EIS and GITT studies on oxide cathodes, O2-Li(2/3)+x(Co0.15Mn0.85)O2 (x=0 and 1/3)

Li ion kinetics in the O2-phase layered manganese oxides, Li_2/3(Co_0.15Mn_0.85)O₂ (O2(Li)) and Li_(2/3)+x(Co_0.15Mn_0.85)O₂ (x=1/3 (O2(Li+x))), has been studied by the electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) at room temperature and the results were correlated with the observed cathodic behaviour. Both compounds show a reversible capacity of ∼180 mA h/g at low current density (∼5 mA/g). EIS studies as a function of cycle number show an increased contribution of resistance associated with surface film formation and bulk contribution which is in agreement with the increased capacity fading observed in O2(Li+x) after 10-15 cycles. The Li ion diffusion coefficient (D_Li) vs voltage plots show minima during the first charge cycle coinciding with the irreversible plateau of the voltage vs capacity profiles reflecting the irreversible phase change in both the compounds. The values of D_Li (GITT method) observed for the second and subsequent cycles (≤6) in the full voltage range (3.0-4.4 V) are 2×10⁻¹¹-10×10⁻¹¹ cm²/s for O2(Li+x) and 0.5×10⁻¹⁰-3.0×10⁻¹⁰ cm²/s for O2(Li). Variation of D_Li as a function of cycle number (up to 35) indicates that, in addition to the interface kinetics, changes in the D_Li values with cycling also contribute to the capacity fading of the compounds, especially in O2(Li+x).     

Mass transfer simulation of biodiesel synthesis in microreactors

A coupled nonlinear mathematical model for the mass transfer of the species involved in the transesterification reaction between soybean oil and methanol in a parallel plates geometry microreactor is presented. The set of partial differential equations that governs the concentration profile of these species were obtained from the general mass balance equation for the case of isothermal flow and steady state with constant physical properties. The velocity profile was obtained from the Navier-Stokes equations assuming fully developed stratified laminar flow for two immiscible Newtonian fluids, with a plane interface between them, based on experimental observation of this flow pattern. The second order kinetic equations for the species were developed assuming homogeneous and reversible chemical reactions and these equations were written as source terms in the main equations. The mathematical model was solved using the hybrid method known as Generalized Integral Transform Technique (GITT). The simulation results were critically compared with those obtained by using the COMSOL multiphysics platform, showing a good agreement between the hybrid and fully numerical simulations. The effects of governing parameters such as residence time, temperature and microreactor dimensions were investigated. It was observed that higher triglycerides conversion rates occurred at higher temperatures and residence times and lower microreactor depths.     

FeSb合金的吸放锂行为

采用悬浮熔炼方法合成了FeSb合金，并研究了其作为锂离子电池负极材料的电化学性能。结果发现，虽然FeSb的容量低于纯锑。但其循环稳定性要优于纯锑。本实验采用恒电流间隙滴定技术(GITT)测定了在嵌锂过程中锂离子的化学扩散系数，在30℃时，该值在10^-12cm^2s^-1～10^-10cm^2s^-1范围内。     

Surface-oxidized tungsten for energy-storable dye-sensitized solar cells

Tungsten oxide electrodes were investigated as charge-storage materials for energy-storable dye-sensitized solar cells (ES-DSSCs). The electrochemical and structural properties of the surface-oxidized tungsten (so-WO_3 − x) and monoclinic nanocrystalline WO₃ (nc-WO₃) were studied on the difference of the charge-discharge properties. Although, the electromotive force (EMF) curve of the so-WO_3 − x was associated with structural change, the so-WO_3 − x did not show the significant structural change indicated by X-ray diffraction (XRD) patterns. On the other hand, the nc-WO₃ showed crystal transformation from monoclinic phase to tetragonal phase. The Li⁺ diffusion coefficients of the so-WO_3 − x with different Li⁺ content ratios obtained by the galvanostatic intermittent titration technique (GITT) did not fall down up to 0.3 of Li/W ratio, whereas the diffusion coefficients of nc-WO₃ decreased about two orders of magnitude in the vicinity of phase transitions. The different electrochemical properties could be explained by the less structural change of so-WO_3 − x compared with the nc-WO₃. The large-sized ES-DSSCs with the so-WO_3 − x were fabricated for the first time, and their photocharge-discharge performances were studied.     

Improved electrochemical properties of Li1+x(Ni0.3Co0.4Mn0.3)O2−δ (x = 0, 0.03 and 0.06) with lithium excess composition prepared by a spray drying method

Layered Li_1+x(Ni_0.3Co_0.4Mn_0.3)O_2−δ (x = 0, 0.03 and 0.06) materials were synthesized through the different calcination times using the spray-dried precursor with the molar ratio of Li/Me = 1.25 (Me = transition metals). The physical and electrochemical properties of the lithium excess and the stoichiometric materials were examined using XRD, AAS, BET and galvanostatic electrochemical method. As results, the lithium excess Li_1.06(Ni_0.3Co_0.4Mn_0.3)O_2−δ could show better electrochemical properties, such as discharge capacity, capacity retention and C rate ability, than those of the stoichiometric Li_1.00(Ni_0.3Co_0.4Mn_0.3)O_2−δ. In this paper, the effect of excess lithium on the electrochemical properties of Li_1+x(Ni_0.3Co_0.4Mn_0.3)O_2−δ materials will be discussed based on the experimental results of ex situ X-ray diffraction, transmission electron microscopy (TEM) and galvanostatic intermittent titration technique (GITT)     

