Si–Ni alloy–graphite composite synthesized by arc-melting and high-energy mechanical milling for use as an anode in lithium-ion batteries

A Si–Ni alloy and graphite composite is synthesized by arc-melting followed by high-energy mechanical milling. The alloy particles consist of an electrochemically active silicon phase with inactive phases such as NiSi₂ and NiSi distributed uniformly on the surface of the graphite. The inactive phases can accommodate the large volume changes of Si during cycling of the composite as an anode material for lithium batteries. The cycle-life of the composite increases with increase in Si content. A large reversible capacity (about 800 mAh g⁻¹) and good cycleability suggest that the composite may prove to be an alternative to conventional graphite-based anode materials for lithium-ion secondary batteries.     

Carbon anode for dry-polymer electrolyte lithium batteries

A spherical carbon material of meso-carbon microbead (MCMB) was examined as an anode in a polyethylene oxide (PEO) based polymer electrolyte lithium battery. The electrochemical performance of the carbon electrode with the polymer electrolyte depended on the electrode thickness and the particle size of MCMB. The 30 μm-thick electrode of MCMB with the particle size of 20–30 μm showed a reversible capacity comparable with that in a liquid electrolyte, but the 100 μm-thick electrode showed a half of the 30 μm-thick electrode. The smaller particle size of 5–8 μm exhibited a high irreversible capacity at the first charge–discharge cycle. The reaction heat between MCMB and the polymer electrolyte was 0.5 J mAh^?1, which was much lower compared to those between lithium metal and the polymer electrolyte, 1.2 J mAh^?1, and MCMB and conventional liquid electrolyte, 4.3 J mAh^?1.     

Cu5Si–Si/C composites for lithium-ion battery anodes

Cu₅Si–Si/C composites with precursor atomic ratio of Si:Cu = 1, 2 and 4.5 have been produced by high-energy ball-milling of a mixture of copper–silicon alloy and graphite powder for anode materials of lithium-ion battery. X-ray diffraction and scanning electron microscope measurements show that Cu₅Si alloy is formed after the intensive ball milling and alloy particles along with low-crystallite Si are interspersed in graphite uniformly. Cu₅Si–Si/C composite electrodes deliver a larger reversible capacity than commercialized graphite and better cyclability than silicon. The increase of copper amount in the composites decreases reversible capacity but improves cycling performance. Cu₅Si–Si/C composite with Si:Cu = 1 demonstrates an initial reversible capacity of 612 mAh g⁻¹ at 0.2 mA cm⁻² in the voltage range from 0.02 to 1.5 V. The capacity retention is respectively 74.5 and 70.0% at the 40th cycle at the current density of 0.2 and 1 mA cm⁻².     

Formation and characterization of Cu–Si nanocomposite electrodes for rechargeable Li batteries

We have investigated the structural and electrochemical properties of Cu–Si nanocomposite electrode fabricated by co-sputtering method. Reversible capacity of an amorphous Si electrode is degraded continuously with increasing cycle number up to 40 cycles. However, a Cu–Si nanocomposite electrode, where Cu nano-dots are embedded in an amorphous Si matrix, shows an excellent reversible capacity with a stable value of ca. 400 μA h cm⁻² μm⁻¹ up to 40 cycles. The improved reversible capacity of the Cu–Si nanocomposite electrodes is attributed to the enhanced structural stability of the electrodes due to the presence of the Cu nano-dots evenly distributed throughout the Si matrix.     

A thin film silicon anode for Li-ion batteries having a very large specific capacity and long cycle life

A thin film of Si was vacuum-deposited onto a 30 μm thick Ni foil from a source of n-type of Si, the film thickness examined being 200–1500 Å. Li insertion/extraction evaluation was performed mainly with cyclic voltammetry (CV) and constant current charge/discharge cycling in propylene carbonate (PC) containing 1 M LiClO₄ at ambient temperature. The cycleability and the Li accommodation capacity were found to depend on the film thickness. Thinner films gave larger accommodation capacity. A 500 Å thick Si film gave a charge capacity over 3500 mAh g⁻¹ being maintained during 200 cycles under 2 C charge/discharge rate, while a 1500 Å film revealed around 2200 mAh g⁻¹ during 200 cycles under 1 C rate. The initial charge loss could not be ignored but it could be reduced by controlling the deposition conditions.     

