Nickel-Hydrogen Batteries: Principles and Practice
Albert Zimmerman
Chapter 2: The Historical Evolution of Nickel-Hydrogen Cell Designs
2.1 Background of Nickel-Hydrogen Development
The technological background of the nickel-hydrogen cell centers on a number of developments in the late 1960s. In this time period significant efforts were under way to develop truly regenerative fuel cells to power electric vehicles, support load leveling for power utilities, and support long-term power needs in high-technology areas such as spacecraft. Much of this development effort was stimulated by concerns about the continued supply of inexpensive petroleum-based fuels. For space power systems, regenerative fuel cells were envisioned that could store energy and cycle reliably in closed systems for decades. Such systems could provide power for satellites operating in low orbits, midaltitude orbits, or the higher geosynchronous orbits; for space launch systems such as the (then conceptual) space shuttle, space stations, and interplanetary probes and spacecraft; and for planetary exploration stations on the moon, Mars, or other bodies in the solar system. With extensive support from the National Aeronautics and Space Administration (NASA), a number of practical fuel cell systems were developed in the United States for space use.
However, the ultimate goal of a long-lived and reliable fully regenerative system remained very elusive. The concept of a regenerative hydrogen-oxygen fuel cell power system also attracted commercial visionaries who saw numerous terrestrial applications for a highly efficient and pollution-free energy source. The "hydrogen economy" was seen by some to be the ultimate future for world energy consumption, in some scenarios replacing the existing petroleum-based economy by the early twenty-first century. For space power, regenerative hydrogen-oxygen fuel cells were attractive because of the simple chemistry, which involved no toxic components or emissions, and the potential for integrating such systems into the life-support systems of astronauts or the fuel needs of rocket thrusters.
Two major problems have plagued these developments, problems that have not been fully resolved to this day. The first is a straightforward engineering issue that is extremely difficult to handle in long-lived space systems. The pumps, motors, and plumbing required for fuel cell operation have to be extremely reliable if these systems are to be depended on in long-term space missions. System engineering solutions to this problem can be devised in terms of high-reliability parts and redundant systems. However, such solutions significantly increase the size, weight, complexity, and cost of the system. The second problem has had no clear technical solution. The oxygen electrodes used in all these systems to regenerate the oxygen fuel are unstable relative to electrochemical oxidation. Even the best catalysts used for oxygen regeneration slowly oxidize, leading to chemical or physical changes that cause an eventual loss of the necessary catalytic activity. This problem persists for regenerative fuel cells to this very day.
However, the development activities that began in the late 1960s led ultimately to the development of robust and highly reliable primary fuel cells, which remain the electrical generation system of choice in short-duration space missions requiring more energy than can be collected from solar panels, such as the space shuttle missions. Just as important for space power, the problems in developing long-lived regenerative fuel cells led to the concept of a nickel-hydrogen battery cell as a viable alternative to the fully regenerative hydrogen-oxygen fuel cell.
The earliest concept of a nickel-hydrogen battery cell was put forward in Germany in 1970, as part of an effort to use hydrogen-based fuel cells to power electric vehicles. The concept offered a brilliant alternative to deal with the stability problems of oxygen regeneration electrodes in fuel cells. The nickel electrode, which had been used for decades in nickel-cadmium and nickel-iron battery cells, provided a surrogate oxygen regeneration electrode. Not only did the nickel electrode reversibly cycle oxygen, but it also stored the oxygen in a metastable solid matrix of nickel oxyhydroxide. (The detailed chemistry of the nickel electrode will be discussed in chapter 4.) The alkaline nickel electrode could make the oxidative power of the oxygen-rich oxyhydroxide matrix available using readily reversible reactions for many thousands of energy-storage cycles.
The nickel-cadmium cell, which also uses the nickel electrode, suffers from the relatively poor lifetime and reliability of the cadmium electrode. The nickel-hydrogen cell concept combined the highly reliable nickel electrode with the advanced hydrogen electrode concepts that came from fuel cell development programs. Because both the nickel and the hydrogen electrodes were seen as very robust, early nickel-hydrogen cell development efforts identified few failure modes that were expected to limit the life of these battery cells. Ultimately, through very long-term testing and usage, a number of key degradation and failure modes of nickel-hydrogen cells have been identified, and these are discussed in detail in chapter 12.
Unfortunately for the original commercial developers of the nickel-hydrogen cell concept, an important advantage of the fuel cell concept had been lost. The nickel-hydrogen cell now had to carry its own fuel in the form of hydrogen gas and oxygen-rich nickel hydroxides, which limited the energy density, volume, and scale-up of the nickel-hydrogen system. Recharging involved more than refueling with fresh hydrogen and oxygen. In addition, nickel-hydrogen cells used expensive catalysts and required the safe containment of large amounts of high-pressure hydrogen gas. These drawbacks made the nickel-hydrogen cell concept less than economically feasible for most commercial applications, including electric vehicles and large-scale load leveling.
However, space applications provided an almost ideal market for such a battery cell. High cost was a minor detail for a technological innovation that was capable of significantly extending the life and performance of satellites costing hundreds of millions of dollars. In the late 1960s and the 1970s, the improved power system life and reliability possible with nickel-hydrogen batteries was seen as a truly enabling technology for future generations of long-lived space power systems. It is in this technology climate of the early 1970s that the story of nickel-hydrogen batteries begins, within the space programs of the United States, the Soviet Union, Japan, and Europe.
2.2 Overview of Nickel-Hydrogen Development History
The history of nickel-hydrogen technology presented here covers the initial design period of the early 1970s, through numerous technology improvements and design changes, up to a relatively mature technology at the start of the twenty-first century. This thirty-year period covers not only the maturation of the technology, but also the entry of new cell producers and the variety of novel cell concepts, both successful and unsuccessful, that have been explored. A historical view of nickel-hydrogen cell technology is provided by the time line shown in Fig. 2.1, which will serve as the basis for the discussions in the following sections.
Figure 2.1. Flowchart and time line showing the development of nickel-hydrogen battery cell technology through 2000. |
In Fig. 2.1, wide lines that have been shaded differently correspond to the major nickel-hydrogen cell design variants that have been used in space missions. These styles allow the evolution of the early COMSAT and AF/HAC (Air Force/Hughes Aircraft Company) cell designs to be easily followed, and the entry of later-generation cell designs, such as the EPI (EaglePicher Industries)/ManTech, to be tracked.
