Dynamic behavior of surface film on LiCoO2 thin film electrode

Electrochemical oxidation behavior of non-aqueous electrolytes on LiCoO₂ thin film electrodes were investigated by in situ polarization modulation Fourier transform infrared (PM-FTIR) spectroscopy, atomic force microscopy and X-ray photoelectron spectroscopy (XPS). LiCoO₂ thin film electrode on gold substrate was prepared by rf-sputtering method. In situ PM-FTIR spectra were obtained at various electrode potentials during cyclic voltammetry measurement between 3.5 V vs. Li/Li⁺ and 4.2 V vs. Li/Li⁺. During anodic polarization, oxidation of non-aqueous electrolyte was observed, and oxidized products remained on the electrode at the potential higher than 3.75 V vs. Li/Li⁺ as a surface film. During cathodic polarization, the stripping of the surface film was observed at the potential lower than 3.9 V vs. Li/Li⁺. Depth profile of XPS also showed that more organic surface film remained on charged LiCoO₂ than that on discharged one. AFM images of charged and discharged electrodes showed that some decomposed products deposited on charged electrode and disappeared from the surface of discharged one. These results indicate that the surface film on LiCoO₂ is not so stable.     

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.     

Electrochemical performance of all-solid-state lithium secondary batteries improved by the coating of Li2O-TiO2 films on LiCoO2 electrode

All-solid-state lithium secondary batteries using LiCoO₂ particles coated with amorphous Li₂O-TiO₂ films as an active material and Li₂S-P₂S₅ glass-ceramics as a solid electrolyte were fabricated; the electrochemical performance of the batteries was investigated. The interfacial resistance between LiCoO₂ and solid electrolyte was decreased by the coating of Li₂O-TiO₂ films on LiCoO₂ particles. The rate capability of the batteries using the LiCoO₂ coated with Li₂Ti₂O₅ (Li₂O·2TiO₂) film was improved because of the decrease of the interfacial resistance of the batteries. The cycle performance of the all-solid-state batteries under a high cutoff voltage of 4.6 V vs. Li was highly improved by using LiCoO₂ coated with Li₂Ti₂O₅ film. The oxide coatings are effective in suppressing the resistance increase between LiCoO₂ and the solid electrolyte during cycling. The battery with the LiCoO₂ coated with Li₂Ti₂O₅ film showed a large initial discharge capacity of 130 mAh/g and good capacity retention without resistance increase after 50 cycles at the current density of 0.13 mA/cm².     

Synthesis of LiCoO2 thin films by sol/gel process

LiCoO₂ thin films were synthesized by sol/gel process using acrylic acid (AA) as chelating agent. The gel formulation was optimized by varying solvent (ethylene glycol or water) and precursors molar ratios (Li, Co, AA) in order to obtain a dense film for positive electrode of lithium batteries. The gel was deposited by spin-coating technique on an Au/TiO₂/SiN/SiO₂/Si substrate. Thin films were deposited by either single or multistep process to enhance the density of the thin film and then calcined during 5 h at 800 °C to obtain the R-3m phase (HT-LiCoO₂).A chemical characterization of the solution was realized by Fourier Transform Infrared (FTIR) spectroscopy. Thermal decomposition of precursors and gel was studied by Thermo Gravimetric Analyses (TGA). Further investigations were done to characterize rheologic behaviour of the gel and solvents affinity with the substrate. Crystallinity and morphology were analyzed respectively by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM).The formation of R-3m phase was confirmed by the electrochemical behaviour of the gel derived LiCoO₂. Cyclic voltammograms and galvanostatic cycling show typical curve shape of the HT-LiCoO₂.     

Preparation and electrochemical properties of pure lithium cobalt oxide films by electron cyclotron resonance sputtering

Positive Li–Co–O films for all-solid-state thin-film lithium batteries were prepared by electron cyclotron resonance (ECR) sputtering using Li_xCoO₂ targets (x = 1.0, 1.2, 1.7 and 2.0). The Li–Co–O films prepared using the x = 1.0, 1.2 and 1.7 targets contained a Co₃O₄ impurity phase with high-temperature phase LiCoO₂. The film prepared using the x = 2.0 target was found to contain only a pure LiCoO₂ phase by Raman spectrometry and the inductively coupled plasma/atomic emission spectrometry (ICP/AES) method, and a thin-film battery using this film exhibited good electrochemical properties as a result of the improved utilization of the positive film.     

