Vapor-induced solid-liquid-solid process for silicon-based nanowire growth

Silicon-based nanowires have been grown from commercial silicon powders under conditions with different oxygen and carbon activities. Nanowires grown in the presence of carbon sources consisted of a crystalline SiC core with an amorphous SiO_x shell. The thickness of the SiO_x shell decreased as the oxygen concentration in the precursor gases decreased. Nanowires grown in a carbon-free environment consisted of amorphous silicon oxide with a typical composition of SiO_1.8. The growth rate of nanowires decreased with decreasing oxygen content in the precursor gases. SiO_1.8 nanowires exhibited an initial discharge capacity of ∼1300 mAh g⁻¹ and better stability than those of silicon powders. A vapor-induced solid-liquid-solid (VI-SLS) mechanism is proposed to explain the nanowire growth (including silicon and other metal-based nanowires) from powder sources. In this approach, both a gas source and a solid-powder source are required for nanowire growth. This mechanism is consistent with experimental observations and also can be used to guide the design and growth of other nanowires.     

Nickel oxyhydroxide/manganese dioxide composite as a candidate electrode material for alkaline secondary cells

Nickel hydroxide and manganese dioxide are used in alkaline cells as positive electrode materials. Positive electrodes comprising a nickel oxyhydroxide/manganese dioxide composite, with modification by Bi₂O₃, deliver a combined reversible discharge capacity of 2.25e per metal atom (650 mAh g⁻¹ metal content), which is higher than that realized from electrodes of either component taken singly. The composite discharges with two potential plateaux, the first appearing at 325 mV corresponds to the discharge of the nickel component, whereas the second at −600 mV is due to the manganese component. Composites of NiO(OH)/MnO₂ can be used as a new electrode material with higher discharge capacity than conventional electrodes.     

Mechanism of lithium storage in Si-O-C composite anodes

A Si-O-C composite material is prepared by pyrolyzing a copolymer of phenyl-substituted polysiloxane and divinylbenzene at 800 °C under a hydrogen atmosphere. The material has a high delithiation capacity about 965.3 mA h g⁻¹ in the first cycle and retains 660 mA h g⁻¹ after 40 cycles at 50 mA g⁻¹. The differential capacity curves of the anode show that there are several reduction peaks between 0.2 and 0.6 V existing all the time during repeated cycles. By comparing ²⁹Si nuclear magnetic resonance (²⁹Si MAS NMR), Si (2p) X-ray photoelectron spectroscopy (XPS) of the anode in the original, fully lithiated, and fully delithiated state, the reduction peaks are related to lithium reversible insertion into SiO₂C₂, SiO₃C, and SiO₄ units, respectively. The corresponding ²⁹Si MAS NMR resonances shift to high field and their binding energies of the Si (2p) XPS peak increase in the fully lithiated state, and then both turn to the opposite direction in the fully delithiated state. The SiO₄ units decrease during repeated cycles. The remaining ones can reversibly transform to Li-silicate (Li₂SiO₃) when lithium is inserted, while the lost ones irreversibly transform to Li-silicate (Li₄SiO₄). However, the SiOC₃ units of the material are totally irreversible with lithium because they nearly disappear in the first discharge process, and lead to the formation SiC₄ units.     

Electrodeposition of mesoporous manganese dioxide supercapacitor electrodes through self-assembled triblock copolymer templates

Mesoporous manganese dioxide supercapcitor electrode materials were electrochemically deposited onto silicon substrates coated with Pt using triblock copolymer species (Pluronic P123 and F127) as the structure-directing agents. Deposited electrodes of manganese dioxide film were physically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and were electrochemically characterized by cyclic voltammetry (CV) in 0.5 M Na₂SO₄ electrolyte. Maximum specific capacitance (SC) values of 449 F g⁻¹ was obtained at a scan rate of 10 mV s⁻¹ from F127 templated mesoporous MnO₂.     

