Characterization of sprayed and sputtered thin films for lithium ion microbatteries

We have investigated the preparation of thin films of anode and cathode materials for all oxide solid state lithium ion microbatteries. Thin films of LiCoO₂ and Li_4/3Ti_5/3O₄ have been deposited by both spray pyrolysis and RF magnetron sputtering. The structural and electrochemical properties of high temperature-LiCoO₂ thin films have been determined. Spray pyrolysis prepared higher quality LiCoO₂ thin films. Both sprayed and sputtered Li_4/3Ti_5/3O₄ thin films exhibited interesting lithium intercalation capacity. However, it has been demonstrated that RF magnetron sputtering is more efficient than spray pyrolysis for optimizing the interface between Li_4/3Ti_5/3O₄ and the substrate material. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Investigation of the solid-state electrolyte/cathode LiPON/LiCoO<Subscript>2</Subscript> interface by photoelectron spectroscopy

The electronic structure of a solid electrolyte/solid electrode interface (SESEI) of an all-solid-state thin film battery was investigated. The thin film battery consisted of a LiPON solid electrolyte and a LiCoO₂ cathode. The lithium phosphorus oxynitride (LiPON) electrolyte was RF sputtered in a step-by-step procedure onto the cathode and investigated by photoelectron X-ray-induced spectroscopy after each deposition step. An intermediate layer was found—composed of some new species—that differs in its chemical composition from the cathode as well as the LiPON solid electrolyte material and changes with growing layer thickness. In contrast, the electronic structure of the underlying cathode material remained predominantly unchanged.     

X-RAY PHOTOELECTRON SPECTROSCOPY STUDY ON ELECTROCHROMIC V2O5 THIN FILM

Nanocrystalline V₂O₅ thin films were reactively radio-frequency magnetron-sputtered under optimal deposition parameters. Their electrochemical and electrochromic characteristics were investigated by cyclic voltammetry and in-situ monochromatic transmittance measurements. Upon lithium intercalation, V₂O₅ thin films showed a double electrochromic behavior depending on the wavelength and the intercalation extent. X-ray photoelectron spectroscopy results showed that part of the V⁵⁺ in V₂O₅ was reduced to V⁴⁺ during the Li⁺ intercalation process.     

Iron doped lithium cobalt oxides as lithium intercalating cathode materials

Layered transition metal oxides of the formula LiMO₂ have good lithium insertion properties for which reason LiCoO₂ and LiNiO₂ have been exploited in practical lithium rocking chair batteries. Another member of the LiMO₂ series, LiFeO₂, should be an attractive cathode material considering the cheapness and environment-friendliness of iron compounds. Its rock-salt structure, however, does not allow significant amounts of lithium to be reversibly intercalated in its structure. Synthesis of layered LiFeO₂ and study of its lithium intercalating properties have been of limited success. Therefore, an attempt has been made here to study LiCo_1−yFe_yO₂ solid solutions (0 ≤ y ≤ 0.4) as prospective cathode materials. XRD, FTIR, Atomic absorption spectroscopy, Particle size and Surface area analysis were carried out in this regard towards the physical characterization of the entire series of LiCo_1−yFe_yO₂ compounds. The electrochemical discharge capacity of these materials is explained as a function of the iron content.     

Characterization of a LiCoO2 thin film cathode grown by pulsed laser deposition

A highly (003)-oriented pure LiCoO₂ thin film cathode, without Co₃O₄ impurities, was grown on a stainless steel substrate by pulsed laser deposition and characterized by electrochemical testing, scanning electron microscopy (SEM), ex situ X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS). The initial reversible discharge capacity of the LiCoO₂ thin film cathode reached 52.5?μAh/cm²µm and capacity loss was about 0.18% per cycle at a current density of 12.74?μA/cm². The chemical diffusion coefficient of the Li⁺ ion was estimated to be about 4.7?×?10^?11?cm²/s from cyclic voltammetric (CV) scans. Ex situ XRD revealed that the spacing of crystalline planes expanded about 0.09?Å when charged to 4.2?V, corresponding to Li_0.5CoO₂, lower than the value for composite powder LiCoO₂ electrodes. XPS results showed that the number of low-coordinated oxygen ions increased relative to the removal of Li⁺ ions.     