Effect of FePO4 coating on electrochemical and safety performance of LiCoO2 as cathode material for Li-ion batteries

LiCoO₂ was surface modified by a coprecipitation method followed by a high-temperature treatment in air. FePO₄-coated LiCoO₂ was characterized with various techniques such as X-ray diffraction (XRD), auger electron spectroscopy (AES), field emission scanning electron microscope (FE-SEM), energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), electrochemical impedance spectroscopy (EIS), 3 C overcharge and hot-box safety experiments. For the 14500R-type cell, under a high charge cutoff voltage of 4.3 and 4.4 V, 3 wt.% FePO₄-coated LiCoO₂ exhibits good electrochemical properties with initial discharge specific capacities of 146 and 155 mAh g⁻¹ and capacity retention ratios of 88.7 and 82.5% after 400 cycles, respectively. Moreover, the anti-overcharge and thermal safety performance of LiCoO₂ is greatly enhanced. These improvements are attributed to the FePO₄ coating layer that hinders interaction between LiCoO₂ and electrolyte and stabilizes the structure of LiCoO₂. The FePO₄-coated LiCoO₂ could be a high performance cathode material for lithium-ion battery.     

Characterization of LiCoO2 coated Li1.05Ni0.35Co0.25Mn0.4O2 cathode material for lithium secondary cells

In this study, nano-crystalline LiCoO₂ was coated onto the surface of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ powders via sol–gel method. The influence of the coating on the electrochemical behavior of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ is discussed. The surface morphology was characterized by transmission electron microscopy (TEM). Nano-crystallized LiCoO₂ was clearly observed on the surfaces of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂. The phase and structural changes of the cathode materials before and after coating were revealed by X-ray diffraction spectroscopy (XRD). It was found that LiCoO₂ coated Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ cathode material exhibited distinct surface morphology and lattice constants. Cyclic voltammetry (2.8–4.6 V versus Li/Li⁺) shows that the characteristic voltage transitions on cycling exhibited by the uncoated material are suppressed by the 7 wt.% LiCoO₂ coating. This behavior implies that LiCoO₂ inhibits structural change of Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ or reaction with the electrolyte on cycling. In addition, the LiCoO₂ coating on Li_1.05Ni_0.35Co_0.25Mn_0.4O₂ significantly improves the rate capability over the range 0.1–4.0C. Comparative data for the coated and uncoated materials are presented and discussed.     

Expanding performance limit of lithium-ion batteries simply by mixing Al(OH)3 powder with LiCoO2

Mixing a small amount of Al(OH)₃ powder with a LiCoO₂ cathode material is demonstrated to improve markedly the cycle performance and thermal stability of commercial grade LiCoO₂/graphite lithium-ion batteries. Al(OH)₃-mixed LiCoO₂/graphite prismatic cells exhibit excellent capacity retention as high as 95% after 400 cycles with negligible polarization build-up. Moreover, the thermal stability of the cells is greatly improved by Al(OH)₃ mixing, which is confirmed by higher residual and recovery capacity ratios after storage at 90 °C compared with a pristine cell. The beneficial effects of Al(OH)₃ are found to be related mainly to an improvement of the cathode side, which is ascribed to reduced unwanted side-reactions with the electrolyte.     

Amorphous Si film anode coupled with LiCoO2 cathode in Li-ion cell

An amorphous silicon film with an average thickness of up to 2 μm was deposited on copper foil by direct-circuit (dc) magnetron sputtering and coupled with commercial LiCoO₂ cathode to fabricate cells. Their cycle performance and high rate capability at room temperature have been investigated. In the voltage range 2.5–3.9 V at the current density of 0.2C (0.11 mA cm⁻²), the lithiation and delithiation capacity of this cell was first increased to 0.55 mAh cm⁻² within 80 cycles and maintained stable during the following cycles. After 300 cycles its capacity still retained 0.54 mAh cm⁻². High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) image indicated that the sputtered film could keep an amorphous structure although the volume expansion ratio during the lithiation and delithiation was still up to 300% after 300 cycles observed from scanning electron microscopy (SEM) image. This recovered amorphous structure was believed to be beneficial for the improvement of the cycle life of the cell. Rate performance showed that the cells had a promising high rate capability. At 30C, its lithiation/delithiation capacity remained 25% of that at 0.2C.     

High-rate performance of all-solid-state lithium secondary batteries using Li4Ti5O12 electrode

The all-solid-state Li–In/Li₄Ti₅O₁₂ cell using the 80Li₂S·20P₂S₅ (mol%) solid electrolyte was assembled to investigate rate performances. It was difficult to obtain the stable performance at the charge current density of 3.8 mA cm⁻² in the all-solid-state cell. In order to improve the rate performance, the pulverized Li₄Ti₅O₁₂ particles were applied to the all-solid-state cell, which retained the reversible capacity of about 90 mAh g⁻¹ at 3.8 mA cm⁻². The 70Li₂S·27P₂S₅·3P₂O₅ glass–ceramic, which exhibits the higher lithium ion conductivity than the 80Li₂S·20P₂S₅ solid electrolyte, was also used. The Li–In/70Li₂S·27P₂S₅·3P₂O₅ glass–ceramic/pulverized Li₄Ti₅O₁₂ cell was charged at a current density higher than 3.8 mA cm⁻² and showed the reversible capacity of about 30 mAh g⁻¹ even at 10 mA cm⁻² at room temperature.     

