LiCoO2 Films Research Articles

8Li β-NMR study of epitaxial LixCoO2 films

In order to investigate the diffusive motion of Li+ in a thin film electrode material for Li-ion batteries, we have measured β-NMR spectra of 8Li+ ions implanted into epitaxial films of Li0.7CoO2 and LiCoO2 in the temperature range between 10 and 310 K. Below 100 K, the spin-lattice relaxation rate (1/T1) in the Li0.7CoO2 film increased with decreasing temperature, indicating the appearance and evolution of localized magnetic moments, as observed with μ+SR. As temperature is increased from 100 K, 1/T1 starts to increase above ~ 200 K, where both Li- NMR and μ+SR also sensed an increase in 1/T1 due to Li-diffusion. Interestingly, such diffusive behavior was found to depend on the implantation energy, possibly because the surface of the film is decomposed due to chemical instability of the Li0.7CoO2 phase in air. Such diffusive behavior was not observed for the LiCoO2 film up to 310 K.

Synthesis of LiCoO2 epitaxial thin films using a sol–gel method

Epitaxial LiCoO2 films are synthesized using a sol–gel method. The precursors are aqueous solutions of acetates or nitrates of Li and Co with polyvinylpyrrolidone as a thickener. The LiCoO2 films prepared from the solutions by spin coating are epitaxially grown on sapphire (0001) substrates with c-axis orientation and in-plane alignment of LiCoO2[11¯0]‖sapphire [100]. A two-step heat treatment of the spin-coated films consisting of preheating on a hotplate at the crystallization temperature followed by a high-temperature treatment notably promotes the c-axis orientation. In addition, the crystal orientation is controllable on different planes of the SrTiO3 substrates; the LiCoO2 films are grown with epitaxial relationships of LiCoO2 (001)‖SrTiO3 (111), LiCoO2 (018)‖SrTiO3 (110), and LiCoO2 (104)‖SrTiO3 (100).

LiCoO2 Thin Film Deposition Using a Low-Temperature Electrochemical-Hydrothermal Route

The increasing miniaturization of electronic devices and the demand for stand-alone systems require adapted power sources in terms of dimensions and energy. All-solid-state microbatteries are an interesting solution for new applications (secured smart cards, medical implants, sensors, etc.). They are obtained by successive deposition of thin films of electrode active materials and solid electrolyte. However, these batteries have a rather low capacity (50-200 µAh/cm²) due to the difficulty to use electrodes thicker than 5µm (electronic conductivity, lithium diffusion and mechanical stress issues). Most of commercially available microbatteries use a LiCoO2 positive electrode due to its interesting properties: a 4V/Li+/Li nominal voltage, an excellent cycle life and a high electronic conductivity [1] . Thin films of LiCoO2 are mainly obtained by sputtering [2, 3] , followed by an annealing (500-700°C) necessary to obtain the electrochemically active phase of LiCoO2 (R-3m). This latter step then limits the possible substrates. Moreover the low deposition rate (10nm/mn) and the required equipments and facilities make this process quite costly.To circumvent these drawbacks, thin films of LiCoO2 were synthesized by a novel single-step route: the electrochemical-hydrothermal deposition. This technique combines the effects of the hydrothermal conditions (temperature and pressure) with a galvanostatic electrodeposition. Using ethanol as co-solvent in the aqueous media, we succeded in obtaining a pure and highly-crystalline LiCoO2 films, without any Co3O4 impurity. Moreover, the surface roughness is greatly decreased (fig.1) compared with films obtained in pure aqueous media. The obtained thickness can be increased up to 30µm, equal to a deposition rate of 200nm/mn. Galvanostatic cycling and linear sweep voltammetry experiments were carried out on these electrodes using CR2032 coin cells. The electrochemical properties of these films are very promising and clearly evidence the influence of each deposition parameter (temperature, current density, water/ethanol ratio, etc.) (fig2). [1] Reimers, J.N. and J.R. Dahn, Journal of the Electrochemical Society, 1992. 139(8): p. 2091-2097. [2] Wei, G., T.E. Haas, and R.B. Goldner, Solid State Ionics, 1992. 58(1-2): p. 115-122. [3] Wang, B., et al., Journal of the Electrochemical Society, 1996. 143(10): p. 3203-3213.

Electrochemically tunable thermal conductivity of lithium cobalt oxide.