Preparation and electrochemical/thermal properties of LiNi0.74Co0.26O2 cathode material

Electrochemical and thermal properties of LiNi_0.74Co_0.26O₂ cathode material with 5, 13 and 25 μm-sized particles have been studied by using a coin-type half-cell Li/LiNi_0.74Co_0.26O₂. The specific capacity of the material ranges from 205 to 210 mA h g⁻¹, depending on the particle size or the Brunauer, Emmett and Teller (BET) surface area. Among the particle sizes, the cathode with a particle size of 13 μm shows the highest specific capacity. Even though the material with a particle size of 5 μm exhibits the smallest capacity value of 205 mA h g⁻¹, no capacity fading was observed after 70 cycles between 4.3 and 2.75 V at the 1 C rate. Differential scanning calorimetry (DSC) studies of the charged electrode at 4.3 V show a close relationship between particle size (BET surface area) and thermal stability of the electrode, namely, a larger particle size (smaller BET surface area) leads to a better thermal stability of the LiNi_0.74Co_0.26O₂ cathode.     

A vacuum deposited Si film having a Li extraction capacity over 2000 mAh/g with a long cycle life

We have found that a Si film vacuum deposited on a Ni foil has a Li insertion capacity over 2000 mAh/g with cycleability over 1000 cycles, but a great issue was its difficulty to obtain a sufficiently thicker film capable of high current charge/discharge. In the present paper the examination of the high current charge/discharge performance of thicker Si film in relation to the film formation condition. The electrochemical evaluation was performed with cyclic voltammetry (CV) and constant current charge/discharge test with various loading currents in PC containing 1 M LiClO₄.A Si film prepared with a rapid deposition rate gave a discharge capacity over 2000 mAh/g even with a very high charge/discharge rate over 10 C. In addition, the surface roughening of the substrate foil was found to play an important role to provide a thick film capable of high current performance. The constant discharge curve gave a wide plateau in the potential range between 200 and 500 mV versus Li/Li⁺. The XRD pattern of the deposited film gave no peaks due to Si, indicating the film to be amorphous. The SEM image of the deposited film was rather homogeneous, and after 500 cycles it still covered the entire surface of the Ni substrate though the surface became inhomogeneous.     

Preparation,electrical, mechanical and thermal properties of composite bipolar plate for a fuel cell

A novel composite bipolar plate for a polymer electrolyte fuel cell has been prepared by a bulk-moulding compound (BMC) process. The electrical resistance of the composite material decreases from 20 000 to 5.8 mΩ as the graphite content is increased from 60 to 80 wt.%. Meanwhile, the electrical resistance of composite increases from 6.5 to 25.2 mΩ as the graphite size is decreased from 1000 to 177 μm to less than 53 μm. The thermal decomposition of 5% weight loss of composite bipolar plate is higher than 250 °C. The oxygen permeability of the composite bipolar plate is 5.82×10⁻⁸ (cm³/cm² s) when the graphite content is 75 wt.%, and increases from 6.76×10⁻⁸ to 3.28×10⁻⁵ (cm³/cm² s) as the graphite size is longer or smaller than 75 wt.%. The flexibility of the plate decreases with increasing graphite content. The flexural strength of the plate decreases with decrease in graphite size from 31.25 MPa (1000–177 μm) to 15.96 MPa (53 μm). The flexural modulus decreases with decrease of graphite size from 6923 MPa (1000–177 μm) to 4585 MPa (53 μm). The corrosion currents for plates containing different graphite contents and graphite sizes are all less than 10⁻⁷ A cm⁻². The composite bipolar plates with different graphite contents and graphite sizes meet UL-94V-0 tests, and the limiting oxygen contents are higher than 50. Testing show that composite bipolar plates with optimum composition are very similar to that of the graphite bipolar plate.     