Charge–discharge characteristics of all-solid-state thin-filmed lithium-ion batteries using amorphous Nb2O5 negative electrodes

All-solid-state thin-filmed lithium-ion rechargeable batteries composed of amorphous Nb₂O₅ negative electrode with the thickness of 50–300 nm and amorphous Li₂Mn₂O₄ positive electrode with a constant thickness of 200 nm, and amorphous Li₃PO_4−xN_x electrolyte (100 nm thickness), have been fabricated on glass substrates with a 50 mm × 50 mm size by a sputtering method, and their electrochemical characteristics were investigated. The charge–discharge capacity based on the volume of positive electrode increased with increasing thickness of negative electrode, reaching about 600 mAh cm⁻³ for the battery with the negative electrode thickness of 200 nm. But the capacity based on the volume of both the positive and negative electrodes was the maximum value of about 310 mAh cm⁻³ for the battery with the negative electrode thickness of 100 nm. The shape of charge–discharge curve consisted of a two-step for the batteries with the negative electrode thickness more than 200 nm, but that with the thickness of 100 nm was a smooth S-shape curve during 500 cycles.     

Electrochemical studies on LiCoO2 surface coated with Y3Al5O12 for lithium-ion cells

Synthesized yttrium aluminum garnet (YAG) sol was coated on the surface of the LiCoO₂ cathode particles by an in situ sol–gel process, followed by calcination at 923 K for 10 h in air. Based on XRD, TEM, and ESCA data, a compact YAG kernel with an average thickness of ∼20 nm was formed on the surface of the core LiCoO₂ particles, which ranged from ∼90 to 120 nm in size. The charge–discharge cycling studies for the coated materials suggest that 0.3 wt.% YAG-coated LiCoO₂ heated at 923 K for 10 h in air, delivered a discharge capacity of 167 mAh g⁻¹ and a cycle stability of about 164 cycles with a fading rate of 0.2 mAh cycle⁻¹ at a 0.2C-rate between 2.75 and 4.40 V vs. Li/Li⁺. The differential capacity plots revealed that impedance growth was slower for YAG surface treated LiCoO₂, when cells were charged at 4.40 V. DSC results exemplified that the exothermic peak at ∼468 K corresponded to the release of much less oxygen and greater thermal-stability.     

All-solid-state lithium secondary batteries using LiCoO2 particles with pulsed laser deposition coatings of Li2S-P2S5 solid electrolytes

Electrode-electrolyte composite materials were prepared by coating a highly conductive Li₂S-P₂S₅ solid electrolyte onto LiCoO₂ electrode particles using pulsed laser deposition (PLD). Cross-sections of the composite electrode layers of the all-solid-state cells were observed using a transmission electron microscope to investigate the packing morphology of the LiCoO₂ particles and the distribution of solid electrolyte in the composite electrode. All-solid-state cells based on a composite electrode composed entirely of solid-electrolyte-coated LiCoO₂ were fabricated, and their performance was investigated. The coating amounts of Li₂S-P₂S₅ solid electrolytes on LiCoO₂ particles and the conductivity of the coating material were controlled to increase the capacity of the resulting all-solid-state cells. All-solid-state cells using LiCoO₂ with thick solid electrolyte coatings, grown over 120 min, had a capacity of 65 mAh g⁻¹, without any addition of Li₂S-P₂S₅ solid electrolyte particles to the composite electrode. The capacity of the all-solid-state cell increased further after increasing the conductivity of the Li₂S-P₂S₅ solid electrolyte coating by heat treatment at 200 °C. Furthermore, an all-solid-state cell based on a composite electrode using both a solid electrolyte coating and added solid electrolyte particles was fabricated, and the capacity of the resulting all-solid-state cell increased to 95 mAh g⁻¹.     

Characterization of CuInS2 thin films prepared by electrodeposition and sulfurization with photoluminescence spectroscopy

Potentiostatic electrodeposition and sulfurization techniques were used to prepare polycrystalline CuInS₂ thin films. X-ray diffraction and photoresponse measurements in a photoelectrochemical cell (PEC) revealed that photoactive polycrystalline CuInS₂ films can be deposited on Ti substrate. Photoluminescence (PL) spectroscopy was used to investigate the prepared thin films and optically characterize them. PL spectra revealed the defect structure of the samples with an acceptor energy level at 109 meV above the valance band and a donor energy level at 71 meV below the conduction band. The CuInS₂ thin films prepared in this investigation are observed to be In-rich material with n-type electrical conductivity.     

