S. Di Stefano

Organic cathode materials in sodium batteries

In order to circumvent the corrosion problems prevalent in many existing electrochemical couples using the Na/β″-alumina half cell, a new class of high energy density organic materials was studied as cathode materials. In particular, one material tetracyanoethylene (TCNE), has favourable electrochemical characteristics with a potential >3.0 V against Na+/Na and energy density ∼620 Wh kg−1. Adopting a cell designed to permit sealing the anode half cell, the performance of TCNE was evaluated under various experimental conditions, that is, at different concentrations of TCNE in the catholyte and with different current collectors. The electrochemical behaviour of the TCNE cathode and the kinetics of TCNE reduction were examined. The kinetic parameters, exchange current density and diffusion coefficient, were determined from different a.c. and d.c. electrochemical techniques and evaluated with respect to the changes in TCNE concentrations in the catholyte. A chemical transformation occurring in the cell operating conditions which does not reduce the electrochemical activity of TCNE was identified from FTIR spectra. Finally, possible approaches to the use of TCNE or other organic materials in sodium or lithium rechargeable batteries are outlined.

Advanced rechargeable sodium batteries with novel cathodes

Abstract Various high energy density rechargeable batteries are being considered for future space applications. Of these, the sodium-sulfur battery is one of the leading candidates. The primary advantage is the high energy density (760 W h kg −1 theoretical). Energy densities in excess of 180 W h kg −1 have been realized in practical batteries. More recently, cathodes other than sulfur are being evaluated. We, at JPL, are evaluating various new cathode materials for use in high energy density sodium batteries for advanced space applications. Our approach is to carry out basic electrochemical studies of these materials in a sodium cell configuration in order to understand their fundamental behaviors. Thus far, our studies have focussed on alternative metal chlorides such as CuCl 2 and organic cathode materials such as TCNE.

A.c. impedance of niobium triselenide cathode in secondary lithium cells

Niobium triselenide is one of the cathode materials being evaluated at JPL for ambient temperature secondary lithium batteries for space applications. The mechanism of reduction of NbSe3 involves two steps, 1 mole of Li intercalating in the first step and 2 moles of Li intercalating at lower potentials in the second step. The a.c. impedance behaviour of the NbSe3 cathode under various conditions, i.e. at different reduction potentials, after discharges at various d.c. potentials and after charge/discharge cycling is discussed.

Determination of the Si-conducting polymer interfacial properties using A C impedanc techniques

A study was made of the interfacial properties of Poly(pyrrole) (PP) deposited electrochemically onto single crystal p-Si surfaces. The interfacial properties are dependent upon the counterions. The formation of “Quasi-Ohmic” and “non-Ohmic” contacts, respectively, of PP(C1O4) and PP films doped with other counterions (BF4 and para-toluene sulfonate) with p-Si, are explained in-terms of the conductivity of these films and the flat band potential,Vfb, of PP relative to that ofp-Si. The PP film seems to passivate or block intrinsic surface states present on thep-Si surface. The differences in the impedance behavior of para-toluene sulfonate doped and C1O4 doped PP are compared.

Characterization of nickel-hydrogen 2-cell common pressure vessels for NASA missions

Cells from the flight lot of the Mars Global Surveyor (MGS) battery were configured as a test pack and underwent characterization tests. The MGS battery is based on 20 ampere-hour 2-Cell common pressure vessel (CPV) technology. The primary goal of the current CPV characterization test was to understand how (Ni-H/sub 2/) V/T curves would map onto the Ni-Cd curves existing in spacecraft hardware. To that end, (Ni-H/sub 2/) V/T curves were experimentally determined. The results of that determination are described in this paper.

Performance characteristics of an aerospace nickel hydrogen cell-experimental data and theoretical predictions

Experimental data have been generated on an Eagle-Picher 50 Ah, IPV Ni-H/sub 2/ cell under various test conditions. These conditions included discharges and charges at different rates and temperatures, in order to determine the effect of these on the charge and discharge efficiencies of the Ni-H/sub 2/ cell. Such a database, hither to not available in the literature, should provide considerable support to the modeling effort in terms of identifying appropriate values for the model parameters. The charge behavior points to an interesting possibility of examining the multiple phases of the positive electrode active material. These studies have been augmented by simulations from our mathematical model.

Aerospace applications of sodium batteries using novel cathode materials

Preliminary fundamental investigations aimed at evaluating sodium metal chloride systems for future aerospace applications are described. Since the sodium metal chloride systems are relatively new, the approach has been to characterize their fundamental properties in order to understand their limitations. To this end, a series of fundamental electrochemical investigations has been carried out, the results of which are reported here. The metal chloride cathodes show high exchange current densities which corroborate their good reversibility in a battery application. The reduction mechanisms appear to be complex and involve multielectron transfer steps and intermediates. Such intermediates in the reaction mechanism have already been identified in the case of FeCl/sub 2/. Similar mechanisms may be operative in the case of NiCl/sub 2/. CuCl/sub 2/, however, exhibits a second relaxation loop in the impedance plot at low frequencies and also a sloping discharge curve, unlike FeCl/sub 2/ and NiCl/sub 2/, which may indicate the existence of monovalent copper in the reduction mechanism.<<ETX>>

Characterization of Thermal and Optical Properties of Polymers by Thermal Lensing Technique

When a light beam passes through a material, some of the absorbed light energy may be converted into heat, generating a time dependent temperature gradient. Increase in temperature causes a change in the index of refraction. Hence, a transient thermal lens is formed in the volume element absorbing the radiation. The temperature rise is typically of the order of 10-2°C within a volume element of less than 10-3 cm3. Several physical and chemical properties of materials such as thermal diffusivity, optical absorption coefficient, multiphoton crossections quantum yield of photoprocesses etc... may be determined by monitoring the time dependence of the amplitude of the thermal lens. This technique is fast, precise and contactless.

Photothermal degradation of ethylene/vinylacetate copolymer

Ethylene/vinyl acetate copolymer (EVA), which has desirable elastomeric properties, low material cost and easy processability, is being considered as a candidate encapsulation material for photovoltaic modules. However, without protection from UV irradiation, EVA is expected to undergo photodegradation similar to that encountered in polyethylene and polyvinylacetate· We have carried out photothermal degradation studies of a “stabilized” formulation of EVA in the temperature range of 25°C to lO5°C under different oxygen environments. At low temperature (25°C), slow photooxidation occurred via electronic energy transfer involving the UV absorber incorporated in the polymer. But no change in physical properties of the bulk polymer can be detected up to 1500 hours of irradiation. At elevated temperature, leaching/evaporation of additives took place leading ultimately to chemical crosslinking of the co-polymer and formation of volatile photoproducts such as acetic acid.