Lithium-air battery technology promising but distant

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PolyPlus Battery Co. has developed a functional lithium electrode that uses a sandwich structure of protective layers and a compliant polymer end sealant to segregate the lithium metal from water.

If a low-cost electrochemical battery the size of a standard gas tank stored enough juice to send a fully electric family sedan a couple hundred miles on a single recharge, few automakers would bother developing hybrid or hydrogen-fuel-cell propulsion technologies. But despite steady performance gains during the last decade, state-of the-art lithium-ion batteries still have 10 times less energy density than a standard tank of gasoline—150 W·h/kg vs. approximately 1500 to 2000 W·h/kg. Energy density determines how much power a storage system can deliver and for how long.

During the past few years, research groups at Scotland’s St. Andrew’s University, Berkeley, CA-based PolyPlus Battery Co., Japan’s National Institute of Advanced Industrial Science & Technology, and elsewhere have attempted to leapfrog today’s technology by focusing on lithium-air cells, which some experts have long viewed as a kind of electrochemical Holy Grail. By storing as much energy as a tank of gasoline of the same size, lithium-air batteries might enable fully capable, affordable EVs.

Earlier this year, IBM partnered with Argonne and Oak Ridge National Laboratories in a research collaboration that aims to exploit nanoscience and technology, as well as supercomputer-enabled materials modeling, to forge a pathway to rechargeable lithium-air batteries.

“Lithium-ion batteries can’t hack it,” Winfried Wilcke, a Senior Manager for nanoscale science and technology at IBM’s Almaden, CA, research center, said in an interview with AEI. “We’ll be lucky to get a doubling of capacity in the next several years.” IBM sees the lack of effective green energy-storage technology for road vehicles to be a key problem for society that it can perhaps help solve and profit from.

A lithium-air cell—which uses a catalytic air cathode that supplies oxygen (similar to that in a zinc-air hearing-aid battery), an electrolyte, and a lightweight lithium-metal anode—embodies a theoretically “ideal” electrochemistry that maximizes specific energy potential. Among practical electrochemistries, “lithium-air is the clear winner, offering potential energy densities from 100 to 200 W·h/kg,” Wilcke said. “That means that an equivalent weight of lithium-air could provide about 1600 W·h/kg,” which places it close to the energy density of gasoline burned in an internal-combustion engine. And like a gasoline engine, a lithium-air battery freely uses the oxygen in the air around it for the oxidation process.

Although the lithium-air battery would be a “game-changer,” the physicist has no illusions about the formidable science and engineering challenges involved. “It’s a Mount Everest kind of problem, but one that deserves a Manhattan Project-like effort,” Wilcke said. If no showstoppers appear after three years, IBM and its collaborators would enter an engineering phase. “But even then, there’s little chance we’ll have it before 2020, and then it’ll take more time to get into regular cars,” he said.

IBM management believes that “we bring a different level of insight into the problem,” said Wilcke. Company researchers think, for example, that they may be able to improve the reaction rates at the active electrode and catalytic surfaces and enlarge the effective surface areas of those interfaces by using advanced techniques such as atomic-force microscopy and nanostructural design.

Noting that new petaflop-scale supercomputers have made it practical to do the immense calculations needed to model catalytic behavior, Wilcke said “we think that we can accelerate progress in the selection of catalysts by picking only the most promising ones for lab tests.” Clever engineering will also be needed to avoid clogging of the ionic pathways in and out of lithium electrodes as the metal reacts.

Considerably less optimistic about the prospects of lithium-air is Jeff Dahn, a solid-state physicist who studies battery materials at Dalhousie University in Halifax, Nova Scotia. “This is a pie-in-the-sky notion—one that is decades off at best,” he told AEI, warning that a raft of difficulties stand in the way of progress.

“Air and lithium do not get along well,” said Dahn. “Air usually contains moisture, which is why the lab samples in chem class are stored in oil.”

Wilcke, on the other hand, maintains that “lithium-air safety looks good, as there is no equivalent to thermal runaway and fire.”

Dahn says that with every charge/discharge cycle, the shape and dimensions of the lithium metal changes substantially—alterations that the battery must accommodate. He pointed out that current lithium batteries can lose 1% of their lithium content with each recharge cycle, so such systematic losses must be stemmed. In addition, “the air electrode is not fully reversible; it’s only about 73% efficient, so you lose energy with each cycle,” he said. Dahn acknowledged that reports at the recent meeting of the Electrochemical Society in Vienna indicate that there has been some progress in this area of late.

Meanwhile, proprietary work at PolyPlus Battery seems to have gone some way in addressing certain of these challenges. Researchers there have figured out a way to encapsulate the lithium-metal anode so that a selective barrier material lets lithium through, but not water, explained Steven J. Visco, Chief Technology Officer.

“Our patented cell design sandwiches a sheet of lithium metal between a lithium-nitride ceramic membrane that is transparent to lithium ions but blocks everything else,” he said. “Because the ceramic is not stable when in contact with lithium metal, we insert a special interlayer that is compatible with both” and that serves as an in situ solid electrolyte. The cell accommodates the shape and volume changes of the lithium metal with a compliant polymer sealing the edge of the thin sandwich. PolyPlus is currently developing a primary (one-time), throwaway lithium-air battery but is working on a future rechargeable version as well.

Like the Arthurian Holy Grail, lithium-air technology offers a shining vision for the future, one that has drawn the attention of customers including Toyota and Toshiba Battery Co. But like the quest to find that fabled and elusive icon, the successful pursuit of the lithium-air battery will require heroic efforts. Argonne scientists predict that “transformational progress”—fundamental breakthroughs—in materials design, chemistry, and engineering will be needed to achieve success.

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