Characterization of the interface between LiCoO2 and Li7La3Zr2O12 in an all-solid-state rechargeable lithium battery

The interfacial layer formed between a lithium-ion conducting solid electrolyte, Li₇La₃Zr₂O₁₂ (LLZ), and LiCoO₂ during thin film deposition was characterized using a combination of microscopy and electrochemical measurement techniques. Cyclic voltammetry confirmed that lithium extraction occurs across the interface on the first cycle, although the nonsymmetrical redox peaks indicate poor electrochemical performance. Using analytical transmission electron microscopy, the reaction layer (∼50 nm) was analyzed. Energy dispersive X-ray spectroscopy revealed that the concentrations of some of the elements (Co, La, and Zr) varied gradually across the layer. Nano-beam electron diffraction of this layer revealed that the layer contained neither LiCoO₂ nor LLZ, but some spots corresponded to the crystal structure of La₂CoO₄. It was also demonstrated that reaction phases due to mutual diffusion are easily formed between LLZ and LiCoO₂ at the interface. The reaction layer formed during high temperature processing is likely one of the major reasons for the poor lithium insertion/extraction at LLZ/LiCoO₂ interfaces.     

Characterization of electrolytic Co3O4 thin films as anodes for lithium-ion batteries

The electrolytic deposition of Co₃O₄ thin films on stainless steel was conducted in Co(NO₃)₂ aqueous solution for anodes in lithium-ion thin film batteries. Three major electrochemical reactions during the deposition were discussed. The coated specimens and the coating films carried out at −1.0 V (saturated KCl Ag/AgCl) were subjected to annealing treatments and further characterized by XRD, TGA/DTA, FE-SEM, Raman spectroscopy, cyclic voltammetry (CV) and discharge/charge cyclic tests. The as-coated film was β-Co(OH)₂, condensed into CoO and subsequently oxidized into nano-sized Co₃O₄ particles. The nano-sized Co₃O₄, CoO, Li₂O and Co particles revealed their own characteristics different from micro-sized ones, such as more interfacial effects on chemical bonding and crystallinity. The initial maximum capacity of Co₃O₄ coated specimen was 1930 mAh g⁻¹ which much more than its theoretical value 890 mAh g⁻¹, since the nano-sized particles offered more interfacial bondings for extra sites of Li⁺ insertion. However, a large ratio of them was trapped, resulting in a great part of irreversible capacity during the first charging. Still, it revealed a capacity 500 mAh g⁻¹ after 50 discharged-charged cycles.     

Photoelectrochemical properties of AgInS2 thin films prepared using electrodeposition

Ternary silver-indium-sulfide samples were deposited on fluorine-doped tin oxide (FTO) coated glass substrates using a one-step electrodeposition method. A new procedure for the deposition of AgInS₂ samples is reported. The effect of the [Ag]/[In] molar ratio in solution bath on the structural, morphological, and photoelectrochemical properties of samples was examined. X-ray diffraction patterns of samples show that the films are the AgInS₂ phase. The thickness, direct band gap, and indirect band gap of the films were in the ranges 209-1021 nm, 1.82-1.85 eV, and 1.44-1.51 eV, respectively. The carrier densities and flat-band potentials of films obtained from Mott-Schottky and open-circuit potential measurements were in the ranges of 4.2×10¹⁹-9.5×10¹⁹ cm⁻³ and −0.736 to −0.946 V vs. the normal hydrogen electrode (NHE), respectively. It was found that the samples with molar ratio [Ag]/[In]=0.8 in solution bath had a maximum photocurrent density of 9.28 mA/cm² with an applied bias of +1.0 V vs. an Ag/AgCl electrode in contact with electrolyte containing 0.25 M K₂SO₃ and 0.35 M Na₂S. The results show that high-quality AgInS₂ films can be deposited on FTO-coated glass substrates for photoelectrochemical (PEC) applications.     

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.     

Improvement in dye-sensitized solar cells employing TiO2 electrodes coated with Al2O3 by reactive direct current magnetron sputtering

A novel surface modification method was carried out by reactive dc magnetron sputtering to fabricate TiO₂ electrodes coated with Al₂O₃ for improving the performance of dye-sensitized solar cells (DSSCs). The Al₂O₃-coated TiO₂ electrodes had been characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–vis spectrophotometer, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The study results revealed that the modification to TiO₂ increases dye absorption amount, reduces trap sites on TiO₂, and suppresses interfacial recombination. The impact of sputtering time on photoelectric performance of DSSCs was investigated. Sputtering Al₂O₃ for 4 min on 5-μm thick TiO₂ greatly improves all cell parameters, resulting in enhancing the conversion efficiency from 3.93% to 5.91%. Further increasing sputtering time decreases conversion efficiency.