In general, the history of nickel-hydrogen cell development can be viewed in three phases. The 1970s saw the development, testing, and initial applications of the first-generation cell designs. The 1980s saw the development of second-generation cell designs, experimentation with numerous design variants, the proliferation of nickel-hydrogen technology in space missions, and the entry of numerous new producers into the market. The 1990s marked the maturation of the most successful technologies. During the 1990s, as nickel-hydrogen became the dominant rechargeable space battery technology, test data became available to identify the design features and cell variants that gave the best long-term performance. It is noteworthy that the 1990s were marked by a significant consolidation in the suppliers of nickel-hydrogen cells. The following sections will fill in the details within each of these three decades of nickel-hydrogen technology, and a final discussion will explore the numerous less-successful design variations that have been tried.
2.3 First-Generation Nickel-Hydrogen Cells
2.3.1 The COMSAT Cell Design
The concept of the nickel-hydrogen cell, first conceived of in Germany, was recognized in 1970 by AT&T/Bell Labs in the United States as a potential technology of key value in space power. AT&T/Bell Labs took on the role of evaluating the feasibility of the basic technology for space use, developing initial component and cell design concepts, and transitioning this technology to battery cell manufacturers and satellite contractors for evaluation. This effort also began the initial commercial nickel-hydrogen cell development for nonspace applications. A number of initial test cells were constructed and evaluated. Early in the development effort, the nickel electrode designs available at the time were identified as the life-limiting components in the cells. While historically adequate for nickel-cadmium cells, the existing nickel electrodes could not meet the more demanding performance levels required of nickel-hydrogen cells.
In response to the need for improved nickel electrodes, AT&T/Bell Labs developed the initial version of what is now referred to as the aqueous electrochemical impregnation process. This process, variants of which are still in use today, produced improved sintered nickel electrodes by electrochemically depositing the energy storage material directly within a conductive substrate. It produced significantly improved nickel electrodes when they were combined with improved sintered nickel substrates based on those that had been used for years in nickel-cadmium cells. Testing of the early "proof of concept" cells demonstrated that nickel-hydrogen cells could be used to significant advantage in space systems as a replacement for nickel-cadmium cells.
To transition the new nickel-hydrogen technology into an actual space program, AT&T/Bell Labs initiated a joint effort with Tyco International to manufacture cells containing the new technology, and with the International Telecommunications Satellite Consortium (Intelsat) to use these cells in AT&T/Bell Labs's geosynchronous communications satellites. This general cell design, which was initially acquired and tested by COMSAT Laboratories (the technical arm of Intelsat in the United States), became known in later years as the COMSAT cell design.
The COMSAT cell, which was one of the first viable nickel-hydrogen cells for space use, contained nickel electrodes made by aqueous electrochemical impregnation of nickel/cobalt hydroxides into a 30 mil thick nickel sinter made by a slurry process. The sintered starting substrate had a porosity of about 76%, and the finished nickel electrode had a porosity of about 40–45%. This electrode design offered significant improvements in life cycle capability over the standard chemically loaded or pasted nickel electrodes that had been used for years in nickel-cadmium cells. Two of these nickel electrodes were stacked back-to-back with asbestos cloth separators on each face of the back-to-back electrodes. The asbestos separators, which were borrowed from fuel cell technology, were in contact with the hydrophilic surface of platinum-catalyst-based hydrogen electrodes, which had also been developed for fuel cell use. The hydrophobic side of these catalytic gas electrodes was in contact with a plastic screen that allowed hydrogen gas to flow freely to the catalyst sites in the hydrogen electrode. This stacking unit was then repeated, with all the nickel electrodes connected in parallel to a bus bar on one side of the cell and all the hydrogen electrodes to a similar bus bar on the other side of the cell. This stacking arrangement is illustrated in Fig. 2.2.
Figure 2.2. Stacking of nickel and hydrogen electrodes in the COMSAT design. |
All the components in the COMSAT design shown in Fig. 2.2 were made to fit into a cylindrical pressure vessel having a nominal diameter of 3.5 in. The stack of components was held together by a steel bolt that passed through the center of all the stack components to tightly clamp them together. One end was bolted to a plastic end plate and a metal ring that attaches the stack to the pressure vessel, while the other end of the stack was simply bolted against a plastic end plate. Positive and negative leads connected to the bus bars on each side of the stack carried the current to positive and negative cell terminals sealed into the ends of the pressure vessel. The stack was attached to the pressure vessel by welding the metal ring at one end of the electrode stack to the wall of the pressure vessel. The pressure vessel was sealed (except for an electrolyte fill tube) by welding the two halves of the pressure vessel together. (The halves were typically hydroformed pieces of Inconel 718.) Tungsten inert gas welding was originally used for this purpose but subsequently gave way to laser and e-beam welding. A tube was also welded into one end of the pressure vessel to allow the later addition of electrolyte, a process known as cell activation.
About two years after the joint effort to transition nickel-hydrogen cell technology into space systems was started, Tyco was replaced as the cell manufacturer by EaglePicher Industries of Joplin, Missouri (EPIJ). The aqueous nickel electrode impregnation process developed by AT&T/Bell Labs was scaled up and put into place at EPIJ, and production of nickel-hydrogen cells was begun in 1974. This COMSAT cell was one of the first-generation nickel-hydrogen cell designs, as indicated in the time line in Fig. 2.1. It had some features that remain to this very day in nickel-hydrogen cells, while many of its other design characteristics have since been altered to provide cells that are more robust.
All nickel-hydrogen cell designs are essentially "electrolyte starved," meaning that there is little or no free electrolyte sloshing about in the pressure vessel. All electrolyte is contained within the porous stack components. The early COMSAT cell had a hydrophobic coating over the inner wall of the pressure vessel, the purpose of which was to force any electrolyte that was expelled from the stack during cell operation back into the stack in the zero-gravity environment of space. This hydrophobic coating has been replaced in designs that are more modern by a hydrophilic wall wick that enables electrolyte to move by capillary forces from wet to dry regions of the stack. The electrolyte used in the original COMSAT cell was potassium hydroxide, which, after a process of activation and conditioning, ended up at a typical concentration of 38–40% by weight. Most state-of-the-art cells today use 26 or 31% potassium hydroxide electrolyte. The original COMSAT cell had a hydrogen precharge of 14.7 psi of hydrogen gas, which has been supplanted by nickel precharge in modern nickel-hydrogen cell designs. Later chapters will detail the reasons for these and many other changes that have occurred throughout the years.
