In-situ optical detection of the passive oxide films on iron and titanium was reviewed. The optical techniques such as ellipsometry (i.e., reflection of polarized light), Raman spectroscopy, and potential modulation reflectance (PMR) have been successfully applied to the detection in thickness, composition, and semiconducting property, respectively, of the thin passive oxides under the in-situ condition. The growth mechanism of the passive oxide has been discussed from the precise measurement of thickness as a function of potential and time by a three-parameter ellipsometry. From the in-situ Raman spectra, the composition of the passive oxide has been estimated to be Fe3O4-γ-Fe2O3 for iron and anatase TiO2 for titanium. From PMR, the Mott-Schottky type plot could be drawn in the passive oxide on iron, which indicates that the formation of the space charge layer can be optically seen. From the PMR spectra, one has evaluated light-absorption edge that may correspond to a band-gap energy between the valence and conduction band.
The carbon-coated LiFePO4/C cathode active material was successfully synthesized via a glycine-assisted sol-gel method. Glycine was used both as the chelating reagent and the carbon source. The effect of the carbon source on the structural, morphological and electrochemical properties of LiFePO4 are studied in this paper. The structure, morphology and electrochemical performance were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and galvanostatic charge-discharge test. The results indicated that glycine did not affect the crystal structure, but restrained the particle size of LiFePO4/C. The SEM images revealed that the majority of the particles lay between 900 and 5000 nm for pure LiFePO4, while the carbon-coated LiFePO4 particles were from 400 to 900 nm. The LiFePO4/C sample synthesized at 700°C for 8 h delivered the highest initial discharge specific capacity of 163.5 mAh/g, i.e. 96.2% of the theoretical capacity. It retained a high discharge capacity of 112.3 mAh/g over 20 cycles at 1 C rate. The glycine-assisted sol-gel synthesis is favor to obtain the high electrochemical performance of LiFePO4/C.
A polyaniline (PANi)-coated sulfur cathode was prepared by in-situ polymerization to improve the performance of Li–S batteries and prevent polysulfide dissolution. Because PANi polymerization requires only the addition of sulfur powder, the polymerization method was simple. Battery performance testing was conducted using various sulfur ratios, with the best performance being exhibited by the 31 wt% sulfur PANi composite (approximately 500 mAh g−1 S after 50 cycles from an initial capacity of 903 mAh g−1 S). Transmission electron microscopy images confirmed that sulfur was coated by an approximately 10-nm-thick layer of PANi. Cycle testing was conducted at a rate of 0.2 C with an electrode loading of 2 mg cm−2 of sulfur. Thus, a highly homogenous distribution of sulfur particles in PANi was achieved, resulting in sulfur-coated composite materials and allowing for the preparation of a conductive polymer by a relatively simple sulfur distribution method.
Layered Li2MnO3-LiMn1/3Ni1/3Co1/3O2 solid-solution cathode materials modified with SiO2 were prepared by spray pyrolysis and their charge-discharge properties were investigated. Both the capacity and voltage fades of SiO2-modified Li2MnO3-LiMn1/3Ni1/3Co1/3O2 during charge/discharge cycling were suppressed compared with that of the unmodified sample. It was found that the addition of SiO2 suppressed the dissolution of manganese ions at 60°C from the results of inductively coupled plasma (ICP) emission spectroscopy. X-ray diffraction (XRD) data indicated that weak reflections corresponding to Li2SiO3 appeared as the SiO2 content increases to 5 wt%. Scanning electron microscope (SEM) studies showed that the microstructure of the particles was significantly changed by SiO2 addition. As indicated by transmission electron microscope (TEM) analysis, Si-rich regions were seen on the Li2MnO3-LiMn1/3Ni1/3Co1/3O2 primary particles. The suppression of Mn ions and the improved charge and discharge properties were correlated with the microstructural changes.
Inhibitory effect of sulfur–containing compounds on anodic oxidation of borohydride was investigated. The linear sweep voltammetry technique revealed that sulfur compounds such as thiosulfate and sulfide inhibited anodic oxidation of borohydride not only on a platinum surface but also on a glassy carbon surface which had less catalytic activity of borohydride electro–oxidation. Through electrochemical impedance spectroscopy, it was confirmed the charge transfer resistance of borohydride oxidation at the electrode surface increased in the presence of sulfur compounds, but the capacitance of the electric double layer at the surface did not depend on sulfur compounds and their concentration. The degree of inhibition by each sulfur compound on the electro–oxidation of borohydride in the presence of electrodes and its spontaneous oxidation in the absence of electrodes was similar. Thus the sulfur compound inhibitive influence on borohydride oxidation occurred without adsorbing onto the surface of the electrode. The anodic oxidation of borohydride was suppressed to less than 1/10 of the exchange current in the case of thiosulfate and sulfide. Sulfur compounds inhibited anodic oxidation of hydrazine and dimethylaminoborane as well, while the effect of each sulfur compound varied from that of borohydride.
The surface morphology of the electrodeposited lithium metal from electrolyte solutions containing electrolyte additives: fluoroethylene carbonate (FEC), vinylene carbonate (VC) and lithium bis(oxalate)borate (LiBOB), were investigated. All the film forming additive improved the surface morphology. The FEC especially shows the most uniform surface morphology compared with the other electrolyte additives and the additive-free electrolyte. The surface analyses of the lithium metal were conducted using X-ray photoelectron spectroscopy (XPS) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The analytical study revealed that the FEC suppresses the decomposition of the PF6− anion, resulting in the formation of a thin stable SEI layer on the lithium metal.
