September 22, 2016: Developing a battery consisting of molten sodium was always going to be a challenge. But sodium sulfur and sodium nickel batteries are still around.
Molten salt and zebras…
The spike in oil prices in 1974 sent the developed world into a tizzy. The sudden reality was that the days of cheap energy were over. And an oil price of $12 was the trigger for an entire generation of scientists to start a worldwide quest for alternative energy sources and improved batteries for energy storage.
One of those unsung scientists who were to make a major contribution to battery technology was a South African called Johan Coetzer. After receiving his PhD in 1968 he joined the X-ray crystallography division of the Council for Scientific and Industrial Research (CSIR) in Pretoria. The work involved the study of molecular structures of single crystal materials by means of X-ray investigations.
Coetzer decided to investigate the structure/electrochemical properties of silver iodide-amine iodide solid electrolytes that showed anomalously high Ag+ ion conductivity at room temperature.
This project heralded the start of a 20-year period when CSIR and South Africa would make major contributions to advancing international battery science and technology.
The discovery of the Na+ ion conducting solid electrolyte, ‘-Al2O3’, by Neill Weber and Joseph Kummer at Ford Motor Company in 1967 had opened the door to the possibility of developing a non-aqueous, high energy and high temperature (350°C) sodium-sulfur (Na/S) battery to replace lead-acid and nickel-cadmium batteries, particularly for electric vehicles and stationary energy storage.
By 1975, development of this system was well under way in the US and Europe.
At the same time, another high-temperature battery, based on a lithium aluminium-iron sulphide (LiAl/FeS2) electrochemical couple and a molten salt (LiCl, KCl) electrolyte, was under development at the Argonne National Laboratory in the US.
Would the ultimate answer to energy storage lie in high temperature sodium or lithium based batteries?
Because molten sodium and sulfur are highly reactive and combine violently if the ceramic ‘-Al2O3’ solid electrolyte in Na/S cells ruptures, and because molten sulfur is highly corrosive, Coetzer proposed the idea of using the pores within zeolitic structures to immobilize and contain the sulfur in a solid electrode matrix, thereby enhancing safety and minimizing corrosion.
This concept was first evaluated in high temperature LiAl/LiCl, KCl/Zeolite-sulfur cells using Argonne’s cell configuration.
This study prompted Coetzer to consider alternative electrodes for Argonne’s technology and his thinking moved away from FeS2 and zeolite-sulfur to iron chloride electrodes, the initial studies being conducted on chlorinated iron carbides, ‘FexCCly’ and, subsequently, simply iron dichloride, FeCl2.
The early battery work and the ideas being generated at CSIR did not go unnoticed. In 1976, Coetzer elicited the interest of industry and, in particular, Roger Wedlake of De Beers who, recognizing the future potential of electric vehicles, persuaded senior management at De Beers and Anglo American Corporation to invest in CSIR’s battery initiatives along with the South African Inventions Development Corporation (SAIDCOR) that was affiliated to CSIR.
In 1977, a formal agreement between CSIR, SAIDCOR, De Beers and Anglo American was signed. Significant progress was made and, within two years, several key patents had been filed internationally; potential partners abroad were identified to help drive CSIR’s battery technologies forward.
In 1979, visits were made to Argonne National Laboratory in the US and to the Atomic Energy Research Establishment at Harwell, UK where the Li/FeSx and Na/S technologies, respectively, were in advanced stages of development. Argonne declined the offer to collaborate, ostensibly because of the political sensitivities in South Africa at the time.
However, Ron Dell and Roger Bones, who had participated with British Rail in the development of NaS batteries, and sensing the technological and safety limitations of the NaS system, welcomed the South African delegation in anticipation of developing an alternative system.
A huge advantage of the early collaboration with AERE was that it gave CSIR scientists immediate access to sodium-sulfur technology that enabled the evaluation of CSIR’s zeolite-sulfur and iron-chloride electrodes in the sophisticated sodium-sulfur battery configuration.
The apartheid problems of South Africa and the international boycotts against the country made it difficult for CSIR/De Beers/Anglo American to operate openly with Harwell and Beta R&D. For this reason, the collaboration was undertaken without public exposure.
Code name ‘Zebra’
Dell and Bones code-named the project ‘Zebra’ for ‘Zeolite Battery Research in Africa’.
