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Sodium balling up on the surface of the battery’s electrolyte, left. New electrode almost completely coats electrolyte surface, right. Courtesy of Pacific Northwest National Laboratory.
A new material, developed by the US Department of Energy's Pacific Northwest National Laboratory (PNNL), may allow for a much more advanced and reliable power grid by enhancing the storage of renewable energy.
A major barrier preventing renewable energy sources such as wind and solar power becoming dominant power sources has always been their inefficiency during times of limited wind and sun exposure.
Adversely, this leads to a great deal of energy being wasted during prolonged periods of strong winds and direct sunlight. The new material which could change all of this comes in the form of an electrode made of a sodium-cesium alloy.
This electrode gives sodium-beta batteries the ability to operate at far lower temperatures than currently possible and allows them to experience a greater number of recharge cycles before becoming defunct. The electrode also helps to smooth their production and reduce the risk of accidental fire.
Current Issues
According to Dupont Energy Consulting, large cargo container sized sodium-beta batteries are being used across the USA, Europe and Japan to store over 300 megawatts of energy. However, these batteries have a very high operating temperature, in some cases reaching up to 350 degrees Celsius. This temperature is high enough to damage most materials and so a great deal of investment into expensive materials is required for current sodium-beta batteries and still their operating life is short.
Running at lower temperatures can make a big difference for sodium-beta batteries and may enable batteries to store more renewable energy and strengthen the power grid.
Xiaochuan Lu - Material Scientist at PNNL
The team at PNNL behind the research, which was funded by DOE's Office of Electricity Delivery and Energy Reliability, set out trying to change the operating temperature of the battery and bring it down to a suitable temperature which will allow the battery to last longer and work more efficiently.
Typically, sodium-beta batteries consist of a solid membrane (made from the ceramic beta alumina) surrounded by two electrodes. There are however two different types of these batteries and they are distinguished based on the materials used in their positive electrodes. There are those that use sulfur in the electrode known as sodium-sulfur batteries and those that use nickel chloride named ZEBRA batteries.
The work doesn't end when the operating temperature of the battery is lowered however. When working under 400 degrees Celsius, the negative sodium electrode will not sufficiently coat (or "wet") beta alumina, the ceramic electrolyte. Sodium in this case will curl up into a ball (see left image, top) resulting in a less efficient battery.
A New Electrode - A New Approach
PNNL researchers believed taking a different approach to solving the wettability issue was required and decided to modify the negative electrode. The team tried a variety of sodium alloys or sodium blended with other metals, rather than the conventionally used pure sodium. After experimentation it was found that liquid sodium-caesium alloy almost completely "wets" the beta alumina membrane (see righthand image, top).
Testing of this new battery showed that it can operate at well below 150 degrees Celsius. This came with a power capacity of 420 milliampere-hours per gram, which matches the power capacity of currently designed sodium-beta batteries.
Further to this lower operating temperature, the new alloy electrode is capable of retaining more of its original storage capacity. PNNL tested the new electrode with 100 charge and discharge cycles showing that it maintained 97% of its intial storage capacity compared to 70% after 60 cycles for traditional batteries.
The use of expensive, durable materials in the batteries is also negated thanks to the lower operating temperature, thus lowering the cost and streamlining the manufacturing process for these batteries. This reduction does offset the increased costs using cesium rather than sodium.
The next step for the PNNL research team is building larger electrodes which can be tested in larger batteries which would be closer to the kind of scale that would be required if this was to be used in reneweable energy storage.