Yan Yao

Stable cycling of double-walled silicon nanotube battery anodes through solid-electrolyte interphase control

Although the performance of lithium ion-batteries continues to improve, their energy density and cycle life remain insufficient for applications in consumer electronics, transport and large-scale renewable energy storage. Silicon has a large charge storage capacity and this makes it an attractive anode material, but pulverization during cycling and an unstable solid-electrolyte interphase has limited the cycle life of silicon anodes to hundreds of cycles. Here, we show that anodes consisting of an active silicon nanotube surrounded by an ion-permeable silicon oxide shell can cycle over 6,000 times in half cells while retaining more than 85% of their initial capacity. The outer surface of the silicon nanotube is prevented from expansion by the oxide shell, and the expanding inner surface is not exposed to the electrolyte, resulting in a stable solid-electrolyte interphase. Batteries containing these double-walled silicon nanotube anodes exhibit charge capacities approximately eight times larger than conventional carbon anodes and charging rates of up to 20C (a rate of 1C corresponds to complete charge or discharge in one hour).

A Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Alloy Anodes

Silicon is regarded as one of the most promising anode materials for next generation lithium-ion batteries. For use in practical applications, a Si electrode must have high capacity, long cycle life, high efficiency, and the fabrication must be industrially scalable. Here, we design and fabricate a yolk-shell structure to meet all these needs. The fabrication is carried out without special equipment and mostly at room temperature. Commercially available Si nanoparticles are completely sealed inside conformal, thin, self-supporting carbon shells, with rationally designed void space in between the particles and the shell. The well-defined void space allows the Si particles to expand freely without breaking the outer carbon shell, therefore stabilizing the solid-electrolyte interphase on the shell surface. High capacity (∼2800 mAh/g at C/10), long cycle life (1000 cycles with 74% capacity retention), and high Coulombic efficiency (99.84%) have been realized in this yolk-shell structured Si electrode.

Interconnected silicon hollow nanospheres for lithium-ion battery anodes with long cycle life.

Silicon is a promising candidate for the anode material in lithium-ion batteries due to its high theoretical specific capacity. However, volume changes during cycling cause pulverization and capacity fade, and improving cycle life is a major research challenge. Here, we report a novel interconnected Si hollow nanosphere electrode that is capable of accommodating large volume changes without pulverization during cycling. We achieved the high initial discharge capacity of 2725 mAh g(-1) with less than 8% capacity degradation every hundred cycles for 700 total cycles. Si hollow sphere electrodes also show a Coulombic efficiency of 99.5% in later cycles. Superior rate capability is demonstrated and attributed to fast lithium diffusion in the interconnected Si hollow structure.

Transition metal oxides as the buffer layer for polymer photovoltaic cells

Polymer-based photovoltaic cells have been fabricated by inserting a thin, transparent, transition metal oxide layer between the transparent anode (indium tin oxide) and the polymer layer. Two different transition metal oxides, namely vanadium oxide and molybdenum oxide, were used and the device performance was compared. The surface of the oxide films and the interface between the polymer and the oxide was studied with the help of atomic force microscopy. The effect of the thickness of the oxide layer on electrical characteristics of the device was also studied and optimized thickness was achieved to give high power conversion efficiency of 3.3% under simulated AM1.5G illumination of 100mW∕cm2.

Investigation of annealing effects and film thickness dependence of polymer solar cells based on poly(3-hexylthiophene)

Regioregular poly(3-hexylthiophene) (RR-P3HT) is a promising candidate for polymer photovoltaic research due to its stability and absorption in the red region. In this manuscript, we report polymer photovoltaic devices based on RR-P3HT:methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) 1:1 weight-ratio blend. We studied the effects of annealing temperature and time on the device performance for devices annealed before and after cathode deposition. Thermal annealing shows significant improvement in the performance for both types of annealing conditions, with postproduction annealing being slightly better. For devices with a 43-nm-thick active layer, maximum power conversion efficiency (PCE) of 3.2% and fill factor up to 67% is achieved under Air Mass 1.5, 100‐mW∕cm2 illumination. We performed atomic force microscopy and ultraviolet-visible absorption spectroscopy on the P3HT:PCBM films to explain the effect of thermal annealing. By keeping the optimized thermal annealing condition and by va...

Improving the performance of lithium-sulfur batteries by conductive polymer coating.

