NBBs typically are constructed on a thick tubular electrolyte and operate at relatively high temperatures. They also have a number of disadvantages such as high capital cost and performance/safety issues that limit market penetration of the technology. The PNNL team has been developing a new generation of NBBs that utilizes a planar (flat) design to attempt to address some of these issues.

Tubular design has been the dominate geometry of NBBs since the invention. However, planar design has a number of advantages over the tubular design. First of all, planar design allows thinner cathode and larger active surface area for a given cell volume, which could achieve higher power and energy densities. Secondly, thinner electrolyte (< 1mm) could be employed in planar design compared that in tubular one (1~3 mm), which contributes to the higher power as well. The third one is that planar design simplifies interconnect between single cells and also impacts the overall cell packing efficiency.

—Lu et al. (2010a)

Because the battery’s main components include abundant materials such as alumina, sodium chloride and nickel, they are less expensive to manufacture than lithium-ion batteries, and could still offer the performance necessary to compete for consumers’ interest. In addition, compared to other battery systems, sodium-beta batteries are safer and can help incorporate renewable energy sources into the electrical system easier.

To take advantage of inexpensive materials, the PNNL researchers thought a redesign of the sodium-beta batteries might overcome the technical and cost issues. Cylindrical sodium beta batteries contain a thick, solid electrolyte and cathode that create considerable resistance when the sodium ion travels back and forth between the anode and the cathode while the battery is in use. This resistance reduces the amount of power produced. To lower the resistance, temperature must be elevated. But increasing operation temperature will shorten the battery’s lifespan.

The researchers found that a planar design allows for a thinner cathode and a larger surface area for a given cell volume. Because the ions can flow in a larger area and shorter pathway, they experience lower resistance. Next, the battery’s design incorporates a thin layer of solid electrolytes, which also lowers the resistance. Because of the decrease of resistance, the battery can afford to be operated at a lower temperature while maintaining a power output 30% more than a similar-sized battery with a cylindrical design.

Finally, the battery’s flat components can be stacked in a way that produces a much more compact battery, making it an attractive option for large-scale energy storage, such as on the electrical grid.

Issues remain to be resolved, however, including capacity fade.

The capacity for first cycle was 292 mAh, which was close to the theoretical for the given amount of cathode material (300 mAh for 2 g cathode). The cell capacity decreased significantly in first three cycles and was stabilized at 203 mAh after five cycles. Once the performance was stable, the cell was continuously cycled at around C/3 rate (i.e., 75 mA)...It can be seen that the capacity gradually decreased from 146 to 87 mAh after 20 cycles even with the low current rejuvenation cycles. The capacity loss during cycling is generally diagnosed to be related to growth of either nickel or sodium chloride grains within the cathode.

During charge, small grains are consumed and they tend to redistribute on the surface of large grains during the following discharge. As a result, the large grains become even larger while the small ones disappear with cycling. This process apparently causes more and more materials unavailable for further charge/discharge, i.e., loss of cell capacity with time.

—Lu et al. (2010a)

The researchers have found that the NaCl grain size in the cathode coarsened during cycling by more than 10x (~3 µm to around 30 µm after cycling). More work will be conducted to optimize the cathode chemistry so as to minimize the cell capacity loss.

Researchers at PNNL and EaglePicher LLC received funding from the Advanced Research Projects Agency - Energy (ARPA-E), earlier this year to conduct the research (earlier post), and will work together to improve the battery’s design, lifespan and power capacity. The research was funded by PNNL and by ARPA-E.

Resources

• Xiaochuan Lu, Greg Coffey, Kerry Meinhardt, Vincent Sprenkle, Zhenguo Yang, and John P. Lemmon (2010a) High Power Planar Sodium-Nickel Chloride Battery, ECS Trans. 28, 7 doi: 10.1149/1.3492326, in press

• Xiaochuan Lu, Guanguang Xia, John P. Lemmon and Zhenguo Yang (2010b) Advanced materials for sodium-beta alumina batteries: Status, challenges and perspectives. Journal of Power Sources, Vol 195, Issue 9, Pages 2431-2442 doi: 10.1016/j.jpowsour.2009.11.120