Using time-domain thermoreflectance, the thermal conductivity and elastic properties of a sputter deposited LiCoO2 film, a common lithium-ion cathode material, are measured as a function of the degree of lithiation. Here we report that via in situ measurements during cycling, the thermal conductivity of a LiCoO2 cathode reversibly decreases from ~5.4 to 3.7 W m(-1) K(-1), and its elastic modulus decreases from 325 to 225 GPa, as it is delithiated from Li1.0CoO2 to Li0.6CoO2. The dependence of the thermal conductivity on lithiation appears correlated with the lithiation-dependent phase behaviour. The oxidation-state-dependent thermal conductivity of electrolytically active transition metal oxides provides opportunities for dynamic control of thermal conductivity and is important to understand for thermal management in electrochemical energy storage devices.

Atomic-Level Characterization of Interfaces in LiCoO2

The powerful synergy between atomic-resolution electron microscopy and atomistic simulation techniques encompassing both first-principles and classical force-field calculations is illustrated through a study of grain boundaries, domain boundaries and surfaces of the lithium-ion secondary battery cathode material LiCoO2. Epitaxial thin films of LiCoO2 deposited on sapphire (0001) substrates are used as model samples, and results from simulation and experiment combined to characterize the structures and properties of representative interfaces.

Enhanced electrochemical properties of as grown LiCoO2 film cathodes: Influence of silicon substrate surface texturing

Influence of Si substrate texturing on growth, microstuctural and electrochemical properties of lithium cobalt oxide (LiCoO2) cathode films was investigated. A batch of rf magnetron sputtered LiCoO2 films were prepared on both Au/Ti/SiO2/(polished) Si and Au/Ti/SiO2/(textured) Si substrates and the film properties were studied, systematically. As-grown films on polished Si substrates were exhibited partially ordered rock-salt structure with a very poor electrochemical performance. However, the as-grown films on textured Si substrates exhibited relatively predominant (0 0 3) orientation with R 3¯ m symmetry and exhibited an initial discharge capacity of 57.5 μAhcm−2 μm−1 with an appreciable capacity fade rate of 0.07%, even after 50 cycles. Electrochemical performance of these films was in close comparison with the annealed (650 °C) LiCoO2 films of Au/Ti/SiO2/(polished) Si substrates. The experimental observations were thoroughly demonstrated and suggested that the as-grown LiCoO2 films on textured Si substrates can be useful for all types of 2D solid state Li+-ion microbattery applications.

Crystal orientation of epitaxial LiCoO2 films grown on SrTiO3 substrates

LiCoO2 (LCO) thin films are epitaxially grown on (100), (110) and (111) 0.5 wt% Nb-doped SrTiO3 (Nb:STO) substrates by pulsed laser deposition and their crystalline orientation relationship investigated via X-ray pole figure measurement. This investigation reveals that the LCO films grown on Nb:STO (100) and (110) have a cube-on-cube orientation relationship with the substrates to be (104)- and (018)-oriented, respectively, while the relationship is accompanied with 60° in-plane rotation on (111) Nb:STO to yield a predominantly (001)-oriented LCO film. The (104)-oriented LCO film on (100) Nb:STO substrate exhibits high rate capability up to 100 C.

Investigations On Sputter Deposited LiCoO2 Thin Films From Powder Target

LiCoO2 thin films used as cathode layers in thin-film solid-state batteries have been deposited from LiCoO2 powder target using radio frequency (rf) magnetron sputtering in a cost effective approach in terms of material consumption and processing time. Xray diffraction (XRD) studies of the films after post deposition annealing treatment revealed (104) orientation, which is the characteristic of rf sputtered LiCoO2 films. Raman spectroscopy was used to identify the different modes of vibration of atoms in the film. The surface morphology and cross-section of the samples were characterized using field emission scanning electron microscopy (SEM). The chemical analysis of LiCoO2 thin films was examined by X-ray photoelectron spectroscopy (XPS) showed atomic ratio of Li/Co as 0.9 which is close to the ideal value 1. Electrochemical characterization such as charge – discharge and cyclic voltammetry were conducted for LiCoO2 films deposited on platinized silicon substrates in the potential range 3.0 to 4.2 V vs Li/Li + . The maximum discharge capacity of 64 µAh.µm -1 . cm -2 and 52 µAh. µm -1 .cm -2 were achieved when discharged at a current of 5 µA and 50 µA for the first discharge cycle respectively. Moreover the electrochemical investigations of LiCoO2 thin films on flexible copper substrates also investigated and achieved an initial discharge capacity of 83 mAh/g. The possible reasons for degradation in the electrochemical properties have been discussed. The present work indicates suitability of sputtering from powder targets for thin film battery fabrication. Copyright © 2013 VBRI press.

