EIS study of the influence of BrCl on the behavior of electrodes in Li/SOCl<Subscript>2</Subscript> cells

The influence of BrCl on the impedance response of both the lithium anode and the carbon cathode in Li/SOCl₂ cells was studied. The impedance of the lithium anode increases with storage time while the addition of BrCl to Li/SOCl₂ cells decreases the impedance. However, the porous carbon cathode shows a small film resistance before discharge. The addition of BrCl to Li/SOCl₂ cells also decreases the impedance, especially for that part of the interface reaction resistance R₂. As a rule, the film resistance of the lithium anode decreases sharply during the early period of discharge, while that of the porous carbon cathode rises rapidly. It follows that the porous carbon cathode is the rate controlling electrode during discharge.     

Determination of state-of-discharge of zinc-silver oxide button cells. III. In situ impedance measurements of each electrode

The ac impedance of each electrode of Zn-Ag₂0 button cells (30–50 mA h) has been measured over an extended range of frequencies using a Hg/HgO reference electrode located in a small hole drilled through the positive case terminal. The high frequency impedance spectrum of the Ag₂O cathode is a straight line with a 22.5° slope typical for diffusion at a porous electrode. The low frequency end exhibits a 45° sloped straight line characteristic of diffusion processes at a planar electrode. The deposition process is fast and hence the change transfer resistance is usually not clearly evident in the complex plane impedance plot. The impedance response of the zinc anode shows a capacitive loop at high frequencies and some inductive effects characteristic of adsorption processes. At low frequency the complex plane impedance plot of the total cell is a straight line of slope close to 45° mainly ascribed to the Ag₂0 cathode. At high frequencies equally important contributions from the two electrodes are evident. The main change in impedance which results from discharge is the decrease of the characteristic relaxation frequency of the high frequency capacitive loop of the zinc anode. The determination of the state-of-discharge at a frequency higher than 1 Hz is best realized if (i) the characteristic relaxation frequency of the high frequency capacitive loop occurring at the zinc anode decreases with discharge and (ii) the response of the Ag₂O cathode is quasi-linear over the entire frequency range. Under these conditions the characteristic relaxation frequency at the zinc anode or some related parameters can be clearly seen on the spectrum measured at the two terminals and used as a state-of-discharge indicator.This is the final part of a series of papers dedicated to Professor Einest Yeager on the occasion of his 60th birthday.     

Electrochemical study on orthorhombic LiMnO2 as cathode in rechargeablelithium batteries

Orthorhombic LiMnO₂ was synthesized via a solid-state reaction. Its electrochemical properties as cathode in lithium batteries were examined. It was found that initially, a few cycles are necessary to activate the electrochemical reactivity of o-LiMnO₂, which is related to the transformation from the orthorhombic phase to a spinel-like phase. A maximum discharge capacity of 180–190 mA h g^- for o-LiMnO₂ electrodes was achieved. An electrochemical impedance spectroscopy (EIS) study showed that the charge-transfer resistance (R _CT) for the initial o-LiMnO₂ electrode is much larger than that for the o-LiMnO₂ electrode in the charged state. The o-LiMnO₂ electrode demonstrated a better cyclability than that of the spinel LiMn₂O₄ directly synthesized by solid-state reaction.     

Electrochemical performances of lithium ion battery using alkoxides of group 13 as electrolyte solvent

Tris(methoxy polyethylenglycol) borate ester (B-PEG) and aluminum tris(polyethylenglycoxide) (Al-PEG) were used as electrolyte solvent for lithium ion battery, and the electrochemical property of these electrolytes were investigated. These electrolytes, especially B-PEG, showed poor electrochemical stability, leading to insufficient discharge capacity and rapid degradation with cycling. These observations would be ascribed to the decomposition of electrolyte, causing formation of unstable passive layer on the surface of electrode in lithium ion battery at high voltage. However, significant improvement was observed by the addition of aluminum phosphate (AlPO₄) powder into electrolyte solvent. AC impedance technique revealed that the increase of interfacial resistance of electrode/electrolyte during cycling was suppressed by adding AlPO₄, and this suppression could enhance the cell capabilities. We infer that dissolved AlPO₄ components formed electrochemically stable layer on the surface of electrode.     

