Fabrication and characterization of Li1+x−yNb1−x−3yTix+4yO3 substrates using aqueous tape casting process

An aqueous system for tape casting Li_1+x−yNb_1−x−3yTi_x+4yO₃ (LNT) ceramics was developed using poly(vinyl alcohol) (PVA) binder, ethylene alcohol (EG) plasticizer and ammonium salt of polycarboxylate (PCA-NH₄) dispersant. The zeta potential measurement showed that the isoelectric point of the LNT particles moved slightly toward more acid region after the dispersant absorbed on the particles, while the zeta potential increased significantly. The rheological test indicated that the ceramic slurry exhibited a typical pseudoplastic behavior without thixotropy. The effect of solid loading on the properties of the green tapes was investigated. The increase in the solid loading increased the tensile strength and the green density of the tapes. TGA analysis indicated that the organic additives in the green tapes can be completely removed by heat treatment at 600 °C. SEM micrographs showed that the microstructure of the green and sintered tapes was homogeneous.     

Preparation and cycling performance of Aland F co-substituted compounds Li4AlxTi5−xFyO12−y

Li₄Al_xTi_5−xF_yO_12−y compounds were prepared by a solid-state reaction method. Phase analyses demonstrated that both Al³⁺ and F⁻ ions entered the structure of spinel-type Li₄Ti₅O₁₂. Charge-discharge cycling results at a constant current density of 0.15 mA cm⁻² between the cut-off voltages of 2.5 and 0.5 V showed that the Al³⁺ and F⁻ substitutions improved the first total discharge capacity of Li₄Ti₅O₁₂. However, Al³⁺ substitution greatly increased the reversible capacity and cycling stability of Li₄Ti₅O₁₂ while F⁻ substitution decreased its reversible capacity and cycling stability slightly. The electrochemical performance of the Al³⁺-F⁻-co-substituted specimen was better than the F⁻-substituted one but worse than the Al³⁺-substituted one.     

Investigation of hydrogen transport through Mm(Ni3.6Co0.7Mn0.4Al0.3)1.12 and Zr0.65Ti0.35Ni1.2V0.4Mn0.4 hydride electrodes by analysis of anodic current transient

Hydrogen transport through such metal-hydride electrodes as Mm(Ni_3.6Co_0.7Mn_0.4Al_0.3)_1.12 and Zr_0.65Ti_0.35Ni_1.2V_0.4Mn_0.4 was investigated in 6 M KOH solution by using potentiostatic current transient technique. From the shape of the anodic current transient and the dependence of the initial current density on the discharging potential, the boundary conditions at the electrode surface were established during hydrogen extraction from the as-annealed and as-surface-treated electrodes. Especially, it was experimentally confirmed that the diffusion-limited boundary condition is no longer valid at the electrode surface during hydrogen transport in case hydrogen diffusion is coupled with either the interfacial charge transfer reaction or the hydrogen transfer reaction between adsorbed state on the electrode surface and absorbed state at the electrode sub-surface. From the transition behaviour of the boundary condition, it was further recognised that the boundary condition at the electrode surface during hydrogen transport is not fixed at the specific electrode/electrolyte system by itself, but it is rather simultaneously determined even at any electrode/electrolyte system by the potential step and the nature of the electrode surface, depending upon e.g. the presence or absence of the surface oxide scales.     

Amorphous Ru1−yCryO2 loaded on TiO2 nanotubes for electrochemical capacitors

Amorphous Ru_1−yCr_yO₂/TiO₂ nanotube composites were synthesized by loading different amount of Ru_1−yCr_yO₂ on TiO₂ nanotubes via a reduction reaction of K₂Cr₂O₇ with RuCl₃·nH₂O at pH 8, followed by drying in air at 150 °C. Cyclic voltammetry and galvanostatic charge/discharge tests were applied to investigate the performance of the Ru_1−yCr_yO₂/TiO₂ nanotube composite electrodes. For comparison, the performance of amorphous Ru_1−yCr_yO₂ was also studied. The results demonstrated that the three dimensional nanotube network of TiO₂ offered a solid support structure for active materials Ru_1−yCr_yO₂, allowed the active material to be readily available for electrochemical reactions, and increased the utilization of active materials. A maximum specific capacitance 1272.5 F/g was obtained with the proper amount of Ru_1−yCr_yO₂ loaded on the TiO₂ nanotubes.     

