Branko N. Popov

Electrochemical Determination of the Diffusion Coefficient of Hydrogen Through an LaNi4.25Al0.75 Electrode in Alkaline Aqueous Solution

Metal hydrides are being used as electrodes in nickel/metal-hydride batteries because of their ability to store large quantities of hydrogen and because of their many advantages over conventional lead-acid and nickel-cadmium batteries. The performance of a metal hydride electrode is determined by both the kinetics of the processes occurring at the metal/electrolyte interface and the rate of hydrogen diffusion within the bulk of the metal. The constant potential and constant current discharge techniques were used to determine the hydrogen diffusion coefficients in an LaNi{sub 4.25}Al{sub 0.75} electrode. The values obtained were 2.97 {times} 10{sup {minus}11} and 3.30 {times} 10{sup {minus}11} cm{sup 2}/s, respectively. The advantages and disadvantages of these two techniques are discussed.

Characterization of Sol‐Gel‐Derived Cobalt Oxide Xerogels as Electrochemical Capacitors

Very fine cobalt oxide xerogel powders were prepared using a unique solution chemistry associated with the sol-gel process. The effect of thermal treatment on the surrace area, pore volume, crystallinity, particle structure, and corresponding electrochemical properties of the resulting xerogels was investigated and found to have significant effects on all of these properties. The xerogel remained amorphous as Co(OH) 2 up to 160°C, and exhibited maxima in both the surface area and pore volume at this temperature. With an increase in the temperature above 200°C, both the surface area and pore volume decreased sharply, because the amorphous Co(OH) 2 decomposed to form CoO that was subsequently oxidized to form crystalline Co 3 O 4 In addition, the changes in the surface area, pore volume, crystallinity, and particle structure all had significant but coupled effects on the electrochemical properties of the xerogels. A maximum capacitance of 291 F/g was obtained for an electrode prepared with the CoO x xerogel calcined at 150°C, which was consistent with the maxima exhibited in both the surface area and pore volume; this capacitance was attributed solely to a surface redox mechanism. The cycle life of this electrode was also very stable for many thousands of cycles

Development of First Principles Capacity Fade Model for Li-Ion Cells

A first principles-based model has been developed to simulate the capacity fade of Li-ion batteries. Incorporation of a continuous occurrence of the solvent reduction reaction during constant current and constant voltage (CC-CV) charging explains the capacity fade of the battery. The effect of parameters such as end of charge voltage and depth of discharge, the film resistance, the exchange current density, and the over voltage of the parasitic reaction on the capacity fade and battery performance were studied qualitatively. The parameters that were updated for every cycle as a result of the side reaction were state-of-charge of the electrode materials and the film resistance, both estimated at the end of CC-CV charging. The effect of rate of solvent reduction reaction and the conductivity of the film formed were also studied.

Development of a Titanium Dioxide-Supported Platinum Catalyst with Ultrahigh Stability for Polymer Electrolyte Membrane Fuel Cell Applications

A significant decrease in performance was observed for commercial Pt/C due to electrochemical oxidation of the carbon support and subsequent detachment and agglomeration of Pt particles. The Pt/TiO(2) cathode catalyst exhibited excellent fuel cell performance and ultrahigh stability under accelerated stress test conditions and can be considered as a promising alternative for improving the reliability and durability of PEMFCs.

Capacity fade study of lithium-ion batteries cycled at high discharge rates

Abstract Capacity fade of Sony US 18650 Li-ion batteries cycled using different discharge rates was studied at ambient temperature. The capacity losses were estimated after 300 cycles at 2C and 3C discharge rates and were found to be 13.2 and 16.9% of the initial capacity, respectively. At 1C discharge rate the capacity lost was only 9.5%. The cell cycled at high discharge rate (3C) showed the largest internal resistance increase of 27.7% relative to the resistance of the fresh cells. The rate capability losses were proportional with the increase of discharge rates. Half-cell study and material and charge balances were used to quantify the capacity fade due to the losses of primary active material (Li+), the secondary active material (LiCoO2/C)) and rate capability losses. It was found that carbon with 10.6% capacity loss after 300 cycles dominates the capacity fade of the whole cell at high discharge rates (3C). A mechanism is proposed which explains the capacity fade at high discharge rates.