Preparation of manganese oxide with high density by decomposition of MnCO3 and its application to synthesis of LiMn2O4

Manganese oxide with high tap density was prepared by decomposition of spherical manganese carbonate, and then LiMn₂O₄ cathode materials were synthesized by solid-state reaction between the manganese oxide and lithium carbonate. Structure and properties of the samples were determined by X-ray diffraction, Brunauer–Emmer–Teller surface area analysis, scanning electron microscope and electrochemical measurements. With increase of the decomposition temperature from 350 °C to 900 °C, the tap density of the manganese oxide rises from 0.91 g cm⁻³ to 2.06 g cm⁻³. Compared with the LiMn₂O₄ cathode made from chemical manganese dioxide or electrolytic manganese dioxide, the LiMn₂O₄ made from manganese oxide of this work has a larger tap density (2.53 g cm⁻³), and better electrochemical performances with an initial discharge capacity of 117 mAh g⁻¹, a capacity retention of 93.5% at the 15th cycle and an irreversible capacity loss of 2.24% after storage at room temperature for 28 days.     

Electrochemical properties of carbon-coated Si/B composite anode for lithium ion batteries

Carbon-coated Si and Si/B composite powders prepared by hydrocarbon gas (argon + 10 mol% propylene) pyrolysis were investigated as the anodes for lithium-ion batteries. Carbon-coated silicon anode demonstrated the first discharge and charge capacity as 1568 mAh g⁻¹ and 1242 mAh g⁻¹, respectively, with good capacity retention for 10 cycles. The capacity fading rate of carbon-coated Si/B composite anode decreased as the amounts of boron increased. In addition, the cycle life of carbon-coated Si/B/graphite composite anode has been significantly improved by using sodium carboxymethyl cellulose (NaCMC) and styrene butadiene rubber (SBR)/NaCMC mixture binders compared to the poly(vinylidene fluoride, PVdF) binder. A reversible capacity of about 550 mAh g⁻¹ has been achieved at 0.05 mAm g⁻¹ rate and its capacity could be maintained up to 450 mAh g⁻¹ at high rate of 0.2 mAm g⁻¹ even after 30 cycles. The improvement of the cycling performance is attributed to the lower interfacial resistance due to good electric contact between silicon particles and copper substrate.     

TiO2(B) as a promising high potential negative electrode for large-size lithium-ion batteries

Needle-like TiO₂(B) powder was obtained from K₂Ti₄O₉ precursor by ion exchange to protons, followed by dehydration. The charge and discharge characteristics of the TiO₂(B) powder were investigated as a high potential negative electrode in lithium-ion batteries. It had a high discharge capacity of 200–250 mAh g⁻¹ at around 1.6 V vs. Li/Li⁺, which was comparable with that of TiO₂(B) nanowires and nanotubes prepared via a hydrothermal reaction in alkaline solution. It showed very good cycleability, and gave a discharge capacity of 170 mAh g⁻¹ even in the 650th cycle. It also had a high rate capability, and gave a discharge capacity of 106 mAh g⁻¹ even at 10 °C.     

Sol–gel synthesis of Mg1.03Mn0.97SiO4 and its electrochemical intercalation behavior

Magnesium manganese silicate (Mg_1.03Mn_0.97SiO₄) was prepared by a sol–gel method and evaluated as an intercalation electrode material for rechargeable magnesium batteries. The crystalline Mg_1.03Mn_0.97SiO₄ phase was obtained after heating at 900 °C and its electrochemical performance was characterized at room temperature. The pure magnesium manganese silicate exhibits a relatively low reversible specific capacity in the electrolyte comprising 0.25 mol L⁻¹ Mg(AlCl₂EtBu)₂/THF owing to its poor electronic conductivity. Using a ball mill in the presence of acetylene black, and in situ carbon coating, the resulting composites present an improved discharge voltage plateau (1.6 V vs. Mg/Mg²⁺) and increased discharge specific capacity (92.9 mAh g⁻¹ at a C/50 rate). The Mg lower price and its feasibility for rechargeable batteries make magnesium manganese silicate an attractive candidate for rechargeable magnesium based batteries.     