Electrochemical properties of high rate bias sputtered LiCoO2 thin films in liquid electrolyte

LiCoO2 thin films have been prepared on aluminium substrate with RF sputtering using a bias effect combined with a moderate post-annealing treatment at 500 °C and their electrochemical properties investigated in liquid electrolyte by cyclic voltammetry, galvanostatic experiments in various voltage windows, and ac impedance spectroscopy. The positive effect of bias is shown allowing to minimize the presence of the cubic LiCoO2 phase. The lithium extraction process is highly reversible in the biased LiCoO2 films with charge–discharge curves which really superimpose at C/10 rate. A 450 nm thin film is shown to exhibit an initial charge capacity of ≈60 μAh cm−2 μm−1 at C/10 and a high discharge capacity of ≈50 μAh cm−2 μm−1. The rate capability study demonstrates the film can easily sustain high charge–discharge rates and exhibits good cycling properties with capacity retention of 90% after 50 cycles up to a 2C rate and still 73% of the maximum capacity is recovered at 8C in the 4.2 V–3 V potential window. This constitutes remarkable performances both in terms of rate capability and cycling stability. The charge–discharge behaviour as a function of the film thickness in the range 0.25–3.6 μm is investigated and indicates an efficient and homogeneous electrochemical activity with a linear increase of the capacity with thickness. As a consequence an optimization of the capacity is proposed with the use of the thickest deposit of 3.6 μm leading to the value of 200 μAh cm−2, the best one available for a LiCoO2 film in liquid electrolyte.

Atomic Layer Deposition of LiCoO2 Thin-Film Electrodes for All-Solid-State Li-Ion Micro-Batteries

One of the remaining challenges in the field of portable electronics is the miniaturization of lithium-ion batteries. To prepare all-solid-state batteries with a sufficient high storage capacity it is vital to prepare high quality thin films for battery stacks on 3D-structured substrates. A remote plasma atomic layer deposition (ALD) process has therefore been developed for LiCoO2 which can serve as a cathode material. A combination of CoCp2 as cobalt precursor, LiOtBu as lithium precursor and O2 plasma as oxidant source was used to create super-cycles to deposit LiCoO2 from Co3O4 and Li2CO3 cycles. The thin films were deposited at a temperature of 325°C and showed linear growth with a rate of 0.06 nm/cycle. After annealing the samples at 700°C for 6 minutes high temperature phase LiCoO2 was obtained, as was demonstrated by XRD and Raman spectroscopy. A new procedure was proposed to obtain the composition of all three chemical elements in the LiCoO2 films. Elastic Backscattering Spectroscopy (EBS) measurements turned out to be very convenient and reliable to obtain the quantities of all chemical elements, including lithium. Moreover, the ALD-deposited LiCoO2 thin film electrodes were electrochemically characterized, revealing good electrochemical performance. To the best of our knowledge this paper provides the first evidence that electrochemically active LiCoO2 can be deposited by ALD.

Epitaxial growth of (104)- and (018)-oriented LiCoO2 films on MgO single crystals prepared by chemical vapor deposition

The lithium cobaltate (LiCoO2) films were deposited onto single-crystal magnesium oxide (MgO) substrates by metal organic chemical vapor deposition, and the in-plane orientation relationship, morphology and deposition rates were investigated. Hetero-epitaxial LiCoO2 films oriented to (104) and (018) were obtained on (100) and (110) MgO substrates. The surface morphology of the (104) LiCoO2 film showed diagonally arrayed rectangular facets, whereas the (018) LiCoO2 film exhibited a roof-like structure extended unidirectionally to [1¯10] MgO. Twin structures formed with in-plane relationships of [010] LiCoO2 // [01¯0] LiCoO2 and [421¯] LiCoO2 // [010] LiCoO2 in the (104) LiCoO2 film and with in-plane relationships of [100] LiCoO2 // [1¯00] LiCoO2 in the (018) LiCoO2 film. The deposition rates of the (104)- and (018)-oriented LiCoO2 films were 10.8μmh−1 and 8.8μmh−1, respectively, which were almost 100 times greater than those reported for LiCoO2 epitaxial films fabricated by pulsed laser deposition.