Thin yttria-stabilized zirconia electrolyte and transition layers fabricated by particle suspension spray

In order to develop high performance intermediate temperature (<800 °C) solid oxide fuel cells (SOFCs) with a lower fabrication cost, a pressurized spray process of ceramic suspensions has been established to prepare both dense yttria-stabilized zirconia (YSZ) electrolyte membranes and transition anode layers on NiO + YSZ anode supports. A single cell with 10 μm thick YSZ electrolyte on a porous anode support and ∼20 μm thick cathode layer showed peak power densities of only 212 mW cm⁻² at 700 °C and 407 mW cm⁻² for 800 °C. While a cell with 10 μm thick YSZ electrolyte and a transition layer on the porous anode support using a ultra-fine NiO + YSZ powder showed peak power densities of 346 and 837 mW cm⁻² at 700 and 800 °C, respectively. The dramatic improvement of cell performance was attributed to the much improved anode microstructure that was confirmed by both scanning electron microscopes (SEM) observation and impedance spectroscopy. The results have demonstrated that a pressurized spray coating is a suitable technique to fabricate high performance SOFCs and at lower cost.     

Enhancement of capacity of carbon-coated Si–Cu3Si composite anode using metal–organic compound for lithium-ion batteries

A carbon-coated Si–Cu₃Si composite material is prepared using silicon and copper(II) d-gluconate powders by simple mechanical milling and pyrolysis, and is investigated as an anode material for lithium-ion batteries. In this process, the Cu₃Si and pyrolyzed carbon uniformly adhere to the surface of the silicon particles. The cycling performance of the composite material exhibits a stable capacity of 850 mAh g⁻¹ for 30 cycles. The improved cycling performance is attributed to the fact that the copper silicide and pyrolyzed carbon provide both a better electrical contact with the current–collector and a buffering effect for the volume expansion–contraction during cycling.     

Four volts class solid lithium polymer batteries with a composite polymer electrolyte

Polyethylene oxide (PEO)-based polymer electrolytes with BaTiO₃ as a filler have been examined as electrolytes in 4 V class lithium polymer secondary batteries. A mixture of 90 wt.% LiN(CF₃SO₂)₂–10 wt.% LiPF₆ was found to be the best candidate as the salt in PEO, and showed high electrical conductivity, good corrosion resistance to the aluminum current collector and low interfacial resistance between the lithium metal anode and the polymer electrolyte. The cyclic performance of the cell, Li/[PEO₁₀–(LiN(CF₃SO₂)₂–10 wt.% LiPF₆)]–10 wt.% BaTiO₃/LiNi_0.8Co_0.2O₂/Al, showed good charge–discharge cycling performance. The observed capacity fading on charging up to 4.2 V at 80 °C in the cell was about 0.28% per cycle in the first 30 cycles, compared to that of 0.5% for the polymer electrolyte without LiPF₆ in the lithium salt.     

Electrical characterization of all-solid-state thin film batteries

All-solid-state thin film micro-batteries comprised of a lithium anode, lithium phosphorus oxy-nitride (LiPON) solid electrolyte and Li_xCoO₂ cathode were evaluated at different temperatures from −50 to 80 °C for electrical behavior and impedance raise. The cell dimensions were ∼2 cm long, ∼1.5 cm wide and ∼15 μm thick. The rated capacity of the cells was about 400 μAh. The cells were cycled (charge/discharge) at room temperature over 100 times at a 0.25C rate. The charge and discharge cut-off voltages were 4.2 and 3.0 V, respectively. The cells did not show any capacity decay over 100 cycles. The measured capacity was 400 μAh. The coulombic efficiency was 1, which suggests that the cell reaction is free from any parasitic side reactions and the lithium intercalation and de-intercalation reaction is completely and totally reversible. These cells also have good high-rate performance at room temperature. For example, these cells discharged at a 2.5C rate delivered ∼90% of the capacity at a 0.25C rate. However, the delivered capacities even at a 0.25C rate at 80 and −50 °C were much lower than the room temperature capacity. Cells soaked at −50 °C were not damaged permanently as seen by the near normal behavior when returned to room temperature. However, cells heated to 80 °C were permanently damaged as seen by the lack of normal performance back at room temperature. Cell impedance was measured before and after cycling at different temperatures. The high-frequency resistance (generally ascribed to the electrolyte and other resistances in series with the electrolyte resistance) decreased with decreasing temperature. However, the interfacial resistance increased significantly with decreasing temperature. Further, the electrolyte resistance accounted for ∼2% of the total cell resistance. The cycled cells showed higher impedance than the uncycled cells.     