New manganese dioxides for lithium batteries

Lithium/manganese dioxide primary batteries use heat treated manganese dioxide (HEMD), a defect pyrolusite structure material as the cathode active material. Ion exchange of the structural protons in electrolytic manganese dioxide (EMD) with lithium before heating results in formation of a lithium containing γ-MnO₂. Increased lithium hydroxide concentration and increased temperature lead to increased lithium levels. At 80 °C with a combination of LiOH and LiBr, almost all of the structural protons in MnO₂ are replaced by lithium resulting in a γ-MnO₂ phase substantially free of protons and containing about 1.8% Li. This highly substituted lithium containing MnO₂ is reduced at between 3.5 and 1.8 V and has a capacity of 250 mAh g⁻¹. There are two reduction processes, one at 3.25 and the other at 2.9 V. TGA studies reveal two processes during heat treatment. Heating the lithium substituted MnO₂ to 350–400 °C results in a partially ordered HEMD-like MnO₂ (LiMD) phase with higher running voltage and superior discharge kinetics. Continued heating of the lithiated manganese dioxide to 450–480 °C under oxygen partial pressure can result in formation of a mixed phase containing both HEMD and a new, ordered MnO₂ phase (OMD). The intimately mixed HEMD/OMD composition has a discharge voltage near 2.9 V with a capacity about 220 mAh g⁻¹. Heating exhaustively lithiated MnO₂ to 350–400 °C results in formation of the partially ordered LiMD MnO₂ phase as with the previous partially lithium substituted MnO₂. Additional heating of the highly lithium substituted MnO₂ to 450–480 °C under oxygen results in formation of the new OMD phase in substantially pure form. Discharge of the new OMD phase shows it has a discharge capacity near 200 mAh g⁻¹ between 3.4 and 2.4 V versus lithium in a single, well-defined discharge process. OMD demonstrated good cycling against Li with no indication of formation of LiMn₂O₄ spinel after 80 deep discharge cycles.     

Preparation and characterization of a new nanosized silicon–nickel–graphite composite as anode material for lithium ion batteries

A new type of nanosized silicon–nickel–graphite (Si–Ni–G) composite was prepared by high energy mechanical milling (HEMM) and pyrolysis using SiO as the precursor of Si for the first time. X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM) and scanning electron microscopy (SEM) were used to determine the phases obtained and to observe the microstructure and distribution of the composite. The composite powders consisted of Si, Ni, SiO₂, NiO and a series of Si–Ni alloys. The formation of the inactive SiO₂ and Si–Ni alloy phases could accommodate the large volume changes of the active particles during cycling. In addition, cyclic voltammetry (CV) and galvanostatic discharge/charge tests were carried out to characterize the electrochemical properties of the composite. The composite electrodes exhibited an initial discharge and charge capacity of 1450.3 and 956.4 mAh g⁻¹, respectively, maintaining a reversible capacity of above 900 mAh g⁻¹ for nearly 60 cycles.     

Supercapacitive behaviors and their temperature dependence of sol–gel synthesized nanostructured manganese dioxide in lithium hydroxide electrolyte

In the present work, a nanostructured manganese dioxide material was synthesized by a sol–gel method starting with manganese acetate (MnAc₂·4H₂O) and citric acid (C₆H₈O₇·H₂O) raw materials, and characterized by X-ray diffraction, infrared spectroscopic and transmission electron microscope techniques. The electrochemical properties and the influence of temperature on supercapacitive behaviors of the nano-MnO₂ electrode in 1 M LiOH electrolyte were investigated using electrochemical methods. Experimental results show that the MnO₂ electrode can exhibit an excellent pseudocapacitive behavior in 1 M LiOH electrolyte, and a high specific capacitance of 317 F g⁻¹ can be obtained at a charge/discharge current rate of 100 mA g⁻¹ and at the temperature of 25 °C. We found that temperature has a crucial influence on the discharge specific capacitance of the electrode. The specific capacitance at 25 °C is higher than that at 15 or 35 °C.     