Spurious chemical diffusion coefficients of Li in electrode materials evaluated with GITT

The galvanostatic intermittent titration technique (GITT) has been used as a standard method for evaluating chemical diffusion coefficients in electrode materials in the last three decades. It will now be demonstrated that these chemical diffusion coefficients evaluated with GITT are spurious as any reaction kinetics is neglected in the GITT theory. The neglect of the reaction kinetics leads to a spurious potential dependence of the GITT diffusion coefficients with minima at those potentials where the slow scan rate cyclic voltammogram or differential capacity plot exhibits peaks even in case where the true chemical diffusion coefficient is constant. This will be demonstrated by the evaluation of GITT diffusion coefficients from numerically generated GITT experiments calculated with a constant chemical diffusion coefficient on the example of a spinel-type LiMn₂O₄ electrode.     

Electrochemical modeling of intercalation processes with phase field models

In this paper, we apply phase field models to move beyond Fick’s law in describing Li diffusion in secondary battery electrodes. Phase field models are potentially more accurate and allow simpler tracking of phase boundaries than Fick’s equation. The phase field models are implemented using the highly accurate but fast Chebyshev-spectral method. Using the phase field we investigate to what extent non-Fickian behavior can affect results from experimental techniques for measuring diffusion coefficients, such as Galvanostatic Intermittent Titration Technique (GITT) and Potentiostatic Intermittent Titration Technique (PITT). We show that GITT and PITT can still accurately measure the diffusion coefficient in systems described by phase field models even when significant gradient energy terms are present.     

Comparison between Cottrell diffusion and moving boundary models for determination of the chemical diffusion coefficients in ion-insertion electrodes

Using PITT technique, the various shapes of the Cottrellian diffusion plots, It^1/2 versus log t, were studied as a function of the electrode potential for the Li ions insertion into graphite electrodes. Single maximum appeared on the It^1/2 versus log t plots for stoichiometries close to pure staged phases, whereas two maxima were always observed in the two-phase coexistence domains. Slow nucleation rate during phase 2/phase 1 transition was revealed by the characteristic shape of the initial time domains of both chronoamperometric and chronovoltammetric curves. Corrected for nucleation, the longer time domain of the response was treated chronocoulometrically in the framework of two alternative approaches using the moving boundary and the classical Cottrellian models. The alternative approaches resulted in virtually the same value of D. The diffusion of Li ions during phase 2/phase 1 transition is largerly affected by slow interfacial charge transfer. In this case, the spinodal domain in the free-energy curve is effectively degenerated even at small overvoltages. This substantiates the formal application of the Cottrellian diffusion approach to determine D during the processes of intercalation and deintercalation, accompanied by first-order transitions; however, involvement of the slow interfacial kinetics in the measured response should be adequatly taken into account.     

Electrochemical Performance of Na3V2(PO4)2F3 Electrode Material in a Symmetric Cell

A NASICON-based Na₃V₂(PO₄)₂F₃ (NVPF) cathode material is reported herein as a potential symmetric cell electrode material. The symmetric cell was active from 0 to 3.5 V and showed a capacity of 85 mAh/g at 0.1 C. With cycling, the NVPF symmetric cell showed a very long and stable cycle life, having a capacity retention of 61% after 1000 cycles at 1 C. The diffusion coefficient calculated from cyclic voltammetry (CV) and the galvanostatic intermittent titration technique (GITT) was found to be ~10⁻⁹–10⁻¹¹, suggesting a smooth diffusion of Na⁺ in the NVPF symmetric cell. The electrochemical impedance spectroscopy (EIS) carried out during cycling showed increases in bulk resistance, solid electrolyte interphase (SEI) resistance, and charge transfer resistance with the number of cycles, explaining the origin of capacity fade in the NVPF symmetric cell. Finally, the postmortem analysis of the symmetric cell after 1000 cycles at a 1 C rate indicated that the intercalation/de-intercalation of sodium into/from the host structure occurred without any major structural destabilization in both the cathode and anode. However, there was slight distortion in the cathode structure observed, which resulted in capacity loss of the symmetric cell. The promising electrochemical performance of NVPF in the symmetric cell makes it attractive for developing long-life and cost-effective batteries.     

Effect of adsorbed water vapor on Mg intercalation in electrochromic a-MoO3 films

The nature of interaction between the water vapor and MoO₃ thin films has been investigated using infrared spectroscopy technique. On prolonged exposure to high humidity conditions, presence of free water vapor and formation of Mo–OH bonds was detected in MoO₃ films. Films exposed to high humidity for various durations and freshly deposited were intercalated with Mg ions under identical experimental conditions. Free water vapor and Mo–OH bonds did not prevent/hinder the intercalation process. The diffusivity (D) of Mg ions in MoO₃ films has been estimated using Galvanostatic Intermittent Titration Technique (GITT) as a function Mg concentration. It is observed that D values are lesser in case of films exposed to humidity. Cyclic voltammetry studies carried out on films revealed reduced cyclability in exposed films compared to that of fresh films.