Wenyue Li

Core–Shell Si/C Nanospheres Embedded in Bubble Sheet‐like Carbon Film with Enhanced Performance as Lithium Ion Battery Anodes

Due to its high theoretical capacity and low lithium insertion voltage plateau, silicon has been considered one of the most promising anodes for high energy and high power density lithium ion batteries (LIBs). However, its rapid capacity degradation, mainly caused by huge volume changes during lithium insertion/extraction processes, remains a significant challenge to its practical application. Engineering Si anodes with abundant free spaces and stabilizing them by incorporating carbon materials has been found to be effective to address the above problems. Using sodium chloride (NaCl) as a template, bubble sheet-like carbon film supported core-shell Si/C composites are prepared for the first time by a facile magnesium thermal reduction/glucose carbonization process. The capacity retention achieves up to 93.6% (about 1018 mAh g(-1)) after 200 cycles at 1 A g(-1). The good performance is attributed to synergistic effects of the conductive carbon film and the hollow structure of the core-shell nanospheres, which provide an ideal conductive matrix and buffer spaces for respectively electron transfer and Si expansion during lithiation process. This unique structure decreases the charge transfer resistance and suppresses the cracking/pulverization of Si, leading to the enhanced cycling performance of bubble sheet-like composite.

Synthesis of Honeycomb‐like Mesoporous Pyrite FeS2 Microspheres as Efficient Counter Electrode in Quantum Dots Sensitized Solar Cells

Honeycomb-like mesoporous pyrite FeS2 microspheres, with diameters of 500-800 nm and pore sizes of 25-30 nm, are synthesized by a simple solvothermal approach. The mesoporous FeS2 microspheres are demonstrated to be an outstanding counter electrode (CE) material in quantum dot sensitized solar cells (QDSSCs) for electrocatalyzing polysulfide electrolyte regeneration. The cell using mesoporous FeS2 microspheres as CE shows 86.6% enhancement in power conversion efficiency (PCE) than the cell using traditional noble Pt CE. Furthermore, it also shows 11.4% enhancement in PCE than the cell using solid FeS2 microspheres as CE, due to the mesoporous structure facilitating better contact with polysulfide electrolyte and fast diffusion of redox couple species in electrolyte.

Polymer electrolytes from PEO and novel quaternary ammonium iodides for dye-sensitized solar cells

Polymer electrolytes were prepared by blending high molecular weight poly(ethylene oxide) (PEO) and a series of novel quaternary ammonium iodides, the polysiloxanes with oligo(oxyethylene) side chains and quaternary ammonium groups. X-ray diffraction (XRD) measurements ensured relatively low crystallinity when the quaternary ammonium iodides were incorporated into the PEO host. The ionic conductivity of these complexes was improved with the addition of plasticizers. The improvement in the ionic conductivity was determined by the polarity, viscosity and amounts of plasticizers. A plasticized electrolyte containing the novel quaternary ammonium iodide was successfully used in fabricating a quasi-solid-state dye-sensitized solar cell for the first time. The fill factor and energy conversion efficiency of the cell were calculated to be 0.68 and 1.39%, respectively.

Iron(II) molybdate (FeMoO4) nanorods as a high-performance anode for lithium ion batteries: structural and chemical evolution upon cycling

FeMoO4 nanorods were synthesized by a one-step solvothermal method and demonstrated to have attractive performance as an anode material in lithium ion batteries (LIBs). The specific capacity of the electrode exhibited an initial fading in the first 50 cycles and subsequently recovered to 1265 mA h g−1 at about the 500th cycle at a rate of 1C, after that, the capacity remained stable around 1110 mA h g−1 until the 1000th cycle. Based on comprehensive analysis of the structural and chemical evolution at each stage of capacity variation, we illustrated that the FeMoO4 nanorods were converted to a Fe2O3/MoO3 mixture after the first cycle and they experienced gradual structural variation of grain refinement and amorphization with their morphology transformed from nanorods to nanosheets upon cycling. Such changes in the chemical composition and microstructure of nanorods led to larger effective surface area, improved electrochemical reaction kinetics, and capacity retention capability. As a similar tendency of the specific capacity upon cycling has been widely observed for metal oxide anodes, studies on structural and chemical evolution of electrode materials during the whole cyclic life will be helpful for understanding their electrochemical reaction mechanism and provide guidance to material design and structural optimization of electrodes.

Layer-stacked cobalt ferrite (CoFe2O4) mesoporous platelets for high-performance lithium ion battery anodes

The extensive volume change and continuous consumption of active electrode materials due to the repeated formation of a solid electrolyte interface (SEI) layer during charge–discharge cycles are two important topics to be considered for the development of new nanostructured electrodes for high-performance lithium ion batteries (LIBs). In this work, layer-stacked cobalt ferrite (CoFe2O4) mesoporous platelets with two different thicknesses are synthesized, and their electrochemical performance as anodes for LIBs is evaluated. We find that the thickness of the platelets has a great impact on the specific capacity and stability. The thicker platelets (∼2 μm) enable a reduction of SEI-induced consumption of active materials and lead to an overall electrochemical performance superior to that of thinner ones. At a high rate of 5 A g−1, after an initial drop, the capacity of thicker platelets continuously increases in the following 500 cycles and reaches saturation around 950 mA h g−1, then gradually decreases and remains at 580 mA h g−1 after 2000 cycles. The high capacitance, outstanding rate performance and stability of thick platelets can be attributed to the special configuration of the layer-stacked mesoporous platelets which provides sufficient interlayer space for volume expansion, and enables the formation of a stable SEI layer during the cycling.

