Li-ion diffusion kinetics in LiFePO4 thin film prepared by radio frequency magnetron sputtering

LiFePO₄ thin films were prepared by radio frequency (RF) magnetron sputtering and were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and atomic force microscope (AFM). Li-ion chemical diffusion coefficients, , were measured by potentiostatic intermittent titration technique (PITT), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). The effects of Ag content, film thickness, and film orientation on the electrochemical performance and Li-ion chemical diffusion coefficients of the LiFePO₄ thin films were investigated. values were measured using the liquid electrolyte and the solid electrolyte, and the obtained values were discussed. The values by PITT and EIS were in the range of 10⁻¹⁴ to 10⁻¹² and 10⁻¹⁵ to 10⁻¹² cm² s⁻¹, respectively and that by CV was in the order of 10⁻¹⁴ cm² s⁻¹.     

Analysis of the chemical diffusion coefficient of lithium ions in Li3V2(PO4)3 cathode material

The chemical diffusion coefficients of lithium ions (DLi⁺) in Li₃V₂(PO₄)₃ between 3.0 and 4.8 V are systematically determined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT). The DLi⁺ values are found to be dependent on the voltage state of charge and discharge. Based on the results from all the three techniques, the true diffusion coefficients measured in single-phase region are in the range of 10⁻⁹ to 10⁻¹⁰ cm² s⁻¹. Its apparent diffusion coefficients measured in two-phase regions by CV and GITT range from 10⁻¹⁰ to 10⁻¹¹ cm² s⁻¹ and 10⁻⁸ to 10⁻¹³ cm² s⁻¹, respectively, depending on the potentials. By the GITT, the DLi⁺ varies non-linearly in a “W” shape with the charge-discharge voltage, which is ascribed to the strong interactions of Li⁺ with surrounding ions. Finally, the chemical diffusion coefficients of lithium ions measured by CV, EIS and GITT are compared to each other.     

Kinetic analysis on LiFePO4 thin films by CV, GITT, and EIS

LiFePO₄ thin films were deposited on Ti substrates by pulsed laser deposition (PLD). The apparent chemical diffusion coefficients of lithium in the films, , were measured by cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS). The average values calculated from CV results were in the order of 10⁻¹⁴ cm² s^–1. The values obtained by GITT, and EIS techniques were in the range of 10^–14–10^–18 cm² s^–1, 10^–14–10^–18 cm² s^–1, respectively. The values obtained by the two methods show a minimum point at x ∼ 0.5 for Li_1−xFePO₄. However, the overpotential values of the LiFePO₄ thin film electrodes obtained from the GITT results and the diffusion impedance deduced from the impedance spectra also show the minimum values at x ∼ 0.5 for Li_1–xFePO₄. This contradict could be caused by the improper use of GITT and EIS techniques for measuring the chemical diffusion coefficient of Li in Li_1–xFePO₄ which constitutes two phase, i.e., LiFePO₄ and FePO₄ in this region.     

Evaluation of electrical energy per order (EEO) with kinetic modeling on the removal of Malachite Green by US/UV/H2O2 process

The kinetics of decolorization of Malachite Green (MG) as a model cationic dye from textile industry, by US/UV/H₂O₂ process, was investigated with nonlinear regression analysis. The experimental results indicated that the decolorization kinetics of MG in this process fit well by pseudo-first order kinetics. With nonlinear regression analysis a model was developed for pseudo-first order constant (k_ap) as a function of operational parameters such as initial concentrations of H₂O₂ (25-600 mg l^− 1) and MG (1.82-9.87 mg l^− 1), temperature (294-307 K) and power density (0.049-0.16 W ml^− 1) as following:

     

Investigation on diffusion behavior of Li in LiFePO4 by capacity intermittent titration technique (CITT)

