Nano-sized copper tungstate thin films as positive electrodes for rechargeable Li batteries

Nano-sized CuWO₄ thin films have been fabricated by radio-frequency (R.F.) sputtering deposition, and are used as positive electrode with both LiClO₄ liquid electrolyte and LiPON solid electrolyte in rechargeable lithium batteries. An initial discharge capacity of 192 and 210 mAh/g is obtainable for CuWO₄ film electrode with and without coated LiPON in liquid electrolyte, respectively. An all-solid-state cell with Li/LiPON/CuWO₄ layers shows a high-volume rate capacity of 145 μAh/cm² μm in first discharge, and overcomes the unfavorable electrochemical degradation observed in liquid electrolyte system. A two-step reactive mechanism is investigated by both transmission electron microscopy and selected area electron diffraction techniques. Apart from the extrusion and injection of Cu²⁺/Cu⁰, additional capacity can be achieved by the reversible reactivity of (WO₄)²⁻ framework. The chemical diffusion coefficients of Li intercalation/deintercalation are estimated by cyclic voltammetry. Nano-CuWO₄ thin film is expected to be a promising positive electrode material for high-performance rechargeable thin-film lithium batteries.     

As-deposited LiCoO2 thin film cathodes prepared by rf magnetron sputtering

LiCoO₂ thin films were deposited using radio frequency (rf) magnetron sputtering system on stainless steel substrates. Different rf powers, up to 150 W, were applied during deposition. The as-deposited films exhibited (1 0 1) and (1 0 4) preferred orientation and the nanocrystalline film structure was enhanced with increasing rf power. The film crystallinity was examined using X-ray diffraction, Raman scattering spectroscopy and transmission electron microscopy. The compositions of the films were determined by inductively coupled plasma-mass spectroscopy. The average discharge capacity of as-deposited films is about 59 μAh/(cm² μm) for cut-off voltage range of 4.2 and 3.0 V. From the electrochemical cycling data, it is suggested that as-deposited LiCoO₂ films with a nanocrystalline structure and a favorable preferred orientation, e.g. (1 0 1) or (1 0 4) texture, can be used without post-annealing at high temperatures for solid-state thin film batteries.     

Anodically synthesized titania films for lithium batteries: Effect of titanium substrate and surface treatment

A number of titania films have been produced through anodising high purity titanium from different suppliers in either the as-received state or following polishing and etching. Anodising was carried out galvanostatically for a period of 10 min in 0.2 M H₂SO₄. The performance of the films was then evaluated as potential anode materials for lithium batteries. Raman spectroscopy showed these films had spectra characteristic of anatase with the presence of some rutile whilst the spectra of the lithiated state was characteristic of the orthorhombic phase of Li_xTiO₂.The surface condition in particular was found to have an effect on the electrochemical performance and properties of the films most notably on capacity fade. Whilst the electrodes produced from as-received titanium demonstrated stable cycle capacities after the initial few cycles, those on polished and etched substrates faded over 50 cycles. The best performing films offered a capacity of at least ∼48 μAh cm⁻² over the 50 cycles. All the electrodes examined however did show signs of the film having being damaged as a result of electrochemical cycling. With the wide range of anodising parameters that can be altered there is considerable scope for optimising the electrochemical performance of films produced through such a technique.     

Structural and electrochemical characterization of SnOx thin films for Li-ion microbattery

SnO_x thin films were prepared by reactive radio frequency magnetron sputtering with different sputtering powers. X-ray photoelectron spectroscopy suggested that all the films have similar chemical stoichiometry as SnO_1.5. X-ray diffraction and transmission electro microscopy results showed that crystal size of the SnO_x thin films gradually increases with increase of sputtering power from 50 to 150 W. Cyclic voltammetry and galvanostatic charge/discharge cycling measurements indicated that the electrochemical properties of SnO_x films strongly rely on their crystal sizes as well as surface morphologies. The SnO_x film deposited at sputtering power of 120 W exhibits the best electrochemical performances. It could deliver a reversible capacity of 670 μAh cm⁻² μm⁻¹ at 50 μA cm⁻² in the voltage range of 0.1-1.2 V up to 50 cycles.     

