Short-term transients, including those related to wind and solar sources, present challenges to the electrical grid. Stationary energy storage systems that can operate for many cycles, at high power, with high round-trip energy efficiency, and at low cost are required. Existing energy storage technologies cannot satisfy these requirements.

Here we show that crystalline nanoparticles of copper hexacyanoferrate, which has an ultra-low strain open framework structure, can be operated as a battery electrode in inexpensive aqueous electrolytes...the low-cost, scalable, room-temperature co-precipitation synthesis and excellent electrode performance of copper hexacyanoferrate make it attractive for large-scale energy storage systems.

—Wessells et al.

At a rate of several cycles per day, the new electrode could offer a good 30 years of useful life on the electrical grid, said Colin Wessells, lead author of the paper.

The electrode’s durability derives from the atomic structure of the crystalline copper hexacyanoferrate used to make it, the team said. The crystals have an open framework that allows ions to move in and out without damaging the electrode. Because the ions can move so freely, the electrode’s cycle of charging and discharging is extremely fast.

To maximize the benefit of the open structure, the researchers needed to use ions that fit; hydrated potassium ions proved to be a much better fit compared with other hydrated ions such as sodium and lithium.

It fits perfectly— really, really nicely. Potassium will just zoom in and zoom out, so you can have an extremely high-power battery.

—Yi Cui

The speed of the electrode is further enhanced because of the size of the nanoparticle electrode material that Wessells synthesized: 100 atoms across. As a result, the ions don’t have to travel very far into the electrode to react with active sites in a particle to charge the electrode to its maximum capacity, or to get back out during discharge.

Much recent research on batteries, including other work done by Cui’s research group, has focused on lithium-ion batteries, which have a high energy density; however, energy density really doesn’t matter for storage on the power grid. Cost is a greater concern.

We decided we needed to develop a new chemistry if we were going to make low-cost batteries and battery electrodes for the power grid.

—Colin Wessells

The researchers chose to use a water-based electrolyte. The battery materials are made from readily available precursors such as iron, copper, carbon and nitrogen—all of which are extremely inexpensive compared with lithium.

The researchers need to find another material to use for the anode before they can build an actual battery. Cui said they have already been investigating various materials for an anode and have some promising candidates.

Even though they haven’t yet constructed a full battery, the performance of the new electrode is such that Robert Huggins, an emeritus professor of materials science and engineering who worked on the project, said the electrode “leads to a promising electrochemical solution to the extremely important problem of the large number of sharp drop-offs in the output of wind and solar systems”.

Cui and Wessells noted that other electrode materials have been developed that show tremendous promise in laboratory testing but would be difficult to produce commercially. That should not be a problem with their electrode. Wessells has been able to readily synthesize the electrode material in gram quantities in the lab. He said the process should easily be scaled up to commercial levels of production.

We put chemicals in a flask and you get this electrode material. You can do that on any scale. There are no technical challenges to producing this on a big-enough scale to actually build a real battery.

—Colin Wessells

Funding for the research was provided by the US Department of Energy and the King Abdullah University of Science and Technology.

Resources

• Colin D. Wessells, Robert A. Huggins & Yi Cui (2011) Copper hexacyanoferrate battery electrodes with long cycle life and high power. Nature Communications 2, Article number: 550 doi: 10.1038/ncomms1563