The basic COMSAT cell with some design variants was in production at EPIJ until about 1997. Production of this "heritage" cell ended because it became clear that this cell was not as capable of high-rate performance and long life as were the improved second-generation cell designs. Major factors in the demise of this cell line were the lack of high-quality asbestos separator and the liability issues associated with the handling and use of asbestos.
Nickel-hydrogen cells of the COMSAT design were extensively tested for many years by COMSAT Laboratories and other aerospace contractors, giving performance that was generally superior to that of nickel-cadmium cells. Some of these test data will be reviewed in chapter 11. COMSAT batteries were flown in two spaceflight experiments in 1977. The NTS-2 satellite flew nickel-hydrogen batteries successfully in geosynchronous orbit for 5 years. Nickel-hydrogen batteries also performed well for about 8 months in the low Earth orbiting Air Force Flight Experiment satellite. COMSAT batteries were subsequently flown on satellites of the Intelsat IV and Intelsat V design during the next 10–12 years.
In general, the nickel-hydrogen batteries performed well in these geosynchronous satellites for much beyond 10 years. Some issues were encountered with increasing cell impedance, which was attributed to asbestos degradation, as well as movement of electrolyte and its loss from the cell stack in response to thermal gradients. These observations were important for the future design and handling of nickel-hydrogen cells and batteries, because they showed how sensitive the performance of the cells could be to electrolyte distribution. Later cell designs have invariably included features to manage the distribution of electrolyte in the cell stack, and thermal control systems have been designed to keep the thermal gradients within an operating cell relatively small.
2.3.2 The First-Generation AF/HAC Cell Design
In parallel with the development of the COMSAT cell design, the U.S. Air Force (USAF) began supporting the development of a nickel-hydrogen cell design that would be more robust in a low Earth orbit environment, where more than 50,000 cycles could be experienced over a 10-year period. In 1971, this development began under the jurisdiction of Wright-Patterson Air Force Laboratories (WAFL). Hughes Aircraft Company (HAC) was chosen as the cell and battery manufacturer in this development effort; thus the resulting cell design is termed the AF/HAC design.
Like AT&T/Bell Labs, WAFL found that a high-quality nickel electrode was critical for obtaining a long-lived nickel-hydrogen cell. An electrochemical impregnation method was also chosen to make sintered nickel electrodes at WAFL. However, rather than using a sinter made by a slurry process, WAFL developed a process by which the substrate was made by sintering a layer of dry nickel powder to a nickel screen using no binders or additives. This is still referred to as the "dry powder" process. To reduce corrosion of the nickel sinter during the subsequent electrochemical impregnation process, the temperature of the highly acidic aqueous bath used by AT&T/Bell Labs was reduced. Because a water/ethanol mixture was used to control the reduced temperature, this process is referred to as the "alcoholic impregnation" or Pickett process. This process incorporates a cobalt additive deposited at about a 10% level with the nickel hydroxide, which is about twice the 5% cobalt additive level used in the aqueous process developed at AT&T/Bell Labs.
The alcoholic nickel electrode impregnation process was put first into pilot plant, and later into full-scale production, at the EaglePicher Industries facility in Colorado Springs, Colorado. The nickel electrodes fabricated using the early alcoholic impregnation processes were built into a new nickel-hydrogen cell design by developers at HAC with support from USAF. This cell used the "recirculating stack" design illustrated in Fig. 2.3.
Figure 2.3. Arrangement of stack components in a recirculating-stack nickel-hydrogen cell. Reprinted courtesy of NASA. |
In this stacking arrangement, each nickel electrode had one face in contact with a gas screen and the hydrophobic side of the hydrogen electrode, and the other face in contact with a separator that in turn was in contact with the hydrophilic side of the hydrogen electrode. The gas screen was an open-mesh polypropylene screen that allowed oxygen formed at the nickel electrode to freely flow across to the hydrogen electrode, where it could catalytically recombine with hydrogen gas. All ionic current flow passed only through the side of the nickel electrode in contact with the separator. Thus, the recirculating stack design tended to pump electrolyte from one end of the stack to the other, leading to a natural concentration gradient across the stack. The cell design also instituted a porous wall wick consisting of a thin flame-sprayed zirconium oxide layer coating the inner wall of the pressure vessel. The wall wick was also in contact with the edges of the separator, which protruded slightly from the stack. The function of the wall wick was to diffusively redistribute the electrolyte gradient set up by the recirculation pattern established by the stack design.
The AF/HAC cell design also used a novel stack geometry that has been referred to as the "pineapple slice" stack. As indicated in Fig. 2.4, all the stack components—nickel electrodes, separators, hydrogen electrodes, and gas screens—are shaped like thin pineapple slices. In this design, leads that run through the center hole of the pineapple slices make the positive and negative connections to the electrodes. This configuration maximizes the contact area between the pressure vessel wall and the edge of the stack to allow optimal thermal conduction to the pressure vessel wall and the thermal sleeve in which the pressure vessel is mounted.
Figure 2.4. Typical pineapple-slice nickel-hydrogen cell components, with a partially assembled stack. |
The nickel electrodes in this design were nominally 3.5 in. in diameter and 30 mils thick. The separator material consisted of two layers of a zirconium oxide fabric, either woven or knit. The hydrogen electrodes were fuel-cell-quality, platinum-catalyst-based electrodes with hydrophilic platinum catalyst on the side facing the separator, and the side facing the gas screen consisting of a hydrophobic porous Teflon layer. The electrolyte was aqueous potassium hydroxide, 31% by weight. The cell components were stacked on a polysulfone core that fit into the center hole of the pineapple slice components. The electrode leads were routed through compartments inside the core, and passed out the top and bottom of the core as lead bundles that were welded onto the cell terminals. The stack was compressed between polysulfone end plates using a polysulfone nut threaded onto one end of the core. Belleville washers were used with the end nut to provide a controlled force to the stack, and to allow the stack to swell. A weld ring at one end of the stack was attached to a hydroformed pressure vessel made from Inconel 718. The cell terminals were sealed in the pressure vessel dome using Teflon compression seals. A 1/8 in. electrolyte fill tube was typically provided in the pressure vessel dome for activating the cell with electrolyte after it was fully assembled. Figure 2.5 shows an example of an AF/HAC cell before being built into a battery. The finished nickel-hydrogen cell shown in this figure, like all variations of this basic design, physically appears as a sealed pressure vessel with two electrical terminals to pass current.