LiCoO2 thin film cathode fabrication by rapid thermal annealing for micro power sources

The rapid thermal annealing (RTA) process was employed to obtain crystalline LiCoO₂ thin films. XRD analyses of the LiCoO₂ thin film show increased crystallinity with an increase in the RTA time. The Auger electron spectroscopic analysis of the LiCoO₂ film strongly suggests that the RTA process is more advantageous to obtain a stable inter layer between the substrate and the deposited film and between each deposited layer than the conventional annealing process. All-solid-state thin film cells composed of Li/lithium phosphorous oxynitride (Lipon)/LiCoO₂ systems were fabricated using the LiCoO₂ cathode treated with RTA. The optimum condition of RTA would be 900 s at 650 °C, which exhibited a good rate capability for high power applications. Two cells were connected in parallel to obtain a higher discharge current, and they showed a specific capacity of 38.4 μAh cm⁻² μm⁻¹ even at a 25C rate (current density: 7.96 mA cm⁻²).     

Electrochemical characterization of amorphous LiFe(WO4)2 thin films as positive electrodes for rechargeable lithium batteries

Amorphous LiFe(WO₄)₂ thin films have been fabricated by radio-frequency (R.F.) sputtering deposition at room temperature. The as-deposited and electrochemically cycled thin films are, respectively, characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectra techniques. An initial discharge capacity of 198 mAh/g in Li/LiFe(WO₄)₂ cells is obtained, and the electrochemical behavior is mostly preserved in the following cycling. These results identified the electrochemical reactivity of two redox couples, Fe³⁺/Fe²⁺ and W⁶⁺/W^x+ (x = 4 or 5). The kinetic parameters and chemical diffusion coefficients of Li intercalation/deintercalation are estimated by cyclic voltammetry and alternate-current (AC) impedance measurements. All-solid-state thin film lithium batteries with Li/LiPON/LiFe(WO₄)₂ layers are fabricated and show high capacity of 104 μAh/cm² μm in the first discharge. As-deposited LiFe(WO₄)₂ thin film is expected to be a promising positive electrode material for future rechargeable thin film batteries due to its large volumetric rate capacity, low-temperature fabrication and good electrode/electrolyte interface.     

Li-intercalation behaviour of vanadium oxide thin film prepared by thermal oxidation of vanadium metal

In order to produce thin films of crystalline V₂O₅, vanadium metal was thermally oxidised at 500 °C under oxygen pressures between 250 and 1000 mbar for 1-5 min. The oxide films were characterised by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). The lithium intercalation performance of the oxide films was investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS). It was shown that the composition, the crystallinity and the related lithium intercalation properties of the thin oxide films were critically dependent on the oxidation conditions. The formation of crystalline V₂O₅ films was stimulated by higher oxygen pressure and longer oxidation time. Exposure for 5 min at 750 mbar O₂ at 500 °C resulted in a surface oxide film composed of V₂O_5, and consisting of crystallites up to 200 nm in lateral size. The thickness of the layer was about 100 nm. This V₂O₅ oxide film was found to have good cycling performance in a potential window between 3.8 and 2.8 V, with a stable capacity of 117 ± 10 mAh/g at an applied current density of 3.4 μA/cm². The diffusion coefficients corresponding to the two plateaus at 3.4 and 3.2 V were determined from the impedance measurements to (5.2 and 3.0) × 10⁻¹³ cm² s⁻¹, respectively. Beneath the V₂O₅ layer, lower oxides (mainly VO₂) were found close to the metal. At lower oxygen pressure and shorter exposure times, the oxide films were less crystalline and the amount of V⁴⁺ increased in the surface oxide film, as revealed by XPS. At intermediate oxygen pressures and exposure times a mixture of crystalline V₂O₅ and V₆O₁₃ was found in the oxide film.     