Electrochemical properties of manganese oxide coated onto carbon nanotubes for energy-storage applications

Birnessite-type manganese dioxide (MnO₂) is coated uniformly on carbon nanotubes (CNTs) by employing a spontaneous direct redox reaction between the CNTs and permanganate ions (MnO₄⁻). The initial specific capacitance of the MnO₂/CNT nanocomposite in an organic electrolyte at a large current density of 1 A g⁻¹ is 250 F g⁻¹. This is equivalent to 139 mAh g⁻¹ based on the total weight of the electrode material that includes the electroactive material, conducting agent and binder. The specific capacitance of the MnO₂ in the MnO₂/CNT nanocomposite is as high as 580 F g⁻¹ (320 mAh g⁻¹), indicating excellent electrochemical utilization of the MnO₂. The addition of CNTs as a conducting agent improves the high-rate capability of the MnO₂/CNT nanocomposite considerably. The in situ X-ray absorption near-edge structure (XANES) shows improvement in the structural and electrochemical reversibility of the MnO₂/CNT nanocomposite after heat-treatment.     

Synthesis and capacitive property of hierarchical hollow manganese oxide nanospheres with large specific surface area

The hierarchical hollow manganese oxide nanospheres with both a large surface area and a layered structure have been successfully prepared by a templating-assisted hydrothermal process at 150 °C for 48 h. SiO₂ template spheres are dispersed in KMnO₄ solution, and then followed by hydrothermal treatment to forming silica/manganese oxide nanospheres with a core-shell structure. The core-shell nanospheres are etched in a NaOH solution (20 wt.%), so that the SiO₂ core is removed, and the hierarchical hollow manganese oxide nanospheres are obtained. The as-synthesized hierarchical hollow manganese oxide nanospheres present a birnessite-type manganese oxide phase with a chemical composition of Na_0.38MnO_2.14·13H₂O, and a specific surface area of 253 m² g⁻¹. The prepared materials exhibit an ideal capacitive behavior and good cycling stability in a neutral electrolyte system and the initial capacitance value is 299 F g⁻¹. Some preparation conditions including the hydrothermal temperature, dwell time and concentration of template have been also investigated.     

Novel polymer electrolytes based on triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) with ionically active SiO2

A novel polymer electrolyte based on triblock copolymer of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) with ionically active SiO₂ inclusions has been designed. The electrolyte shows favorable features for ion migration such as low glass transition temperature and high concentration of amorphous phase. Combined with the effect of active SiO₂, its ionic conductivity is about 8.0 × 10⁻⁵ S cm⁻¹ at 30 °C, which exceeds that for the PEO-based systems. As applying them to cells with LiFePO₄-type cathodes, a capacity of about 147.0 mAh g⁻¹ is obtained at 60 °C, which is retained by more than 90% after 40 charge/discharge cycles. Moreover, about 100 mAh g⁻¹ could still be delivered as temperature decreases to 30 °C.     

Effects of roasting temperature and modification on properties of Li2FeSiO4/C cathode

Li₂FeSiO₄/C cathodes were synthesized by combination of wet-process method and solid-state reaction at high temperature, and effects of roasting temperature and modification on properties of the Li₂FeSiO₄/C cathode were investigated. The XRD patterns of the Li₂FeSiO₄/C samples indicate that all the samples are of good crystallinity, and a little Fe₃O₄ impurity was observed in them. The primary particle size rises as the roasting temperature increases from 600 to 750 °C. The Li₂FeSiO₄/C sample synthesized at 650 °C has good electrochemical performances with an initial discharge capacity of 144.9 mAh g⁻¹ and the discharge capacity remains 136.5 mAh g⁻¹ after 10 cycles. The performance of Li₂FeSiO₄/C cathode is further improved by modification of Ni substitution. The Li₂Fe_0.9Ni_0.1SiO₄/C composite cathode has an initial discharge capacity of 160.1 mAh g⁻¹, and the discharge capacity remains 153.9 mAh g⁻¹ after 10 cycles. The diffusion coefficient of lithium in Li₂FeSiO₄/C is 1.38 × 10⁻¹² cm² s⁻¹ while that in Li₂Fe_0.9Ni_0.1SiO₄/C reaches 3.34 × 10⁻¹² cm² s⁻¹.     

