Albert H. Zimmerman

Technological implications in studies of nickel electrode performance and degradation

Abstract Several processes that are significant in sintered nickel electrode performance and degradation are addressed. The buildup of residual capacity is discussed in terms of the formation of a boundary layer between the current collector and the undischarged active material. The resistance characteristics that accompany the formation of the layer are presented, and the effects of changes in active material resistance and structure are correlated with electrode performance. Processes that can control nickel electrode recharge efficiency are also discussed, and data are presented indicating the kind of performance problems or changes that can be caused by changes in the interfacial morphology and physical structure of the active material deposit. Data are also presented indicating how variations in the composition of the active material can result during electrochemical deposition of active material into nickel electrodes.

Electrolyte Management Considerations in Modern Nickel Hydrogen and Nickel Cadmium Cell and Battery Designs

In the early 1980s, the battery group at the NASA Lewis Research Center (LeRC) reviewed the design issues associated with nickel/hydrogen cells for low-earth orbit applications. In 1984, these issues included gas management, liquid management, plate expansion, and the recombination of oxygen during overcharge. The design effort by that group followed principles set forth in an earlier LeRC paper that introduced the topic of pore size engineering. Also in 1984, the beneficial effect of lower electrolyte concentrations on cycle life was verified by Hughes Aircraft as part of a LeRC-funded study. Subsequent life cycle tests of these concepts have been carried out that essentially have verified all of this earlier work. During the past decade, some of the mysteries involved in the active material of the nickel electrode have been resolved by careful research done at several laboratories. While attention has been paid to understanding and modeling abnormal nickel/hydrogen cell behaviors, not enough attention has been paid to the potassium ion content in these cells, and more recently, in batteries. Examining the potassium ion content of different portions of the cell or battery is a convenient way of following the conductivity, mass transport properties, and electrolyte volume in each of the cell or battery portions under consideration. Several of the consequences of solvent and solute changes within fuel cells have been well known for some time. However, only recently have these consequences been applied to nickel/hydrogen and nickel/cadmium cell designs. As a result of these studies, several unusual cell performance signatures can now be satisfactorily explained in terms of movement of the solvent and solute components in the electrolyte. This paper will review three general areas where the potassium ion content can impact the performance and life of nickel/hydrogen and nickel/cadmium cells. Sample calculations of the concentration or volume changes that can take place within operating cells will be presented. With the aid of an accurate model of an operating cell or battery, the impact of changes of potassium ion content within a potential cell design can be estimated. All three of these areas are directly related to the volume tolerance and pore size engineering aspects of the components used in the cell or battery design. The three areas follow. (i) The gamma phase uptake of potassium ion can result in a lowering of the electrolyte concentration. This leads to a higher electrolyte resistance as well as electrolyte diffusional limitations on the discharge rate. This phenomenon also impacts the response of the cell to a reconditioning cycle. (ii) The transport of water vapor from a warmer to a cooler portion of the cell or battery under the driving force of a vapor pressure gradient has already impacted cells when water vapor condenses on a colder cell wall. This paper will explore the convective and diffusive movement of gases saturated with water vapor from a warmer plate pack to a cooler one, both with and without liquid communication. (iii) The impact of low-level shunt currents in multicell configurations results in the net movement of potassium hydroxide from one part of the battery to another. This movement impacts the electrolyte volume/vapor pressure relationships within the cell or battery.

The effects of platinum on nickel electrodes in the nickel hydrogen cell

Abstract Under conditions of nickel precharge in nickelhydrogen cells, it is possible for platinum complex ions to form at the platinum catalyst electrode when no hydrogen is present. Platinum complex ions have been shown to interact with the active material in the nickel electrode to catalyze the formation of a nickelcobalt compound within the active material lattice. This compound is readily identified by its characteristic voltage signature. A mechanism for the formation of this compound is proposed, and the effects that this compound has on the performance of the nickel electrode in the nickelhydrogen cell are described.

Techniques to improve the usability of nickel–hydrogen cells

The voltages at which the different charging and discharging peaks occur in a nickel electrode were investigated for a set of six representative electrodes. The impact of cycling temperature, electrolyte concentration, and cycling history on these different electrodes resulted in alterations in the usable capacities of the electrodes and/or the efficiency of the charging reaction. The cobalt level in the active material, the KOH concentration of the electrolyte, and the cycling temperature were all found to impact the position of the charging peaks of the beta and the gamma charging reactions. The position of the oxygen evolution characteristic was found to be mainly a function of the cycling temperature. The findings of these studies resulted in recommendations for selecting certain cell design factors and charging protocols that will lead to improved cycle lives and higher levels of usable cell capacities.

