LiCoO2 Films Research Articles

Lithium cobalt oxide cathode film prepared by rf sputtering

Thin films of LiCoO2 prepared by radio frequency magnetron sputtering on Pt-coated silicon are investigated under various deposited parameters such as working pressure, gas flow rate of Ar to O2, and heat-treatment temperature. The as-deposited film was a nanocrystalline structure with (1 0 4) preferred orientation. After annealing at 500–700 °C, single-phase LiCoO2 is obtained when the film is originally deposited under an oxygen partial pressure (PO2) from 5 to 10 mTorr. When the sputtering process is performed outside these PO2 values, a second phase of Co3O4 is formed in addition to the HT-LiCoO2 phase. The degree of crystallization of the LiCoO2 films is strongly affected by the annealing temperature; a higher temperature enhances the crystallization of the deposited LiCoO2 film. The grain sizes of LiCoO2 films annealed at 500, 600 and 700 °C are about 60, 95, and 125 nm, respectively. Cyclic voltammograms display well-defined redox peaks. LiCoO2 films deposited by rf sputtering are electrochemically active. The first discharge capacity of thin LiCoO2 films annealed at 500, 600 and 700 °C is about 41.77, 50.62 and 61.16 μAh/(cm2 μm), respectively. The corresponding 50th discharge capacities are 58.1, 72.2 and 74.9% of the first discharge capacity.

Electrochemical Stability of Thin-Film LiCoO2 Cathodes by Aluminum-Oxide Coating

High-performance thin-film LiCoO2 cathodes were successfully fabricated by aluminum-oxide coating. Both the galvanostatic charge−discharge experiments and the cyclic voltammograms (CVs) showed enhanced electrochemical properties in the Al2O3-coated LiCoO2 films compared to those in the uncoated ones. The improved cycling behaviors in the coated samples are caused by the suppression of cobalt dissolution from the LiCoO2 thin films, with the formation of an aluminum-oxide solid electrolyte residing between the LiCoO2 cathode and liquid electrolyte. Galvanostatic intermittent titration technique (GITT) results clearly showed that the Al2O3-coated samples had higher Li diffusivities than the uncoated ones after 80 cycles. The effect of Al2O3 thickness on the electrochemical properties up to 300 nm was also studied.

Direct Electroplating of Lithium Cobalt Oxide Film on Platinum Substrate in 100°–200°C Aqueous Solution

Well crystallized, shape‐formed, and electrochemically active lithium cobalt oxide (LiCoO2) films are electroplated directly on an electron‐conducting substrate in an aqueous solution using a soft solution processing that is economical, consumes less energy consuming, and is environmentally friendly. Although LiCoO2 films are easily and economically prepared without any post‐synthesis heat treatment, the estimated film properties show a possibility of using the deposited films as a cathode film for lithium rechargeable microbatteries. In addition, the soft solution processing reveals that an exact understanding of chemical reactions and the proper combination of the chemical reactions can create an advanced synthetic procedure.

Fabrication of Patterned LiCoO2 Films Deposited on the Membrane Surface of a Paper-like Substrate

The deposition of LiCoO2 films on a flexible paper-like Teflon substrate has been demonstrated. Two variations of a single-step solution process are reported, the first by electrochemical/hydrothermal, and the second by hydrothermal only, dissolution of Co metal in LiOH and deposition on the Teflon membrane. The Figure show a laser microscopy image of the patterned LiCoO2 membrane film.

Direct Fabrication of Patterned Functional Ceramic Films by Soft Solution Processing without Post-Firing