Synthesis of pyrochlore tungsten trioxide thin film and electrochemical lithium intercalation

The pyrochlore tungstate thin film has been prepared by an autoclave hydrothermal method at a temperature of 200 °C. The film was characterized by X-ray diffractometry, X-ray photoelectron spectroscopy and scanning electron micrograph measurements, showing that pure pyrochlore phase of sodium tungstate containing a small amount of water was formed by heating the film at a temperature of 350 °C. The cyclic voltammetric and galvanostic measurements revealed that a reversible electrochemical lithium intercalation into the crystal lattice of pyrochlore tungstate film takes place with charge-discharge cycling. Furthermore, the thermodynamics and kinetics of electrochemical lithium intercalation into the pyrochlore sodium tungstate film were also studied.     

Role of mesopores on the electrochemical performance of LiCoO<Subscript>2</Subscript> composite cathodes for lithium ion batteries

Mesoporous carbon (MC) was utilized to increase the mesoporosity of LiCoO₂ composite cathode. Graphite powder (GP) was chosen as a standard of comparison because of its very low mesoporosity. Compared with MC, GP has similar particle size, lower specific surface area, and higher electronic conductivity. Acetylene black (AB) exists in the form of chains of nanoparticles. With all other factors held constant, the mixture of AB and MC (ABMC)-loaded LiCoO₂ composite cathode (ABMC cathode) was superior to the mixture of AB and GP (ABGP)-loaded LiCoO₂ composite cathode (ABGP cathode). The reason is described as follows. Both GP and MC form a conductive network with AB chains. ABGP cathode has higher electronic conductivity than ABMC cathode. But the ionic conductivity of the ABMC cathode is more easily enhanced than the ABGP cathode because the former has much greater mesoporosity. In addition, the mesopores absorb and retain electrolyte solution and then provide buffer lithium ions for quick electrochemical reactions, so shortening the lithium ion transfer path in the composite cathode.     

Recent developments in the doping and surface modification of LiFePO4 as cathode material for power lithium ion battery

Lithium ion batteries have become attractive for portable devices due to their higher energy density compared to other systems. With a growing interest to develop rechargeable batteries for electric vehicles, lithium iron phosphate (LiFePO₄) is considered to replace the currently used LiCoO₂ cathodes in lithium ion cells. LiFePO₄ is a technically important cathode material for new-generation power lithium ion battery applications because of its abundance in raw materials, environmental friendliness, perfect cycling performance, and safety characteristics. However, the commercial use of LiFePO₄ cathode material has been hindered to date by their low electronic conductivity. This review highlights the recent progress in improving and understanding the electrochemical performance like the rate ability and cycling performance of LiFePO₄ cathode. This review sums up some important researches related to LiFePO₄ cathode material, including doping and coating on surface. Doping elements with coating conductive film is an effective way to improve its rate ability.     

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. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Electrochemical behaviour of sputtered c-V2O5 and LiCoO2 thin films for solid state lithium microbatteries

Here are reported for the first time electrochemical data on all-solid-state lithium microbatteries using crystalline sputtered V₂O₅ thin films as cathode materials and LiPON as solid electrolyte. The stable specific capacity of 30 µAh/cm² found with a 2.4 µm thick film competes very well with the best values obtained for solid state microbatteries using amorphous films. With the challenge of decreasing the temperature of heat treatment for sputtered LiCoO₂ thin films, we show that a temperature of 500 °C combined with an optimized bias sputtering (-50 V) allows to get highly crystalline deposits, to minimize the presence of Co₃O₄ and to suppress any trace of the cubic phase. At the same time the theoretical specific capacity is reached in the 4.2 V-3 V range and a good cycling behaviour is achieved with a high capacity of 50 µAh/cm²/µm after 140 cycles at 10 µA.cm².     

V2O5 aerogel-like lithium intercalation host

Thin self-standing films and powders of highly amorphous V₂O₅ have been prepared via a combined sol-gel and solvent exchange procedure. The amorphous V₂O₅ is a highly interconnected porous material with very thin solid walls and shows a unique lithium intercalation behavior. Electrochemical tests performed on composite cathodes made by mixing the material with carbon particles demonstrated a very high lithium insertion capacity.     

Effect of mesoporous carbon containing binary conductive additives in lithium ion batteries on the electrochemical performance of the LiCoO2 composite cathodes

Binary conductive additives (BCA), formed by sonication of mesoporous carbon (MC) and acetylene black (AB), were used as conductive additives to improve the electrochemical performance of a LiCoO₂ composite cathode. The electrochemical performance of the LiCoO₂ composite cathode dispersed with BCA was investigated. The results showed that the electrochemical performance (including the discharge capacity, the discharge voltage and the total internal resistance) of a BCA loaded LiCoO₂ composite cathode was better than that of a cathode loaded with AB. The possible mechanism is that the MC in BCA can adsorb and retain electrolyte solution, which allows an intimate contact between the lithium ions and the cathode active material LiCoO₂ due to its large mesopore specific surface area. A simplified model was also proposed.     