Eight reference electrodes (Li/Li+) were embedded around a square (30 mm × 30 mm) lithium-ion battery (LIB) cell (Type A or Type B), and the electrical potential changes of the positive and negative electrodes against the reference electrodes (Rx; x-1–8) during charge and discharge were studied. Type A has a jutting positive-side (JP-side) and a jutting negative-side (JN-side). During the charge and discharge of Type A, significant potential shifts (>1.3 V) in the horizontal plane of the cell were observed near the JP-side and JN-side, but which were relieved after several hours. The potential shifts in the horizontal plane of the cell (the difference between the maximum and the minimum of P-Rx (x = 1–8) or N-Rx (x = 1–8)) were caused by the relaxation current between P and JP-side or between N and JN-side. AC impedance measurements also were conducted with the cells several hours after they reached a specific SOC level. Linear relations were obtained at wide frequencies between the ratio of P-Rx (x = 1–8) and Z′ (Real) or Z′′ (Imaginary) resistance. The sharing rate of P-Rx (x = 1–8) in Z′ (Real) or Z′′ (Imaginary) resistance was mainly determined by the position of the reference electrode which was close to the edge of the positive electrode or that of the negative one. It was found that when a reference electrode is installed outside of the cell, it is difficult to divide the impedance into both electrodes by the AC impedance measurements with the reference electrode.
In this study, a new electrolyte urea/1-ethyl-3-methylimidazolium chloride was developed to replace traditional sulfuric acid in the zinc electroplating process using zinc oxide as a zinc source. The electrochemical behavior of zinc from zinc oxide was investigated in the urea/1-ethyl-3-methylimidazolium chloride electrolyte using chronoamperometric and cyclic voltammetric techniques. The cyclic voltammograms for this electrolyte illustrated that the zinc reduction is a diffusion–controlled irreversible process via a single-step two-electron transfer procedure. Chronoamperometric measurements suggested that the zinc electrodeposition on a tungsten electrode followed a three-dimensional instantaneous nucleation with a diffusion-controlled growth model at 353 K. Electrodeposits from Chronoamperometric measurements were analyzed with various characterization techniques. SEM images show that electrodeposits were flakes-like at low cathodic potentials and became clusters at more negative cathodic potentials. The obtained electrodeposits are metallic zinc, confirmed by XRD and EDS.
Triclinic LiTiOPO4 fine powders were synthesized via low-temperature thermal treatment. First, amorphous fine powders were prepared by mechanical milling (MM) treatment of a mixture of the starting materials (Li2CO3, TiO2, and P2O5) at room temperature. The mechanically milled amorphous powder formed a triclinic LiTiOPO4 crystal below 700°C in air. The mechanically milled powders before heat treatment barely worked as an active material at ca. 0.1 C rate in the potential range of 1.0 to 3.0 V versus Li/Li+ in lithium cells. On the other hand, the triclinic LiTiOPO4 compounds obtained by the crystallization of the mechanically milled amorphous powders at 700°C in air worked as anode materials in lithium cells. The triclinic LiTiOPO4 electrode materials exhibited capacities of around 100 mAh g−1. The average operation potential was around 1.7 V (vs. Li/Li+). The triclinic LiTiOPO4 pristine compound when used as an anode caused a phase transition into an orthorhombic compound during lithium insertion (charge process). The orthorhombic compound changed into a triclinic phase during lithium extraction (discharge process). The triclinic LiTiOPO4 electrodes exhibited a good cycle performance in the potential range of 1.0 to 3.0 V versus Li/Li+. It is suggested that the crystalline phase changes reversibly during charge/discharge.
Keggin-type polyoxometalates (POMs), which possess multiple redox centers, were investigated as bidirectional redox mediators in rechargeable batteries. A series of POMs have been synthesized and employed in sulfur electrodes where neither the active material nor the discharge product were electrically conductive. POMs were found to have multiple redox potentials covering the range of the equilibrium potentials of the redox reactions of sulfur, which consequently facilitated both charge and discharge reactions. In particular, [SiMo12O40]4− offered a large discharge capacity of 1270 mAh g−1 by accelerating the reduction of shorter, less soluble polysulfides, leading to a higher cycling performance. The mediator role was confirmed via an X-ray photoelectron spectroscopy study on the cycled cathodes. Density functional theory calculations showed that the redox potentials of POMs are tunable, allowing selective design of suitable POM molecules for specific battery electrodes.
A lithium-sulfur cell was developed, consisting of a Si nanoflake negative electrode, a Li2S/graphene positive electrode, and a non-flammable solvate ionic liquid electrolyte. This cell configuration allows us to avoid the use of Li metal electrode and the concomitant dendritic deposition of Li metal at the negative electrode during charging, thereby rendering the cell operation highly safe and stable. The solvate ionic liquid electrolyte solution was composed of tetraglyme, lithium bis(trifluoromethanesulfonyl)amide, and a hydrofluoroether. The solubility of lithium polysulfide (reaction intermediate of the positive electrode) is very low, resulting in high Coulombic efficiency of discharge/charge and long cycle life of the Si/Li2S cell.