Capacity intermittent titration technique (CITT) was used to investigate the chemical diffusion coefficient () of lithium-ion in LiFePO₄ cathode material. The values of at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 8.8 × 10⁻¹⁶ to 8.9 × 10⁻¹⁴ cm² s⁻¹ and from 1.2 × 10⁻¹⁶ to 8.5 × 10⁻¹⁴ cm² s⁻¹ in the voltage range from 3.2 to 4 V (vs. Li⁺/Li). The transfer coefficients of cathode (0.32-0.42) and anodic (0.26-0.3), and the standard rate constant (1.58 × 10⁻⁹ to 1.30 × 10⁻⁸ cm s⁻¹) were measured from the Tafel plots of LiFePO₄ in the equilibrium potential range from 3.06 to 3.45 V. From these kinetic parameters, the finite kinetics at interface was taken into account to revise the above values of . The revised values of at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 2.44 × 10⁻¹⁵ to 2.21 × 10⁻¹³ cm² s⁻¹ and from 5.81 × 10⁻¹⁶ to 3.22 × 10⁻¹³ cm² s⁻¹ in the voltage range from 3.2 to 4 V. Results show that the approximation of infinite charge-transfer kinetics leads to a spurious value of which is lower than the revised value, and the spurious extent depends on the galvano-charge current of CITT experiment.     

The effect of tert-butanol adsorption on the two-step electroreduction of Zn and its dependence on the NaClO4 concentration

The influence of water molecules on the adsorption of organic substances and kinetics of electroreduction of inorganic depolarizers is very often overlooked. The electroreduction of Zn²⁺ is a typical example of a reaction controlled by both diffusion and charge transfer. This process in 1.0, 0.5 and 0.1 M NaClO₄ solutions at a mercury electrode in the presence of tert-butanol is expected to involve two consecutive one-electron transfer steps in the overall reaction. Solutions of tert-butanol were prepared to cover the concentration range from 0.01 to 0.3 M. Measurements were performed using an impedance method for a wide range of both the potential and frequency. The difference between the potential of the anodic and cathodic peaks, which was obtained from the cyclic voltammetry method, increases with an increase in the concentration of both tert-butanol and NaClO₄. From the dependences of , the true standard rate constants and the constants and characterizing the stage of the first electron transfer and the stage of the second electron exchange, respectively, were determined. The obtained results indicate that the inhibiting properties of tert-butanol are the weakest in 0.1 M NaClO₄. In addition the stage of the second electron transfer is less sensitive to the inhibiting effect of tert-butanol than the stage of the first electron exchange.     

Anomalous IR optical properties of aggregates of Pd nanoparticles induced through electrochemical cyclic voltammetry

IR optical properties of Pd nanoparticles with different size and aggregation state were studied in the current paper. The dispersed Pd nanoparticles () stabilized with poly(N-vinylpyrrolidone) (PVP) were synthesized by the seeding growth method, in which the seeds were formed step by step through reducing H₂PdCl₄ with ethanol. The dispersed Pd nanoparticles of much large size () were grown from the by keeping the colloid of undisturbed for 150 days at room temperature around 20 °C. The aggregates of () were prepared through an agglomeration process induced during a potential cyclic scanning between −0.25 V and 1.25 V for 20 min at a scan rate of 50 mV s⁻¹. Scanning electron microscope (SEM) patterns confirmed such aggregation of . Fourier transform infrared (FTIR) spectroscopy together with CO adsorption as probe reaction was employed in studies of IR optical properties of the prepared Pd nanoparticles. The results demonstrated that CO adsorbed on films substrated on CaF₂ IR window or glassy carbon (GC) electrode yielded two strong IR absorption bands around 1970 cm⁻¹ and 1910 cm⁻¹, which were assigned to IR absorption of CO bonded on asymmetric and symmetric bridge sites, respectively. Similar IR bands were observed in spectra of CO adsorbed on films, except the IR bands were much weak, whereas CO adsorbed on film produced an IR absorption band near 1906 cm⁻¹, and an anomalous IR absorption band whose direction has been completely inverted around 1956 cm⁻¹. The direction inversion of the IR band of CO bonded to asymmetric bridge sites on was ascribed to the interaction between Pd nanoparticles inside the aggregates. Based on FTIR spectroscopic and cyclic voltammetric results, the aggregation mechanism of Pd nanoparticles from to has been suggested that the agglomeration of Pd nanoparticles was driven by the alteration of electric field across electrode-electrolyte interface, when the PVP stabilizer was stripped via oxidation during cyclic voltammetry.     