Rechargeable lithium–sulfur (Li–S) batteries hold great potential for next-generation high-performance energy storage systems because of their high theoretical specific energy, low materials cost, and environmental safety. One of the major obstacles for its commercialization is the rapid capacity fading due to polysulfide dissolution and uncontrolled redeposition. Various porous carbon structures have been used to improve the performance of Li–S batteries, as polysulfides could be trapped inside the carbon matrix. However, polysulfides still diffuse out for a prolonged time if there is no effective capping layer surrounding the carbon/sulfur particles. Here we explore the application of conducting polymer to minimize the diffusion of polysulfides out of the mesoporous carbon matrix by coating poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) onto mesoporous carbon/sulfur particles. After surface coating, coulomb efficiency of the sulfur electrode was improved from 93% to 97%, and capaci...

Symmetrical MnO2-carbon nanotube-textile nanostructures for wearable pseudocapacitors with high mass loading

While MnO(2) is a promising material for pseudocapacitor applications due to its high specific capacity and low cost, MnO(2) electrodes suffer from their low electrical and ionic conductivities. In this article, we report a structure where MnO(2) nanoflowers were conformally electrodeposited onto carbon nanotube (CNT)-enabled conductive textile fibers. Such nanostructures effectively decrease the ion diffusion and charge transport resistance in the electrode. For a given areal mass loading, the thickness of MnO(2) on conductive textile fibers is much smaller than that on a flat metal substrate. Such a porous structure also allows a large mass loading, up to 8.3 mg/cm(2), which leads to a high areal capacitance of 2.8 F/cm(2) at a scan rate of 0.05 mV/s. Full cells were demonstrated, where the MnO(2)-CNT-textile was used as a positive electrode, reduced MnO(2)-CNT-textile as a negative electrode, and 0.5 M Na(2)SO(4) in water as the electrolyte. The resulting pseudocapacitor shows promising results as a low-cost energy storage solution and an attractive wearable power.

Manipulating regioregular poly (3 -hexylthiophene ) : [6,6]-phenyl-C61-butyric acid methyl ester blends-route towards high efficiency polymer solar cells

Polymer or “plastics” solar cells have been an intensively studied area since the discovery of efficient electron transfer between polymers and fullerenes and the introduction of the bulk-heterojunction concept. The last few years have seen significant improvement in plastic solar cell performance through aggressive research on the regioregular poly(3-hexylthiophene) (RR-P3HT) : [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) system. The morphology of the system is controlled through two major strategies which have proven effective in improving the device efficiency—thermal annealing and solvent annealing (slow growth). In this Feature Article, we review the recent progress on this material system. A detailed discussion on thermal annealing and solvent annealing approaches to improve device performance is presented, including a comparison between the two strategies. The effects of these two approaches on improving polymer crystallinity, light absorption in the polymer, carrier transport, blend film nano-morphology, etc. are summarized. We also include a brief discussion on accurate measurement and characterization techniques for polymer solar cells to correctly determine the efficiency by applying spectral mismatch factors. Future directions and challenges on polymer solar cell development are also discussed.

Effect of self-organization in polymer/fullerene bulk heterojunctions on solar cell performance

The authors investigate the effect of self-organization by controlling the growth rate on the performance of polymer/fullerene bulk-heterojunction solar cells. The effect of growth rate on the morphology of the active layer is studied by atomic force microscopy technique. The electrical characterization by dark current and photocurrent measurements is performed. The hole mobility in the polymer increases by about two orders in magnitude and the carrier transport becomes highly balanced. Increased exciton generation rate, more efficient electron-hole pair dissociation, higher carrier mobility, and balanced carrier transport in the active layer explain the enhancement in the short-circuit current and fill factor.

Efficient light harvesting in multiple-device stacked structure for polymer solar cells

We demonstrate a multiple-device stacked structure of polymer solar cells for efficient light harvesting. Two polymer photovoltaic cells are stacked together and connected in series or in parallel to achieve a tandem structure. In this two-cell structure, a multilayer semitransparent electrode, made of lithium fluoride (LiF)/aluminum (Al)/gold (Au), is used as the top contact in the bottom cell to efficiently transmit the unabsorbed photons to the upper cell. Maximum transparency of up to 80% is achieved for the semitransparent cathode. Upon stacking, the open-circuit voltage and the short-circuit current are almost doubled compared to a single cell.