Amorphous LiCoO2 thin films on Li1+x+yAlxTi2−xSiyP3−yO12 prepared by radio frequency magnetron sputtering for all-solid-state Li-ion batteries

Amorphous LiCoO₂ thin films were deposited on the NASICON-type glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering below 180 °C. The as-deposited LiCoO₂ thin films were characterized by X-ray diffraction, scanning electron microscopy and atomic force microscope. All-solid-state Li/PEO₁₈-Li (CF₃SO₂)₂N/LATSP/LiCoO₂/Au cells were fabricated using the amorphous film. The electrochemical performance of the cells was investigated by galvanostatic cycling, cyclic voltammetry, potentiostatic intermittent titration technique and electrochemical impedance spectroscopy. It was found that the amorphous LiCoO₂ thin film shows a promising electrochemical performance, making it a potential application in microbatteries for microelectronic devices.     

Effects of B-site ion (M) substitution on the ionic conductivity of (Li3xLa2/3−x)1+y/2(MyTi1−y)O3 (M=Al, Cr)

Effect on the ionic conductivity of B-site ion (M) substitution in (Li_3xLa_2/3−x)_1+y/2M_yTi_1−yO₃ (M=Al, Cr) has been investigated. It has been found that partial substitution of smaller Al³⁺ for Ti⁴⁺ is effective to enhance the ionic conductivity of Li_3xLa_2/3−xTiO₃. At 300 K, the maximum bulk conductivity of (1.58±0.01)×10⁻³ S cm⁻¹ is observed from the composition of (Li_0.39La_0.54)_1−y/2Al_yTi_1−yO₃ with y=0.02 (x=0.13), that is the highest yet reported for known perovskite solutions at room temperature. The conductivity enhancement is interpreted as being due to the substitution-induced bond-strength change rather than due to bottleneck size change for Li migration, TiO₆-octahedron tilting or A-site cation ordering.     

Structural, electrochemical and thermal properties of LiNi0.8−yTiyCo0.2O2 as cathode materials for lithium ion battery

Multiple substitution compounds with the formula LiNi_0.8−yTi_yCo_0.2O₂ (0≤y≤0.1) were synthesized by sol-gel method using citric acid as a chelating agent. The effects of titanium substitution on the structural, electrochemical and thermal properties of the cathode materials are investigated. A solid solution phase (R-3m) is observed in the range of 0≤y≤0.1 for the titanium-doped materials. X-ray photoelectron spectroscopy (XPS) shows that there are Ni³⁺, Ni²⁺, Co³⁺, Co²⁺ and Ti⁴⁺ five transition metal ions in titanium-doped materials. Rietveld refinement of X-ray diffraction (XRD) patterns indicates that titanium substitution changes the materials’ structure with different cationic distribution. An increase of the Ni/Co amount in the 3a Li site is found with the addition of titanium amount. An improved cycling performance is observed for titanium-doped cathode materials, which is interpreted to a significant suppression of phase transitions and lattice changes during cycling. The thermal stability of titanium-doped materials is also improved, which can be attributed to its lower oxidation ability and enhanced structural stability at delithiated state.     