Investigation of the synergetic effects of LiBF<Subscript>4</Subscript> and LiODFB as wide-temperature electrolyte salts in lithium-ion batteries

Herein, we present the use of lithium tetrafluoroborate (LiBF₄) as an electrolyte salt for wide-temperature electrolytes in lithium-ion batteries. The research focused on the application of blend salts to exhibit their synergistic effect especially in a wide temperature range. In the study, LiCoO₂ was employed as the cathode material; LiBF₄ and lithium difluoro(oxalate)borate (LiODFB) were added to an electrolyte consisting of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC). The electrochemical performance of the resulting electrolyte was evaluated through various analytical techniques. Analysis of the electrical conductivity showed the relationship among solution conductivity, the electrolyte composition, and temperature. Cyclic voltammetry (CV), charge-discharge cycling, and AC impedance measurements were used to investigate the capacity and cycling stability of the LiCoO₂ cathode in different electrolyte systems and at different temperatures. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were applied to analyze the surface properties of the LiCoO₂ cathode after cycling. The results indicated that the addition of a small amount of LiODFB into the LiBF₄-based electrolyte system (LiBF₄/LiODFB of 8:2) may enhance the electrochemical performance of the LiCoO₂ cell over a relatively wide temperature range and improve the cyclability of the LiCoO₂ cell at 60 °C.     

4-Volt cathode materials for rechargeable lithium batteries wet-chemistry synthesis, structure and electrochemistry

Lithium transition-metal oxides used as intercalation compounds for rechargeable lithium batteries are widely studied in search of structural stability and improved electrochemical performance. Cathode materials belonging to the 4-volt class electrodes were synthesized by wet-chemistry methods, i.e., sol-gel, combustion or co-precipitation techniques. It is shown that synthesis greatly affects the electrochemistry and cycle life characteristics of the cathodes. Extensive damage including local strain variation, nanodomain formation, and changes in cation ordering, has been observed by local probes such as Raman and FTIR spectroscopy. In this work we wish to show the relationship between the local cationic environment and electrochemical characteristics of the 4-volt cathodes. Materials such as LiMn₂O₄, LiCoO₂, LiNi_1−yCo_yO₂, LiNi_1−yCo_yVO₄, and LiMoVO₆ are investigated.     

Electrochemical and structural properties of MoO3–V2O5 nanocomposite thin film electrodes for lithium rechargeable batteries

We have investigated the electrochemical properties of V₂O₅-based thin film electrodes as a function of the amount of MoO₃ by means of X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and transmission electron microscopy (TEM). XRD results show that the V₂O₅-based thin film electrodes give an amorphous characteristic. XPS results reveal the formation of V₂O₅ and MoO₃ phases. TEM results show that MoO₃ dots (5–30 nm in size) are embedded in the amorphous V₂O₅ matrix. It is further shown that cells fabricated with the MoO₃–V₂O₅ nanocomposite thin film electrodes give better cycling performance than those made with the single V₂O₅ thin film electrodes. A possible explanation for the MoO₃ nano-dot dependence on the cycling performance of the V₂O₅-based thin film electrodes is described.     

Electrochemical characteristics of sulfur composite cathode for reversible lithium storage

The electrochemical characteristics of the sulfur composite cathode for reversible lithium storage were investigated. The sulfur composites showed novel electrochemical characteristics as well as high specific capacity and good cycleability. The sulfur composite presented the average discharge voltage of 1.9 V, which was just the half of conventional LiCoO₂ cathode materials, indicating that the double cells in series presented the same working voltage as conventional LiCoO₂ cells and meaning that the sulfur composite cells will have good interchangeability with conventional LiCoO₂ cells. The overcharge test showed that the sulfur composite cell cannot be charged over 5.0 V, indicating that the sulfur composite cell presented the intrinsic safety for overcharge. Overcharge can cause serious problems for the conventional Li ion cells. The overcharge test also showed that the sulfur composite cell was destroyed when the cell was charged over 4.0 V, resulting in that the cell cannot normally be discharged again. It is found, however, that the sulfur composite cell can be discharged again at very low current density of a 0.002-C rate after the cell was overcharged. Being much safer than lithium metal anode, the graphite anode was used to fabricate sulfur composite/graphite lithium ion cells with a prelithiated sulfur composite cathode, which was produced by electrochemical lithiation. The charge/discharge and cycling characteristics of the sulfur composite/graphite cell was investigated. The result showed that the sulfur composite/graphite cells can be normally cycled and showed the different voltages from that of the cell with the lithium metal anode. This paves the effective way to fabricate safer sulfur composite/graphite lithium ion cells.     

