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Copper Compound Nanoparticles Advance Battery Electrodes

Wind turbines at dusk (NREL)

(National Renewable Energy Laboratory)

Materials scientists at Stanford University in California have developed a new, longer-lasting battery electrode with crystalline nanoparticles of a copper compound. Their discovery, with implications for solar and wind energy storage on the power grid, is described this week in the journal Nature Communications (paid subscription required).

According to the authors — materials science professor Yi Cui and graduate student Colin Wessells — the new electrode survived 40,000 cycles of charging and discharging, and it could still be charged to more than 80 percent of its original capacity. The average lithium ion battery, by comparison, can handle about 400 charge/discharge cycles before it deteriorates beyond practical use.

The authors attribute the electrode’s durability to the atomic structure of its crystalline copper hexacyanoferrate material. The crystals have an open framework that allows the ions that charge or discharge a battery to easily go in and out without damaging the electrode. Most current batteries fail because of accumulated damage to an electrode’s crystal structure.

The electrode’s cycle of charging and discharging is also extremely fast. The speed that the ions move in a battery is a critical factor, since the power delivered by a battery is proportional to the speed of discharging the electrode.

To maximize the benefit of the open framework, the researchers needed to use the right size ions, to pass through the structure and not get lodged or lost. The right-sized ion turned out to be hydrated potassium that fit better than other hydrated ions such as sodium and lithium.

The material’s synthesized particles were delivered at nanoscale, only 100 atoms across. The extremely small particle size also helped boost the speed of ions in the electrode, since they don’t have to travel very far to react with active sites in a particle to charge the electrode to its maximum capacity, or to get back out during discharge.

Much of the research on batteries focuses on lithium-ion batteries, because of their use in portable electronics and electric cars. Lithium-ion batteries need to be high in density to fit into the small spaces in these devices or vehicles. Energy density and battery size are much less of a concern, however, when dealing with electricity storage on the power grid. Size matters less for this purpose than cost.

Storage on the power grid is a concern for intermittent renewable sources such as solar or wind power, where excess power generated during windy or sunny days can be stored for calm or cloudy periods. And lower costs are always a factor in attracting users.

The researchers chose to use a water-based electrolyte, much lower in cost than the organic electrolytes found in lithium ion batteries.  The team also made the battery electric materials from readily available iron, copper, carbon, and nitrogen, all of which are less inexpensive and more plentiful than lithium.

A key constraint, however, is that the high voltage produced by the electrode would work for only for the battery’s cathode. The battery also needs a low voltage anode, to create the voltage difference that produces electricity.  The researchers need to find another material for the anode before they can build an actual battery; Cui says the team has identified some promising candidates.

Wessells says he synthesized the electrode material in gram quantities in the lab, but should be able to scale up quantities to commercial levels for production. “We put chemicals in a flask and you get this electrode material,” says Wessells. “There are no technical challenges to producing this on a big-enough scale to actually build a real battery.”

Read more.

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