In situ incorporation of FeS nanoparticles/carbon nanosheets composite with an interconnected porous structure as a high-performance anode for lithium ion batteries

Interconnected porous FeS/C composite consisting of FeS nanoparticles (∼20 nm) homogeneously embedded in carbon nanosheets was synthesized via a facile freeze-drying/carbonization method using a NaCl template. As an anode for LIBs, this composite shows significantly enhanced electrochemical performance due to the synergistic effects of the conductive carbon film and the porous structure, which provides an ideal conductive matrix and buffer spaces for electron/ion transfer and FeS expansion, respectively, during lithiation processes. This composite exhibits reversible capacities of ∼703 mA h g−1 over 150 cycles at 1 A g−1 and a high-rate capability of ∼530 mA h g−1 even at 5 A g−1, which is among the best reported electrochemical performances for FeS-based materials thus far. With a long cycling life and high power density, this composite demonstrates its potential application in LIBs.

Facile fabrication and electrochemical properties of high-quality reduced graphene oxide/cobalt sulfide composite as anode material for lithium-ion batteries

A reduced graphene oxide (rGO)/cobalt sulfide composite is synthesized with a simple and efficient ultrasound-assisted wet chemical method. The morphology and microstructure of the composite are examined with field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. The results confirm that cobalt sulfide nanoparticles are homogeneously and tightly attached on the surfaces of rGO. As an anode material for lithium-ion batteries, this composite delivers a high reversible capacity of 994 mA h g−1 after 150 cycles at a current density of 200 mA g−1. A synergistic effect combining the merits of rGO and cobalt sulfide nanoparticles endows the composite with superior electrochemical performances over those of pure cobalt sulfide.

Copper substituted P2-type Na0.67CuxMn1−xO2: a stable high-power sodium-ion battery cathode

While sodium-ion batteries (SIBs) are considered as a next-generation energy storage device because of the higher abundance and lower cost of sodium compared to those of lithium, developing high-power and stable cathode materials remains a great challenge. Here, micron-sized plate-like copper-substituted layered P2-type Na0.67CuxMn1−xO2 is demonstrated to rapidly charge and discharge within 5 minutes while giving a capacity of more than 90 mA h g−1, corresponding to a half-cell energy density of 260 W h (kg cathode)−1 at a power density of 3000 W (kg cathode)−1, which is comparable to that of high-power lithium-ion cathodes. The materials show excellent stability, retaining more than 70% of the initial capacity after 500 cycles at 1000 mA g−1. The good cycle and rate performances of the materials are attributed to copper in the lattice, which stabilizes the crystal structure, increases the average discharge potential and improves sodium transport. This makes Na0.67CuxMn1−xO2 an ideal choice as a cathode for high-power sodium-ion batteries.

Gel polymer electrolytes based on a novel quaternary ammonium salt for dye-sensitized solar cells

Gel polymer electrolytes were prepared with polyacrylonitrile (PAN) and solutions of a novel quaternary ammonium salt, polysiloxane with quaternary ammonium side groups (PSQAS), in a mixture of ethylene carbonate (EC) and propylene carbonate (PC). The influences of PAN content and salt concentration on the ionic conductivity have been investigated. The ionic conductivity can be further improved with the use of the mixtures of KI and PSQAS, which can be expected as inorganic-organic salts. The gel polymer electrolytes were used in the fabrication of the dye-sensitized solar cells with a nanoporous TiO2 working electrode, cis-di(thiocyanato)-N,N′-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) complex dye and a counter electrode based on platinized conducting glass. The cells showed open-circuit voltages (Voc) around 0.6 V and short-circuit current densities (Jsc) larger than 7.5 mA cm−2 under 60 mW cm−2 irradiation. The fill factors (FF) and energy conversion efficiencies (η) of the cells were calculated to be higher than 0.56 and 4.4%, respectively.

Hollow nanospheres of loosely packed Si/SiOx nanoparticles encapsulated in carbon shells with enhanced performance as lithium ion battery anodes

Silicon materials are considered as the new generation of high specific energy and energy density anodes for rechargeable lithium ion batteries, but the silicon pulverization during lithium insertion hinders their commercial implementation. Although extensive effort has been put on addressing these problems, the microstructure of the silicon material still needs to be well engineered in order to improve the stability of the anode materials and simplify the synthesis procedure using a scalable and easy available silicon source without any toxicity. In this work, a novel hollow nanosphere with Si/SiOx nanoparticles incompactly distributed within a spherical carbon shell was successfully fabricated via a facile in situ carbonization/reduction method. With enhanced electrode conductivity and sufficient free space for silicon expansion during the lithiation process, this material shows much better cycle life and rate capability than directly reduced silicon nanoparticles.