Figure 2.5. Typical 50 A h AF/HAC nickel-hydrogen cell. The white tie-wrap holds a thermistor in place. |
Cells of the AF/HAC design were originally activated using a precharge of 50 psi of hydrogen gas, thus enabling the full capacity of the nickel electrodes to be used. In the early 1980s it was found that the hydrogen precharge reacted with and degraded the nickel electrodes during storage in the fully discharged state, leading to capacity loss by a mechanism termed "hydrogen sickness." This degradation mode is described in detail in chapter 12. Starting in about 1984, a nickel precharge was adopted as standard in the AF/HAC design.
Early AF/HAC cells provided a robust design that was tolerant of reversal, withstood overcharge even at high rates, and provided good long-term cycle life performance. Testing in standard low Earth orbit 90 min cycling regimes typically gave 7000 to 9000 cycles at 80% depth of discharge, and more than 60,000 to 70,000 cycles at 40% depth of discharge.
2.4 The ManTech Program
In 1981, Yardney was awarded the Nickel-hydrogen Manufacturing Technology (ManTech) Program to develop a lower-cost design and improved manufacturing methods for nickel-hydrogen cells. From this program emerged what is referred to today as the ManTech cell. The ManTech cell design and manufacturing methods developed by Yardney are described in detail in Bentley and Denoncourt2.1 and are summarized in Dunlop et al.2.2 These changes include the following.
- The Inconel 718 pressure vessels are hydroformed to a uniform thickness, with no chemical milling.
- A machined terminal boss is e-beam welded to the pressure vessel dome.
- E-beam girth welding is used rather than tungsten inert gas welding.
- The fill tube is made an integral part of the cell terminal.
- A Teflon compression seal is baselined for the terminals; however it is not clear whether this was a major improvement over the nylon Zeigler compression seal used in the COMSAT design.
- Molded polysulfone end plates are used at each end of the stack, rather than Inconel end plates.
- A modified molded polysulfone core is used, along with dual Belleville washers to maintain stack compression on a floating core.
- A dual-layer separator consisting of one layer of Zircar and one layer of asbestos is used, with the asbestos layer facing the nickel electrode surface. This design change did not prove best compared to dual-layer Zircar in life tests.
Each of the other cell manufacturers developed their own versions of the ManTech cell design that used many of the key changes developed in the Yardney ManTech Program, but included other major improvements specific to each manufacturer. The following section describes many of these second-generation cell designs.
2.5 Second-Generation Cell Designs
The AF/HAC cell design, which was largely being produced by EaglePicher Industries in Colorado Springs by this time, underwent some major changes in the mid-1980s. The cells began to be made in a larger, 4.5 in. diameter size, primarily to support higher-power satellites having a single battery. The single-battery power system provided a somewhat higher energy density than could be achieved with multiple batteries employing smaller cells. This was an important advantage for the geosynchronous satellites in which these cells and batteries were primarily finding applications. In addition, the HAC cell design adopted the back-to-back stacking arrangement that was pioneered in the COMSAT design, and recommended with the pineapple slice components by the ManTech program. The arrangement of the stack units in this back-to-back configuration, which is common to many modern nickel-hydrogen cell designs, is shown in Fig. 2.6.
Figure 2.6. Arrangement of stack components in a typical back-to-back ManTech nickel-hydrogen cell. Reprinted courtesy of NASA. |
Other changes in the AF/HAC cell design included the adoption of a single layer of Zircar (a zirconium oxide fabric) separator and the use of 26% potassium hydroxide electrolyte. While the decreased amount of electrolyte retained by the single layer of separator clearly made the cell less robust in terms of susceptibility to dry-out, the use of 26% electrolyte compensated by making the stack less prone to dimensional changes throughout its life. The nickel electrode design and fabrication methods used in the earlier cells were retained, although the thickness of the nickel electrode was increased from 30 mils to 35 mils. Cell activation techniques that left free electrolyte in the cell tended to be the major problem throughout the long term, causing popping, occasional stack damage, and a few short-circuit failures. These problems have been dealt with by improved activation methods that assure all free electrolyte is drained from the cells. The methods have included better drain procedures, and have included centrifuging cells to remove excess electrolyte. This cell design, while perhaps not optimum for the more than 40,000 cycles needed in a low-orbit satellite, was perfectly appropriate for a lightweight geosynchronous satellite power system.
The second-generation HAC (later Hughes Space and Communications and Boeing Satellite Systems) cell design has been utilized extensively in geosynchronous satellites during the past 10–15 years. Beginning in the late 1990s a new 5.5 in. diameter cell design was developed by HSC for higher-power satellites, providing cells having capacities up to 350 A h. These cells were built at the Hughes Space and Communications facility in Torrance, California, as well as in Colorado Springs. Along with the larger cell capacities, improved thermal control systems were developed to help dissipate the increased heat generated in the cells. The improvements seen in these systems included carbon composite thermal sleeves that offered better thermal conductivity with lighter weight, and the use of heat pipes to provide active cooling to cells. In 2000, the Hughes Space and Communications production facilities were acquired by Boeing Satellite Systems. Since 2000, Boeing has developed a low Earth orbit version of their standard cell, which goes back to the dual-layer Zircar separator and 31% electrolyte that was used in the earliest HAC cells.