Preparation and electrochemical performance of In-doped ZnO as anode material for Ni–Zn secondary cells

In-doped ZnO (IZO) samples were synthesized by a simple co-precipitation method. X-ray diffraction (XRD) patterns, Raman spectra and scanning electron microscopy (SEM) images show that IZO with 2.5 wt% In₂O₃ has a pure wurtzite structure and a plate-like morphology. IZO with 16.3 wt% In₂O₃ (theoretical value) mainly shows a wurtzite structure. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge–discharge measurement were utilized to examine the electrochemical performances of IZO with 2.5 wt% In₂O₃ as anode material for Ni–Zn simulated cells. Compared with the physical mixture of ZnO with In₂O₃, IZO increases the charge-transfer resistance of zinc electrode. Furthermore, the initial discharge capacity of IZO is 569 mAh g⁻¹, and the discharge capacity decays slightly with the capacity retention ratio of 95.2% over 73 cycles, which is much higher than that of the physical mixture of ZnO with In₂O₃.     

Structural and electrochemical features of 3D nanoporous platinum electrodes

The morphologies, roughness factors, and thicknesses of 3D nanoporous Pt (3D-npPt) films were investigated in terms of electroplating conditions. The electrochemical behaviors of 3D-npPt films with regard to electrochemical glucose oxidation, O₂ reduction, and H₂O₂ reduction were investigated as a function of roughness factors (Rf). Close comparison of glucose oxidation on 3D-npPt and 1D nanoporous Pt (1D-npPt) showed that the overall electrode activity of 3D-npPt is significantly higher than that of 1D-npPt. Electrochemical impedance analysis based on transmission line theory confirmed a substantially low pore resistance of 3D-npPt, which may account for the superior electrode response of this material.     

Effect of ZnO modification on the performance of LiNi0.5Co0.25Mn0.25O2 cathode material

ZnO was coated on LiNi_0.5Co_0.25Mn_0.25O₂ cathode (positive electrode) material for lithium ion battery via sol–gel method to improve the performance of LiNi_0.5Co_0.25Mn_0.25O₂. The X-ray diffraction (XRD) results indicated that the lattice structure of LiNi_0.5Co_0.25Mn_0.25O₂ was not changed distinctly after surface coating and part of Zn²⁺ might dope into the lattice of the material. Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) proved that ZnO existed on the surface of LiNi_0.5Co_0.25Mn_0.25O₂. Charge and discharge tests showed that the cycle performance and rate capability were improved by ZnO coating, however, the initial capacity decreased dramatically with increasing the amount of ZnO. Differential scanning calorimetry (DSC) results showed that thermal stability of the materials was improved. The XPS spectra after charge–discharge cycles showed that ZnO coated on LiNi_0.5Co_0.25Mn_0.25O₂ promoted the decomposition of the electrolyte at the early stage of charge–discharge cycle to form more stable SEI layer, and it also can scavenge the free acidic HF species from the electrolyte. The electrochemical impedance spectroscopy (EIS) results showed ZnO coating could suppress the augment of charge transfer resistance upon cycling.     

Study on synthesis routes and their influences on chemical and electrochemical performances of Li3V2(PO4)3/carbon

In this study, Li₃V₂(PO₄)₃/carbon samples were synthesized by two different synthesis routes. Their influence on chemical and electrochemical performances of Li₃V₂(PO₄)₃/carbon as cathode materials for lithium-ion batteries was investigated. The structure and morphology of Li₃V₂(PO₄)₃/carbon were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM) measurements. TEM revealed that the Li₃V₂(PO₄)₃ grains synthesized through the sol-gel route had a depressed grain size. Electrochemical behaviors were characterized by galvanostatic charge/discharge, cyclic voltammetry and AC impedance measurements. Li₃V₂(PO₄)₃/carbon with smaller grain size showed better performances in terms of the discharge capacity and cycle stability. The improved electrochemical properties of the Li₃V₂(PO₄)₃/carbon were attributed to the depressed grain size and enhanced electrical contacts produced via the sol-gel route. AC impedance measurements also showed that the sol-gel route significantly decreased the charge-transfer resistance and shortened the migration distance of lithium ion.     