Lattice orientation control of lithium cobalt oxide cathode film for all-solid-state thin film batteries

For the application to all-solid-state thin film batteries, LiCoO2 thin films are deposited by RF-sputtering with controlling the lattice orientations to maximize lithium-ion diffusivity in film textures. The nano-sized crystalline grains grow up with the (003) preferred orientation parallel to the substrates at room temperature due to the lowest surface energy of this atomic plane. However, because the surface energy difference of atomic planes of LiCoO2 reduces with increasing substrate temperature, the influence of surface energy becomes weaker at high temperature. The LiCoO2 thin films with the (110) preferred orientation are obtained at higher temperatures by dominant influence from the lowest the volume strain energy of this orientation. To take advance of this orientation effect in full cell, the influences of the metallic current collector layer on the structural properties of sputtered cathode are investigated. It turns out that the Li2O buffer layers between the cathode films and the metallic current collector layers can suppress the formation of lithium-deficient phase, Co3O4, and the growth of (003) plane by reducing the lattice match between of LiCoO2 plane and Al(111) plane. The LiCoO2 films with the controlled orientation show enhanced rate performance owing to improved interfacial resistance and lithium-ion conductivity.

Interface characterization of MgF2-coated LiCoO2 thin films

The electrochemical properties and interface reactions of pristine and coated LiCoO2 films were characterized. MgF2 was used as a new coating material to improve the electrochemical properties of the LiCoO2 film electrode. To directly observe the interface reaction between the surface of the cathode and the electrolyte, a film electrode with a wide, flat surface was introduced in this study. The rate capability, cyclic performance, and stability at high temperature of the LiCoO2 thin film were improved by MgF2 coating. The concentration–depth profile and surface image of the LiCoO2 film were investigated by SEM and XPS analyses before and after cycling. Pristine film showed significant Co dissolution and surface damage on cycling at high temperature (45°C). However, both were efficiently depressed by the introduction of the MgF2 coating. These results confirm that the MgF2 coating layer successfully protected the surface of the cathode from the reactive electrolyte, leading to enhanced electrochemical properties for MgF2-coated LiCoO2 films.

The Effects of Substrate and Annealing on Structural and Electrochemical Properties in LiCoO2 Thin Films Prepared by DC Magnetron Sputtering

LiCoO2 thin films were fabricated by direct current magnetron sputtering method on STS304 and Ti substrates. The effects of substrate and annealing on their structural and electrochemical properties of LiCoO2 thin film cathode were studied. Crystal structures and surface morphologies of the deposited films were investigated by X-ray diffraction and field emission scanning electron microscopy. The as-deposited films on both substrates have amorphous structure. The (104) oriented perfect crystallization was obtained by annealing over 600 degrees C in STS304 substrate. The LiCoO2 thin film deposited on Ti substrate shows the (003) texture after annealing at 700 degrees C. The electrochemical properties were investigated by the cyclic voltammetry and charge-discharge measurement. The 600 degrees C-annealed LiCoO2 film deposited on STS304 substrate exhibits the inithial discharge capacity of 22 uAh/cm2 and the 96% capacity retention rate at 50th cycles. The electrochemical measurement on annealed films over 600 degrees C was impossible due to the formed TiO2 insulator layer using Ti substrate. As a result, it was found that the STS304 substrate seems to be more suitable material than the Ti substrate in fabricating LiCoO2 thin film cathode.

RF-sputtered LiCoO2 thick films: microstructure and electrochemical performance as cathodes in aqueous and nonaqueous microbatteries

Films of LiCoO2 are prepared on metallized silicon substrates using RF-magnetron sputtering technique. The microstructural properties of the films are investigated by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. The films deposited at a substrate temperature of 250 °C with subsequent annealing at 650 °C exhibited hexagonal layered structure with R $$ \overline 3 $$ m symmetry. The kinetics of lithium ions in LiCoO2 film cathode host matrix and its cycleability are studied in aqueous Pt//LiCoO2 and nonaqueous Li//LiCoO2 cell. Both the electrochemical cells at same current density of 50 μA cm−2 delivered the same initial discharge capacity of about 60 μA h cm−2 μm−1 with a chemical diffusion coefficient of ca. 10−11 cm2 s−1 for Li+ ions. The capacity fade rates for the Pt//LiCoO2 and Li//LiCoO2 cells, in average are 3.0 and 0.15 % per cycle, respectively, for the first 20 cycles. The Pt//LiCoO2 cell is found to be advantageous for small number of cycles and is cost effective than the Li//LiCoO2 cell.