In the mid-1980s, Gates Energy Products developed a nickel-hydrogen cell line that was produced in the company's Gainesville, Florida, facility. This design was similar to that of the AF/HAC cell in that it used a recirculating stack and dry sinter in the nickel electrodes. An aqueous electrochemical impregnation process was developed using 5% cobalt additive in the nickel hydroxide. The cells used two layers of Zircar separator, 31% potassium hydroxide electrolyte, and a nickel precharge. The seals used in the pressure vessels were ceramic-to-metal seals similar to those used for years by Gates Energy Products in its sealed nickel-cadmium satellite cells. Performance problems associated with the recirculation pattern of electrolyte in the relatively long 3.5 in. diameter cells eventually led to a shift to the back-to-back design that had been adopted by the rest of the nickel-hydrogen industry by this time.
The sale of Gates Energy Products to the French company Saft in 1994 enabled Saft to offer the Gates Energy Products cell design to its customers, a design that soon replaced the existing Saft nickel-hydrogen cell designs. Saft had developed a nickel-hydrogen cell design using the nylon separators and chemically impregnated nickel electrodes used in its nickel-cadmium cell lines. While performing reasonably well, the existing Saft design had exhibited problems associated with the relatively compressible and thermally reactive nylon separator. Several ongoing research programs to develop improved separator materials, such as Zirfon, to replace nylon, lost much of their impetus after Saft obtained the Gates Energy Products cell designs. As was found by others attempting to develop improved separators, it was difficult to identify materials that worked better than Zircar.
In parallel with its highly successful ManTech program, Yardney developed the capability to produce nickel-hydrogen cells that essentially followed the features specified by that program. Yardney developed a facility for the aqueous electrochemical impregnation of nickel electrodes, using 5% cobalt additive and a slurry-based nickel sinter. This facility is the only one that has ever made nickel electrodes individually. Each nickel electrode was impregnated within an individual cell through which solution from a common manifold was flowing. This facility was capable of producing some electrodes of very high quality, but it also suffered from variability in quality of electrodes made at different points in time, or from different production cells. The nickel electrodes were stacked in a back-to-back configuration using a novel composite separator. The separator consisted of a layer of Zircar that supported a layer of asbestos. This separator was supposed to combine the incompressible physical structure and electrolyte retention characteristics of Zircar with the ability of asbestos to channel oxygen into the gas spaces at the edge of the cell. The Yardney cell also used a Teflon compression seal to seal the terminals into the pressure vessel, and it was activated with one atmosphere of hydrogen precharge. Cells made by Yardney were placed into a number of life tests. These cells were generally marked by a significant spread in their performance. Some cells gave excellent performance, while other cells gave much poorer performance. In about 1983, Yardney ceased to produce nickel-hydrogen cells.
Following the ManTech program, EPIJ developed a ManTech cell line to go along with the standard COMSAT cell line that they already produced. This cell design used the electrochemical impregnation method developed for nickel electrodes in the COMSAT cell design, employing either slurry sinter or dry sinter. The nominal thickness of the nickel electrodes was 35 mils, as opposed to the 30-mil thickness of the electrodes used in the COMSAT cells. The cells were built with a back-to-back stacking arrangement that included two layers of Zircar separator, and they used 31% potassium hydroxide electrolyte. They were built with about a 15% nickel precharge. As in the COMSAT cell, a nylon compression seal was used to seal the terminals into the pressure vessel. A flame-sprayed zirconium oxide wall wick was included in the design to allow electrolyte gradients to equilibrate by diffusion. Extensive electrolyte draining procedures were employed to ensure that no free electrolyte existed in the cell, because free electrolyte was found to result in popping damage and internal cell short circuits.
The initial cells in the EPIJ ManTech line were in the 35–50 A h capacity range and were 3.5 in. in diameter. The length of the stack that could be supported in a pressure vessel while meeting vibration and mechanical safety margins limited the maximum cell capacity. To satisfy the need for cells in the 60 to 100 A h capacity range, EPIJ developed a dual-stack cell design that enclosed two stacks within the same pressure vessel. The two stacks were attached to a weld ring that was welded to the center of the cylindrical portion of the pressure vessel, thus providing mass balance about the point where the stacks were supported. Electrical connection of the two stacks in parallel provided twice the capacity that could be achieved from a single stack. Because the wall wick on the inner wall of the pressure vessel could not extend across the weld region at the center of the cell, Zircar wicking assemblies through the weld ring were provided to enable electrolyte diffusion between the two stacks. The dual stack design has achieved cell level energy densities up to 60 W h/kg based on capacity at 10°C, which is a record for 3.5 in. diameter nickel-hydrogen cells containing two layers of Zircar separator. The ManTech cell design containing a dual stack has performed quite well in a wide range of life tests, and in a large number of orbital satellites.
The successful dual stack nickel-hydrogen cell suggested a design alternative that could reduce the number of pressure vessels required in some power systems. Rather than connecting the two stacks in the pressure vessel in parallel, the two stacks could be connected in series to provide a unit having twice the voltage but only half the capacity. This configuration was termed a common pressure vessel, which refers specifically to two cells connected in series within the same pressure vessel. This is quite different from the single pressure vessel configuration, which involves a large number of cells that are series-connected within the same pressure vessel, or the individual pressure vessel configuration, in which all electrodes are connected in parallel.
While in principle the common pressure vessel design should perform as reliably as the individual pressure vessel cells, an additional degradation mode does exist for the common pressure vessel. Any electrolyte bridging that occurs between the two stacks will result in ionic migration (driven by the voltage difference between the cells) and the development of an electrolyte gradient. A hydrophobic barrier strip on the pressure vessel wall between the two stacks is provided to minimize this possibility. In addition, if oxygen that is generated in one stack recombines in the other stack, an electrolyte volume gradient can develop. These issues are likely to be driven by significant thermal gradients across the cells, or by excessive overcharge. These cells have not yet been tested as extensively as the standard individual pressure vessel cells; however, the test data that are available do not show premature degradation modes.
A number of other nickel-hydrogen cell design variants have been developed by EPIJ. Cells have been built in both a smaller-diameter (2.5 in.) size, as well as in larger sizes (4.5 and 5.5 in. diameter). These other cell sizes are essentially scale versions of the 3.5 in. diameter ManTech cell design. The 2.5 in. diameter cell has been built in capacities ranging from 6 to 32 A h. It is intended primarily for small satellites, or any other applications needing high reliability and long cycle life. The 2.5 in. diameter cell has been built in both the common pressure vessel as well as the individual pressure vessel configuration. Cells having a 4.5 in. diameter typically cover the capacity range from 100 to 220 A h, and the 5.5 in. diameter cells have a capacity of 250 to 350 A h.
