A high performance silicon/carbon composite anode with carbon nanofiber for lithium-ion batteries

The electrochemical performance of a composite of nano-Si powder and a pyrolytic carbon of polyvinyl chloride (PVC) with carbon nanofiber (CNF) was examined as an anode for lithium-ion batteries. CNF was incorporated into the composite by two methods; direct mixing of CNF with the nano-Si powder coated with carbon produced by pyrolysis of PVC (referred to as Si/C/CNF-1) and mixing of CNF, nano-Si powder, and PVC with subsequent firing (referred to as Si/C/CNF-2). The external Brunauer-Emmett-Teller (BET) surface area of Si/C/CNF-1 was comparable to that of Si/C/CNF-2. The micropore BET surface area of Si/C/CNF-2 (73.86 m² g⁻¹) was extremely higher than that of Si/C/CNF-1 (0.74 m² g⁻¹). The composites prepared by both methods exhibited high capacity and excellent cycling stability for lithium insertion and extraction. A capacity of more than 900 mA h g⁻¹ was maintained after 30 cycles. The coulombic efficiency of the first cycle for Si/C/CNF-1 was as low as 53%, compared with 73% for Si/C/CNF-2. Impedance analysis of cells containing these anode materials suggested that the charge transfer resistance for Si/C/CNF-1 was not changed by cycling, but that Si/C/CNF-2 had high charge transfer resistance after cycling. A composite electrode prepared by mixing Si/C/CNF-2 and CNF exhibited a high reversible capacity at high rate, excellent cycling performance, and a high coulombic efficiency during the first lithium insertion and extraction cycles.     

Electrochemical performance of a tin-coated carbon fibre electrode for rechargeable lithium-ion batteries

An Sn-carbon fibre composite electrode is fabricated by electrodepositing a thin film (0.5 ± 0.1 μm) of Sn with an ultrafine grain size (350 ± 50 nm) on the 7.5 ± 1.5 μm diameter fibres of a carbon fibre paper (CFP). The electrochemical performance of the Sn-CFP composite being considered as an anode material for rechargeable Li-ion batteries is evaluated by conducting galvanostatic charge-discharge cycling tests. The Sn-CFP electrode displays a reversible planar capacity of 2.96 mAh cm⁻² with a capacity retention of 50% after twenty cycles, compared to the 23% measured for a 2.2 ± 0.2 μm thick Sn coating deposited on a Cu foil. The enhanced cycling performance of the Sn-CFP electrode is attributed to the double role played by carbon fibres, which act as randomly oriented current collectors in addition to being an active material. The small thickness and large surface area of the Sn coating on the carbon fibres enhances the coating's chemical reactivity and tolerance for volume change. It is suggested that transforming Sn to Sn oxides in Sn-CFP electrodes may improve the cycling performance of these composites as anode materials for rechargeable Li-ion batteries.     

High performance Si/C@CNF composite anode for solid-polymer lithium-ion batteries

The electrochemical performance of a composite of nano-Si powder and a pyrolytic carbon of polyvinyl chloride (PVC) with carbon nanofiber (CNF) was examined as an anode for solid-polymer lithium-ion batteries. Nano-Si powder was firstly coated with carbon by pyrolysis of PVC and then mixed with CNF (referred to as Si/C@CNF) using a rotation mixer. The composite exhibited good cycling performance, but suffered from a large irreversible capacity loss of which the retention was less than 60%. In order to reduce the loss, a thin lithium sheet was attached to the Si/C@CNF electrode surface as a reducing agent. The irreversible capacity of the first cycle was lowered to as much as 0 mAh g⁻¹ and after the third cycle, the lithium insertion and extraction efficiency was almost 100%. A reversible capacity of more than 1000 mAh g⁻¹ was still maintained after 40 cycles.     

Surface modification of aluminum with tin oxide coating

Tin oxide (SnO) was coated on the surface of aluminum spherules with an average particle size of 37 μm by a chemical deposition method to improve the electrochemical properties. The samples were characterized by particle size analysis, X-ray diffraction (XRD), scanning electron microscope (SEM), ac impedance spectroscopy and galvanostatic cycling. Pure aluminum electrode delivers an initial reversible capacity of 779 mAh g⁻¹, whose capacity loss is 58% after 10 cycles. In comparison, 10 wt% SnO–Al composite delivers an initial reversible capacity of 806 mAh g⁻¹ with the capacity loss of 28% after 10 cycles. Results show that SnO coating plays an important role in the improvement of the electrochemical performances. It could not only reinforce the mechanical stability of aluminum particles, but also provide better electronic contacts to the electrode.     

