Scientists at the DoE’s Oak Ridge National Laboratory (ORNL) have struck the battery mother lode: They’ve created an all-solid lithium-sulfur battery that is cheaper, less flammable, and has four times the energy density of conventional lithium-ion batteries. Beyond the obvious gains from a four-fold increase in energy density, these sulfur-based batteries could play a key role in electric vehicles and airplanes, where the flammability of lithium-ion batteries is a serious concern.
We have known about the potential of lithium-sulfur batteries for decades. Theoretically, sulfur-lithium battery chemistry can have an energy density (watt-hours per gram) that is 10 times that of conventional lithium-ion batteries. In small-scale testing, sulfur-lithium has been shown to have an energy density that is four times that of lithium-ion. Another advantage is that sulfur cathode is a lot cheaper than the lithium-based cathode normally used in a lithium-ion battery. Sulfur, by virtue of being a byproduct of petroleum processing, is almost free.
The problem with conventional lithium-sulfur battery chemistry, though, is that the liquid electrolyte — which is present in just about every commercial battery chemistry — essentially burns through the sulfur cathode, resulting in a battery that’s only good for a few charge-discharge cycles. The scientists at ORNL have solved this problem in two ways: They’ve developed a new, more-rugged sulfur-based material for the cathode — and they’ve also introduced a solid electrolyte, further reducing the wear and tear on the cathode. This new cathode material, fashioned out of lithium polysulfidophosphates, has a massive capacity of 1,200 milliamp-hours (mAh) per gram after 300 charge/discharge cycles; a lithium-ion cathode, on the other hand, has a capacity of just 140-170 mAh/g.
Combined with the solid electrolyte, and after factoring in lithium-sulfur’s lower voltage, the ORNL battery has four times the energy density of the lithium-ion battery in your smartphone. In other words, if your current smartphone battery gets you eight hours of use, a lithium-sulfur battery should get you 32 hours.
Beyond its increased power density, the use of a solid electrolyte instead of flammable liquid electrolyte should significantly reduce the chance of combustion. This is good news for Boeing, which recently had its fleet of 787 Dreamliners grounded due to some on-board battery fires, and for electric vehicle makers.
Moving forward, ORNL’s new battery is still in the demonstration stage, but the team has filed for a patent. It’s easy to get bogged down by the seemingly never-ending stream of go-nowhere battery advances, but this one, by virtue of its efficacy, frugality, and relatively simple chemistry, really could come to market in the next few years. Maybe the Department of Energy’s plea for a 5x improvement in chemical batteries within five years will actually come to pass.