Electrodeposition of P-type BixSb2−xTey thermoelectric film from dimethyl sulfoxide solution

The electrochemical behaviors of Bi(III), Te(IV), Sb(III) and their mixtures in DMSO solutions were investigated using cyclic voltammetry and linear sweep voltammetry measurements. On this basis, Bi_xSb_2−xTe_y film thermoelectric materials were prepared by potentiodynamic electrodeposition technique from mixed DMSO solution, and the compositions, structures, morphologies as well as the thermoelectric properties of the deposited films were also analyzed. The results show that Bi_xSb_2−xTe_y compound can be prepared in a very wide potential range by potentiodynamic electrodeposition technique in the mixed DMSO solutions. After anneal treatment, the deposited film prepared in the potential range of −200 to −400 mV shows the highest Seebeck coefficient (185 μV/K), the lowest resistivity (3.34 × 10⁻⁵ Ω m), the smoothest surface, the most compact structure and processes the stoichiometry (Bi_0.49Sb_1.53Te_2.98) approaching to the Bi_0.5Sb_1.5Te₃ ideal material most. This Bi_0.49Sb_1.53Te_2.98 film is a kind of nanocrystalline material and (0 1 5) crystal plane is its preferred orientation.     

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.     

Comments on the review ‘Anomalous behaviour of hydrogen extraction from hydride-forming metals and alloys under impermeable boundary conditions’, by J.-W. Lee, S.-I. Pyun, Electrochim. Acta 50 (2005) 1777-1805

The review by Lee and Pyun, which was published recently [Electrochim. Acta 50 (2005) 1777-1805], provides a comprehensive survey of the anomalous behaviours of hydrogen transport in hydride-forming electrodes, with particular emphasis on hydrogen extraction under the impermeable boundary condition during potentiostatic current transients. Some theoretical derivations presented in the above review, as well as the validity domains of theoretical results obtained by Lee and Pyun are discussed in these comments in terms of potential step amplitude, interfacial reaction mechanism, electrode kinetics and influence of Ohmic drop. In particular, the concepts of ‘constraint by constant concentration’, ‘constraint by Butler-Volmer behaviour’ and ‘constraint by hydrogen transfer of absorbed state to adsorbed state’ used by Lee and Pyun to discriminate between the boundary conditions at the electrode surface during hydrogen extraction are examined thoroughly in these comments. The ‘transition of the boundary condition at the electrode surface during hydrogen extraction’ is also discussed, as well as the influence of Ohmic potential drop on the chronoamperometric responses to large-potential steps.     

The electrochemistry of Pd-coated MgySc(1 − y) thin film electrodes: A thermodynamic and kinetic study

The thermodynamic and kinetic properties of Pd-coated Mg_ySc_(1 − y) thin film electrodes are investigated. These thin film electrodes can be described as a two-layer structure, in which the Pd and Mg_ySc_(1 − y) layer contribute to the overall electrochemical response. In order to identify the response of the Pd layer in the two-layer system, thin films consisting of solely Pd, with identical thickness and orientation, were measured. Based on the fact that the chemical potentials of the individual layers of the Pd-coated Mg_ySc_(1 − y) thin films are equal at equilibrium, the exact hydrogen concentration in each layer could be determined. It is shown that during the major part of the hydrogen extraction process of the Mg_ySc_(1 − y) thin films, the composition of the Pd topcoat is close to PdH_0.001. The kinetics of the surface reactions was investigated using electrochemical impedance spectroscopy and showed that, when cross-correlating the results of Mg_ySc_(1 − y) thin films (y = 0.65 − 0.85) and pure Pd films, the surface kinetics are completely dominated by the Pd topcoat. Additionally, it was shown that the charge transfer reaction, and not the absorption reaction is the rate-determining step. The impedance response, dominating the overall kinetic impedance at the hydrogen-depleted state, could be linked to the transfer of hydrogen across the Pd/Mg_ySc_(1 − y) interface in the two-layer thin film electrode.     