Understanding and managing capacity walkdown in nickel-hydrogen cells and batteries

The findings of a recent study that investigated the impact of cycling temperature, electrolyte concentration, cobalt content of the active material, and the cycling history of the electrode on the charging potential for the beta-phase and gamma-phase materials have been incorporated into earlier studies of the capacity walkdown phenomenon. Significant amounts of capacity walkdown were found to be associated with many of the life cycle testing programs carried out at the Navy facility in Crane, IN (USA). These findings are not only helpful for understanding the mechanisms associated with capacity walkdown, but will be useful in selecting cell design factors and cycling conditions that will allow the capacity loss to be held to an acceptable value.

Electrochemical voltage spectroscopy for analysis of nickel electrodes

Electrochemical voltage spectroscopy (EVS) is a technique that directly measures the density of electrochemically active states in an electrode as a function of the applied voltage. In EVS measurements, the voltage of an electrode is scanned at a rate that is slow enough to maintain the electrode close to thermodynamic equilibrium, over a potential range where electroactive species may be oxidized or reduced. The density of reactive sites is obtained from the Coulombs of charge passed through the electrode per voltage increment, which is essentially differential capacitance. For most electrodes, interest is primarily in the Faradaic components of the EVS spectra, which exhibit sharp peaks at the electrochemical redox potentials, although non-Faradaic components (such as double-layer or surface capacitance) can also be measured. For nickel electrodes, EVS provides an extremely useful method for probing the phase composition of the active material based on subtle differences in redox potentials. Alternatively, EVS can detect trace levels of electroactive contaminants in nickel-hydrogen cells or nickel electrodes by scanning the potential over the redox range for the contaminant of interest. We discuss the use of nickel electrode EVS signatures to indicate cobalt additive levels, sinter corrosion, surface changes, double-layer capacitance, electrode swelling, and other factors influencing the performance of the nickel electrode.

Flooded Utilization and Electrochemical Voltage Spectroscopy Studies on Nickel Electrodes

A group of standard nickel electrodes were evaluated at different test temperatures and in different electrolyte concentrations. These production quality electrodes came from different backgrounds in terms of number of charge/discharge cycles, cycling temperature, electrolyte concentration, and cobalt level in the active material. The results of the matrix of tests using the flooded utilization (FU) technique demonstrated that capacity gains are available when cycling at lower temperatures and when higher concentrations of KOH are used as the electrolyte. Electrochemical voltage spectroscopy (EVS) scans were also taken for the complete matrix of tests. Since the cycling conditions used in the FU technique are much closer to those in actual cell cycling tests, they will be emphasized in this study in regard to the capacity trends. Comparative EVS scans were helpful in displaying the shifting potentials of the charging peaks of the active material relative to the oxygen evolution characteristics of these electrodes. The voltage span between the potential at which the active material is charged and the potential at which the co-evolution of oxygen becomes a significant parallel reaction determines the charging efficiency for the recharge step and is the root cause of the differences in useable electrode capacity.

Dynamic calorimetry for thermal characterization of battery cells

The development of dynamic calorimetry as a technique for characterizing the thermal properties and thermal dissipation of battery cells under operational conditions is presented. This technique is discussed as a method for measuring dissipation, detecting phase transitions, determining thermal gradients in battery cells, evaluating gas management in nickel hydrogen cells, and for measuring unknown thermal resistances from thermal transients.

Scanning porosimetry for characterization of porous electrode structures

A scanning technique has been developed to allow the porous microstructures and pore size distributions within porous electrodes to be directly unaged. The measurements use a piezoelectric-driven scanning probe similar to that used in scanning tunneling microscopy. The probe can map the conductivity through cross sections of porous electrodes, thus recognizing pores by their lack of high conductivity. By fitting the probe with a force gauge, it can simultaneously measure the topography of the surface. This technique can provide high-resolution (to 0.1 micron) images of the cross-section, or alternatively by scanning large numbers of pores it can provide pore-size distributions in localized regions of the electrode structure. Examples are provided of the types of electrode features that can be evaluated using this technique.

Real time charge efficiency monitoring for nickel electrodes in nickel—cadmium and nickel—hydrogen cells

Abstract This work demonstrates that the charging efficiency of both Ni/Cd and Ni/H 2 cells can be determined by cell voltage monitoring. This will allow better charge control in flight.