Phosphorus-doped glass proton exchange membranes for low temperature direct methanol fuel cells

Phosphorus-doped silicon dioxide thin films were used as ion exchange membranes in low temperature proton exchange membrane fuel cells. Phosphorus-doped silicon dioxide glass (PSG) was deposited via plasma-enhanced chemical vapor deposition (PECVD). The plasma deposition of PSG films allows for low temperature fabrication that is compatible with current microelectronic industrial processing. SiH₄, PH₃ and N₂O were used as the reactant gases. The effect of plasma deposition parameters, substrate temperature, RF power, and chamber pressure, on the ionic conductivity of the PSG films is elucidated. PSG conductivities as high as 2.54 × 10⁻⁴ S cm⁻¹ were realized, which is 250 times higher than the conductivity of pure SiO₂ films (1 × 10⁻⁶ S cm⁻¹) under identical deposition conditions. The higher conductivity films were deposited at low temperature, moderate pressure, limited reactant gas flow rate, and high RF power.     

Electrochemical characterizations of multi-layer and composite silicon-germanium anodes for Li-ion batteries using magnetron sputtering

To improve the electrochemical performance of Si film, we investigate the addition of two film forms of Ge. Si/Ge multi-layered and Si-Ge composite electrodes that are fabricated by magnetron sputtering onto Cu current collector substrates are investigated. X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and extended X-ray absorption fine structure (EXAFS) are employed to analyze the structures of the Si-Ge electrodes. When used as an anode electrode for a lithium ion battery, the first discharge capacity of a Si/Ge 150 multi-layer cell with a ratio of Si 15 nm/Ge 3 nm is 2099 mAh g⁻¹ between 1.1 and 0.01 V. A stable reversible capacity of 1559 mAh g⁻¹ is maintained after 100 cycles with a capacity retention rate of 74.25%. Additionally, the Si_0.84Ge_0.16 composite has an initial discharge capacity of 1915 mAh g⁻¹ and a capacity retention of 74.25%. In full cell tests of Si-Ge electrodes, the Si_0.84Ge_0.16/LiCoO₂ cell delivers a specific capacity of approximatly 160 mAh g⁻¹ and a capacity retention of 52.4% after 100 cycles. The results reveal that these two systems of sputtered Si-Ge electrodes can be used as anodes in lithium ion batteries with higher energy densities.     

Anodic deposition of porous RuO2 on stainless steel for supercapacitor studies at high current densities

Ruthenium dioxide is deposited on stainless steel (SS) substrate by galvanostatic oxidation of Ru³⁺. At high current densities employed for this purpose, there is oxidation of water to oxygen, which occurs in parallel with Ru³⁺ oxidation. The oxygen evolution consumes a major portion of the charge. The oxygen evolution generates a high porosity to RuO₂ films, which is evident from scanning electron microscopy studies. RuO₂ is identified by X-ray photoelectron spectroscopy. Cyclic voltammetry and galvanostatic charge–discharge cycling studies indicate that RuO₂/SS electrodes possess good capacitance properties. Specific capacitance of 276 F g⁻¹ is obtained at current densities as high as 20 mA cm⁻² (13.33 A g⁻¹). Porous nature of RuO₂ facilitates passing of high currents during charge–discharge cycling. RuO₂/SS electrodes are thus useful for high power supercapacitor applications.     

High energy density capacitor using coal tar pitch derived nanoporous carbon/MnO2 electrodes in aqueous electrolytes

Asymmetric aqueous electrochemical capacitors with energy densities as high as 22 Wh kg⁻¹, power densities of 11 kW kg⁻¹ and a cell voltage of 2 V were fabricated using cost effective, high surface carbon derived from coal tar pitch and manganese dioxide. The narrow pore size distribution of the activated carbon (mean pore size ∼0.8 nm) resulted in strong electroadsorption of protons making them suitable for use as negative electrodes. Amorphous manganese dioxide anodes were synthesized by chemical precipitation method with high specific capacitance (300 F g⁻¹) in aqueous electrolytes containing bivalent cations. The fabricated capacitors demonstrated excellent cyclability with no signs of capacitance fading even after 1000 cycles.