Durability study and lifetime prediction of baseline proton exchange membrane fuel cell under severe operating conditions

Comparative studies of mechanical and electrochemical properties of Nafion^®- and sulfonated polyetheretherketone polymer-type membranes are carried out under severe fuel cell conditions required by industrials, within stationary and cycling electric load profiles. These membranes are proposed to be used in PEM between 70 and 90 °C as fluorinated or non-fluorinated baseline membranes, respectively. Thus, thought the performance of both membranes remains suitable, Nafion^® backbone brought better mechanical properties and higher electrochemical stabilities than sulfonated polyetheretherketone backbone. The performance stability and the mechanical strength of the membrane–electrode assembly were shown to be influenced by several intrinsic properties of the membrane (e.g., thermal pre-treatment, thickness) and external conditions (fuel cell operating temperature, relative humidity). Finally, a lifetime prediction for membranes under stationary conditions is proposed depending on the operation temperature. At equivalent thicknesses (i.e. 50 μm), Nafion^® membranes were estimated able to operate into the 80–90 °C range while sulfonated polyetheretherketone would be limited into the 70–80 °C range. This approach brings baseline information about the capability of these types of polymer electrolyte membrane under fuel cell critical operations. Finally, it is revealed as a potential tool for the selection of the most promising advanced polymers for the ensuing research phase.     

Solvothermal synthesis of hierarchical LiFePO4 microflowers as cathode materials for lithium ion batteries

Hierarchical LiFePO₄ microflowers have been successfully synthesized via a solvothermal reaction in ethanol solvent with the self-prepared ammonium iron phosphate rectangular nanoplates as a precursor, which is obtained by a simple water evaporation method beforehand. The hierarchical LiFePO₄ microflowers are self-assemblies of a number of stacked rectangular nanoplates with length of 6-8 μm, width of 1-2 μm and thickness of around 50 nm. When ethanol is replaced with the water-ethanol mixed solvent in the solvothermal reaction, LiFePO₄ micro-octahedrons instead of hierarchical microflowers can be prepared. Then both of them are respectively modified with carbon coating through a post-heat treatment and their morphologies are retained. As a cathode material for rechargeable lithium ion batteries, the carbon-coated hierarchical LiFePO₄ microflowers deliver high initial discharge capacity (162 mAh g⁻¹ at 0.1 C), excellent high-rate discharge capability (101 mAh g⁻¹ at 10 C), and cycling stability, which exhibits better electrochemical performances than carbon-coated LiFePO₄ micro-octahedrons. These enhanced electrochemical properties can be attributed to the hierarchical micro/nanostructures, which can take advantage of structure stability of micromaterials for long-term cycling. Furthermore the rectangular nanoplates as the building blocks can improve the electrochemical reaction kinetics and finally promote the rate performance.     

Lithium-ion batteries based on carbon–silicon–graphite composite anodes

The paper is devoted to the development of lithium-ion battery grade negative electrode active materials with higher reversible capacity than that offered by conventional graphite. The authors report on results of their experiments as related to the electrochemical performance of silicon-based materials for lithium-ion batteries. A commercial grade of spherically shaped natural graphite (FormulaBT™ SLA1025) was modified in a number of different ways with nano-sized silicon. The reversible capacity of SLA1025 modified by 9.2 wt% of the nano-sized amorphous silicon was seen to be as high as 590 mAh g⁻¹. The irreversible capacity loss with this compound was 20%. Lithium-ion batteries using such material were observed to display sharp capacity decay during prolonged cycling. In contrast, the reversible capacity of another experimental grade, the SLA1025 modified by 7.9 wt% of the carbon-coated Si was as high as 604 mAh g⁻¹. The irreversible capacity loss with this material was as low as 8.1%. This grade, also, was seen to display much better cycling performance than the baseline natural graphite.     

