Li-ion diffusion kinetics in LiCoPO4 thin films deposited on NASICON-type glass ceramic electrolytes by magnetron sputtering

LiCoPO₄ thin films were deposited on Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP) solid electrolyte by radio frequency magnetron sputtering and were characterized by X-ray diffraction and scanning electron microscope. The films show a (1 1 1) preferred orientation upon annealing and are chemically stable with LATSP up to 600 °C in air. An all-solid-state Li/PEO₁₈-Li(CF₃SO₂)₂N/LATSP/LiCoPO₄/Au cell was fabricated to investigate the electrochemical performance and Li-ion chemical diffusion coefficients, , of the LiCoPO₄ thin films. The potential dependence of values of the LiCoPO₄ thin film was investigated by potentiostatic intermittent titration technique and was compared with those of the LiFePO₄ thin film. These results showed that the intercalation mechanism of Li-ion in LiCoPO₄ is different from that in LiFePO₄.     

Li-ion transport in all-solid-state lithium batteries with LiCoO2 using NASICON-type glass ceramic electrolytes

LiCoO₂ thin films were deposited on the NASICON-type glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂, by radio frequency (RF) magnetron sputtering and were annealed at different temperatures. The as-deposited and the annealed LiCoO₂ thin films were characterized by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). It was found that the films exhibited a (1 0 4) preferred orientation after annealing and Co₃O₄ was observed by annealing over 500 °C due to the reaction between the LiCoO₂ and the glass ceramics. The effect of annealing temperature on the interfacial resistance of glass ceramics/LiCoO₂ and Li-ion transport in the bulk LiCoO₂ thin film was investigated by galvanostatic cycling, cyclic voltammetry (CV), potentiostatic intermittent titration technique (PITT) and electrochemical impedance spectroscopy (EIS) with the Li/PEO/glass ceramics/LiCoO₂ cell. The cell performance was limited by the Li-ion diffusion resistance in Ohara/LiCoO₂ interface as well as in bulk LiCoO₂.     

Synthesis and characterization of Nd doped LiMn2O4 cathode for Li-ion rechargeable batteries

Spinel powders of LiMn_1.99Nd_0.01O₄ have been synthesized by chemical synthesis route to prepare cathodes for Li-ion coin cells. The structural and electrochemical properties of these cathodes were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, cyclic voltammetry, and charge-discharge studies. The cyclic voltammetry of the cathodes revealed the reversible nature of Li-ion intercalation and deintercalation in the electrochemical cell. The charge-discharge characteristics for LiMn_1.99Nd_0.01O₄ cathode materials were obtained in 3.4–4.3 V voltage range and the initial discharge capacity of this material were found to be about 149 mAh g⁻¹. The coin cells were tested for up to 25 charge-discharge cycles. The results show that by doping with small concentration of rare-earth element Nd, the capacity fading is considerably reduced as compared to the pure LiMn₂O₄ cathodes, making it suitable for Li-ion battery applications.     

Interfacial lithium-ion transfer at the LiMn2O4 thin film electrode/aqueous solution interface

Interfacial lithium-ion transfer at the LiMn₂O₄ thin film electrode/aqueous solution was investigated. The cyclic voltammograms of the film electrode conducted in the aqueous solution was similar to an adsorption-type voltammogram of reversible system, suggesting that fast charge transfer reaction proceed in the aqueous solution system. We found that the activation energy for this interfacial lithium-ion transfer reaction obtains 23–25 kJ mol⁻¹, which is much smaller than that in the propylene carbonate solution (50 kJ mol⁻¹). This small activation energy will be responsible for the fast interfacial lithium-ion transfer reaction in the aqueous solution. These results suggest that fast lithium insertion/extraction reaction can be realized by decreasing the activation energy for interfacial lithium-ion transfer reaction.     

