There's no doubt that sources of renewable energy such as wind and solar are critical to a clean energy future, but just as important is a way to store the energy generated for use when the sun isn't shining and the wind isn't blowing. Researchers at Stanford University are reporting the development of a new high-power electrode that is so cheap, durable and efficient that it could enable the creation of batteries that are big enough and economical enough for large-scale storage of renewable energy on the grid.
The new electrode was made using crystalline nanoparticles of a copper compound called copper hexacyanoferrate. Most batteries fail because of accumulated damage to an electrode's crystal structure caused as ions - the electrically charged particles whose movements either charge or discharge a battery - move in and out of the electrode. In comparison, the atomic structure of the crystals found in the new electrode have an open framework that allows ions to easily move in and out without damaging the electrode.
Laboratory tests saw the electrode survive 40,000 charging/discharging cycles, after which it could still be charged to 80 percent of its original capacity. This is a huge improvement over an average lithium-ion battery, which can handle around 400 charge/discharge cycles before it deteriorates too much for practical use. And because the ions can move so freely, the charging and discharging cycles of the new electrode are extremely fast.
"At a rate of several cycles per day, this electrode would have a good 30 years of useful life on the electrical grid," said Colin Wessells, a graduate student in materials science and engineering who is the lead author of the study.
"That is a breakthrough performance - a battery that will keep running for tens of thousands of cycles and never fail," added Yi Cui, an associate professor of materials science and engineering, who is Wessell's adviser and a coauthor of the paper.
To maximize the benefit of the crystal's open structure, the researchers had to find ions of just the right size. Too large and they would get stuck and damage the crystal structure and too small might see them sticking to one side of the open spaces between atoms instead of passing through. Hydrated potassium turned out to be the perfect fit.
Additionally, because the particles of the electrode material are a mere 100 atoms across, the ions don't have far to travel to react with active sites in a particle to charge the electrode to its maximum capacity, or to get back out during discharge, which further enhances the speed of the electrode.
While energy density has been a major focus for researchers working to build a better lithium-ion based battery for use in portable electronic devices, such as mobile phones and laptop computers, such concerns aren't as important as cost when it comes to developing an energy storage solution for the gird. Since the battery doesn't need to be portable, it can be big, as long as it is cheap.
As some of the components used in lithium-ion batteries are expensive, scaling them up to a point where they could be used in the power grid isn't economical. Instead of the organic electrolyte used in lithium-ion batteries, the researchers opted for a water-based electrolyte, which Wessels says is "basically free." They also used readily available precursors such as iron, copper, carbon and nitrogen to make the battery electric materials, which are all much cheaper than lithium.
But it's not all good news for the new electrode. Its chemical properties make it only usable as a high voltage electrode and every battery needs two electrodes - a high voltage electrode for the cathode and a low voltage electrode for the anode - in order to create the voltage difference that produces electricity. This means the researchers will have to find another material to use for the anode before they can build an actual battery. However, the team says they are already investigating various materials and have some promising candidates.
Cui and Wessells admit that other electrode materials have been developed that show great promise in the laboratory but say these would be difficult to produce commercially - unlike their electrode material which they've already synthesized in gram quantities in the lab in a process they say should be easy to scale 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," said Wessells. "There are no technical challenges to producing this on a big-enough scale to actually build a real battery."