“Perhaps a fitting description of the Zebra can be the following: a unique and robust creature that is equally at home in the dry, scorching plains of Central Africa, where the temperatures can reach over 40°C in summer, as in the desolate, cold mountains of the Southern Cape, where sub-zero temperatures are common during the winter months,” says Coetzer. “And then it has a mighty good kick too!”
Because the zeolite-sulfur electrode was solid, a molten salt NaAlCl4 electrolyte (melting point of 155°C) was added to the electrode compartment to enable rapid Na+-ion diffusion between the zeolite-sulfur and sodium electrodes via the solid ‘-Al2O3’ electrolyte. The early results on Na/ Zeolite-sulfur cells were not promising, largely because the zeolite component added considerable extra weight to the system, thereby yielding lower energy per unit mass compared with the pure NaS battery.
Fortunately, the sodium-sulfur battery configuration was also suitable for evaluating the iron chloride electrodes being developed by Coetzer and his team for the Argonne-type high temperature lithium battery. In the sodium cell configuration, the reaction is simply:
2 Na + FeCl2 = 2 NaCl + Fe
It was Roy Galloway at CSIR who first realized and demonstrated that, unlike LiAl/FeSx, LiAl/FeCl2 and Na/S cells that are assembled in the charged state with highly reactive LiAl and Na negative electrodes (anodes), CSIR’s sodium-iron chloride cells could be assembled in the discharged state using a simple mixture of table salt (NaCl) and iron metal powders in the positive electrode (cathode), thereby circumventing the difficulty and hazards of handling LiAl alloy or metallic sodium.
Galloway also showed that the Na/ NiCl2 electrochemical couple offered a slightly higher cell voltage (2.58 V) and was more stable than the Na/ FeCl2 couple (2.35 V) to electrochemical cycling, making it the preferred system.
Despite the demise of CSIR’s Na/ Zeolite-sulfur technology, the name ‘Zebra’ persisted and is still in use today to describe sodium-metal chloride batteries, although the acronym was later changed to represent ‘Zero Emission Battery Research Activity’.
Significant progress was made by CSIR and Harwell in the early 1980s in demonstrating the feasibility of sodium/metal chloride battery technology.
In 1982, recognizing the need to scale up the production and expedite the evaluation of Zebra batteries, Anglo American acquired facilities in Derby, UK and established the company Beta R&D to manufacture ‘-Al2O3’ tubes, cells and batteries under the management of Jim Sudworth, a pioneer of Na/S technology from British Rail. By 1984, a multi kWh Zebra battery had been built and demonstrated in an electric test vehicle.
In 1986, CSIR divested from the Zebra project with most of the CSIR team joining Anglo American and moving to new facilities Zebra Power Systems Pty Ltd outside Pretoria. Johan Coetzer was appointed managing director and later a director of the holding company, Dynamic Power Systems, which formed part of the Anglo American Industrial Corporation (AMIC).
The first car tested at Zebra Power Systems was a converted Suzuki minibus powered by a locally assembled 40kWh battery consisting of all-iron Zebra electrodes.
The following year, Coetzer was awarded the gold medal of the South African Academy of Science and Arts for his pioneering contribution to battery technology.
Over the next 10 to 15 years in a joint effort between Zebra Power Systems, Harwell, Beta R&D and Daimler Benz, Germany, outstanding progress was made in optimizing Zebra battery technology. For example, to increase the power-to-energy ratio of the cell, in 1991, Coetzer and Tony Meintjes developed a cell with a convoluted beta alumina tube which both increased the surface area and reduced the thickness of the positive electrode. This became known as the monolith cell.
In the meantime energy storage had moved from the theoretically practical to the commercially possible. By 1998 AAB had taken the development of the Zebra battery to the point where it was ready to be put into production. Pilot lines in Derby and Berlin were producing batteries at the rate of up to 20 per month.
Johan Coetzer remained involved with the programme at Beta R&D in Derby until 2001 as a part-time consultant.
In 2002 he left the battery scene to concentrate on his farming activities, which had continued in parallel over the years.
The contributions made by Johan Coetzer and his colleagues to advancing battery technology have largely been forgotten by the mainstream energy storage industry but his legacy lives on. Major advances continue to be made in molten salt battery application and design, although their commercial value is still in doubt.
While entire fleets of electric vehicles are still on the road powered by Coetzer’s Zebra battery, others are taking the battery chemistry to yet more exciting places. Italian battery manufacturer FIAMM has continued the exploration into molten salt batteries with its FZSoNick (sodium nickel) battery range.