ABSTRACTWe are proposing an innovative concept and technology, Soft Solution Processing (SSP) for ceramics, which aims to achieve direct fabrication of shaped, sized, located, oriented ceramic materials from solutions without firing and/or sintering. We have successfully fabricated thin and thick films of BaTiO3, SrTiO3, BaWO4, SrMoO4, LiCoO2, and LiNiO2 by SSP in aqueous solutions from room temperature to 200 °C. In these experiments, interfacial reactions between a solid reactant (substrate) and component(s) in a solution have been designed and realized. By locally activating the reaction and moving the reaction point dynamically in these reactions, we can produce patterned ceramics directly in solution without masking, etching, pattern forming, or any post-heating such as firing or sintering. In this paper we present recent results for patterned ceramic films of PbS, CdS, and LiCoO2. The processes used to produce these films are entirely new, and represent the first examples of successful direct patterning of ceramics from solutions. In previous reports, heating processes have been essential for synthesis and/or sintering of powders and precursors to obtain patterns in ceramic materials. Such processes inevitably cost environmentally and economically. In contrast, our method, where no firing is needed, provides an environmentally and economically less expensive alternative.

Direct Fabrication of LiCoO2 Films on Various Substrates in Flowing Aqueous Solutions at 150°C

In this paper, a newly developed idea for direct fabrication of double oxide films on various substrates, the “dual anode system,” is described. Films of LiCoO2 were directly fabricated on Pt, Ni, and graphite substrates by a hydrothermal–electrochemical method at 125–175°C for 2 hours in a flow cell using a cobalt metal anode as a cobalt ion source and a 4M LiOH aqueous solution as a lithium source, without any subsequent firing or annealing. The target substrate (Pt, Ni, or graphite) was also connected to the anode in the dual anode system. The addition of an oxidant, H2O2, in the flowing LiOH solution has produced LiCoO2 films in a short reaction time of 2 h using the flow cell. The H2O2 seems to accelerate the oxidation of Co2+ into Co3+ in cobalt species during the reactions, dissolution–oxidation–precipitation. The thickness and the microstructure of the films could be controlled. In optimum conditions, the film consisting of plate-like LiCoO2 crystals at most a few micrometers thick could be obtained. Micro-Raman and X-ray diffraction studies demonstrated that increasing the fabrication temperature produces a phase change in LiCoO2 from spinel to hexagonal. Spinel phase LiCoO2 is obtained around 125°C and hexagonal phase appears above 125°C. Above 150°C, the redissolution rate of formed crystals seems to dominate the formation of crystals; thus the grain size decreases with increasing temperature.

Structure and electrochemistry of thin-film oxides grown by laser-pulsed deposition

Thin films of V2O5, LiCoO2 and LiMn2O4 were grown by pulsed-laser deposition in the view of their use in lithium microbatteries. Lithiated polycrystalline crystalline thin dense films grown without post-deposition annealing were formed onto substrates maintained at low temperature (300 °C) from a sintered composite target including Li2O as additive. The structural characterizations of these films have been carried out by X-ray diffraction and Raman scattering spectroscopy. The electrochemical features of thin films are investigated by cyclic voltammetry and their charge-discharge profiles in lithium microbatteries are shown.

Fabrication of LiCoO2 thin-film cathodes for rechargeable lithium microbatteries

Abstract Polycrystalline thin films of lithium cobalt oxide were grown by pulsed-laser deposition (PLD) technique. Films of LiCoO2 were deposited onto Si substrates heated at temperature lower than 300°C from a sintered composite target (LiCoO2+Li2O) irradiated with a Nd:YAG laser. The structural characterizations of these films were carried out by X-ray diffraction and Raman scattering spectroscopy, which probe the local environment of cations in the LiCoO2 framework. Raman spectra of PLD LiCoO2 films were investigated as a function of various growth conditions as substrate temperature, partial oxygen pressure in the deposition chamber, and target composition. PLD LiCoO2 films obtained with a polycrystalline morphology were successfully used as cathode materials in lithium microbatteries. The Li//LiCoO2 cells were tested by cyclic voltammetry and galvanostatic charge–discharge techniques in the potential range 2.0–4.2 V. Specific capacity as high as 195 mC cm−2 μm was measured on polycrystalline films.