Efforts to make lighter-weight nickel-hydrogen cells at EPIJ have led to the use of single layers of Zircar separator, as well as the use of larger cell sizes. The highest energy density reported for nickel-hydrogen cells to date is 70 W h/kg for a 4.5 in. diameter cell design using a single layer of Zircar separator. Some efforts to make lighter cells have also involved the use of lightweight composite pressure vessels. Generally, when the weight of nickel-hydrogen cells has been reduced to give energy densities above about 55–60 W h/kg, the weight reduction has been accompanied by some reduction in cell robustness. The 70 W h/kg cell cited above is not likely to be the best cell for a low Earth orbit satellite that must operate throughout 40,000 or more eclipse cycles, but is likely to be adequate for a geosynchronous satellite requiring 88 cycles per year. As will be seen in later sections of this chapter, a number of other design changes intended to reduce cell and battery weight have been explored. Almost invariably, the weight reductions are accompanied by reduced life or reduced reliability.
One innovation that has been pursued by EPIJ for improving cell performance during overcharge is the use of catalytic strips on the wall wick in the ManTech-type cell. These catalytic wall wicks have spiral strips of platinum catalyst firmly embedded in the zirconium oxide wall wick. The catalyst sites on the wall of the pressure vessel allow the direct recombination on the cell wall of any oxygen that is generated during overcharge and that finds its way to the edge of the stack. This deposits a portion of the heat from overcharge directly on the cell wall where it can be most easily dissipated by the thermal control system, allowing cells to run cooler and better tolerate overcharge. Life tests of cells containing this feature have demonstrated significantly improved cycle life when compared to cells cycled in the same test that did not have catalytic wall wicks. This design feature has only slowly been accepted in the aerospace industry, and it is now used in the battery cells that power several satellites.
A final design variant that was pioneered in nickel-hydrogen cells manufactured by EPIJ is one that uses what is known as "rabbit ear" terminals. In this design, both the positive and negative terminals are situated in the top of the cell, emerging from opposing sides of the top dome of the pressure vessel. The lead bundles from both the positive and the negative plates are run up through the core to the top dome of the cell, where they are welded to the terminals. This cell design allows cells to be efficiently wired together in series strings by positioning the negative terminal of one cell immediately adjacent to the positive terminal of the next cell in the string. These terminals can then be connected together by an extremely short length of conductor, thus allowing a significant reduction in the contribution of the intercell conductors to battery impedance. A typical ManTech cell with the rabbit-ear terminal configuration is shown in Fig. 2.7.
Figure 2.7. ManTech 100 A h nickel-hydrogen cell with a rabbit-ear terminal configuration. |
While the rabbit ear design has been extremely popular, it can have a negative impact on cell performance. With this design, the resistance is now greater to the electrodes at the bottom of the cell because of the longer leads, thus imposing a gradient in depth of discharge, particularly at high discharge rates. In addition, heating is greater at the top of the cell because of the greater local discharge near the top of the cell, as well as the flow of both current and heat upwards through the lead bundles. These issues make these cells somewhat more sensitive to the thermal environment and the charge control system used for maintaining cell state of charge.
2.6 NASA Advanced Design Matrix
In 1988 NASA entered the nickel-hydrogen development arena by initiating a program intended to acquire both standard and (several) advanced cell designs from each cell manufacturer for the purpose of conducting life tests on these cell designs. The manufacturers providing cells for this program were Gates Energy Products, Saft, Yardney, and EPIJ. This test program was intended to evaluate nickel-hydrogen designs for future NASA missions, including the space station (initially Space Station Freedom, SSF, and later the International Space Station), which was planned to use nickel-hydrogen batteries for secondary power. Each of the manufacturers involved in this program constructed a range of design variations for life testing,2.3 as shown in Table 2.1, In this table, the stack types are back-to-back (BB) or recirculating (Rec), and the separator is either dual-layer Zircar (ZZ) or a Zircar/asbestos composite (ZA).
Life testing of the cells in the advanced design matrix continued many years. One somewhat surprising conclusion became evident from this test program. Of the cell designs under test, very few of the advanced designs functioned even as well as the standard designs submitted by the manufacturers. It appeared that the advanced designs were simply experiments with new features having promise of improvement. To actually realize the hoped-for performance improvements, an iterative redesign and test program was likely to be needed. These results drive home an important point that has been demonstrated by numerous tests of nickel-hydrogen cells. The majority of the stresses in the cells that ultimately limit performance and life arise from strong interactions between a large number of processes that affect the dynamics between all the cell components. Simply changing one component or adding a new design feature can have a nonlinear effect on the dynamics of cell operation. Design improvements must encompass the entire cell dynamic, not just the performance of isolated components.
The International Space Station cell-testing program became a significant additional test effort supported by NASA. Cells from Gates Energy Products were tested as the first generation of nickel-hydrogen cells planned for the International Space Station. Cells from EPIJ were tested as an alternative after Gates Energy Products was sold to Saft. The International Space Station program ultimately used the standard cell designs produced by these manufacturers, probably because of the NASA experience with advanced designs. The standard ManTech designs used a back-to-back stack with dual-layer Zircar separator and 31% electrolyte. The batteries eventually used on the International Space Station contained 81 A h cells made by EPIJ.
2.7 Single Pressure Vessel Cells and Batteries
Nickel-hydrogen cells have a relatively poor volumetric energy density, which partially results from the difficulty in closely packing cylindrical domed devices into a battery. The single pressure vessel nickel-hydrogen battery encloses a large number of individual cells within a single large pressure vessel. In addition to minimizing the volume of the battery, the single pressure vessel battery can have significantly reduced impedance, because the cells can be directly connected together within the pressure vessel, eliminating the need for interconnecting cell wires.