Synthesis and electrochemical performance of novel loose structured submicro/micro-sized NiSnx alloy composites for lithium batteries

An effective method of carbothermal reduction was employed to prepare spherical microcrystal NiSn_x alloy powders from oxides of Sn and Ni used as anode materials for Li-ion battery. According to XRD, SEM and TEM analysis, the synthesized spherical NiSn_x powders show a loose submicro/micro-sized structure and a multi-phase composition. The prepared NiSn_x alloy composite electrode exhibits a stable discharge capacity of electrode is ca. 380 mAh g⁻¹ at constant current density of 50 mA g⁻¹, and can be retained at 350 mAh g⁻¹ after 25 cycles. Moreover, NiSn_x alloys exhibit excellent high rate performance, i.e. stable discharge capacities of 300-310 mAh g⁻¹ and the coulombic efficiencies of 97.5-99.5% have been obtained at the current density of 500 mA g⁻¹. The loose submicro-sized particle structural characteristic and the Ni addition in Sn matrix should be responsible for the improvement of cycling stability of NiSn_x electrode. The carbothermal reduction method is simple, low-cost and mass-productive, which should be viable to other alloy composite materials system of rechargeable lithium ion batteries.     

Quantitative study of pH change during LaNi5−xAlx (x = 0, 0.3) discharge process by SECM

Oxygen reduction reaction (ORR) on Pt microelectrode was used for developing a micro pH sensor for scanning electrochemical microscopy (SECM) study in this work. When the potential of Pt microelectrode was held constant in ORR region, the ORR current (cathodic current) increased with decreasing solution pH and vice versa. The response time of the ORR current to pH changes was measured to be ca. 30 ms which implies that the pH response is fast enough for monitoring the temporal pH changes. Furthermore, a fine linear relationship was found to exist between the half wave potential of ORR (E_1/2) and the solution pH value, and the slope is −46 mV/pH. The Pt micro pH sensor was located 1 μm above the LaNi_5−xAl_x (x = 0, 0.3) substrate electrode surface in pH = 9 KOH solution to perform the tip-substrate voltammetry of SECM. In tip voltammogram, the ORR tip current qualitatively reflects the transit solution pH changes during LaNi_5−xAl_x discharge reaction. Also, the minimum values of the solution pH near LaNi₅ and LaNi_4.7Al_0.3 surface during the discharge reaction were quantitatively detected; they were 7.17 and 7.57, respectively. The result indicates that Al partial substitution for Ni degrades the maximum discharge ability of the alloy and decreases the hydrogen diffusion coefficient in alloy bulk.     

Bioelectrochemistry and enzymatic activity of glucose oxidase immobilized onto the bamboo-shaped CNx nanotubes

The novel bamboo-shaped CN_x nanotubes, synthesized by nitrogen atoms doping into carbon nanotubes, were used for the immobilization of a relatively large enzyme glucose oxidase (GOx) and its bioelectrochemical studies. The morphologies and adsorptions of GOx immobilization onto CN_x nanotubes were clearly observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Electrochemical impedance spectroscopy (EIS) was also used to feature the GOx adsorbed onto the surface of CN_x nanotubes. The immobilized GOx incorporated into CN_x nanotubes films exhibited a well-defined nearly reversible cyclic voltammetric peaks for the electroactive centers of GOx and a fast heterogeneous electron transfer rate with the rate constant (K_s) of 1.96 s⁻¹. The immobilized GOx onto the CN_x nanotubes exhibited its bioelectrocatalytic activity for the oxidation of glucose. The obtained results suggest that with a large amount of defective/active sites on the tube surfaces, a special bamboo structure and a suitable C-N microenvironment introduced by nitrogen doping, CN_x nanotubes could not only facilitate the direct electron transfer between the enzyme and electrode, but also retain the high enzyme loading and the enzymatic bioactivity.     