Synthesis and electrochemical property of LiCoO2 thin film coated on the surface of carbon and anatase TiO2 powders

For the high rate discharge capability, LiCoO2 with the layered rock salt structure thin films were coated on the surface of carbon materials using the nanosheet restacking method and hydrothermal treatment. LiCoO2 thin films consisted of nano-particles with the size of ca.30nm are synthesized through the electrophorestic deposition of cobalt hydroxide nanosheet precursor on the carbon material and the subsequent hydrothermal treatment. To optimize the synthesis condition, LiCoO2 film without any binders and conductive additives was synthesized on the glassy carbon substrate. An excellent high rate discharge capability, 72% of cathode utilization efficiency at 300C-rate compared with that at 30C-rate, was obtained using the thinnest film with 5min deposition time. In the LiCoO2 coating on the surface of powder supports (Toka Black, Denka Black and anatase TiO2), LiCoO2 coated on Denka Black showed good charge–discharge performance. From the surface observation by SEM, LiCoO2 nano-particles are found to be coated on the surface of Denka Black homogeneously, while LiCoO2 nano-powder was aggregated in the system of Toka Black.LiCoO2 coated on Denka Black showed a good high rate charge–discharge capability: 66% of the capacity retained at 17C in comparison with that at 1.7C, which is superior to the reports of other bulk materials.

Suppression of interface reaction of LiCoO2 thin films by Al2O3-coating

Al2O3-coated LiCoO2 thin films were prepared to characterize the effects of coating in detail. The LiCoO2 film electrodes served as effective pristine cathodes to investigate the interface reaction between a cathode and electrolyte because they have wide and smooth surface and do not contain conducting agents (such as carbon) or binders. The electrochemical properties, surface morphology, and concentration profile (vs. depth) were characterized to identify the effects of coating. The Al2O3-coated film showed a much improved rate capability and cyclic performance compared with those of the pristine film. Specifically, based on the results of SIMS analyses and SEM images, dissolution of transition-metal ions and surface damage during storage at high temperature were suppressed by Al2O3 coating. This result directly shows the successful protective effect of surface coating, which is associated with enhanced electrochemical properties of the coated film.

Fabrication of LiCoO2 thin film cathodes by DC magnetron sputtering method

LiCoO2 thin films were fabricated on Al substrate by direct current magnetron sputtering method. The effects of Ar/O2 gas rates and annealing temperatures were investigated. Crystal structures and surface morphologies of the deposited films were investigated by X-ray diffraction, Raman scattering spectroscopy and field emission scanning electron microscopy. The as-deposited LiCoO2 thin films exhibited amorphous structure. The crystallization starts at the annealing temperature over 400°C. However, the annealed films have the partially disordered structure without completely ordered crystalline structure even at 600°C annealing. The electrochemical properties of the LiCoO2 films were investigated by the charge–discharge and cycle measurements. The 500°C annealing film has the highest capacity retention rate of 78.2% at 100th cycles.

Orientation and Morphology of LiCoO<sub>2</sub> Prepared by Chemical Vapor Deposition on Al<sub>2</sub>O<sub>3</sub> Single Crystal

Lithium Cobaltate (LiCoO2) Films Were Prepared on the (001), (110), (110) and (112) Planes of Al2O3 Single Crystals Substrates by Metal Organic Chemical Vapor Deposition, and the Phases, Orientated Textures and Surface Morphologies Were Examined. (001)-Oriented LiCoO2 Films Were Obtained on (001) and (110) Al2O3 Substrates, while (018)- and (104)-Oriented LiCoO2 Films Were Grown on (110) and (112) Al2O3 Substrate. Triangular and Elongated Rectangular Faceted Structures Were Directionally Aligned, and (001)- and (018)-Oriented Grains Were Epitaxially Grown on (001) and (110) Al2O3 Substrates. Randomly Arranged Polygonal Faceted Structures Were Observed in the (001)-Oriented Licoo2 Film on (110) Al2O3 Substrate, while Locally Inhomogeneous Grains Were Observed in the (104)-Oriented LiCoO2 Film on (1_,12) Al2O3 Substrate.

Spatially-Resolved Electrochemical Strain Microscopy Simulation of Polycrystalline LiCoO2 Films

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