Performance evaluation of printed LiCoO2 cathodes with PVDF-HFP gel electrolyte for lithium ion microbatteries

In order to improve the discharge capacity in lithium ion microbatteries, a thick-film cathode was fabricated by a screen printing using LiCoO₂ pastes. The printed cathode showed a different discharge curves when the cell was tested using various (liquid, gel and solid-state) electrolytes. When a cell test was performed with organic liquid electrolyte, the maximum discharge capacity was 200 μAh cm⁻², which corresponded to approximately 133 mAh g⁻¹ when the loading weight of LiCoO₂ was calculated. An all-solid-state microbattery could be assembled using sputtered LiPON electrolyte, an evaporated Li anode, and printed LiCoO₂ cathode films without delamination or electrical problems. However, the highest discharge capacity showed a very small value (7 μAh cm⁻²). This problem could be improved using a poly(vinylidene fluoride-hexafluoro propylene) (PVDF-HFP) gel electrolyte, which enhanced the contact area and adhesion force between cathode and electrolyte. The discharge value of this cell was measured as approximately 164 μAh cm⁻² (≈110 mAh g⁻¹). As the PVDF-HFP electrolyte had a relatively soft contact property with higher ionic conductance, the cell performance was improved. In addition, the cell can be fabricated in a leakage-free process, which can resolve many safety problems. According to these results, there is a significant possibility that a film prepared using the aforementioned paste with screen printing and PVDF-HFP gel electrolyte is feasible for a microbattery.     

Investigation of thin sputtered Mn films for electrochemical capacitors

Pseudocapacitive manganese oxide films have been synthesized by anodic oxidation of metallic films deposited by sputtering. Results are presented from an electrochemical investigation into properties of these thin sputtered manganese films. Mn films with thickness ranging from 20 to 200 nm have been sputtered onto Pt coated Si wafers in an Argon atmosphere. Electrochemical oxidation converts the metal film into a porous, dendritic structure which displays significant pseudocapacitance. We have observed a specific capacitance (C_s) of 700 F/g when cycled very slowly at a constant current density of 160 μA/cm². The same films probed by cyclic voltammetry (CV) at a rate of 5 mV/s yielded a lower specific capacitance of 400-450 F/g. Post-oxidation material loading was measured to be in the range of 25-75 μg/cm².     

Redox behavior and optical response of nanostructured poly(3,4-ethylenedioxythiophene) films grown in a camphorsulfonic acid based micellar solution

Poly(3,4-ethylenedioxythiophene) (PEDOT) films have been electropolymerized from an aqueous micellar solution comprising camphorsulfonic acid (CSA), lithium trifluoromethanesulfonate (LiCF₃SO₃) and EDOT. The inclusion of the dopants CS⁻ and CF₃SO₃⁻ in the polymer structure and an unusually high doping level of 0.54 have been ascertained by the X-ray photoelectron spectroscopy. Transmission electron microscopy and atomic force microscopy studies show that the micellar effect of CSA leads to a morphology wherein polymer particles link together to form elongated shapes and also endows the film with a surface roughness of 25-30 nm. These nanostructures permit a facile intercalation-deintercalation of anions in the film during redox cycling. Electrochemical impedance spectroscopy show that the charge transfer phenomenon at the PEDOT-electrolyte interface is dominant in the high frequency region and diffusion controlled ionic movement prevails in the low frequency regime. The use of these films as potential cathodes in electrochromic windows is rationalized not only on the basis of their high scalability and ease of processing but also due to their large coloration efficiency (123 cm² C⁻¹) and transmission modulation (50%) at a photopic wavelength of 550 nm. But further improvement in color-bleach kinetics and reproducibility of redox behavior is desirable to broaden their spectrum of utility.     