Electrochemistry of LiMn2O4 epitaxial films deposited on various single crystal substrates

LiMn₂O₄ epitaxial thin films were synthesized on SrTiO₃:Nb(1 1 1) and Al₂O₃(0 0 1) single crystal substrates by pulsed laser deposition (PLD) method and the electrochemical properties were discussed comparing with that of amorphous LiMn₂O₄ film on polycrystalline Au substrate. LiMn₂O₄ epitaxial film showed only a single plateau in charge–discharge curves and a single redox peak at the corresponding voltage of cyclic voltammograms. This phenomenon seems to originate from the effect of the epitaxy: the film is directly connected with the substrate by the chemical bond and this connection would suppress the phase transition of Li_xMn₂O₄ film during lithium (de-)intercalation. The discharge voltage of LiMn₂O₄ epitaxial film on SrTiO₃ was lower than that of LiMn₂O₄ film on Al₂O₃. This lowered discharge voltage may be caused by the electronic interaction between LiMn₂O₄ film and SrTiO₃:Nb n-type semiconductor substrate.     

Thin nanostructured LiMn2O4 films by flame spray deposition and in situ annealing method

A new approach has been developed to rapidly synthesize nanostructured LiMn₂O₄ thin films by flame spray deposition (FSD) and in situ annealing. A precursor solution of lithium acetylacetonate and manganese acetylacetonate in an organic solution was supplied through a flame spray pyrolysis (FSP) reactor. The liquid solution spray was ignited and stabilized by a premixed methane/oxygen flame ring surrounding the FSP nozzle. Thus, LiMn₂O₄ nanoparticles were formed by combustion and deposited onto a current collector followed by in situ annealing. Two different types of current collectors, i.e. stainless steel and aluminum coated with carbon-based primer were tested. The prepared thin films were characterized by X-ray diffraction and field-emission scanning electron microscopy. The electrochemical properties of the thin films were evaluated by cyclic voltammetry and galvanostatic cycling. The LiMn₂O₄ films exhibited good cyclability. Films that underwent sintering and crystal growth during in situ annealing developed more robust film structures on the current collector surface and exhibited better electrochemical performance than poorly adhered films.     

Structural, morphological and electrochemical properties of nanocrystalline V2O5 thin films deposited by means of radiofrequency magnetron sputtering

Due to easiness of preparation and high energy density, V₂O₅ nanocrystalline thin films are particularly attractive as cathode materials for all-solid-state rechargeable lithium microbatteries. However, their electrochemical performances are strictly related to the film microstructure, which, in turn, is related to the nature and parameters of the deposition technique. For this reason, the preparation of thin films with reproducible electrochemical properties is still an open problem.Here, we report on the deposition of V₂O₅ crystalline thin films by means of reactive radiofrequency (r.f.) magnetron sputtering, using vanadium metal as the target. Different deposition times and substrate temperatures were adopted. X-ray powder diffraction (XRD) and atomic force microscopy were used to investigate the structural and morphological features of the films. In particular, XRD analysis revealed that the deposition parameters affect the crystallographic orientation of the films. A h 0 0 orientation is observed in case of thin samples (about 100 nm) prepared at 300 °C, whereas a 1 1 0 preferential growth is obtained for thicker films. Films deposited at 500 °C display a 0 0 1 orientation irrespective on the deposition time.Reversible Li intercalation/deintercalation processes and high specific capacity are observed for the h 0 0-oriented V₂O₅ thinner films, with the ab plane arranged perpendicular to the substrate. In this case, the cycling behaviour is very promising, and a stable capacity higher than 300 mAh g⁻¹ was delivered in the potential range 3.8-1.5 V at 1C rate over at least 70 cycles.     

Electrochemical studies of low-temperature processed nano-crystalline LiMn2O4 thin film cathode at 55 °C

LiMn₂O₄ thin films with nano-crystals less than 100 nm were successfully grown on polished stainless steel substrates at 400 °C and 200 m Torr of oxygen by pulsed laser deposition. A maximum discharge capacity of 62.4 μAh cm⁻² μm⁻¹ cycled between 3.0 and 4.5 V with a current density of 20 μAh cm⁻² was achieved. The effect of several overdischarge cycles was negligible, and both the effect of Jahn–Teller distortion at low potentials on capacity loss and structure instability at high potentials were effectively inhibited in this nano-crystalline film, resulting in an excellent cycling stability with a very low fading rate of capacity up to 500 cycles at 55 °C.     