Mathematical modeling of the capacity fade of Li-ion cells

A capacity fade prediction model has been developed for Li-ion cells based on a semi-empirical approach. Correlations for variation of capacity fade parameters with cycling were obtained with two different approaches. The first approach takes into account only the active material loss, while the second approach includes rate capability losses too. Both methods use correlations for variation of the film resistance with cycling. The state of charge (SOC) of the limiting electrode accounts for the active material loss. The diffusion coefficient of the limiting electrode was the parameter to account for the rate capability losses during cycling.

Cycle Life Modeling of Lithium-Ion Batteries

A first-principles-based charge-discharge model was developed to simulate the capacity fade of Li-ion batteries. The model is based on the loss of active lithium ions due to solvent reduction reaction and on the rise of the anode film resistance. The effect of parameters such as exchange current density, depth of discharge (DOD), end of charge voltage, film resistance, and the overvoltage of parasitic reaction were studied quantitatively. The model controls the required DOD by controlling the discharge time and estimates the end of discharge voltages as a function of cycle number.

Synthesis and Characterization of MnO2-Based Mixed Oxides as Supercapacitors

Mn/Pb and Mn/Ni mixed oxide were prepared at ambient temperature by reduction of KMnO4 with Mn, Pb, and Ni salts. This low-temperature approach provides amorphous structure of the active material. The specific capacitance of pure MnO 2 was estimated to be 166 F/g and increased to 210 and 185 F/g for Mn/Ni and Mn/Pb oxides, respectively. The carbon loading was optimized at 20 wt %. Based on a single electrode, the Mn/Ni mixed oxide showed a high rate capability of 3.12 Wh/kg at constant power discharge of 1 kW/kg.

Electrochemical Investigations of Cobalt-Doped LiMn 2 O 4 as Cathode Material for Lithium-Ion Batteries

A wide range (y = 0.05-0.33) of Co-doped LiCo y Mn 2-y O 4 spinels were synthesized and electrochemically characterized. These Co-doped spinels showed improved specific capacity and capacity retention over pure spinels. Electrochemical impedance spectroscopy and the linear polarization resistance technique were used to determine the transport and electrochemical kinetic parameters of Co-doped spinels. The presence of Co in the spinel inhibits the passivation process occurring on the surface of the cathode. Also, Co increases the exchange current density and facilitates the charge-transfer reaction of the active material. The lower self-discharge observed for Co-doped spinels was attributed to their low surface areas. The cumulative capacity loss estimated for a pure spinel resulting from self-discharge in the first 30 h was 3 and 6 times larger than those estimated for Co-doped spinels with y = 0.05 and y = 0.16 in LiCo y Mn 2-y O 4 , respectively.

Studies on Capacity Fade of Lithium-Ion Batteries

The capacity fade of Sony 18650S Li-ion cells has been analyzed using cyclic voltammetry, impedance spectroscopy and electron . . . probe microscopic analysis EPMA . The surface resistance at both the positive LiCoO and negative carbon electrodes were found to 2 increase with cycling. This increase in resistance contributes to decreased capacity. Impedance data reveal that the interfacial resistance at . LiCoO electrode is larger than that at the carbon electrode. The impedance of the positive electrode LiCoO dominates the total cell 2 2 resistance initially and also after 800 charge-discharge cycles. EPMA analysis on carbon electrodes taken from the fresh and cycled cell show the presence of oxidation products in the case of cycled cells. No change in the electrolyte resistance is seen with cycling. q 2000 Elsevier Science S.A. All rights reserved.