The structure and electronic properties of passive and prepassive films of iron in borate buffer

The films that form on pure iron during potentiodynamic anodic polarization in aqueous borate buffer were investigated by surface enhanced Raman spectroscopy (SERS), and by electrochemical impedance spectroscopy and Mott-Schottky analysis at selected potentials. According to SERS, the passive film is a bilayer film with an outer layer of an as yet undetermined Fe(III)oxide/hydroxide, identified by a strong Raman peak at 560 cm⁻¹. The inner layer was a spinel compound. The capacitances of passive iron were frequency dependent and a constant phase element (CPE) best described the frequency dispersion. Current increases in cathodic polarization scans confirmed the accuracy of flatband potentials calculated from Mott-Schottky tests at two different film formation potentials. Both films were found to be n-type and flatband potentials of −846 and −95 mV vs. SHE and carrier densities of 1.6 × 10²² and 8.3 × 10²⁰/cm³ were found for films grown at −500 and +1000 mV, respectively. The cathodic polarization curve of passivated iron exhibited a complex shape that was explained by the electronic properties of iron's passive and prepassive films. The reductive dissolution of the films abruptly began when the potential was lowered below their flatband potentials. It is suggested that the cathodic polarization behavior contributes to iron's susceptibility to localized corrosion.     

Li-intercalation electrochemical/electrochromic properties of vanadium pentoxide films by sol electrophoretic deposition

Thin films of orthorhombic V₂O₅ have been prepared by sol electrophoretic deposition (EPD) followed by post-treatment at 500 °C. Their electrochemical and optical performances have been investigated for possible applications in electrochemical/electrochromic devices. Li⁺-intercalation properties of the films have been explored in two voltage ranges: 0.4 to −1.1 V and 0.4 to −1.6 V versus Ag/Ag⁺, respectively. High capacities of over 300 mAh/g are acquired in the wider voltage range at a current density of 50 μA/cm² and moderate capacities of 140 and 110 mAh/g are obtained in the narrower voltage range at a current density of 25 and 50 μA/cm², respectively. Electrochemical measurements have shown that the films demonstrate good cyclability in both voltage ranges. X-ray diffraction, scanning electron microscopy and optical spectra have been used to examine the changes in crystallinity, microstructure, morphology and transmittance of the films during cycling. Films cycled to a deeper voltage of −1.6 V versus Ag/Ag⁺ deliver higher capacity with appreciable morphological change, while films cycled in the narrower voltage range show moderate capacity and maintain the morphology, optical responses and crystalline structure. Voltage range can be optimized in between to acquire both high capacity and stability in structure, electrochemical and optical properties. High Li⁺-intercalation capacity and good cyclic stability are attributed to the porous structure of V₂O₅ films prepared by EPD.     

Towards the conception of an amperometric sensor of l-tyrosine based on Hemin/PAMAM/MWCNT modified glassy carbon electrode

A novel amperometric sensor was fabricated based on the immobilization of hemin onto the poly (amidoamine)/multi-walled carbon nanotube (PAMAM/MWCNT) nanocomposite film modified glassy carbon electrode (GCE). Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and ultraviolet visible (UV-vis) adsorption spectroscopy were used to investigate the possible state and electrochemical activity of the immobilized hemin. In the Hemin/PAMAM/MWCNT nanocomposite film, MWCNT layer possessed excellent inherent conductivity to enhance the electron transfer rate, while the layer of PAMAM greatly enlarged the surface average concentration of hemin (Γ) on the modified electrode. Therefore, the nanocomposite film showed enhanced electrocatalytical activity towards the oxidation of l-tyrosine. The kinetic parameters of the modified electrode were investigated. In pH 7.0 phosphate buffer solution (PBS), the sensor exhibits a wide linear range from 0.1 μM to 28.8 μM l-tyrosine with a detection limit of 0.01 μM and a high sensitivity of 0.31 μA μM⁻¹ cm⁻². In addition, the response time of the l-tyrosine sensor is less than 5 s. The excellent performance of the sensor is largely attributed to the electro-generated high reactive oxoiron (IV) porphyrin (O = Fe^IV-P) which effectively catalyzed the oxidation of l-tyrosine. A mechanism was herein proposed for the catalytic oxidation of l-tyrosine by oxoiron (IV) porphyrin complexes.     

