LBL Researchers Work on New Generation of Batteries

Spring, 1990

By Jeffery Kahn ([email protected])

The battery, the slow-maturing kid among a large family of power sources, is growing up. Showing great promise more than a century ago -- it once was a contender for powering the automobile but lost out to the internal combustion engine -- the battery yet may prove to be a late bloomer.

Battery-powered tools, camcorders, and computers are among a proliferation of emerging uses for the electric cell. But, as has been the case throughout their history, the role of batteries is limited by their performance. Batteries won't become major sources of energy until more powerful and enduring cells are developed.

Creating electric cells capable of becoming mainstream energy sources is the job of Lawrence Berkeley Laboratory's electrochemical research program. Some 50 researchers are involved in this wide-ranging multi-divisional effort. Recognizing the laboratory's leadership in the field, the federal government has assigned LBL major national research and management responsibilities. LBL, along with Sandia National Laboratory, oversees the Department of Energy's Electrochemical Energy Storage Program.

LBL's program, which is directed by Applied Science Division Director Elton Cairns, is responsible for discovering and identifying promising new electrochemical systems. Technologies under development include rechargeable batteries and fuel cells. Researchers at LBL and elsewhere, who are funded through this LBL managed program, perform initial experiments, evaluating and developing cells that are candidates for commercial development.

Technologies are invented and ripened in the LBL program and then passed to the companion federal program at Sandia, or directly to industry. Subsequently, mass production and manufacturing details are examined. Performance, cost, and the logic of the market ultimately dictate whether "the best and the brightest" cells to come out of LBL's labs ever see the light of day.

One way to win a place in the market is to invent batteries that make new applications not only feasible but desirable. Batteries to power electric vehicles, allowing them to accelerate and cruise like gasoline-fueled cars, and batteries to supplement power plants, augmenting electrical supplies during the hours of peak customer demand, are the principal goals of LBL's research program, says Cairns.

Batteries that assist power plants -- electrochemical load leveling systems is the technical nomenclature -- and electric vehicle batteries share several prospective attributes. Cairns says that in both cases, they would provide cheaper power as well as reduce air pollution.

Load leveling systems can stretch the use of existing generating facilities, and avert the need to build new power plants, saving billions of dollars, says Cairns. Nationally, power plants are throttled down at night to an average of only 50-55 percent of their peak daytime output. All that unused night-time capacity can be put to work charging the pods of batteries which make up a load leveling system. During peak daytime demand periods, these batteries can assist with the load. As for feasibility, a pilot 40 megawatt-hour load leveling system is operating in Chino, California, and other systems have been tested in Japan and Germany.

Air quality should be improved by load leveling systems in that they could supplant auxiliary power generators that utilities crank up to satisfy peak demands for power. These auxiliary plants customarily are dirtier burning and less fuel efficient than the primary generators.

Deteriorating air quality -- nearly 60 percent of the U.S. population lives in areas that do not meet federal clean air standards -- is a prime incentive behind the effort to develop practical electric vehicle batteries.

"The electric vehicle itself is non-polluting," says Cairns. "The batteries do not emit sulfur dioxide, nitrogen oxides, or carbon dioxide. Electric cars, which are quieter, can be charged at night, and the pollution associated with them occurs at the power plant. It is more cost effective and more efficient to build one central pollution control system for a power plant rather than multiple vehicular pollution control systems."

Though load leveling systems are being tested and though electric vehicles have been sold for years, they remain immature technologies. Moving from the realm of the experimental to the commonplace will require better batteries.

Batteries being developed by the Cairns group -- it includes staff scientist Frank McLarnon, research associate Tom Adler and graduate student Ken Miller -- and LBL Materials and Chemical Sciences Division scientist Phil Ross are representative of the substantial progress made by LBL researchers.

Cairns and company are developing a zinc/nickel-oxide battery, and Ross has devised a zinc/air battery. Cairns' cell is multi- purpose, whereas Ross' is engineered strictly for an electric vehicle. Though the cells have fundamental differences, both rely on zinc to serve as the reactant in the negative electrode.

Zinc is bestowed with propitious electrochemical characteristics. It is cheap, plentiful, has rapid electrochemical reaction rates, and gives good cell voltages. Compared, for instance, to the cadmium/nickel-oxide cell, zinc cells are made of less expensive materials, and do not contain cadmium, which is a toxic environmental hazard. Too, zinc can provide higher voltages; cadmium/nickel-oxide cells give 1.2 volts whereas zinc/nickel-oxide provides in excess of 1.7 volts.

Standard flashlight batteries have used zinc electrodes for years, but they have a major shortcoming. Up until now, commercial zinc batteries have not been rechargeable. The inability to recycle a zinc cell, to extend its life by recharging it hundreds of times, has been its Achilles heel. Though a rechargeable zinc cell recently has been introduced in Japan, its number of cycles still falls far short of the cadmium/nickel-oxide cell.

