This is the first in a series of three blog posts about promising laboratory experiments that might show up as products in the battery industry in the coming years.
There are plenty of spectacular experimental battery cathode materials that have excellent voltage, cycling, or cost specifications. There are none (yet) that boast all three. That is what is so promising about a new technology out of Stanford. If the battery can successfully be made into a mass manufactured product, it holds the promise to be high-powered, durable, and cheap.
This paper is out of the Stanford laboratories of Robert Huggins and Yi Cui; Cui is famous for being one of the most prolific battery scientists alive. His lab has been described to me by a battery scientist as “a factory of useful patents.” Huggins is also well respected in the materials science community as an innovative and rigorous researcher.
The problem that Huggins was trying to solve when he began his research was how to make an aqueous electrolyte that worked without any of the expensive and toxic solvents that are required to make traditional battery electrolytes work. He stumbled upon an odd candidate for a cathode that would work with a water-based electrolyte: Prussian Blue. The compound’s true name is hexacyanoferrate, but it’s better known to lab technicians as the dye you use to turn an iron-rich culture into a deep blue that’s easier to view under a microscope. The compound works so well as a dye because it has such a rigid crystalline structure that consistently bends light in the proper direction to make the color blue. For a battery cathode the color doesn’t matter, but a rigid porous structure does.
It’s the cost requirement that sets the Prussian Blue battery apart: most exotic cathodes cost a fortune to make. Battery scientists often wave away the business strategists who question the economic viability of a technology by saying, “Someone will come up with a way to make this material cheaper.” Huggins and Cui don’t have to make that argument. I can buy a metric ton of Prussian Blue on the Internet for the equivalent of $2.60 per kilogram (kg). That results in about $5 in cathode costs per kilowatt (kW) of power capacity, assuming that the end product can match the hypothetical specific power of 100 W/kg of the battery (the initial paper showed a maximum of 45 W/kg). Compare that to the cathode cost of a Nissan LEAF lithium manganese spinel battery, the cheapest large format battery in production today, which Pike Research estimates to be at least $58 per kilogram. Likewise, the material inputs for a lithium titanate battery, which better compares to the high power capabilities of the Prussian Blue battery, are probably upwards of $500 per kilogram.
The initial results of the Prussian Blue battery aren’t all so rosy. The data for the initial experiments shows that the battery has a specific energy rating of only 5 watt-hours (Wh) per kilogram (versus more than 100 Wh/kg for most currently mass produced lithium ion batteries). This is not a great long-term energy storage vessel. However, the authors expect to improve on that number as the ingredients are fine-tuned. Even incremental gains in that area will allow the chemistry to compete with ultracapacitors, which are extremely expensive.
The scientists behind the Prussian Blue battery have already formed a company to develop it commercially called Alveo Energy. That company has already scored a major grant: a $4 million Advanced Research Projects Agency-Energy (ARPA-E) project to develop the Prussian Blue battery. While it’s still the early days for the technology, this is certainly one to keep an eye on.