Kinetic behavior of LiFePO4/C cathode material for lithium-ion batteries

The composite cathode materials of LiFePO₄/C were synthesized by spray-drying and post-annealing method. The crystalline structure and morphology of products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The charge-discharge kinetics of LiFePO₄/C electrode was investigated using electrochemical impedance spectroscopy (EIS). The results show that the increment of the resistance has a close relation to the appearance of the FePO₄ phase during charge-discharge course, and that the ohmic resistance, charge transfer resistance and lithium-ion diffusion coefficients of the LiFePO₄/C electrode do not change much by extended cycling tests, implying a relatively superior cyclability of the battery. The effect of cell temperature on the electrochemical reaction behaviors of LiFePO₄/C electrode was also investigated using the EIS. It is confirmed that the effect of the cell temperature on the impedance results mainly from the enhancement of the lithium-ion diffusion at elevated temperatures.     

Electrochemical stability of thin film LiMn2O4 cathode in organic electrolyte solutions with different compositions at 55 °C

A survey of the electrochemical stability of electrostatic spray deposited thin film of LiMn₂O₄ was performed in LiClO₄-EC-PC, LiBF₄-EC-PC, and LiPF₆-EC-PC solutions at 55 °C. The solution resistance, the surface film resistance, and the charge-transfer resistance were all found to depend on the electrolyte composition. Among the LiX-salts studied, the lowest charge transfer-resistance, and surface layer resistance were obtained in LiBF₄-EC-PC solution. There is no major influence of the electrolyte solution compositions upon lithium ion transport in the LiMn₂O₄ bulk at 55 °C. The diffusion coefficient of lithium in the solid phase varied within 10⁻¹⁰-10⁻⁸ cm² s⁻¹ in the three solutions. In general, it seems that in LiBF₄ solutions, the surface chemistry is the most stable in the three solutions examined, and hence the electrode impedance in LiBF₄ solutions was the lowest. In LiPF₆ solutions, HF seems to play an important role, and thus, the electrode impedance is relatively high due to the precipitation of surface LiF.     

Electrochemical characterization of amorphous LiFe(WO4)2 thin films as positive electrodes for rechargeable lithium batteries

Amorphous LiFe(WO₄)₂ thin films have been fabricated by radio-frequency (R.F.) sputtering deposition at room temperature. The as-deposited and electrochemically cycled thin films are, respectively, characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectra techniques. An initial discharge capacity of 198 mAh/g in Li/LiFe(WO₄)₂ cells is obtained, and the electrochemical behavior is mostly preserved in the following cycling. These results identified the electrochemical reactivity of two redox couples, Fe³⁺/Fe²⁺ and W⁶⁺/W^x+ (x = 4 or 5). The kinetic parameters and chemical diffusion coefficients of Li intercalation/deintercalation are estimated by cyclic voltammetry and alternate-current (AC) impedance measurements. All-solid-state thin film lithium batteries with Li/LiPON/LiFe(WO₄)₂ layers are fabricated and show high capacity of 104 μAh/cm² μm in the first discharge. As-deposited LiFe(WO₄)₂ thin film is expected to be a promising positive electrode material for future rechargeable thin film batteries due to its large volumetric rate capacity, low-temperature fabrication and good electrode/electrolyte interface.     

Effect of copper additive on Zr0.9Ti0.1V0.2Mn0.6Cr0.05Co0.05Ni1.2 alloy anode for nickel–metal hydride batteries

Zr_1–x Ti _x V_0.2Mn_0.6Cr_0.05Co_0.05Ni_1.2 (0 x 0.3) alloys have been characterized as metal–hydride electrodes for nickel–metal hydride batteries. Although the alloy electrodes with no Ti substitution in place of Zr exhibit a specific capacity value of 375 mA h g^–1, it has been possible to enhance the specific capacity of the electrodes to 395 mA h g^–1 by substituting 10% Ti in place of Zr, that is, with Zr_0.9Ti_0.1V_0.2Mn_0.6Cr_0.05Co_0.05Ni_1.2 alloy. The specific capacity value of Zr_0.9Ti_0.1V_0.2Mn_0.6Cr_0.05Co_0.05Ni_1.2 alloy was further enhanced to 415 mA h g^–1 on copper powder addition. Interestingly, the discharge curves for the latter electrode are quite flat thus providing an advantage of constant specific energy output over the entire regime of electrode discharge. Both a.c. impedance and d.c. linear polarization studies conducted on these electrodes lead to a lower charge-transfer resistance value for the metal-hydride electrode with copper additive suggesting the electrode with copper powder additive to have a higher catalytic activity than those without copper. The electrode with the copper additive also exhibits little change in its capacity over about 100 charge–discharge cycles.     