Towards a rechargeable alcohol biobattery

This research focused on the transition of biofuel cell technology to rechargeable biobatteries. The bioanode compartment of the biobattery consisted of NAD-dependent alcohol dehydrogenase (ADH) immobilized into a carbon composite paste with butyl-3-methylimidazolium chloride (BMIMCl) ionic liquid serving as the electrolyte. Ferrocene was added to shuttle electrons to/from the electrode surface/current collector. The bioanode catalyzed the oxidation of ethanol to acetaldehyde in discharge mode. This bioanode was coupled to a cathode that consisted of Prussian Blue in a carbon composite paste with Nafion 212 acting as the separator between the two compartments. The biobattery can be fabricated in a charged mode with ethanol and have an open circuit potential of 0.8 V in the original state prior to charging or in the discharged mode with acetaldehyde and have an open circuit potential of 0.05 V. After charging it has an open circuit potential of 1.2 V and a maximum power density of 13.0 μW cm⁻³ and a maximum current density of 35.0 μA cm⁻³, respectively. The stability and efficiency of the biobattery were studied by cycling continuously at a discharging current of 0.4 mA and the results obtained showed reasonable stability over 50 cycles. This is a new type of secondary battery inspired by the metabolic processes of the living cell, which is an effective energy conversion system.     

Structural and electrochemical characterization of tin-containing graphite compounds used as anodes for Li-ion batteries

Two tin–graphite composites (“core-shell” structures) with different metal content (80 wt% and 20 wt%) as well as their structural and electrochemical characteristics are presented. Mitsubishi's synthetic carbon was used as starting material for the modification experiments. Chemical reduction was applied for the coating process, which was carried out under inert argon atmosphere. Although a homogeneous film of the nanoscale tin particles (∼60 nm) have been achieved, the electrochemical performance improvement strongly depends on the thickness of the “shell’ layer and the progressively increased active surface area together with the tin metal contents. The electrode with low metal concentration displayed both improved cycling performance and stable discharge capacity of 435 Ah kg⁻¹ compared with untreated graphite electrode. The tin-rich composite shows a higher medial discharge capacity (540 mAh g⁻¹) but increased capacity fading, while higher metal contents lead to bulk-coated film with disassociated and agglomerated tin nanoparticles as well as higher surface area and likely presence of oxide impurities.     

Electrodeposited porous-microspheres Li-Si films as negative electrodes in lithium-ion batteries

A porous-microspheres Li-Si film (PMLSF) is prepared by multi-step constant current (MSCC) electrodeposition on Cu foil. Its structure and morphology are characterized using X-ray diffraction (XRD) and scanning electron microscope (SEM). As negative electrodes of lithium-ion batteries, the PMLSF electrode delivers the first gravimetric and geometric charge capacities of 2805.7 mA h g⁻¹ and 621.9 μA h cm⁻² at the current density of 25.5 μA cm⁻², and its initial coulombic efficiency is as high as 98.2%. When the PMLSF electrode is cycled in VC-containing electrolyte, the superior cycling performance can be obtained. After 50 cycles, 96.0% of its initial capacity is retained at the current density of 50.0 μA cm⁻². Electrochemical impedance spectra (EIS) research confirms the positive effect of VC additive on the behavior of the PMLSF electrode.     

Increase of positive active material utilization in lead-acid batteries using diatomaceous earth additives

In this study we examined the use of diatomites to improve the discharge capacity and utilization of the positive electrode of the lead-acid battery. A large fraction of the positive electrode performance of this battery system (half-reaction shown below) is based on the ionic conduction of sulfuric acid through the plate.

PbO₂(s) + HSO₄⁻ + 3H⁺ + 2e⁻ → PbSO₄(s) + 2H₂O

Anode properties of thick-film electrodes prepared by gas deposition of Ni-coated Si particles

Thick-film electrodes of Si particles coated with Ni, Ni-Sn, and Ni-P were fabricated by electroless deposition followed by gas deposition to form the anode of a Li-ion battery. The electrode of Ni-coated Si showed remarkably improved cycling performance with a discharge capacity of 580 mA h g⁻¹ at the 1000th cycle, which is possibly caused by its higher elastic modulus than that of the uncoated Si electrode. The electrode of Si coated with Ni-P, which consisted of Ni₃P, with the lower coating amount exhibited a higher initial capacity and excellent cycling performance with a capacity of 790 mA h g⁻¹ at the 1000th cycle, whereas poor performance was obtained for the electrode of Si coated with Ni-Sn. The excellent performance in the case of Ni-P coating is attributed to the smaller amount of coating, the high elastic modulus, and the lower reactivity of Ni₃P with Li ions in comparison with Ni₃Sn in Ni-Sn.     