Electroreduction of peroxycitric acid coexisting with hydrogen peroxide in aqueous solution

The electrochemical reduction of peroxycitric acid (PCA) coexisting with citric acid and hydrogen peroxide (H₂O₂) in the equilibrium mixture was extensively studied at a gold electrode in acetate buffer solutions containing 0.1 M Na₂SO₄ (pH 2.0-6.0) using cyclic and hydrodynamic voltammetric, and hydrodynamic chronocoulometric measurements. The reduction of PCA was characterized to be an irreversible, diffusion-controlled process, and the cyclic voltammetric reduction peak potential () was found to be more positive by ca. 1.0 V than that of the coexisting H₂O₂, e.g., the values obtained at 0.1 V s⁻¹ for PCA and H₂O₂ were 0.35 and −0.35 V, respectively, vs. Ag|AgCl|KCl (sat.) at pH 3.3. The of PCA was found to depend on pH, i.e., at pH > 4.5, the plot of vs. pH gave the slope (−64 mV decade⁻¹) which is close to the theoretical value (−59 mV decade⁻¹) for an electrode process involving the equal number of electron and proton in the rate-determining step, while at pH < 4.5, the was almost independent of pH. The relevant electrochemical parameters, Tafel slope, number of electrons, formal potential (E⁰′), cathodic transfer coefficient and standard heterogeneous rate constant (k⁰′) for the reduction of PCA and the diffusion coefficient of PCA were determined to be ca. 100 mV decade⁻¹, 2, 1.53 V (at pH 2.6), 0.29, 1.2 × 10⁻¹² cm s⁻¹ and 0.29 × 10⁻⁵ cm² s⁻¹, respectively, and except for E⁰′, the obtained values were almost independent of the solution pH. The overall mechanism of the reduction of PCA was discussed.     

Oxygen reduction mechanism on copper in a 0.5 M H2SO4

The mechanism of the oxygen reduction reaction (ORR) in a naturally aerated stagnant 0.5 M H₂SO₄ was studied using electrochemical methods. The cathodic polarization curve showed three different regions; electrochemical impedance spectroscopy (EIS) measurement was used accordingly. The EIS data were analyzed, and the mechanism for the ORR was proposed consequently. The three regions include a limiting current density region with the main transfer of 4e⁻ controlled by diffusion (−0.50 V < E < −0.40 V), a combined kinetic-diffusion region (−0.40 V < E < −0.20 V) with an additional 2e⁻ transfer due to the adsorption of the anions, and a hump phenomenon region (−0.20 V < E < −0.05 V), in which the chemical redox between the anodic intermediate and the cathodic intermediate , together with the electrochemical reaction, synergistically results in the acceleration of the ORR. Therefore, a coupled electrochemical/chemical process (the EC mechanism) in the hump phenomenon region was proposed, and a good agreement was found between the experimental and fitted results. The EC mechanism was confirmed by the deaerated experiments.     

Synthesis, characterization and thermodynamic properties of poly(3-mesityl-2-hydroxypropyl methacrylate-co-N-vinyl-2-pyrrolidone)

Poly(3-mesityl-2-hydroxypropyl methacrylate-co-N-vinyl-2-pyrrolidone) P(MHPMA-co-VP) was synthesized in 1, 4-dioxane solution using benzoyl peroxide (BPO) as initiator at 60 °C. The copolymer was characterized by ¹H ¹³C NMR, FT-IR, DSC, TGA, size exclusion chromatography analysis (SEC) and elemental analysis techniques. According to SEC, the number-average molecular weight (M_n), weight-average molecular weight (M_w) and polydispersity index (PDI) values of PMHPMA-co-VP were found to be 58,000, 481,000 g/mol and 8.26, respectively. According to TGA, carbonaceous residue value of PMHPMA-co-VP was found to be 6% at 500 °C. Also, some thermodynamic properties of PMHPMA-co-VP such as the adsorption enthalpy, ΔH_a, molar evaporation enthalpy, ΔH_v, the sorption enthalpy, , sorption free energy, , sorption entropy, , the partial molar free energy, , the partial molar heat of mixing, , at infinite dilution was determined for the interactions of PMHPMA-co-VP with selected alcohols and alkanes by inverse gas chromatography (IGC) method in the temperature range of 323-463 K. According to the specific retention volumes, , the weight fraction activity coefficients of solute probes at infinite dilution, , and Flory-Huggins interaction parameters, between PMHPMA-co-VP-solvents were determined in 413-453 K. According to and , selected alcohols and alkanes were found to be non-solvent for PMHPMA-co-VP at 413-453 K. The glass transition temperature, T_g, of the PMHPMA-co-VP found to be 370 and 363 K, respectively, by IGC and DSC techniques, respectively.     

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.