An investigation of cell-impedance-controlled lithium transport through LiCoO2/Li1−δMn2O4 bilayer film electrode prepared by rf magnetron sputtering

Lithium transport through LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode prepared by radio-frequency (rf) magnetron sputtering was investigated in a 1 M solution of LiClO₄ in propylene carbonate. From the analyses of the AC-impedance spectra experimentally measured from the Li_1−δMn₂O₄ single-layer and LiCoO₂/Li_1−δMn₂O₄ bilayer film specimens, the internal cell resistance of the LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode was determined to be smaller in value than that of the Li_1−δMn₂O₄ single-layer film electrode over the whole potential range, which can be accounted for by the kinetic facility for the interfacial charge-transfer reaction in the presence of the more conductive LiCoO₂ surface film. Moreover, from the analyses of the anodic current transients measured from both the film specimens, it was suggested that the cell-impedance-controlled constraint at the electrode surface is changed to the diffusion-controlled constraint simultaneously characterised by the large potential step and the small amount of lithium transferred during lithium transport. In addition, in the case of the LiCoO₂/Li_1−δMn₂O₄ bilayer film electrode, it was found that the critical value of the applied potential step needed for the mechanism transition is reduced, which strongly indicates that the internal cell resistance plays a significant role in determining the cell-impedance-controlled lithium transport. Furthermore, from the comparison of the cathodic current transients measured on the Li_1−δMn₂O₄ single-layer film specimens with various thicknesses, it was experimentally verified that the diffusion resistance is explicitly distinguished from the internal cell resistance.     

Study on the surface reaction of LaNi5 alloy during discharge process in KOH solution

A new method for studying surface reaction of LaNi₅ absorbing alloy in KOH solution (pH 12) was established. It is based on tip-substrate voltammetry of scanning electrochemical microscopy (SECM) where the tip faradic current is recorded while scanning the substrate potential. The Pt electrode is selected as tip electrode, and the Pt oxide formation-reduction is used as a pH-dependent reaction while the tip potential is held at a constant value. As substrate surface reactions proceed, the pH of solution can be changed, and then the tip faradic current is recorded. The mechanism of discharge process of LaNi₅ alloy was analyzed by comparing the tip current (I_tip) versus substrate potential (E_sub) curve, which reflects the exchange of H⁺ or OH⁻ between the alloy surface and the solution, with the substrate current (I_sub) versus substrate potential (E_sub) curve, which reflects the exchange of electron on the LaNi₅ alloy surface. The results showed that the OH⁻ adsorption process is occurred before the electron transfer process during discharge process, and the adsorptive OH⁻ helps the oxidation of adsorbed hydrogen atom on the alloy surface. A quantitative assessment for the maximum changes of pH during discharge process is also proposed, and the variation as large as 2.65 pH unit was detected.     

Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5)

A series of Ni substituted spinel LiNi_xMn_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNi_xMn_2−xO₄ (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni²⁺ cations were partially oxidized to Ni³⁺. The local structures of LiNi_xMn_2−xO₄ were studied by analyzing the and A_1g Raman bands. The most compact [Mn(Ni)O₆] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi_0.2Mn_1.8O₄, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm⁻¹. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni³⁺ in the materials.     

Electrochemical characterizations of Fe-substituted LiNiO2 synthesized in air by the combustion method

The phases that appear in the intermediate reaction steps for the formation of lithium nickel oxide were deduced from XRD and DTA analyses. XRD analysis and electrochemical measurements were performed for LiNi_1−yFe_yO₂ (0.000 ≤ y ≤ 0.300) samples calcined in air after preheating in air at 400 °C for 30 min. Rietveld refinement of the LiNi_1−yFe_yO₂ XRD patterns (0.000 < y ≤ 0.100) was carried out from a [Li,Ni]_3b[Li,Ni,Fe]_3a[O₂]_6c starting structure model. The samples of LiNi_1−yFe_yO₂ with y = 0.025 and 0.050 had higher first discharge capacities when compared with LiNiO₂ and exhibited better or similar cycling performance at a 0.1 C rate in the voltage range of 2.7–4.2 V. The LiNi_0.975Fe_0.025O₂ sample had the highest first discharge capacity of 176.5 mAh/g and a discharge capacity of 121.0 mAh/g at n = 100. With the exception of Co-substituted LiNiO₂, such a high first discharge capacity has not been previously reported.