In the early 1980s, Johnson Controls began a program to develop large single pressure vessel nickel-hydrogen battery systems for terrestrial uses. This development effort was highly dependent on the battery- and cell-packaging technologies that Johnson Controls had developed for lead-acid batteries. While a few test batteries were built and tested with some success, the commercial market for nickel-hydrogen batteries never really developed. By the late 1980s Johnson Controls was targeting satellites as the most realistic market for single pressure vessel nickel-hydrogen batteries. During the next several years a number of single pressure vessel nickel-hydrogen battery variations were produced, each providing somewhat better performance than the last. The technical issues in making a single pressure vessel battery operate reliably over a long cycle life were daunting. These issues included removing heat, maintaining all electrolyte within each cell while allowing hydrogen to flow into and out of each cell, recombining all oxygen within the cell in which it was generated, handling popping in the cells, and minimizing the impacts of a leak in the single pressure vessel. By the early 1990s, Johnson Controls had developed a design that worked reasonably well and was targeted for the large satellite constellations planned for the mid-1990s, such as Iridium, Teledesic, etc.
EPIJ had also been working for some years on a single pressure vessel design, which also targeted the large-satellite-constellation market. In 1992 EaglePicher acquired the battery division of Johnson Controls and transferred all the single pressure vessel technology to the Joplin facility. The company possessed the facilities to produce the single pressure vessel batteries in the volume that would be required for large constellations of satellites, and the single pressure vessel battery system could be produced much more rapidly and cheaply than the individual pressure vessel batteries traditionally used in satellites. EaglePicher manufactured the single pressure vessel batteries that were flown in the Iridium constellation of 88 satellites (98 satellites including orbital spares). While these batteries have functioned reasonably well, some difficulties have been encountered in maintaining all the cells at a similar state of charge when they share a common hydrogen reservoir. This problem stems from subtle differences in loss rates between individual cells combined with the limited overcharge to which these batteries are subjected, resulting from concerns with oxygen recombination and movement of oxygen between the cells. However, these single pressure vessel batteries continue to perform well in orbit, and ground tests suggest they can provide 75–80% of the lifetime expected from individual pressure vessel batteries.
2.8 Nickel-Hydrogen Cell Design Variants
A number of other viable nickel-hydrogen cell designs have been produced and tested, and have given good life test results, but have not been selected for actual flight use in satellites. For a design variant to be selected for satellite use, it generally must meet some key need; otherwise it is not selected in place of standard designs for which there is a large body of experience and test data. Thus, these variants may be as capable as the more standard designs, or even better in some cases. They simply need more testing, or a satellite program that matches their capabilities.
2.8.1 The Dependent Pressure Vessel Cell Design
The first design variant is the dependent pressure vessel cell design. This cell design uses a flattened pressure vessel, giving the cell the shape of a discus. The flattened surface alone is not capable of supporting the internal cell pressure; thus multiple cells must be mechanically tied together (i.e., using tie-rods) to hold the pressure. The internal cell stack is in thermal contact with the flattened face of the cell, thus providing a large thermal dissipation surface. In addition to providing better heat dissipation, these cells can be more closely packed to give a battery better volumetric energy density. Because this cell design is dependent on the pressure in adjacent cells or a battery end plate for support, internal structural accommodation must be provided to prevent an individual cell from being crushed by its adjacent cells if it loses pressure for any reason while the other cells are charged. Such pressure loss could occur if a cell fails by a short circuit, if a leak occurs, or if a sufficiently large imbalance in state of charge develops. Dependent pressure vessel cells have been built and tested at EPIJ, providing good performance.
2.8.2 The Dual-Anode Design
A second design variant is termed the dual-anode design. It uses a stack design that is a variation of the commonly used standard back-to-back design. In the dual-anode cell, each face of each nickel electrode is in contact with a separator layer, which in turn is in contact with a hydrogen electrode. Thus, it uses twice as many layers of separator and twice as many negative electrodes. If a single thickness of Zircar separator is used for each separator layer, then the weight of separator and electrolyte is similar to that in a back-to-back cell with two layers of Zircar separator. The extra negative electrodes make the dual-anode design somewhat larger and heavier than the standard back-to-back design.
However, the dual-anode design offers some significant performance advantages that can more than offset the extra size and weight. Both faces of each nickel electrode are fully and symmetrically used, making the superficial current density half of that in standard designs. The distance for ionic current flow and mass transport through the nickel electrode and separator is about 50% of that in standard designs, significantly decreasing internal impedance and improving oxygen recombination pathways. These changes result in significant decreases in recharge voltage and increases in discharge voltage, as well as improved capacity and reduced degradation rates. Cells of this design are being tested at The Aerospace Corporation to evaluate their capability for long-term cycling at 60% depth of discharge. If the cells can reliably operate for 60,000 cycles or more at 60% depth of discharge, their improved capability, as compared to that of standard cells, would more than compensate for the slightly higher weight. These cells may be appropriate for future high-power low Earth orbiting satellites. As of late 2008, these cells have demonstrated 40,000 cycles at 60% depth of discharge with little evidence of cell wear.
2.9 Experimental Nickel-Hydrogen Cell Designs
A number of nickel-hydrogen cell design and component design variations have been tried over the years. While many have functioned adequately, they have not been introduced into production, because they provided insufficient advantage over existing designs, or in some cases did not perform as well as existing designs. These are referred to as experimental cell designs, and they are documented here to illustrate the range of design options that have been pursued.
2.9.1 Bipolar Cells
Bipolar nickel-hydrogen cells have been explored in the context of single pressure vessel systems. In a bipolar nickel-hydrogen cell, cells are stacked up in a single pressure vessel, with a solid conduction plate providing direct electrical contact between the positive electrode of one cell and the negative electrode of the adjacent cell. The reliable management of both gas and electrolyte in these systems, while assuring that no electrolyte bridges occur between cells, is not simple. Because of these issues and assembly problems, successful long-term operation of a bipolar nickel-hydrogen cell has never been demonstrated.
2.9.2 Separators for Cells
Most cell developers have explored the development of improved separators for nickel-hydrogen cells. The benchmark separator material that has successfully been used is Zircar, a material that is relatively incompressible, is chemically stable, and holds electrolyte within relatively large pores compared to the positive and negative electrodes. Zircar is costly to make, and it is brittle and difficult to handle. Many other materials have been evaluated to replace it, but in no cases have any performed better than or even as well as Zircar. Alternative separator materials have included asbestos, asbestos composites, Zirfon (a zirconium oxide polysulfone blend), nonwoven nylon, and a range of polymer composites.