Crystallization of LiMn2O4 observed with high temperature X-ray diffraction

We investigated the formation of LiMn₂O₄ phases by calcinating a stoichiometric mixture of Li₂CO₃ and various manganese compounds with high temperature X-ray diffraction (HT-XRD) technique to understand the influence of starting materials on the electrochemical performance. XRD measurements were carried out during heating processes from room temperature to 700 °C. In case of Li₂CO₃/electrolytic manganese dioxide and Li₂CO₃/MnCO₃ mixtures used as starting materials, Li_0.33MnO₂ phase and low crystalline phase, respectively, appeared as intermediate products during heating process followed by the crystallization into the spinel. HT-XRD observation confirmed that the LiMn₂O₄ phase was directly formed from starting Li₂CO₃/Mn₂O₃ and Li₂CO₃/Mn₃O₄ mixtures. The reactivity of the mixture, meant by the lower reaction temperature between Li and Mn compounds and the faster evolution of Li–Mn–O phase, depended on manganese compounds. The purity and stoichiometry of spinel type LiMn₂O₄ was not achieved only by the higher reactivity. From these results, the dependence of reversible capacities and cycleability of synthesized LiMn₂O₄s on the formation process which varied with the starting materials was discussed.     

Synthesis of spinel LiMn2O4 nanoparticles through one-step hydrothermal reaction

Phase pure spinel LiMn₂O₄ nanoparticles can be directly synthesized by one-step hydrothermal reaction of γ-MnO₂ with LiOH in an initial Li/Mn ratio of 1 at 200 °C. The reaction might involve a redox reaction between Mn⁴⁺ and OH⁻, and the formation of LiMn₂O₄ at the same time under the proposed hydrothermal conditions. This hydrothermal process is simple since only γ-MnO₂ powders are used as the Mn source, whereas without use of any oxidants, reductants, or low valence Mn source. The electrochemical performance of the as-synthesized LiMn₂O₄ nanoparticles towards Li⁺ insertion/extraction was examined. Rather good capacity and cycle performance, and an especially excellent high rate capability, were observed for the sample that was hydrothermally reacted for 3 days.     

Morphology and electrochemistry of LiMn2O4 optimized by using different Mn-sources

Spinel-typed LiMn₂O₄ cathode active materials have been prepared for different microstructures by the melt-impregnation method using different forms of manganese. The effect of the starting materials on the microstructure and electrochemical properties of LiMn₂O₄ is investigated by X-ray diffraction, scanning electron microscopy, and electrochemical measurements. The powder prepared from nanostructured γ-MnOOH, with good crystallinity and a regular cubic spinel shape, provided an initial discharge capacity of 114 mAh g⁻¹ with excellent rate and high capacity retention. These advantages render LiMn₂O₄ attractive for practical and large-scale applications in mobile equipment.     

Improvement of capacity fading resistance of LiMn2O4 by amphoteric oxides

The capacity fading of LiMn₂O₄ is improved by adding amphoteric oxides such as Al₂O₃, ZnO, SnO₂, and ZrO₂ to the cathode slurry. The effectiveness of the amphoteric oxides on the fade resistance of LiMn₂O₄ is compared by measuring the capability of scavenging hydrofluoric acid (HF) in the electrolyte by the oxides using a pH meter and by BET surface analysis. Results suggest that the capacity fading is determined by the reactivity of oxides with HF and the effective surface-area of the oxide particles when they were mixed in the slurry. Zinc oxide is the most effective of the oxides in scavenging HF.     