The basic elements of a cell consist of positive and negative electrodes which are immersed in an electrolyte solution, and which together, produce electricity through an electrochemical process. The process begins when reactant metals covering the negative electrode (anode) oxidize, giving up electrons. As free electrons are formed, atoms with a positive charge called ions also are liberated. The ions move away from the anode toward the positive electrode (cathode), flowing through the conductive electrolyte. Electrons, meanwhile, are left behind on the anode, causing it to become more negatively charged than the cathode.

If the cell is connected to an external circuit such as in a flashlight, the anode's excess electrons flow through the circuit and light the bulb, and then return to the cathode. This sustained flow of electrons causes the compound(s) that compose the cathode to be altered by reduction reactions.

After the reactant metal coating the anode has been stripped away and redistributed during this process, the cell is said to be discharged. In a rechargeable cell, it can be recharged by introducing an external charge which is run through the cell in the opposite direction. This reconstitutes the cathode, and pushes ions and electrons back into the anode, replating it with the reactant metal.

That's the chain of electrochemical events in theory. In the case of the zinc cell, the weak link has been the zinc electrode. Over the course of many recharges, the zinc electrode progressively changes shape. As it deforms, the power output of the cell declines until it ultimately becomes useless.

Current, commercial zinc batteries are powerful, but discardable after a single use. If the life of the zinc electrode can be extended, a formidable rechargeable battery could result. Zinc cells, which have been the equivalent of sprinters, also could become marathoners.

Both Cairns and Ross have succeeded in making long distance, rechargeable zinc cells, but through separate approaches.

Cairns began by dealing with one of the known causes of the breakdown of the zinc anode. Part of the zinc electrode's shape change is attributable to its high current density, or high current output per unit of area.

To reduce current densities, Cairns fabricated an anode made of porous zinc oxide. The microscopic pores enlarged the total surface area, creating a larger interface between the zinc and the electrolyte. Current was generated from a greater total surface area, allowing the cell to meet the load demands placed on it with a lower current output per unit of area. Thus, without any loss of power, the pores extended the cell's cycle life.

The pores mitigated but did not eliminate the deformation of the zinc anode during repeated recharging or cycling.

Cairns explained that as a cell is discharged, soluble zinc ions are created which move into the electrolyte. Charged particles, these ions are moved around by the electric field within the cell. And, because they are soluble, the zinc ions also are moved by the convection currents within the electrolyte. "When you go to recharge the zinc electrode," said Cairns, "the zinc ions aren't where they started out; they have moved somewhere else. They deposit then as zinc metal on the area of the zinc anode that is nearest. So the electrode starts changing shape. With repeated charging and discharging and the motion of the dissolved zinc ions, you end up with strange and unpredictable patterns of zinc deposits when you recharge the zinc electrode. The zinc can all move to the middle, or all move to the edges of the electrode."

The evolving kinetic sculpting of the anode saps life from the battery.

Cairns explained that the anode starts out as a uniform porous plate that faces a parallel nickel-oxide cathode. Ions take the shortest path between electrodes, so as long as the electrodes are parallel, ions are exchanged along their entire lengths. That broad, well-distributed flow of ions provides optimal electrochemical performance.

When soluble ions gradually cause the anode to disfigure -- swelling in the middle, for instance, and shrinking at the edges -- then only the center of the electrodes will participate in the electrochemical reaction. Such a cell retains only a fraction of its original storage capability.

To defeat the zinc redistribution problem, Cairns looked for a way to reduce the solubility of the zinc ion.

"If I could do that," says Cairns, "the battery would have less zinc in solution, and when the electrolyte moves, less zinc would be carried along with it. The result should be a longer lived zinc electrode."

One approach was to change the composition of the electrolyte.

Standard electrolyte compounds use hydroxide ions to accomplish their function of conducting the internal flow of ions. Hydroxide ions, however, substantially increase the solubility of zinc. Electrochemists have quantified this effect. If the hydroxide concentration is cut by a factor of two, the solubility of zinc is reduced by a factor of four.

Cairns experimented with ionic compounds that act as charge carriers, yet do not have a solubilizing effect on zinc. Fluoride and borate did the job, but carbonate compounds performed particularly well. Tests showed that replacing half the hydroxide with carbonate extended the cycle life of the zinc electrode with essentially no loss in performance.

Seeking further improvements, Cairns altered the composition of the zinc electrode by the addition of calcium oxide. This supplemental compound reacted with the soluble zinc ions in the electrolyte, binding them to the electrode. Again, the cycle life of the zinc electrode was extended by minimizing the amount of zinc in the electrolyte.

Unfortunately, the benefits obtained from the two approaches -- new electrolytic ions and the calcium oxide -- so far cannot be combined. The substitute ions react with calcium oxide, rendering the calcium in the electrode ineffective. Efforts are continuing to identify electrolytic ions that will not react with calcium oxide or zinc.