Study on the surface modification of metal hydride electrodes with cobalt

A novel method has been applied to the surface modification of the metal hydride (MH) electrode of the MH/Ni batteries. Both sides of the electrode were plated with a thin cobalt film about 0.15 μm using vacuum evaporate plating technology and the effect of the electrode on the performance of the MH/Ni batteries was examined. It was found that the surface modification could enhance the electrode conductivity and decrease the battery ohmic resistance. After surface modification, the discharge capacity at 5C (8.5 A) was increased by 115 mAh and discharge voltage was increased by 0.04 V, the resistance of the batteries was also decreased by 18%. The batteries with modified electrode exhibited satisfactory durability. The remaining capacity of the modified batteries was 93% of the initial capacity even after 500 cycles. The inner pressure of the batteries during overcharging was lowered and the charging efficiency of the batteries was improved.     

Improvement of cyclability of Si as anode for Li-ion batteries

Silicon working as anode for Li-ion batteries has attracted much attention due to its high capacity (∼4200 mAh g⁻¹). However, due to the large volume expansion during lithiation, the capacity of silicon fades very fast. In this systematic study, we focus on the issue to fight the capacity fading. Results show that Si with sodium carboxymethyl cellulose (Na-CMC) as a polymer binder exhibits a better cyclability than that with poly(vinylidene fluoride) (PVDF). Yet differing from the system used in PVDF, the addition of vinylene carbonate (VC) does not improve or even worsens the performance of the system using Na-CMC. In addition, the small particle size of Si, a large amount of carbon black (CB), the good choice of electrolyte/conducting salt and charge-discharge window also play important roles to enhance the cyclability of Si. It is found that electrode consisting of 40 wt.% nano-Si, 40 wt.% carbon black and 20 wt.% Na-CMC (pH 3.5) displays the best cyclability, and in the voltage range from 0 to 0.8 V, after 200 cycles, its capacity can still keep 738 mAh g⁻¹ (C/2, in 1 M LiPF₆ ethylene carbonate/diethyl carbonate electrolyte, with VC-free), almost twice as that of graphite.     

NH4V3O8/carbon nanotubes composite cathode material with high capacity and good rate capability

NH₄V₃O₈/carbon nanotubes (CNTs) composites are synthesized by one-step hydrothermal method. All the samples show the flake-like morphology with the width of up to 5 μm and thickness of 500 nm and the CNTs are clearly observed on the surface of modified NH₄V₃O₈. It is found that incorporation of 0.5 wt% CNTs into NH₄V₃O₈ could greatly improve its discharge capacity and cycling stability. It delivers a maximum discharge capacity of 358.7 mAh g⁻¹ at 30 mA g⁻¹, 55 mAh g⁻¹ larger than that of the pristine one. At 150 mA g⁻¹, the composite shows 226.2 mAh g⁻¹ discharge capacity with excellent capacity retention of 97% after 100 cycles. The much improved electrochemical performance of NH₄V₃O₈ is attributed to incorporation of CNTs, which facilitates the interface charge transfer and Li⁺ diffusion.     

Spherical silicon/graphite/carbon composites as anode material for lithium-ion batteries

A spherical nanostructured Si/graphite/carbon composite is synthesized by pelletizing a mixture of nano-Si/graphite/petroleum pitch powders, followed by heat treatment at 1000 °C under an argon atmosphere. The structure of the composite sphere is examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDAX). The resultant composite sphere consists of nanosized silicon and flaked graphite embedded in a carbon matrix pyrolyzed from petroleum pitch, in which the flaked graphite sheets are concentrically distributed in a parallel orientation. The composite material exhibits good electrochemical properties, a high reversible specific capacity of ∼700 mAh g⁻¹, a high coulombic efficiency of 86% on the first cycle, and a stable capacity retention. The enhanced electrochemical performance is attributed to the structural stability of the composite sphere during the charging–discharging process.     