Effect of 20°–200oC Fabrication Temperature on Microstructure of Hydrothermally Prepared LiCoO2 Films

Films of LiCoO2 were directly fabricated by hydrothermal treatment of a cobalt metal plate in a 4M LiOH aqueous solution at 20°–200°C, with no subsequent annealing, and the effect of fabrication temperature on the film microstructure was investigated for the films. Micro‐Raman studies have indicated that increasing the fabrication temperature produces a phase change in LiCoO2 from spinel to hexagonal. This change is revealed by a variation in the film thickness and the film surface morphology, as seen in the micrographs. The present scanning electron microscopy results showed a growth of spinel LiCoO2 particles up to 125°C and the formation of hexagonal particles at >125°C, in good agreement with the Raman and X‐ray photoemission spectroscopy results. A film‐formation mechanism based on the dissolution of cobalt metal, followed by precipitation, as LiCoO2, onto the cobalt substrate, is proposed. The mechanism is supported by experimental data, such as the one‐step potential evolution (0 → 0.6 V, with respect to the Ag/AgCl reference electrode) of the cobalt electrode during hydrothermal treatment and the detection of dissolved cobalt species by atomic absorption and ultraviolet–visible‐light absorption spectroscopic analyses. Apparently, the evolution of the film structure arises from different nucleation and growth rates of LiCoO2 particles on the film, caused by the dissolution–precipitation mechanism, and a phase selection of spinel or hexagonal as the fabrication temperature increases.

Preparation and Characterization of LiCoO2 and LiMg0.05Co0.95O2 Thin Films on Porous Ni/NiO Cathodes for MCFC by Complex Sol-Gel Process (CSGP)

AbstractThe major disadvantage of Ni/NiO cathodes for a Molten Carbonate Fuel Cells (MCFC) application is dissolution of NiO in K/Li electrolyte that significantly decreases the cell lifetime. Thin films of LiCoO2 or LiMg0.05Co0.9502 were prepared on a cathode body in order to protect them against dissolution. For preparation of starting sols the Complex Sol-Gel Process (CSGP) has been applied. These sols have been prepared by adding of LiOH to aq. acetates solution of Co 2+(Mg2+) with ascorbic acid and then by alkalizing them with aqueous ammonia to pH=8. The cathode plates of various dimensions (to several hundreds cm2) have been dipped in these sols and withdrawn at rate a 1.7 cm/s. Commercially sintered Ni plates were always initially oxidized by heating at various temperatures. Their microstructure and mechanical properties as a function of temperature were observed. Heat treatment should be carried out under the dead load of the ceramic plates in order to avoid their waving. The best non-folded plates were obtained by treating them for lh at 550°C. The covered substrates were calcined for lh at 650°C, using low heating ratel°C/min. The presence of LiCoO2 in a deposited coating has been proved by EDS patterns. The resultant film thicknesses were measured by scanning electron microscopy (SEM) on the fractured cross-sections; they ranged from 0.5 to 2νm and depended on sol concentration and viscosity. A 350 hundred hours test in molten carbonates, proved that the cathode bodies covered with LiCoO2 are completely prevented from dissolution of Ni in a molten K/Li electrolyte. Dissolution of LiCoO2 coating was not observed as well. After treatment in a molten electrolyte SEM observations did not show any changes in microstructures and morphology of the covered cathodes.

Preferred Orientation of Polycrystalline LiCoO2 Films

Polycrystalline films of deposited by radio frequency magnetron sputtering exhibited a strong preferred orientation or texturing after annealing at 700°C. For films thicker than about 1 μm, more than 90% of the grains were oriented with their (101) and (104) planes parallel to the substrate and less than 10% with their (003) planes parallel to the substrate. As the film thickness decreased below 1 μm, the percentage of (003)‐oriented grains increased until at a thickness of about 0.05 μm, 100% of the grains were (003) oriented. These extremes in texturing were caused by the tendency to minimize volume strain energy for the thicker films or the surface energy for the very thin films. Films were deposited using different process gas mixtures and pressures, deposition rates, substrate temperatures, and substrate bias. Of these variables, only changes in substrate temperature could cause large changes in texturing of thick films from predominately (101)–(104) to (003). Although lithium ion diffusion should be much faster through cathodes with a high percentage of (101)‐ and (104)‐oriented grains than through cathodes with predominately (003)‐oriented grains, it was not possible to verify this expectation because the resistance of most cells was dominated by the electrolyte and electrolyte‐cathode interface. Nonetheless, cells with cathodes thicker than about 2 μm could deliver more than 50% of their maximum energies at discharge rates of or higher. © 2000 The Electrochemical Society. All rights reserved.

