battery performance in terms of capacity and cyclability of each structure is evaluated.

A generic and facile method of coating graphene oxide (GO) on particles is reported, with sulfur/GO core-shell particles demonstrated as an example for lithium-sulfur (Li-S) battery application with superior performance. Particles of different diameters (ranging from 100 nm to 10 μm), geometries, and compositions (sulfur, silicon, and carbon) are successfully wrapped up by GO, by engineering the ionic strength in solutions. Importantly, our method does not involve any chemical reaction between GO and the wrapped particles, and therefore, it can be extended to vast kinds of functional particles. The applications of sulfur/GO core-shell particles as Li-S battery cathode materials are further investigated, and the results show that sulfur/GO exhibit significant improvements over bare sulfur particles without coating. Galvanic charge-discharge test using GO/sulfur particles shows a specific capacity of 800 mAh/g is retained after 1000 cycles at 1 A/g current rate if only the mass of sulfur is taken into calculation, and 400 mAh/g if the total mass of sulfur/GO is considered. Most importantly, the capacity decay over 1000 cycles is less than 0.02% per cycle. The coating method developed in this study is facile, robust, and versatile and is expected to have wide range of applications in improving the properties of particle materials.

There is a great deal of interest in developing next-generation lithium ion (Li-ion) batteries with higher energy capacity and longer cycle life for a diverse range of applications such as portable electronic devices, satellites, and next-generation electric vehicles. Silicon (Si) is an attractive anode material that is being closely scrutinized for use in Li-ion batteries because of its highest-known theoretical charge capacity of 4200 mAh g −1 .[1] The development of Si-anode Li-ion batteries has been hindered, however, mostly because of the large volumetric changes (up to 400%) that occur upon insertion and extraction of Li ions, and in turn the large electrochemically related stress, which results in electrode pulverization, loss of electrical contact, and early capacity fading of battery cells. [2][3][4][5] Despite this challenge, the extraordinarily high energy capacity of Si in its own right has motivated researchers to develop new techniques that reduce the limitations of Si as a practical anode material. Ultrathin Si films down to 50 nm in thickness have been reported for successful antipulverization and capacity nondegradation over two thousand charge/discharge cycles on roughened current collectors. [6] This result, together with a surge of work on improving the capacity retention of Si anodes such as nanoparticles [7,8] and/or composites, [9][10][11][12] nanowires, [13][14][15] or nanotubes [16,17] have shown improved performances, where the nanoforms of materials can offer expansion spaces during lithium insertion/extraction ( Figure 1A). However, some degree of capacity fading still exists due to the limited space for accommodating the facile strain expansion as well as decreased accessibility of the electrolyte to the solid -electrolyte interphase (SEI) between the silicon nanostructures and electrolyte. Here, we present a new strategy of stress relaxation for Si films using an elastomeric substrate that will establish an alternative route for new electrode design. In addition, the design of the anodes offers more efficient ion and electron transport than the reported work that uses nanoparticles, nanowires, or nanotubes.The general concept of stress relaxation can be understood using an eigen strain analogy. It is well-known that the eigen deformation of a free-standing material does not lead to mechanical stress, but only to self-compatible deformations, and eigen-strain-induced stresses are generated when the eigen strain is constrained. Consequently, the stress can be released by removing these constraints (e.g., stainless steel [13] and rough substrates [6] ). Herein, we report an approach in which the rigid substrates (e.g., current collectors) that constrain the "free" expansion/contraction of the Si anodes during charge/ discharge are replaced by soft substrates. The mechanism for stress relaxation is that the volumetric strain in Si that is induced by charge/discharge cycling can buckle the flat Si thin films when they are on soft substrates ( Figure 1B), which in turn releases the stress ...