Electrochemical properties of silicon deposited on patterned wafer

An amorphous silicon thin-film deposited on a patterned wafer is prepared by radio-frequency (rf) magnetron sputtering and is characterized by X-ray diffraction, galvanostatic cycle testing and field emission scanning electron microscopy. The specimen is assembled in cell of configuration: silicon working electrode/1 M LiPF₆ in EC/DMC, electrolyte/lithium metal, counter electrode (EC = ethylenecarbonate; DMC = dimethyl carbonate). A patterned silicon (1 0 0) wafer prepared by photolithography and KOH etching is used as the electrode substrate. The size of the patterns, which are composed of arrays of the negative square pyramids, is 5 μm/side.The patterned specimen (silicon film on patterned substrate) is compared with a normal specimen (silicon deposited on a flat substrate). The rate of capacity fade on cycling is monitored as a function of the voltage window and current density. The patterned specimen displays better cycle behaviour at a high current density (high C-rate).During the cycle tests at 200 μA cm⁻², the silicon electrodes yield an initial capacity of 327 μAh (cm² μm)⁻¹. After 100 cycles, the capacity is 285 μAh (cm² μm)⁻¹ and the capacity retention is 86%. Capacity retention is 76 and 61% at cycles 200 and 300, respectively.     

Carbon-coated MoO3 nanobelts as anode materials for lithium-ion batteries

MoO₃ nanobelts are synthesized by a simple hydrothermal route followed by carbon coating. The effects of the carbon coating on the nanobelts are investigated by Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS), a transmission electron microscope (TEM), and galvanostatic cycling. As observed from the TEM and SEM images, the C-MoO₃ nanobelts have a diameter of 150 nm and a length of 5-8 μm. In the electrochemical results, the C-MoO₃ nanobelts exhibit excellent cycling stability after being cycled at a current rate of 0.1 C, maintaining their capacity at 1064 mAh g⁻¹ after 50 cycles. These results are better than those for a bare MoO₃ nanobelt electrode. The excellent electrochemical performance of the C-MoO₃ nanobelts can be attributed to the effects of the carbon coating which stabilizes the structure of the MoO₃, enhances the ionic/electrical conductivity, and moreover, can serve as a buffering agent to absorb the volume expansion during the Li⁺ intercalation process.     

Integration of silicon micro-hole arrays as a gas diffusion layer in a micro-fuel cell

This work examines silicon micro-hole arrays (Si-MHA) as a gas diffusion layer (GDL) in a micro-fuel cell that was fabricated using micro-electro-mechanical systems (MEMS) fabrication technique. Pt was deposited on the surface of the Si-MHA, to increase the conductivity of the micro-fuel cell. The Si-MHA with three micro-holes, replaces the traditional GDL, and the performance of the micro-proton exchange membrane fuel cell was discussed. Wet etching was performed on a 500 μm-thick layer of silicon to yield fuel channels with a depth of 450 μm and a width of 200 μm. The Si-MHA formed by deep reactive ion etching (DRIE) in the fabricated structure had diameters of 10 μm, 30 μm and 50 μm; the thickness of the structure was 50 μm.     

Bilayered nanofilm of polypyrrole and poly(DMcT) for high-performance battery cathodes

Bilayered nanofilm electrodes made of polypyrrole (Ppy) and poly-2,5-dimercapto-1,3,4-thiadiazole (poly-DMcT) are produced by electrochemical means onto a carbon-fiber substrate, with the goal of preventing the loss of the electroactive mass of the disulfide and improving the electrode stability during the charge/discharge cycling process. Lightweight and high surface area composites are obtained by potentiostatically depositing a nanometric layer of Ppy onto a carbon fiber piece already supporting a layer of a potentiodynamically obtained poly(DMcT) film. The growth charge/mass ratio for the bilayered polymeric electrode is optimized, leading to a high electrochemical performance cathode with a stable specific capacity of ∼320 mA h g⁻¹ after 100 cycles.