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Physical Sciences Division
Research Highlights

May 2009

A Jolt of Energy Use

Making progress in energy storage applications for batteries

Advanced battery constructions have appeared since the 1980s, and are making great strides in research today according to Dr. Anil Virkar, an internationally recognized researcher in the field of solid oxide fuel cell technology and ceramic materials at the University of Utah. Dr. Virkar presented "The Sodium-Sulfur Storage Battery: History, Current Status, Challenges, and Opportunities for R&D and Commercialization," at the Frontiers in Materials Sciences Seminar Series held April 10, 2009, at the Pacific Northwest National Laboratory. The series features innovative and strategic speakers from industry, government, and academia to discuss novel ideas and advancements in the material sciences.

The sodium sulfur battery is one of the most promising candidates for energy storage applications developed since the 1980s, explained Virkar. It works based on the electrochemical reaction between sodium and sulfur and the formation of sodium polysulfide. This type of battery exhibits high power and energy density, and temperature stability. Moreover the low-cost of raw materials and suitability for high volume mass production makes this a potentially low cost investment.

History of the sodium sulfur battery: Great advances have been made during the last two decades, especially under the collaboration between Tokyo Electric Power Company and NGK Insulator, Ltd. The batteries have been applied in various ways such as emergency and uninterruptible power supplies. The markets covered include industrial, commercial owners and wind power generating systems. At this moment, the main obstacles for the large-scale applications of sodium sulfur battery are its high production cost which depends greatly on the scale of the battery production.

Virkar described the cell of the battery, which is usually made in a tall cylindrical configuration. The entire cell is enclosed by a steel or aluminum casing that is protected, usually by chromium and molybdenum, from corrosion on the inside. This outside container serves as the positive electrode, while the liquid sodium serves as the negative electrode. The container is sealed at the top with an airtight lid

An essential part of the cell is the presence of a beta-alumina solid electrolyte (BASE), membrane, which selectively conducts sodium ions or Na+. The cell becomes more economical with increasing size. In commercial applications, the cells are arranged in blocks for better conservation of heat and are encased in a vacuum-insulated box.

The sodium sulfur battery is environmentally benign, because the battery is completely sealed and allows no emissions during operation. Also,  99% of the battery materials can be recycled and only sodium must be handled as a hazardous material.

The ZEBRA battery: Over the years, another battery system using BASE and sodium anode but with a metal chloride cathode has evolved, the ZEBRA battery. A small amount of NaAlCl4 (sodium chloroaluminate) is added to the cathode. This battery was invented in 1985 by the Zeolite Battery Research Africa Project, or ZEBRA, Group led by Dr. Johan Coetzer at the Council for Scientific and Industrial Research in South Africa, hence the name ZEBRA battery. In 2009, the battery had been under development for more than 20 years.

The ZEBRA battery has an attractive specific energy and power: 90 Wh/kg and 150 W/kg. The ZEBRA's liquid electrolyte freezes at 157 °C, and the normal operating temperature range is 270-350 °C. The β-alumina solid electrolyte that has been developed for this system is very stable, both to sodium metal and the sodium chloroaluminate. The primary elements used in the manufacture of ZEBRA batteries, have much higher worldwide reserves and annual production than lithium batteries.

New discoveries:  The heart of both of battery systems is the solid electrolyte, sodium beta alumina electrolyte (BASE). The state-of-the-art NaS battery uses one endclosed BASE tubes made by isostatic pressing followed by transient liquid phase sintering. The composition and microstructure of BASE made by the conventional process must be carefully controlled to obtain high conductivity, high strength and low sensitivity to attack by moisture. In fact, despite various treatments, the BASE made by conventional process degrades in humid air.

Virkar and his colleagues developed a vapor phase process for the fabrication of high conductivity, high strength, water-resistant BASE containing ceramics. The vapor phase processed BASE is highly resistant to moisture attack. So resistant is this BASE to water attack that it can actually be used in some aqueous applications. The origin of the resistance to moisture attack of the vapor phase processed BASE lies in the fundamental thermodynamics and phase diagrams. BASE discs made by the vapor phase process are used in preliminary studies on flat plate batteries.

Dr. Virkar is a prolific researcher with more than 200 publications, 35 patents, and 200+ invited presentations. He is internationally recognized in the field of solid oxide fuel cell technology and ceramic materials.

 


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