Performance of Ni/ScSZ cermet anode modified by coating with Gd0.2Ce0.8O2 for an SOFC running on methane fuel

A Ni/scandia-stabilized zirconia (ScSZ) cermet anode was modified by coating with nano-sized gadolinium-doped ceria (GDC, Gd_0.2Ce_0.8O₂) prepared using a simple combustion process within the pores of the anode for a solid oxide fuel cell (SOFC) running on methane fuel. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed in the anode characterizations. Then, the short-term stability for the cells with the Ni/ScSZ and 2.0 wt.%GDC-coated Ni/ScSZ anodes in 97%CH₄/3%H₂O at 700 °C was checked over a relative long period of operation. Open circuit voltages (OCVs) increased from 1.098 to 1.179 V, and power densities increased from 224 to 848 mW cm⁻², as the operating temperature of an SOFC with 2.0 wt.%GDC-coated Ni/ScSZ anode was increased from 700 to 850 °C in humidified methane. The coating of nano-sized Gd_0.2Ce_0.8O₂ particle within the pores of the porous Ni/ScSZ anode significantly improved the performance of anode supported cells. Electrochemical impedance spectra (EIS) illustrated that the cell with Ni/ScSZ anode exhibited far greater impedances than the cell with 2.0 wt.%GDC-coated Ni/ScSZ anode. Introduction of nano-sized GDC particles into the pores of porous Ni/ScSZ anode will result in a substantial increase in the ionic conductivity of the anode and increase the triple phase boundary region expanding the number of sites available for electrochemical activity. No significant degradation in performance has been observed after 84 h of cell testing when 2.0 wt.%GDC-coated Ni/ScSZ anode was exposed to 97%CH₄/3%H₂O at 700 °C. Very little carbon was detected on the anodes, suggesting that carbon deposition was limited during cell operation. Consequently, the GDC coating on the pores of anode made it possible to have good stability for long-term operation due to low carbon deposition.     

Electrochemical properties of the Li-ion polymer batteries with P(VdF-co-HFP)-based gel polymer electrolyte

Gel polymer electrolytes consisting of 25 wt.% P(VdF-co-HFP), 65 wt.% ethylene carbonate + propylene carbonate and 10 wt.% LiN(CF₃SO₂)₂ are prepared using by a solvent-casting technique. The electrodes are for use in lithium-ion polymer batteries. The electrochemical characteristics of the gel polymer electrolytes are evaluated by means of ac impedance and cyclic voltammetry. The charge–discharge performance of lithium polymer and lithium-ion polymer batteries is examined. A LiCoO₂ | gel polymer electrolyte (GPE) | mesocarbon microbeads (MCMB) cell delivers a discharge capacity of 146.8 and 144.5 mAh g⁻¹ on the first and the 20th cycle, respectively. The specific discharge capacity is greater than 140 mAh g⁻¹ for up to 20 cycle at all the current densities examined.     

High capacity Si/C nanocomposite anodes for Li-ion batteries

Nanocomposites of Si/C were synthesized from Si and polystyrene (PS) resin using high-energy mechanical milling (HEMM) followed by subsequent heat-treatment. The resultant nanocomposites are comprised of amorphous carbon and nanocrystalline silicon as verified by X-ray diffraction (XRD). The XRD results also indicate the presence of iron silicide (FeSi) arising as a contaminant during HEMM. The Si/C nanocomposite corresponding to Si:C = 1:2 composition obtained after milling in two stages of 12 h each for a total time period of 24 h shows a capacity as high as ∼850 mAh/g with reasonable capacity retention (∼1.1% loss/cycle). The increase in either heat-treatment temperature or milling time renders the nanocomposites more stable at the expense of capacity. Transmission electron microscopy (TEM) analysis shows that the HEMM derived Si nanocrystallites <50 nm in size are distributed homogeneously within the amorphous carbon matrix.     

