Ion C. Halalay

Revisiting LiClO4 as an Electrolyte for Rechargeable Lithium-Ion Batteries

In this work, LiClO 4 was revisited and explored as a possible electrolyte in Li-ion batteries. LiClO 4 and LiPF 6 solutions in alkyl carbonate solvent mixtures were compared in several aspects: electrochemical windows with noble metal and aluminum electrodes, anodic stability, surface chemistry developed on negative electrodes (Li, Li-graphite, Li-Si), the electrochemical behavior of graphite anodes and LiMn 1/3 Ni 1/3 Co 1/3 O 2 cathodes, and thermal behavior (solutions alone and mixtures of solutions and electrode materials). The anodic stability and the aluminum passivation are much better in LiPF 6 solutions than in LiClO 4 solutions. However, HF contamination in the former solutions worsens the passivation of negative electrodes due to reactions with surface ROCO 2 Li and ROLi species. Thermal reactions of LiClO 4 produce more specific heat than LiPF 6 solutions. However, in terms of onset temperatures for thermal runaway, the two electrolytes are equivalent. In conclusion, LiClO 4 is still an electrolyte that may be considered for use in lithium-ion batteries.

On the Study of Electrolyte Solutions for Li-Ion Batteries That Can Work Over a Wide Temperature Range

Based on previous data and an understanding of possible reactions with electrodes, we selected five electrolyte solutions as promising components for Li-ion batteries that can operate down to -40°C, consisting of solutions of LiPF 6 or LiTFSI electrolytes in optimized ternary carbonate solvent mixtures and quaternary solvent mixtures containing esters, both with and without vinylene carbonate and LiBOB salt as additives. The main criteria for selecting these solutions were a specific conductivity ≥1 mS/cm at -40°C and the ability to work well with a wide variety of electrode materials (for example, transition metal oxides and phosphoolivine cathodes, lithiated titanium oxide and carbon anodes) over a temperature range of -40 to + 60°C. As a first step in the selection of battery materials for operation at low temperatures, we focused our work on the negative electrodes and tested three types of graphite electrodes with these electrolyte solutions. In general, practical graphite electrodes can work reasonably well only at temperatures above -20°C. A limited improvement of their low temperature performance can be achieved by increasing the surface area (i.e., decreasing the particle size) of the active material at the expense of high initial irreversible capacity.

Visualization of Steady-State Ionic Concentration Profiles Formed in Electrolytes during Li-Ion Battery Operation and Determination of Mass-Transport Properties by in Situ Magnetic Resonance Imaging

Accurate modeling of Li-ion batteries performance, particularly during the transient conditions experienced in automotive applications, requires knowledge of electrolyte transport properties (ionic conductivity κ, salt diffusivity D, and lithium ion transference number t(+)) over a wide range of salt concentrations and temperatures. While specific conductivity data can be easily obtained with modern computerized instrumentation, this is not the case for D and t(+). A combination of NMR and MRI techniques was used to solve the problem. The main advantage of such an approach over classical electrochemical methods is its ability to provide spatially resolved details regarding the chemical and dynamic features of charged species in solution, hence the ability to present a more accurate characterization of processes in an electrolyte under operational conditions. We demonstrate herein data on ion transport properties (D and t(+)) of concentrated LiPF6 solutions in a binary ethylene carbonate (EC)-dimethyl carbonate (DMC) 1:1 v/v solvent mixture, obtained by the proposed technique. The buildup of steady-state (time-invariant) ion concentration profiles during galvanostatic experiments with graphite-lithium metal cells containing the electrolyte was monitored by pure phase-encoding single point imaging MRI. We then derived the salt diffusivity and Li(+) transference number over the salt concentration range 0.78-1.27 M from a pseudo-3D combined PFG-NMR and MRI technique. The results obtained with our novel methodology agree with those obtained by electrochemical methods, but in contrast to them, the concentration dependences of salt diffusivity and Li(+) transference number were obtained simultaneously within the single in situ experiment.

Accurate Characterization of Ion Transport Properties in Binary Symmetric Electrolytes Using In Situ NMR Imaging and Inverse Modeling

We used NMR imaging (MRI) combined with data analysis based on inverse modeling of the mass transport problem to determine ionic diffusion coefficients and transference numbers in electrolyte solutions of interest for Li-ion batteries. Sensitivity analyses have shown that accurate estimates of these parameters (as a function of concentration) are critical to the reliability of the predictions provided by models of porous electrodes. The inverse modeling (IM) solution was generated with an extension of the Planck-Nernst model for the transport of ionic species in electrolyte solutions. Concentration-dependent diffusion coefficients and transference numbers were derived using concentration profiles obtained from in situ (19)F MRI measurements. Material properties were reconstructed under minimal assumptions using methods of variational optimization to minimize the least-squares deviation between experimental and simulated concentration values with uncertainty of the reconstructions quantified using a Monte Carlo analysis. The diffusion coefficients obtained by pulsed field gradient NMR (PFG-NMR) fall within the 95% confidence bounds for the diffusion coefficient values obtained by the MRI+IM method. The MRI+IM method also yields the concentration dependence of the Li(+) transference number in agreement with trends obtained by electrochemical methods for similar systems and with predictions of theoretical models for concentrated electrolyte solutions, in marked contrast to the salt concentration dependence of transport numbers determined from PFG-NMR data.

