Argonne Settles On Two Most Promising Successors To Lithium Ion Battery

After studying more than 22,500 ingredients for batteries, Energy Department researchers have settled on two prototypes they believe can surpass lithium-ion at much lower cost, the project's director announced.
The Joint Center for Energy Storage Research (JCESR), an innovation hub based at Argonne National Laboratory, will build an organic flow prototype for grid storage and a perfected lithium-sulfur prototype for transportation. JCESR will test the prototypes to prove they can achieve the project's founding goal: five times the energy density of commercial batteries at one-fifth their 2011 cost.
The new batteries will have to cost less than $100/kWh when scaled for commercialization.
"Full cell testing of each of these concepts is now underway, and proof-of-principle prototypes will soon be evaluated," JCESR Director George Crabtree said in a "Director's Message" published last week on the project's website.
Grid Storage
The stakes are high for grid-level storage. With viable grid-level batteries, utilities could smooth out the variability of wind and solar power without having to rely on natural gas or nuclear plants. To meet those demands, a battery has to store large amounts of energy inexpensively. The cost of lithium-ion has fallen precipitously, making it competitive in some markets, but JCESR is aspiring to a fraction of those costs.
The redox-flow prototype replaces the solids in lithium-ion batteries with liquids infused with organic molecules that carry a charge as they flow through the battery. The organic molecules are inexpensive, recyclable, and harmless to the environment, Crabtree said.
"The big advantage of this so-called flow battery is that it's scalable, so if I make that tank of active ions 10 times larger I can store 10 times the energy density," Crabtree said in a recent appearance at Carnegie-Mellon university. "You can't do that with a lithium-ion battery."
The organic molecules will link together to form particles large enough to be blocked by a porous membrane, which will keep the charged particles in the liquid separate from the uncharged.
"If there's crossover you're basically shorting out the battery," he said. "So you always have to (keep them separate). And if the molecule is big enough, you can do it with a very cheap porous filter."
Lithium-sulfur batteries are known already for surpassing the energy density of lithium-ion and for being lighter, which makes them promising for transportation. Electric cars expend too much energy transporting the weight of their batteries, and airplanes require light batteries for flight.
But the lithium-sulfur batteries that exist today suffer from low cycle life—the number of times they can be recharged before they fail.
JCESR set out to increase cycle life while optimizing lithium-sulfur's virtues—improving energy density and making the battery lighter still.
“The way we approach this challenge is to think about redesigning these electrolytes essentially from the ground up.” said Kevin Zavadil, a member of the technical staff, in an Argonne video. “What we’re specifically after is creating electrolytes capable of supporting the types of rates, the power that we need, and the stored energy content that we’re trying to achieve.”
A successful Lithium-Sulfur battery can hold five times the theoretical energy density of lithium-ion, Crabtree said, or 1o times the actual energy density of the lithium-ion batteries in use today.
The Runners Up
In an appearance a year ago at the University of Chicago, Crabtree revealed during Q&A that JCESR had narrowed its search from thousands of possibilities to just four batteries. He was still speaking of four in his appearance in October at Carnegie Mellon University.
The JCESR team believes the two selected prototypes have the best chance of achieving "the target of $100/kWh at the pack level … by the end of JCESR’s five-year charter," he said last week, but the two runners-up remain promising.
The runners-up are an air-breathing aqueous sulfur battery for grid storage and a multivalent magnesium battery for transportation.
Crabtree called the aqueous sulfur battery "very clever."
"The idea of this battery is, it’s cheap. So it uses sulfur actually as the anode in this case, not the cathode. Sulfur is extremely cheap and abundant. And it uses water as the electrolyte. Nothing is cheaper than water. So the fact that this is so cheap means it might be able to compete with pumped hydro for the cost of storage, and indeed it might be so cheap that it could enable seasonal energy storage, so you store solar energy in the summer and you use it in the winter, for example."
The aqueous sulfur battery was a recent addition to JCESR's five-year mission, coming under scrutiny in the team's labs late in 2015.
The multivalent magnesium battery operates on the theory that if lithium holds one positive ion, an element that holds two or three will have two or three times the energy storage capacity. Scientists have experimented with batteries based on calcium and zinc, and JCESR scientists believe magnesium is most promising. But none have yet overcome the challenges.
"Only a few operate at sufficient voltage, capacity, and working ion mobility to meet JCESR’s performance targets," Crabtree said.
So JCESR plans to concentrate its efforts in the final year of its five-year mission.
"For the remaining year, JCESR’s research will be directed primarily at improving the prototypes for the redox flow battery for the grid and lithium-sulfur battery for transportation."
In response to a reader inquiry, Crabtree elaborated on his point about the scalability of flow batteries vs. lithium-ion: "While flow batteries are fully scalable (meaning 10x the volume equals 10 times the energy), lithium ion batteries are not. Making lithium ion batteries 10x larger in volume produces much less than 10 times the energy, because the mobility of lithium ions in lithium ion batteries is limited, and a lithium ion will take 10x the time to traverse the cathode or anode. But this greater time interval may exceed the time available for charging or discharging, so that the full volume of the cathode or anode is not accessed. This seem counter-intuitive at first sight, but is common knowledge in the battery community. This scalability is one of the most attractive features of flow batteries for the grid, where large amounts of energy must often be stored."

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