Electrodeposited PbO2 thin film as positive electrode in PbO2/AC hybrid capacitor

Lead dioxide (PbO₂) thin films were prepared on Ti/SnO₂ substrates by means of electrodeposition method. Galvanostatic technique was applied in PbO₂ film formation process, and the effect of deposition current on morphology and crystalline form of the PbO₂ thin films was studied by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). The energy storage capacity of the prepared PbO₂ electrode was investigated by means of cyclic voltammetry (CV) and charge/discharge cycles, and a rough surface structure PbO₂ film was selected as positive electrode in the construction of PbO₂/AC hybrid capacitor in a 1.28 g cm⁻³ H₂SO₄ solution. The electrochemical performance was determined by charge/discharge tests and electrochemical impedance spectroscopy (EIS). The results showed that the PbO₂/AC hybrid capacitor exhibited high capacitance, good cycling stability and long cycle life. In the voltage range of 1.8-0.8 V during discharge process, considering the weight of all components of the hybrid capacitor, including the two electrodes, current collectors, H₂SO₄ electrolyte and separator, the specific energy and power of the device were 11.7 Wh kg⁻¹ and 22 W kg⁻¹ at 0.75 mA cm⁻², and 7.8 Wh kg⁻¹ and 258 W kg⁻¹ at 10 mA cm⁻² discharge currents, respectively. The capacity retains 83% of its initial value after 3000 deep cycles at the 4 C rate of charge/discharge.     

TiO2 nanotubes manufactured by anodization of Ti thin films for on-chip Li-ion 2D microbatteries

The use of self-organized TiO₂ nanotube arrays electrochemically grown onto Si is investigated for the fabrication of an alternative electrode dedicated to on-chip Li-ion 2D microbatteries. Discharge/charge curves and cycling performance are studied in lithium-anode electrochemical test cells for both amorphous and crystalline titania nanotubes. At 5 μA cm⁻² amorphous TiO₂ nanotube layers onto Si deliver a maximum areal capacity of 89 μAh cm⁻² in the first reversible discharge and 56 μAh cm⁻² over 50 cycles. We demonstrate that these nanostructured thin film electrodes showing such electrochemical performances are compatible with IC technology.     

Cathodoluminescence and electron field emission of boron-doped a-C:N films

The optical and electrical properties of so-called carbon nitride films (a-C:N) and boron doped so-called carbon nitride films (a-C:N:B) are studied with cathodoluminescence (CL) spectroscopy and electron field emission measurement. The a-C:N films were first deposited on Si by a filtered cathodic arc plasma system, and then boron ions (∼1 × 10¹⁶ cm⁻²) were implanted into the a-C:N films to form a-C:N:B films by a medium current implanter. The structural and morphological properties of a-C:N and a-C:N:B films were then analyzed using secondary ion mass spectrometer, X-ray photoelectron spectroscopy, FT-IR spectra, Raman spectroscopy and atomic force microscopy. The a-C:N film exhibits luminescence of blue light (∼2.67 eV) and red light (∼1.91 eV), and the a-C:N:B film displays luminescence of blue light (∼2.67 eV) in CL spectra measured at 300 K. Furthermore, the incorporated boron atoms change the electron field emission property, which shows a higher turn on field for the a-C:N:B film (3.6 V/μm) than that for the a-C:N film (2.8 V/μm).     