Electrochemical performance of spinel LiMn2O4 cathode materials made by flame-assisted spray technology

Spinel lithium manganese oxide LiMn₂O₄ powders were synthesized by a flame-assisted spray technology (FAST) with a precursor solution consisting of stoichiometric amounts of LiNO₃ and Mn(NO₃)₂·4H₂O dissolved in methanol. The as-synthesized LiMn₂O₄ particles were non-agglomerated, and nanocrystalline. A small amount of Mn₃O₄was detected in the as-synthesized powder due to the decomposition of spinel LiMn₂O₄ at the high flame temperature. The impurity phase was removed with a post-annealing heat-treatment wherein the grain size of the annealed powder was 33 nm. The charge/discharge curves of both powders matched the characteristic plateaus of spinel LiMn₂O₄ at 3 V and 4 V vs. Li. However, the annealed powder showed a higher initial discharge capacity of 115 mAh g⁻¹ at 4 V. The test cell with annealed powder showed good rate capability between a voltage of 3.0 and 4.3 V and a first cycle coulombic efficiency of 96%. The low coulombic efficiency from capacity fading may be due to oxygen defects in the annealed powder. The results suggest that FAST holds potential for rapid production of uniform cathode materials with low-cost nitrate precursors and minimal energy input.     

Electrochemical performance of nanostructured spinel LiMn2O4 in different aqueous electrolytes

A nanostructured spinel LiMn₂O₄ electrode material was prepared via a room-temperature solid-state grinding reaction route starting with hydrated lithium acetate (LiAc·2H₂O), manganese acetate (MnAc₂·4H₂O) and citric acid (C₆H₈O₇·H₂O) raw materials, followed by calcination of the precursor at 500 °C. The material was characterized by X-ray diffraction (XRD) and transmission electron microscope techniques. The electrochemical performance of the LiMn₂O₄ electrodes in 2 M Li₂SO₄, 1 M LiNO₃, 5 M LiNO₃ and 9 M LiNO₃ aqueous electrolytes was studied using cyclic voltammetry, ac impedance and galvanostatic charge/discharge methods. The LiMn₂O₄ electrode in 5 M LiNO₃ electrolyte exhibited good electrochemical performance in terms of specific capacity, rate dischargeability and charge/discharge cyclability, as evidenced by the charge/discharge results.     

Nano Li4Ti5O12–LiMn2O4 batteries with high power capability and improved cycle-life

We report the effects of electrode thickness, cathode particle size and morphology, cathode carbon coating matching ratio and laminate structure on the electrochemical characteristics of nanosized Li₄Ti₅O₁₂–LiMn₂O₄ batteries. We show that a correct adjustment of these parameters resulted in significant improvements in power capability and cycle-life of such devices, making them competitive, low-cost and safe battery chemistry for next generation Li-ion batteries. In addition, Li₄Ti₅O₁₂ reversible specific capacity beyond three Li-ions intercalation is reported.     

Electrochemical performance of Al-doped LiMn2O4 prepared by different methods in solid-state reaction

Al-doped LiMn₂O₄ cathode materials synthesized by a newly developed wet-milling method and a dry process method using a conventional solid-state reaction were evaluated physicochemically and electrochemically. In the wet-milling method, a precursor was made from the raw materials atomized by a wet milling. A good cyclic performance was obtained for the LiMn₂O₄ samples prepared by the wet-milling method, achieved up to 99% of retention of capacity at 50 °C at the 30th cycle. The precursor obtained by the wet-milling method was well homogenous and highly reactive due to their finely ground particles, giving good crystallinity to LiMn₂O₄ products.     