Prototype electrochromic device and dye sensitized solar cell using spray deposited undoped and ‘Li’ doped V2O5 thin film electrodes

Lithium (Li) (0–5 wt%) doped V₂O₅ thin films were spray deposited at 450 °C onto ITO substrates. Structural analysis using X-ray diffraction and Raman spectroscopy revealed orthorhombic phase of the films. In addition to the V₂O₅ phase, presence of VO₂ peaks due to high deposition temperature is also evident from structural and optical characterization. The non-stoichiometric nature of the films due to loss of the terminal O atom was confirmed from Raman spectroscopy. The direct band gap, indirect bandgap, and phonon energies were also calculated from optical studies. Different charge states of vanadium ions present in the film were identified from X-ray photoelectron spectroscopy study. Results from cyclic voltammetry experiments reflected significant differences between the undoped and Li doped V₂O₅ samples. Transport properties by Hall-effect measured at room temperature indicated significant increase in conductivity, carrier concentration and mobility of V₂O₅ thin films on doping with Li. A Dye Sensitized Solar Cell (DSSC) was fabricated using mobility enhanced 5 wt% Li doped V₂O₅ film as photoanode and its efficiency was found to be 2.7%. A simple electrochromic cell is fabricated using undoped V₂O₅ thin film to demonstrate the colour change.     

From ‘core-shell’ to composite mixed cathode materials for rechargeable lithium batteries by mechanochemical process

Solid state mechanical activation method was applied for surface modification of LiMn₂O₄ by Li-M-O (M = Co, Co+Ni) and for preparation of composite mixed LiMn₂O₄/LiCoO₂ cathode materials. Pristine LiMn₂O₄ was ground with correspondent precursors (for coating) or with LiCoO₂ (for composites) in high-energy planetary mills and then heat treated at different temperatures. As prepared materials were studied by XRD, ⁷Li MAS NMR spectroscopy, XPS, SEM and electrochemical cycling. It has been shown that both ‘core-shell’ and composite materials prepared by mechanochemical process are characterized by superior electrochemical performance due to smaller particles and chemical modification of LiMn₂O₄.     

Pickering emulsion polymerization of polyaniline/LiCoO2 nanoparticles used as cathode materials for lithium batteries

The oil in water (o/w) emulsions were prepared using aniline dissolved in toluene and LiCoO₂ particles as stabilizers (Pickering emulsions). Pickering emulsions are stabilized by adsorbed solid particles instead of emulsifier molecules. The mean droplet diameter of emulsions was controlled by the mass ratio M (oil)/M (solid particles). The emulsions showed great stability during 3 days. The composite materials containing LiCoO₂ and the conductive polymer polyaniline (PANI) have been prepared by means of polymerization of aniline emulsion stabilized by LiCoO₂ particles. The composite materials were characterized by nanosphere and nanofiber-like structures. The nanofiber-like morphology of the powdered material was distinctly different of the morphologies of the parent materials. The electrochemical reactivity of PANI/LiCoO₂ composites as positive electrode in a lithium battery was examined during lithium ion deinsertion and insertion by galvanostatic charge–discharge testing; PANI/LiCoO₂ (1:4) composite materials exhibited the best electrochemical performance by increasing the reaction reversibility and capacity compared to that of the pristine LiCoO₂ cathode. The first discharge capacity of PANI/LiCoO₂ (1:4) was 167 mAh/g, while that of LiCoO₂ was136 mAh/g.     

Synthesis and characterization of oxide cathode materials of the system (1-x-y)LiNiO<Subscript>2</Subscript>·xLi<Subscript>2</Subscript>MnO<Subscript>3</Subscript>·yLiCoO<Subscript>2</Subscript>

Composite cathode materials produced by integrating isostructural (2D-layered) compounds LiNiO₂, LiCoO₂, and Li₂MnO₃ (Li(Li_1/3Mn_2/3)O₂) have been investigated utilizing a compositional phase diagram. The samples were characterized by multiple techniques to establish structure–property relationships. Specifically, for structural characterization, powder X-ray diffraction, scanning electron microscopy, thermo-gravimetric analysis, and X-ray photoelectron spectroscopy were carried out. For properties, electrochemical characterization was carried out. The best composition showed a discharge capacity of 244 mAh/g (C/15 rate) in the testing range of 4.6–2 V, with good coulombic efficiency and cyclability.