Three-dimensional reticular tin-manganese oxide composite anode materials for lithium ion batteries

Tin-manganese oxide film with three-dimensional (3D) reticular structure has been prepared by electrostatic spray deposition (ESD). X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicate that the film is amorphous. X-ray-photoemission spectroscopy (XPS) demonstrates that the 3D grid is composed of tin-manganese oxide. As an anode electrode for the lithium ion battery, the tin-manganese oxide film has 1188.3 mAh g⁻¹ of initial discharge capacity and very good capacity retention of 656.2 mAh g⁻¹ up to the 30th cycle. Such a composite film can be used as an anode for lithium ion batteries with higher energy densities.     

Investigation of the inherent electrical potential of PEO electrolyte films

Measurement of the through-plane potential of PEO-lithium triflate electrolyte films has demonstrated that they possess an inherent potential as cast from an acetonitrile solution onto a Teflon substrate. These films have an inherent potential of around 0.2 V and the cast films display a discharge behavior similar to a double layer capacitor system with a small discharge capacitance of 80 nF cm⁻². It is postulated that electrochemical properties of the films can be attributed to different salt concentration at the two surfaces. This difference in concentration may result from a matching of the surface-free energy of the Teflon substrate side of the film and the side of the film where evaporation occurs with the lithium triflate species in the polymer. Different spherulite morphologies were also observed for each surface. These morphologies can be assigned to spherulites having much different ion concentrations. Attenuated total reflection (ATR) IR spectroscopy was used to investigate the surface concentrations of free ions, ion pairs and ion multiples of both surfaces of the films. AC impedance spectroscopy of the surfaces of the film was also conducted. These data indicated that there is a difference in the surface concentration of each side. The ability of electrolyte films to exhibit a potential as fabricated may have potential applications as an easily manufactured power source for micro and nanodevices.     

Oxide film growth on Al-In alloys in a borate buffer solution in conditions of galvanostatic anodising

Anodic oxide films have been formed galvanostatically on Al-In alloys (containing up to 0.074% In) in a borate buffer solution (pH 7.8) at different current densities (20-100 μA cm⁻²). The mechanism, kinetics of growth and properties of formed oxide films have been investigated. The study of charge curves suggests that the growth of oxide films on Al-In alloys occurs by an activation-controlled ionic conduction under the influence of the high electric field through the oxide film according to an exponential law, like on valve metals. The following parameters have been calculated: the constants of the exponential law, ionic conductivity through the film, the effective activation distance for ion movement and the corresponding field strength. The values for the field strength, of the order of magnitude of 10⁶ V cm⁻¹, justify the application of the high field migration mechanism. Properties of anodic oxide films have been determined by means of electrochemical impedance spectroscopy; the resistance and thickness of the oxide film have been found to increase with the increase in the indium content in the alloy and with increased anodic current density. It has been established that the current efficiency in oxide films formation on Al-In alloys is lower than 100%: the increase of the indium content in the alloy, as well as the increase in anodic current density, increases the value of current efficiency.     

Hydrothermal synthesis of Co3O4 microspheres as anode material for lithium-ion batteries

Co₃O₄ microspheres were synthesized in mass production by a simple hydrothermal treatment. One micrometer-sized spherical particles with well-crystallization could be obtained by XRD and SEM. Higher specific surface area (93.4 m² g⁻¹) and larger pore volume (78.4 cm³ g⁻¹) by BET measurements offered more interfacial bondings for extra sites of Li⁺ insertion, which resulted in the anomalous large initial irreversible capacity and capacity cycling loss due to SEI film formation. The capacity retention of Co₃O₄ microspheres involved first forming acted as Li-ion anode material is almost above 90% from 12th cycle and it retain lithium storage capacity of 550.2 mAh g⁻¹ after 25 cycles, which show good long-life stability. The electrochemical impedance spectroscopy (EIS) tests before and after cyclic voltammetry measurements and charge-discharge experiments were carried out and the corresponding D_Li values were also calculated. The relationship of the ac impedance spectra and the cycling behaviors was discussed. It is found that the decrease of capacity results from the larger Li⁺ charge-transfer impedance and the lower lithium-diffusion processes on cycling, which is in very good agreement with the electrochemical behaviors of Co₃O₄ electrode.