On the Oxidation State of Manganese Ions in Li-Ion Battery Electrolyte Solutions

We demonstrate herein that Mn3+ and not Mn2+, as commonly accepted, is the dominant dissolved manganese cation in LiPF6-based electrolyte solutions of Li-ion batteries with lithium manganate spinel positive and graphite negative electrodes chemistry. The Mn3+ fractions in solution, derived from a combined analysis of electron paramagnetic resonance and inductively coupled plasma spectroscopy data, are ∼80% for either fully discharged (3.0 V hold) or fully charged (4.2 V hold) cells, and ∼60% for galvanostatically cycled cells. These findings agree with the average oxidation state of dissolved Mn ions determined from X-ray absorption near-edge spectroscopy data, as verified through a speciation diagram analysis. We also show that the fractions of Mn3+ in the aprotic nonaqueous electrolyte solution are constant over the duration of our experiments and that disproportionation of Mn3+ occurs at a very slow rate.

Combined Electron Paramagnetic Resonance and Atomic Absorption Spectroscopy/Inductively Coupled Plasma Analysis As Diagnostics for Soluble Manganese Species from Mn-Based Positive Electrode Materials in Li-ion Cells

Manganese dissolution from positive electrodes significantly reduces the durability of lithium-ion batteries. Knowledge of dissolution rates and oxidation states of manganese ions is essential for designing effective mitigation measures for this problem. We show that electron paramagnetic resonance (EPR) combined with atomic absorption spectroscopy (AAS) or inductively coupled plasma (ICP) can determine both manganese dissolution rates and relative Mn(3+) amounts, by comparing the correlation between EPR and AAS/ICP data for Mn(2+) standards with that for samples containing manganese cations dissolved from active materials (LiMn2O4 (LMO) and LiNi(0.5)Mn(1.5)O4 (LNMO)) into the same electrolyte solution. We show that Mn(3+), and not Mn(2+), is the dominant species dissolved from LMO, while Mn(2+) is predominant for LNMO. Although the dissolution rate of LMO varies significantly for the two investigated materials, due to particle morphology and the presence of Cr in one of them, the Mn speciation appears independent of such details. Thus, the relative abundance of dissolved manganese ions in various oxidation states depends mainly on the overall chemical identity of the active material (LMO vs LNMO). We demonstrate the relevance of our methodology for practical batteries with data for graphite-LMO cells after high-temperature cycling or stand at 4.2 V.

Sonochemical method for producing titanium metal powder

We demonstrate a sonochemical method for producing titanium metal powder. The method uses low intensity ultrasound in a hydrocarbon solvent at near-ambient temperatures to first create a colloidal suspension of liquid sodium-potassium alloy in the solvent and then to reduce liquid titanium tetrachloride to titanium metal under cavitation conditions. XRD data collected for the reaction products after the solvent removal show only NaCl and KCl, with no diffraction peaks attributable to titanium metal or other titanium compounds, indicating either the formation of amorphous metal or extremely small crystallite size. TEM micrographs show that hollow spheres formed of halide salts and titanium metal, with diameters with diameters ranging from 100 to 500 nm and a shell thickness of 20 to 40 nm form during the synthesis, suggesting that the sonochemical reaction occurs inside the liquid shell surrounding the cavitation bubbles. Metal particle sizes are estimated to be significantly smaller than 40 nm from TEM data. XRD data of the powder after annealing and prior to removal of the alkali chloride salts provides direct evidence that titanium metal was formed during the sonochemical synthesis.

VERSATILE CELL FOR COMPLEX PERMITTIVITY MEASUREMENTS ON LIQUIDS

A disk type guarded measurement cell is described, which permits the determination of the complex permittivity of liquids with an accuracy of 0.5% or better. The cell has small residual circuit elements (4 nH and 20 mΩ) and behaves like an ideal capacitor for radius-to-electrode spacing ratios ranging from 15 to 135. Ruggedness and ease of use are achieved through a split cell design with a minimal number of parts and threads for assembly, as well as the attachment of fluid lines and electrical connections.

Anode Materials for Mitigating Hydrogen Starvation Effects in PEM Fuel Cells

Localized hydrogen starvation at a polymer electrolyte membrane (PEM) fuel cell anode can lead to the formation of local cells in the membrane electrode assembly, which cause performance degradation at the fuel cell cathode due to carbon corrosion. We propose using hydrogen spillover materials as a hydrogen reservoir in the fuel cell anode in order to compensate for any temporary proton deficit caused by local flooding of the anode channels. We tested composite electrodes containing TiO 2 , WSi 2 , and WO 3 , and compared their behavior to that of commercial Pt/Vulcan XC-72 carbon (Pt/Vu) benchmark catalysts, using gas-diffusion electrodes in a 0.1 M HClO 4 solution and pellet electrodes in a 0.5 M H 2 SO 4 solution. While TiO 2 yields no benefits, both WSi 2 and WO 3 can significantly delay the voltage excursion of the gas-diffusion electrode into the oxygen evolution region upon the cessation of hydrogen flow. X-ray data indicate that the beneficial effect of WSi 2 may be caused by WO 3 , because we observed conversion of WSi 2 to W0 3 during voltage cycling, without a significant loss in the apparent hydrogen adsorption―desorption area. Electrodes with WO 3 yielded the best results, with a hydrogen storage charge higher by a factor of 6 than for the Pt/Vu benchmark.

Testing of mode-coupling theory through impulsive stimulated thermal scattering

Abstract Impulsive stimulated thermal scattering, a time-domain light scattering technique spanning ps-ms time scales, allows a number of experimental tests of mode-coupling theory (MCT) predictions. Time-dependent acoustic responses and also slower relaxational responses to sudden heating are observed. From the acoustic responses, mechanical modulus spectra are deduced which permit testing of MCT predictions for α and β relaxation dynamics. From the relative intensities of acoustic and relaxational signals, the prediction of a cusp in the temperature-dependence of the nonergodicity parameter (Debye-Waller factor) can be tested. ISTS results from a concentrated (13 m%) aqueous LiCl solution, the molten salt [Ca(NO3)2]0.4[KNO3]0.6, and salol are reviewed.