The essential reason why Zircar has performed better than all these alternatives is its incompressibility. As the nickel-hydrogen cell cycles, the nickel electrodes tend to swell until the opposing stack compression, which is set by the Belleville washer and the core elasticity, matches the compressibility of the stack components. Any separator that is much more compressible than the nickel electrode will introduce dimensional instability to the cell stack, which will reduce performance by its increased impedance as it compresses and by dry-out as the expanding nickel electrodes draw in electrolyte from the separator.
2.9.3 Catalysts for Cells
The platinum black catalyst used in the hydrogen electrode is costly. For this reason, significant effort has been devoted to finding cheaper catalysts that will function adequately for use in nickel-hydrogen cells. Standard hydrogen electrodes probably contain ten times the amount of platinum black required for adequate functionality. The most successful efforts for better catalysts have simply reduced the amount of platinum black in each negative electrode. This does increase the polarization of the hydrogen electrode, which does become an important design issue at high currents. In addition, a platinum electrode with lower platinum loading is not as robust for tolerating the decreases in active catalyst surface area that result when the platinum is exposed to an oxygen environment. This happens every time a nickel-precharged cell is fully discharged and put into storage. Because nickel precharge is standard in today's cell designs, most manufacturers have chosen to retain significant platinum loading margin. Catalysts other than platinum have not had the needed catalytic activity, or they had activity that was not stable over long-term operation, or they were chemically unstable.
2.9.4 Energy Density of Cells
The energy density of the nickel-hydrogen cell is strongly influenced by the energy density of the nickel electrode. One technique that has been explored for increasing cell energy density is to use thicker nickel electrodes. Most cells today use 35-mil-thick nickel electrodes placed back-to-back, resembling a single 70-mil-thick nickel electrode. The use of thicker nickel electrodes (45 and 60 mil) has been explored. Clearly in a back-to-back design there will be reduction in usable capacity and increase in impedance that results from the use of thicker nickel electrodes. However, the most significant drawback of thicker nickel electrodes has been the lack of sintering techniques capable of producing the thicker sinter with a uniform porosity and pore-size distribution through its thickness. Sinter having a poor uniformity is difficult to load properly with active material, and when loaded, does not give the expected utilization of its active material.
Increasing the active material loading levels in the nickel electrode has also been evaluated as a method to improve energy density. The standard loading level used in today's nickel-hydrogen cells is 1.6–1.8 g per cubic centimeter of void volume. While higher loading can increase the capacity of the nickel electrode, the stability of the capacity tends to become poorer. The nickel electrodes tend to expand more rapidly as they are cycled, their internal structure suffers more rapid breakdown, and the capacity drops with cycle life. Because most nickel-hydrogen cells are used in applications requiring very long life, decreased capacity stability throughout life has not been found a very productive trade.
Similarly, energy density can be improved by using higher-porosity substrates, whether nickel sinter, fiber, foam, or felt. With all these materials, it has been found that movement towards higher porosity tends to decrease electrode strength and life. The nickel sinter commonly used today for nickel electrodes ranges from 80% porosity (for most slurry sinters) to 84% porosity (for most dry sinters). When the porosity of either sintered or alternative substrates is made much greater than these levels, the utilization of the active material typically drops markedly and the strength of the substrate is not able to keep the electrode from undergoing excessive expansion as it is cycled.
In addition, the felt and fiber substrates tend to have a relatively high compressibility, which can also lead to cell stack dimensional changes with cycling. Although the fiber substrates have lower utilization and life than sintered substrates, they have found a place in commercial nickel-cadmium or nickel-metal-hydride battery cells using pasted nickel electrodes. The large void spaces between the fibers in these materials enable active material slurry to be physically pasted into the substrate structure. The performance of these electrodes is quite adequate for commercial cell applications.
2.9.5 Additives for Improving Cell Performance
Additives of various types have been introduced into nickel electrodes and electrolytes in attempts to improve cell performance. The principal ones are cobalt or cadmium in the nickel electrodes, and lithium in the electrolyte. Of these, cobalt is consistently used at either a 5% or 10% level in the nickel electrodes to provide significantly improved performance and life, as will be discussed in more detail in chapter 4. Cadmium additives improve the charge efficiency of the nickel electrodes by poisoning the oxygen evolution reaction that occurs in overcharge. However, cadmium additives can lead to dendritic short circuits and can poison the platinum catalyst in the hydrogen electrodes. Lithium additives in the electrolyte can increase capacity early in life by enabling the nickel electrodes to be more easily recharged to higher oxidation states. However, the capacity has been found to degrade much more rapidly throughout life with lithium additives, presumably from the greater volume changes and stresses that occur in the nickel electrodes.
2.9.6 Potassium Hydroxide Electrolyte Concentration
The concentration of potassium hydroxide electrolyte has been found to have a significant impact on the capacity and performance of nickel-hydrogen cells. Higher concentrations tend to give higher cell capacities, at the cost of significant reduction in cycle life. Today's nickel-hydrogen cells typically operate with electrolyte concentrations of either 26% or 31% by weight. When test cells of the same design activated with both these electrolyte concentrations have been cycled in the same test regime, the cells with 26% electrolyte have invariably given better cycle life. Cells tested with 35% to 40% electrolyte have given significantly lower cycle life. Cells containing 20% potassium hydroxide electrolyte have exhibited some problems supporting high discharge rates, possibly because of having insufficient ionic strength to support the required ionic current flow.
2.10 References
2.1J. G. Bentley and P. J. Denoncourt, Manufacturing Technology for Nickel/Hydrogen Cells, AFWAL-TR-87-4051, October 1987.
2.2J. D. Dunlop, G. M. Rao, and T. Y. Yi, NASA Handbook for Nickel-Hydrogen Batteries (NASA Ref., Pub. 1314, September 1993), pp. 1–54.
2.3B. A. Moore, H. M. Brown, and T. B. Miller, "International Space Station Nickel Hydrogen Battery Cell Testing at NAVSURFWARCENDIV Crane," Proc. of the 32nd International Energy Conversion and Engineering Conf., Vol. 1 (1997), pp. 174–179.