Unique hierarchical mesoporous LaCrO3 perovskite oxides for highly efficient electrochemical energy storage applications

Following sol-gel route, hierarchical mesoporous nanostructures of lanthanum chromates (LaCrO₃) perovskite oxides are successfully synthesized for supercapacitor applications. The structural behaviors of nano perovskite oxides are investigated using X-rays diffraction, low and high resolution scanning and transmission electron microscopes, photoelectron X-ray spectroscopy, and Brunauer-Emmett-Teller (BET). The as-prepared LaCrO₃ powders is mingled with activated carbon and subsequently glazed on a flexible carbon cloth current collector (LCO@CC). The electrochemical capabilities of LCO@CC based electrode, such as: cyclic voltammetry, galvanostatic charge:discharge and electrochemical impedance spectroscopy are investigated in neutral M LiCl aqueous solutions. Moreover, the fabricated LCO@CC electrode achieves maximum capacitance of 1268 F/g at 2 A/g and retains excellent cyclic ability of 91.5% after 5000 charge:discharge cycles. The efficient electrochemical performances of carbon cloth decorated LaCrO₃ electrode are credited to dynamic charge storage mechanism by fast redox reaction of electrolyte-electrode interactions with extremely low charge transfer resistance.     

Composite Ni/Al2O3 coatings and their corrosion resistance

Composite coatings Ni/Al₂O₃ were electrochemically deposited from a Watts bath. Al₂O₃ powder with particle diameter below 1 μm was codeposited with the metal. The obtained Ni/Al₂O₃ coatings contained 5-6% by weight of corundum. The structure of the coatings was examined by scanning electron microscopy (SEM). It has been found that the codeposition of Al₂O₃ particles with nickel disturbs the nickel coating's regular surface structure, increasing its microcrystallinity and surface roughness. DC and AC electrochemical tests were carried out on such coatings in a 0.5 M solution of Na₂SO₄ in order to evaluate their corrosion resistance. The potentiodynamic tests showed that the corrosion resistance of composite coating Ni/Al₂O₃ is better than that of the standard nickel coating. After 14 days of exposure the nickel coating corrodes three times faster than the Ni/Al₂O₃ coating. The electrochemical behaviour of the coatings in the corrosive solution was investigated by electrochemical impedance spectroscopy (EIS). An equivalent circuit diagram consisting of two RC electric circuits: one for electrode, nickel corrosion processes and the other for processes causing coating surface blockage, were adopted for the analysis of the impedance spectra. The changes in the charge transfer resistance determined from the impedance measurements are comparable with the changes in corrosion resistance determined from potentiodynamic measurements.     

Aging characteristics of high-power lithium-ion cells with LiNi0.8Co0.15Al0.05O2 and Li4/3Ti5/3O4 electrodes

The impedance rise that results from the accelerated aging of high-power lithium-ion cells containing LiNi_0.8Co_0.15Al_0.05O₂-based positive and graphite-based negative electrodes is dominated by contributions from the positive electrode. Data from various diagnostic experiments have indicated that a general degradation of the ionic pathway, apparently caused by surface film formation on the oxide particles, produces the positive electrode interface rise. One mechanistic hypothesis postulates that these surface films are components of the negative electrode solid electrolyte interphase (SEI) layer that migrate through the electrolyte and separator and subsequently coat the positive electrode. This hypothesis is examined in this article by subjecting cells with LiNi_0.8Co_0.15Al_0.05O₂-based positive and Li_4/3Ti_5/3O₄-based negative electrodes to accelerated aging. The impedance rise in these cells was observed to be almost entirely from the positive electrode. Because reduction products are not expected on the 1.55 V Li_4/3Ti_5/3O₄ electrode, the positive electrode impedance cannot be attributed to the migration of SEI-type fragments from the negative electrode. It follows then that the impedance rise results from mechanisms that are “intrinsic” to the positive electrode.     