Independently, each of the two approaches has succeeded in tripling or quadrupling the cycle life of a zinc cell.

"The standard cell that we started with had a life of about 100 cycles. Now," says Cairns, "we can regularly get 300 to 400 cycles. In one case we got some 530 cycles."

Though research is continuing, the progress made by the Cairns group has resulted in plans for either Sandia National Laboratory or private industry to examine any remaining impediments to the commercial manufacture of the cell.

There is more than one way to build a zinc battery. Phil Ross takes a radically different approach.

Whereas Cairns sought to minimize the solubility of zinc, Ross maximized it. Ross' cell consequently includes features engineered to efficiently strip zinc from the anode during discharge and to replate the anode during charging. A new zinc electrode concept, a novel cathode, and an external pump to circulate the electrolyte are integral to the battery.

The anode, differing from the structure in Cairns' cell, is honeycombed or reticulated. This sponge-like configuration extends the capacity of the electrode by increasing the area involved in the reaction. Its open structure also allows the electrolyte to flow through it, circulated by a small, external electrical pump.

The pump stirs and mixes the electrolyte so that even concentrations of ions are maintained throughout the flow. This even flow allows the anode to be replated with zinc in a uniform manner during charging. It also enhances the battery's performance during discharge, allowing ions to flow evenly along the entire front between the electrodes.

After engineering his zinc distribution system, Ross was forced to contend with dendrites, a secondary phenomenon that can cripple a battery. As cells are recharged, dendrites, sharp tree- like growths of zinc, begin to appear. Taking root in the anode, they grow like miniature evergreens, reaching toward the cathode. Ultimately, dendrites can work their way through the separator (a thin barrier between the electrodes), touch the cathode, and short out the cell.

Ross engaged in a trade-off to ward off dendrites. He applied a chemically inert coating on the exterior surface of the sponge- like anode. This inert coating reduced the capacity of the cell, but inhibited the formation of dendrites in places where they would do the most damage. The loss of capacity was easily compensated by the ample maze of zinc surfaces within the interior of the anode.

The air cathode Ross developed for his battery plays a critical role in enhancing its performance.

Metal-air batteries such as Ross' have, in principle, tremendous advantages in terms of weight. An air cathode contains a catalyst, but its active material, oxygen in the air, is outside the battery. The weight savings afforded by an air electrode are critical when it comes to an electric vehicle battery. Both the degree of acceleration and range of travel provided by a battery are directly related to weight.

Theoretically, air electrodes should deliver superior performance. Practically, development of the technology has been impeded by difficulties in developing air electrode materials that work equally well during discharge and charging. Not only must the air electrode consume oxygen during discharge, but it must serve the reverse function of producing oxygen during charging.

Experimenting with possible air electrode materials, Ross examined various carbon composite materials. Inexpensive, light- weight, and easy to fabricate, carbon composites unfortunately also corrode in a battery environment. Ross, however, found a way to resolve this technical challenge. The carbon composite electrode he has developed has sufficient corrosion resistance to meet the lifetime requirements of an electric vehicle battery.

Ross said the process involved in fabricating the air electrode begins with what is essentially cardboard. Cardboard is soaked in a polymer and subjected to very high temperatures, transforming it into a fibrous, graphite-like structure. The resultant material becomes quite conductive.

Next, the top surface of the material is metalized by coating it with a nickel-oxide catalyst which is mixed with Teflon. Teflon is inert, porous, and non-wettable, and creates empty spaces in the metallic coating.

These spaces -- actually, they are channels a millionth of a meter in length -- create the passages through which the cathode interacts with outside air. Filled with electrolyte, the channels allow gas to diffuse and convert to soluble ions as it passes through the air electrode.

When it is fruitful, the experimental cell work performed at LBL ultimately ripens into a commercial engineering project. Ross' zinc-air cell is at this stage. Two commercial firms are continuing development. MATSI, Inc. of Emeryville is refining the zinc anode, and Electromedia, Inc. of New Jersey is supplying air electrodes to MATSI for prototype cells.

Cells like those developed by Cairns and Ross are indicative of the progress made by LBL's battery research program. Cairns takes the long view. He doesn't believe the gasoline engine is endangered at this point, but is confident that electric vehicles gradually will begin to appear on city streets.

Shortly after the oil crisis in the early 1970's, renewed demand surfaced for an electric vehicle. The automotive industry and the federal government suggested performance standards related to range, acceleration, and lifetime. Since then, researchers have striven to meet those then unattained standards.

"After years of improvements," says Cairns, "we are close to satisfying the requirements not only for electric vehicles but for utility load leveling systems. We are on the verge of having batteries that can, in practice, do the job."

Acknowledgement for the range of research covered in this story:

The electrochemical research effort is supported by the Assistant Secretary for Conservation and Renewable Energy, Office of Energy Storage and Distribution, Energy Storage Division of the U.S. Department of Energy.