Heat transfer enhancement in a hybrid microchannel-photovoltaic cell using Boehmite nanofluid

Experiments were conducted to investigate the cooling performance of water-based Boehmite (AlOOH · xH₂O) nanofluid in a hybrid photovoltaic (PV) cell. A Perspex plate consists of 40 parallel rectangular microchannels with a hydraulic diameter of 783 μm, a length of 24 cm, a width of 1.8 mm and a depth of 500 μm attached to the back of the cell. Cooling performances of water, as the base fluid, and three different concentrations of nanofluid (0.01, 0.1 and 0.3 wt.%) were compared. The nanofluid thermal performance has been assessed from the obtained results for outlet flow temperature and the average PV surface temperature. The average PV surface temperature decreased from 62.29 °C to 32.5 °C at zero and 300 ml/min of flow rate for 0.01 wt.% nanofluid, respectively. Moreover, the highest improving in the electrical efficiency was achieved about 27% for 0.01 wt.% concentration of the nanofluid at this flow rate.     

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.     

Electrocatalytic characteristics of the metal hydride electrode for advanced Ni/MH batteries

The electrocatalytic characteristics of a metal hydride (MH) electrode for advanced Ni/MH batteries include the hydrogen adsorption/desorption capability at the electrode/electrolyte interface. The hydrogen reactions at the MH electrode/electrolyte interface are also related to factors such as the surface area of the MH alloy powder and the nature of additives and binder materials. The high-rate discharge capability of the negative electrode in a Ni/MH battery is mainly determined by the mass transfer process in the bulk MH alloy powder and the charge transfer process at the interface between the MH alloy powder and the electrolyte. In this study, an AB₅-type hydrogen-absorbing alloy, Mm (Ni, Co, Al, Mn)_5.02 (where Mm denotes Mischmetal, comprising 43.1 wt.% La, 3.5 wt.% Ce, 13.3 wt.% Pr and 38.9 wt.% Nd), was used as the negative MH electrode material. The MH electrode was charged and discharged for up to 200 cycles. The specific discharge capacity of the alloy electrode decreases from a maximum value of 290–250 mAh g⁻¹ after 200 charge/discharge cycles. A cyclic voltammetry technique is used to analyze the charge transfer reactions at the electrode/electrolyte interface and the hydrogen surface coverage capacity.     

Development of micro direct methanol fuel cells for high power applications

A micro direct methanol fuel cell (μDMFC) with active area of 1.625 cm² has been developed for high power portable applications and its electrochemical characterization carried out in this study. The fragility of the silicon wafer makes it difficult to compress the cell for good sealing and hence to reduce contact resistance in the Si-based μDMFC. We have instead used very thin stainless steel plates as bipolar plates with the flow field machined by photochemical etching technology. For both anode and cathode flow fields, widths of both the channel and rib were 750 μm, with a channel depth of 500 μm. A gold layer was deposited on the stainless steel plate to prevent corrosion. This study used an advanced MEA developed in-house featuring a modified anode backing structure with a compact microporous layer. Maximum power density of the micro DMFC reached 62.5 mW cm⁻² at 40 °C, and 100 mW cm⁻² at 60 °C at atmospheric pressure, which almost doubled the performance of our previous Si-based μDMFC.     

Anode-supported solid oxide fuel cell with yttria-stabilized zirconia/gadolinia-doped ceria bilalyer electrolyte prepared by wet ceramic co-sintering process

To protect the ceria electrolyte from reduction at the anode side, a thin film of yttria-stabilized zirconia (YSZ) is introduced as an electronic blocking layer to anode-supported gadolinia-doped ceria (GDC) electrolyte solid oxide fuel cells (SOFCs). Thin films of YSZ/GDC bilayer electrolyte are deposited onto anode substrates using a simple and cost-effective wet ceramic co-sintering process. A single cell, consisting of a YSZ (∼3 μm)/GDC (∼7 μm) bilayer electrolyte, a La_0.8Sr_0.2Co_0.2Fe_0.8O₃–GDC composite cathode and a Ni–YSZ cermet anode is tested in humidified hydrogen and air. The cell exhibited an open-circuit voltage (OCV) of 1.05 V at 800 °C, compared with 0.59 V for a single cell with a 10-μm GDC film but without a YSZ film. This indicates that the electronic conduction through the GDC electrolyte is successfully blocked by the deposited YSZ film. In spite of the desirable OCVs, the present YSZ/GDC bilayer electrolyte cell achieved a relatively low peak power density of 678 mW cm⁻² at 800 °C. This is attributed to severe mass transport limitations in the thick and low-porosity anode substrate at high current densities.