At any given location, the sun doesn't always shine and the wind doesn't always blow. While there's enough cumulative sunlight and wind to provide for our much of energy needs, their transient nature makes it difficult to rely on these renewable sources of energy as a significant contributor to the grid. Battery technology is still pretty crude and expensive, though some communities and even the military are experimenting with battery storage on the grid. But in order for grid-level storage to make an impact, the cost of batteries needs to come down.
Working with the US Department of Energy and the SLAC National Accelerator Laboratory, Dr. Yi Cui, a professor of materials science and engineering at Stanford University, thinks he has a solution: a lithium- polysulfide flow battery that uses inexpensive materials.
Image credit: Matt Beardsley/SLAC
A typical flow battery consists of two liquids separated by a membrane. The liquids interact in a chamber that contains the membrane. Through the interaction, chemical energy is converted to electricity. The membrane keeps the active ions separated, keeping one side of the battery positive and the other side negative. Unfortunately the liquids and the membrane are made from rare and expensive materials, which makes the cost of the batteries very high.
Image credit: Greg Stewart/SLAC
The new battery design consists of a single liquid made of abundant and cheap chemicals like sulfur and lithium, and doesn't require a membrane at all. In tests, the new battery withstood more than 2000 complete charge-discharge cycles (equivalent to more than 5 years of operation if cycled daily) with no degradation in quality. The new battery's energy density is about 100 Wh/kg, which is less than that of lithium-ion batteries but more than today's flow batteries. If you're wondering why we don't just stick with Li-ion batteries with their higher energy densities, it's because they're more expensive and have shorter lifespans than flow batteries. Although flow batteries are heavier for the same storage capacity, that's not a big factor when it's sitting in a box in your neighborhood. More important is the fact that flow batteries can last up to 20 years, compared to less than ten years for Li-ion.
I'm not a chemist, so I won't attempt to explain how the battery works. If you want the gory details, please click the [Read More...] link or check out the complete research paper in the journal of Energy and Environmental Science. (The journal requires a subscription. The [Read more...] link is free.)