Direct Fabrication of LiCoO2 Film Electrodes Using Soft Solution-Processing in LiOH Solution at 20 - 200°C

AbstractApplication of Soft Solution-Processing, which is defined by environmentally friendly processing using (aqueous) solution, to the field of rechargeable lithium microbattery has been demonstrated by fabricating LiCoO2 film on the cobalt metal substrate in LiOH solution under hydrothermal condition. The film formation mecahnism could be interpreted in terms of chemical dissolution of cobalt metal plate in LiOH solution at fixed temperature of 20 - 200°C, resulting in the formation of H1−nCoO2 where n value increases with fabrication temperature and precipitation as LiCoO2 by heterogeneous nucleation on the substrate followed by crystal growth. The crystal structure and film properties have been characterized by X-ray diffraction, X-ray photoelectron, micro-Raman spectroscopic and scanning electron microscopic analyses. The films exhibited a good crystallinity despite the low reaction temperature without any postsynthesis annealing. The films prepared under different conditions showed different phase selection such as spinel (Fd3m) or hexagonal (R3-m), surface morphology and film thickness. An electrochemical activity of the LiCoO2 films was evidenced by cyclic voltammogram revealing a good reversibility on Li-intercalation and -deintercalation.

Fabrication in a Single Synthetic Step of Electrochemically Active LiMO2 (M = Ni and Co) Thin-Film Electrodes Using Soft Solution Processing at 20−200 °C

Well-crystallized and electrochemically active Li1-xNi1+xO2 and LiCoO2 thin-film electrodes for lithium rechargeable microbatteries can be fabricated in a single synthetic step using an economical, less energy consuming, and environmentally friendly “Soft Solution Processing” method in a concentrated LiOH solution at fixed temperatures between 20 and 200 °C without any postsynthesis annealing. While the purely hydrothermal treatment of cobalt plates directly leads to the formation of LiCoO2 films, Ni(OH)2 films on nickel plates are fabricated under the same hydrothermal conditions. By using the electrochemical−hydrothermal approach under supplementary galvanostatic charge with the same hydrothermal conditions, Li1-xNi1+xO2 films can only be effectively prepared in a single synthetic step from nickel plates. Such different activation methods required for the preparation of LiCoO2 and Li1-xNi1+xO2 films respectively can be ascribed to the different metal cation valencies between dissolved nickel species (that contain divalent nickel) and dissolved cobalt species (that should possess trivalent cobalt).

Lattice model calculation of the strain energy density and other properties of crystalline LiCoO2

The strain energy densities for various crystalline planes of LiCoO2 were calculated from the stiffness tensors obtained from lattice model calculations using the program GULP. In addition to Coulomb and Buckingham potentials, it was necessary to include shell models for the oxygen and cobalt ions in order to obtain acceptable agreement between the observed and calculated structural parameters and high frequency dielectric constant. The strain energy densities u due to differential thermal expansion were calculated using the theoretical stiffness tensors and estimated values for the thermal expansion coefficients of LiCoO2. For a temperature change of 675 °C, these ranged from 0.5 to 1.3×108 erg/cm3 or 5 to 13 J/m2 for 1-μm-thick films on alumina substrates. In particular, the energies for the (003), (101), and (104) planes were ordered as u(003)≫u(104)>u(101). This suggests that the strong (101) preferred orientation of LiCoO2 films (⩾1 μm thick) is due to the tendency to minimize volume strain energy that arises from differential thermal expansion between the film and the substrate. Additional properties obtained from the GULP calculations include the free energy, heat capacity, and the k=0 vibrational modes.