Preparation of NaBiO3 and the electrochemical characteristic of manganese dioxide doped with NaBiO3

NaBiO₃ crystal of high purity has been synthesized through chemical oxidization. The morphology and thermal stability of NaBiO₃ were examined with scanning electron microscope (SEM) and thermogravimetry-differential scanning calorimetry (TG-DSC). The electrochemical properties of MnO₂ electrodes with and without doping NaBiO₃ were studied through galvanostatic charge/discharge and cyclic voltammetry. The results indicate that the MnO₂ electrode doped with NaBiO₃ possesses remarkably higher discharge voltage and capacity and better reversibility than the pure MnO₂ electrode and Bi₂O₃ doping MnO₂ electrode.     

Mechanism transition of cell-impedance-controlled lithium transport through Li1−δMn2O4 composite electrode caused by surface-modification and temperature variation

The mechanism transition of lithium transport through a Li_1−δMn₂O₄ composite electrode caused by the surface-modification and temperature variation was investigated using the galvanostatic intermittent titration technique (GITT), electrochemical impedance spectroscopy (EIS) and the potentiostatic current transient technique. From the analyses of the ac-impedance spectra, experimentally measured from unmodified Li_1−δMn₂O₄ and surface-modified Li_1−δMn₂O₄ with MgO composite electrodes, the internal cell resistance of the MgO-modified Li_1−δMn₂O₄ electrode was determined to be much smaller in value than that of the unmodified electrode over the whole potential range. Moreover, from the analysis of the anodic current transients measured on the MgO-modified Li_1−δMn₂O₄ electrode, it was found that the cell-impedance-controlled constraint at the electrode surface is changed to a diffusion-controlled constraint, which is characterised by a large potential step and simultaneously by a small amount of lithium transferred during lithium transport. This strongly suggests that the internal cell resistance plays a significant role in determining the cell-impedance-controlled lithium transport through the MgO-modified Li_1−δMn₂O₄ electrode. Furthermore, from the temperature dependence of the internal cell resistance and diffusion resistance in the unmodified Li_1−δMn₂O₄ composite electrode measured by GITT and EIS, it was concluded that which mechanism of lithium transport will be operative strongly depends on the diffusion resistance as well as on the internal cell resistance.     

Preparation and electrochemical properties of LiMn2O4 by the microwave-assisted rheological phase method

Pure-phase and well-crystallized spinel LiMn₂O₄ powders as cathode materials for lithium-ion batteries were successfully synthesized by a new simple microwave-assisted rheological phase method, which was a timesaving and efficient method. The physical properties of the as-synthesized samples compared with the pristine LiMn₂O₄ obtained from the rheological phase method were investigated by thermogravimetry analysis (TGA), X-ray diffraction (XRD) and scanning electronic microscope (SEM). The as-prepared powders were used as positive materials for lithium-ion battery, whose charge/discharge properties and cycle performance were examined in detail. The powders resulting from the microwave-assisted rheological phase method were pure, spinel structure LiMn₂O₄ particles of regular shapes with distribution uniformly, and exhibited promising electrochemical properties for battery. Furthermore, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the reactions of Li-ion insertion into and extraction from LiMn₂O₄ electrode.     

Effect of electrochemical preparation methods on structure and properties of PbO2 anodic layer

PbO₂ anodic layers were formed by direct oxidation of lead electrodes galvanostatically (G) and potentiostatically (P), in attempt to study PbO₂ as positive active material for lead-acid batteries. The structural and electrochemical properties of PbO₂ anodic layers, prepared by the two methods, were examined and found to be different. Comparative studies indicated that PbO₂-G electrode exhibited a longer discharge time, corresponding to a higher capacity, and a lower charge transfer resistance than that of PbO₂-P. Through SEM observation it was established that the PbO₂-G had a compact structure, while the PbO₂-P surface showed a porous structure. Discharge capacity was related to the film structure and internal resistance of PbO₂ layer. The compact structure with higher degree of interconnected particles lead to lower internal resistance thus higher capacity, and the porous surface layer would result in high transfer resistance and diminished capacity. It was concluded that preparation methods could controllably affect the structure of PbO₂ films, which further played important roles in discharge capacity.