Characteristics of indium zinc oxide thin films prepared by direct current magnetron sputtering for flexible solar cells

The In₂O₃-ZnO (IZO) thin films were prepared on polyethylene terephthalate substrate at room temperature by direct current (dc) magnetron sputtering. The properties of IZO thin films were studied in terms of O₂ concentration and deposition parameters. As the O₂ concentration in O₂/Ar gas increased, the transmittances of the films were increased up to 90% and the resistivities were decreased. The systematic variation of process parameters including dc power, gas pressure and target-to-substrate distance was performed to examine the properties of the deposited films. It was disclosed that there was an optimum O₂ concentration for high transmittance and low resistivity. With decrease in dc power and gas pressure and increase in target-to-substrate distance, the IZO films with high transmittance and low resistivity were obtained. The observation of the IZO films by atomic force microscopy indicated that the microstructure and surface morphology of the films were responsible for the transmittance. It was demonstrated that IZO films with a resistivity of 5.1×10⁻⁴ Ω cm and an optical transmission of 90% in the visible spectrum could be prepared at room temperature on flexible substrates.     

Visible light-active nitrogen-doped TiO2 thin films prepared by DC magnetron sputtering used as a photocatalyst

The visible light-active nitrogen-doped TiO₂ has been prepared by dc-reactive magnetron sputtering using Ti target in an Ar+O₂/N gas mixture. The preparation of highly crystallized anatase TiO_xN_y thin films with various nitrogen concentrations allowed us to identify the optimum nitrogen flow ratio for the photocatalytic oxidation (PCO) of 2-propanol. At higher nitrogen flow rate, nitrogen is found to be difficult to substitute for oxygen having been predicted to contribute the band gap narrowing, giving rise to undesired deep level defects. In addition, Raman spectroscopy and X-ray diffraction (XRD) studies revealed that highly crystallized anatase growth of nitrogen-doped TiO_xN_y thin films are difficult at higher nitrogen flow rate. The optical band gap was found to be lower for the films deposited at 2 sccm of nitrogen flow rate. The PCO of 2-propanol was studied as a function of nitrogen flow rate using in situ FTIR spectroscopy. The PCO of 2-propanol found to proceed along two routes: one was through the chemisorbed species, 2-propoxide to form the CO₂ directly; the other was through conversion of 2-propanol to acetone, followed by formation of formate species, and finally CO₂.     

Low-temperature synthesis of highly crystallized LiMn2O4 from alpha manganese dioxide nanorods

One-dimensional alpha manganese dioxide (α-MnO₂) nanorods synthesized by a hydrothermal route were explored as the starting material for preparing lithium manganese spinel LiMn₂O₄. Pure and highly crystalline spinel LiMn₂O₄ was easily obtained from α-MnO₂ nanorods through a low-temperature solid-state reaction route, while Mn₂O₃ impurity was present along with the spinel phase when commercial MnO₂ was used as starting material. The particle size of LiMn₂O₄ prepared from α-MnO₂ nanorods was about 100 nm with a homogenous distribution. Electrochemical tests demonstrated that the LiMn₂O₄ thus prepared exhibited a higher capacity than that prepared from commercial MnO₂. Therefore, α-MnO₂ nanorods are proved to be a promising starting material for the preparation of high quality LiMn₂O₄.     

Enhanced high rate performance of LiMn2O4 spinel nanoparticles synthesized by a hard-template route

A nanosized LiMn₂O₄ (nano-LiMn₂O₄) spinel was prepared by a novel route using a porous silica gel as a sacrificial hard template. This material was found to be made up of 8–20 nm nanoparticles with a mean crystallite size of 15 nm. The electrochemical properties of nano-LiMn₂O₄ were tested in lithium cells at different cycling rates and compared to those of microsized LiMn₂O₄ (micro-LiMn₂O₄) obtained by the classical solid state route. Microsized LiMn₂O₄ is formed by 3–20 μm agglomerates, the size of each individual particle being approximately 0.20 μm. The behaviour of nano-LiMn₂O₄ as a positive electrode improves with increasing current densities (from C/20 to 2C). Moreover, it was found to exhibit a noticeably better performance at high rates (2C), with higher initial capacity values and very good retention (only 2% loss after 30 cycles), with respect to micro-LiMn₂O₄, almost certainly due to enhanced lithium diffusion in the small particles.