Physical Properties of Lithium-Cobalt Oxides Grown by Laser Ablation

ABSTRACTWe have studied the growth and the physical properties of LiCoO2 films as a function of formation conditions, i.e., substrate temperature and partial oxygen pressure. These films were grown on various substrates by the laser ablation technique using a Nd:YAG laser at 100 MW/cm2 power density. LiCoO2 films were characterized by XRD, SEM, and vibrational spectroscopy. The synthesized compounds show high homogeneity in composition and in particle dimension. The main advantage of this method is the less time needed for the reaction to occur completely. Raman scattering spectroscopy provides information on the local environment of cations.

Thin Films of Ionic Conductors by Laser Ablation

ABSTRACTFilms of insertion compounds and solid state electrolytes have been synthesized in order to study their applications in the domain of electrochemical ion and gas sensors. Laser ablation was used to deposit lithium metal oxides (LiMO2 where M is a transition metal Co or Mn).The chemical composition in the films has been studied by Rutherford Backscattering spectrometry and nuclear reactions analysis in the case of the light elements (O, Li). For lithium transition metal oxides (LiMO2), the oxygen and lithium contents are determined by a thermodynamical equilibrium between the films and the partial pressures in the chamber. In these cases, laser ablation allows the synthesis of crystalline structures with a large range of oxygen non stoichiometry as compared to solid state reactions. They lead to interesting electrical properties. Using the appropriate temperatures and oxygen pressures, films with the correct stoichiometry could be obtained as polycrystalline onto Si or Si/Pt substrates whereas they exhibit high texturing and epitaxial growth onto MgO or MgO/Pt.The films of LiMn2O4 and LiCoO2 have been used as electrochemical sensors for the measurement of the lithium concentration in solutions. They show a very rapid and selective response.

Thin Film Rechargeable Lithium Batteries for Implantable Devices

Thin films of LiCoO2 have been synthesized in which the strongest x-ray reflection is either weak or missing, indicating a high degree of preferred orientation. Thin film solid state batteries with these textured cathode films can deliver practical capacities at high current densities. For example, for one of the cells, 70% of the maximum capacity between 4.2 and 3 V (∼0.2 mAh/cm2) was delivered at a current of 2 mA/cm2. When cycled at rates of 0.1 mA/cm2, the capacity loss was 0.001%/cycle. The reliability and performance of LiLiCoO2 thin film batteries make them attractive for application in implantable devices such as neural stimulators, pacemakers, and defibrillators.

THIN-FILM RECHARGEABLE LITHIUM BATTERIES FOR IMPLANTABLE DEVICES

Thin films of LiCoO2 have been synthesized in which the strongest x-ray reflection is either weak or missing, indicating a high degree of preferred orientation. Thin film solid state batteries with these textured cathode films can deliver practical capacities at high current densities. For example, for one of the cells, 70% of the maximum capacity between 4.2 and 3 V (approximately 0.2 mAh/cm2) was delivered at a current of 2 mA/cm2. When cycled at rates of 0.1 mA/cm2, the capacity loss was < or = 0.001%/cycle. The reliability and performance of Li-LiCoO2 thin film batteries make them attractive for application in implantable devices such as neural stimulators, pacemakers, and defibrillators.

LiCoO2 Thin Films Grown by Pulsed Laser Deposition on Low Cost Substrates

AbstractWe report on the use of pulsed laser depositon (PLD) to grow thin films of LiCoO2 on a number of low cost substrates including SnO2 coated Upilex, stainless steel and SnO2 coated glass. Highly textured (001) films grown on CVD deposited SnO2 films on 7059 glass, were obtained at 200 to 500 mTorr O2 and a temperature of 500 C. Similar texture was not obtained on the stainless or Upilex however dense films from crystalline to amorphous were obtained. The films were characterized by x-ray diffraction and Raman spectroscopy.

In Situ Conductivity Measurements of LiCoO2 Film during Lithium Insertion/Extraction by Using Interdigitated Microarray Electrodes

In situ conductivity‐potential profiles and cyclic voltammograms of an thin film were obtained simultaneously by using an interdigitated microarray electrode with a constant potential difference. The electronic conductivity of increased exponentially with potential during lithium extraction from 2.7 to 4.0 V. The conductivity changes drastically when very small amounts of Li () are extracted.