Study says lithium-ion batteries should rule for the next decade at least

  • 25-Apr-2013 06:51 EDT
Oxis_nail.jpg

A sample lithium-sulfur pouch cell from Oxis Energy can withstand the classic nail-penetration test.

Despite Boeing’s well publicized problems with the lithium-ion batteries on its new 787 Dreamliner, do not expect any fundamentally different electrical storage technology to replace Li-ion anytime soon.

That is the main message of a new market analysis of next-generation battery technologies recently released by Lux Research, an independent research and consulting firm that focuses on emerging technologies.

Although several much-touted pretenders to the high-performance battery throne—including lithium-air (Li-air), lithium-sulfur (Li-S), solid-state, and zinc-air batteries—offer as much as 10 times better theoretical energy density as Li-ion, most of these technologies are at early stages of development and none is expected compete with Li-ion before 2024, said Cosmin Laslau, research analyst and lead author of the report. “The next generation of batteries is still not yet ready for prime time.

“It’s incredibly difficult to satisfy all the criteria that are demanded from batteries,” Laslau noted. An EV battery needs to have sufficient specific energy density, enough power density to provide IC engine-like acceleration, long cycle life (about 1000 recharging cycles), and bulletproof safety at an affordable cost. “It’s really hard to combine them all in one package,” he said, likening the engineering process to squeezing a balloon only to have it expand elsewhere.

“Despite the current safety concerns, Li-ion has done a fantastic job,” the electrochemist said. In fact, the current dominant battery technology will be hard to displace in transport applications because it offers “a moving target” for the competition, he continued. Improved silicon anode technology will be able to pack in “a lot more lithium ions, which should double Li-ion’s energy density, while high-voltage cathodes will improve overall cell performance.” In the meantime, higher-volume manufacturing should reduce costs.

“But even though Li-ion will get better over time,” Laslau warned, “R&D efforts will yield ever-diminishing returns,” providing an opening for alternatives.

Meanwhile, “the new battery technologies have been getting a lot of hype,” he said. “They typically emphasize one aspect of their performance and ignore the less-attractive aspects.” Nevertheless, private capital and governments continue investing in Li-ion’s competition, said Laslau, citing a recent $120 million U.S. Department of Energy program to support R&D on next-generation energy storage.

Lot of hot air?

One of the chief contenders in the battery race is the Li-air cell, which incorporates a lithium-containing anode and an "air" cathode that brings oxygen from the atmosphere into the cell to take part in the electrochemical reaction, which eliminates components and helps to make the unit lightweight. Li-air, stated the report, “is the most-hyped” because it offers the highest theoretical specific energy (13,000 W·h/kg), but nearly all state-of-the-art prototypes can withstand only a few recharging cycles, while longer-lived versions use impractical materials such as nanoporous gold air cathodes. A feasible Li-air battery will therefore probably only offer a maximum specific energy density of 3500 W·h/kg.

“We’re skeptical about Li-air,” Laslau said. “It doesn’t look like it will have the right mix of advantages in terms of cost, cycle life, and so forth.” Researchers, he said, will have to address the cycle-life problems and develop a fully safe form of metallic lithium anodes because lithium ignites spontaneously in air and reacts with water.

He added that fundamental materials innovation will be needed to come up with an air cathode that can ensure adequate power as well as cost-effective catalytic layers. Also, as the technology is scaled up for automotive use, maintaining sufficient airflow becomes problematic. “How quick can you get the necessary air in there?” he asked. Powerful compressors or blowers as well as filter systems will be needed to supply enough air, potentially adding $1000 or more to a typical automotive application.

Despite the difficulties, the potential payoffs are sufficiently attractive to induce companies such as IBM, Bosch, as well as the team of Toyota and BMW to invest considerable sums in Li-air research.

Long time coming

Another potential Li-ion alternative is the Li-S battery, which is appealing because sulfur is cheaper than standard Li-ion cathode materials and presumably would yield lower-priced products. (The Li-S anode uses metallic lithium.) In addition, Li-S features a high theoretical specific energy (exceeding 2,600 W·h/kg) although developers have struggled to achieve even part of that maximum value.

Plus, fulfillment of the promise of Li-S battery technology is taking a long time. “People have done a lot of work on it for many years without much result,” Laslau noted. “One of the pioneers in this area, Sion Power, has been developing Li-S for 20 years and still has yet to come to market with a product.”

Still, slow-arriving results have not stopped BASF from investing $50 million last year in Sion Power, he added. In the meantime, Oxis Energy, a U.K.-based start-up firm that is part financed by Sasol New Energy, is developing proprietary organic and polymer electrolytes for Li-S. Another contender, PolyPlus, which is supported by the U.S. DOE, has developed a coating for protecting lithium electrodes in Li-air and Li-S.

For widespread application, Li-S must overcome several problems, the Lux report stated. First, the safety of the metallic lithium electrode must be ensured—the same issue faced by Li-air cells. Further, intermediate reaction products called polysulfides tend to migrate away from the Li-S cathode via the electrolyte, resulting in a steady loss of the sulfur needed for operation, which degrades both capacity and cycle life. Protective cathode coatings designed to stem this loss must be flexible enough to cope with a near doubling in volume during recharging.

Dry cells

The other oft-cited alternative to Li-ion batteries is the solid-state type, wherein the electrolyte is a solid material, which eliminates liquid electrolytes and their associated drawbacks. Solid-state batteries combine separators and electrolyte into a single ceramic or polymer, which not only boosts energy density but also improves safety, particularly resistance to thermal runaway. They also feature outstanding cycle life, with some units exhibiting the ability to recharge more than 50,000 times.

Solid-state batteries have been commercialized in large-volume production, although only for small devices used to power microelectronics. Commercial ceramic solid-state batteries offer capacities that are several orders of magnitude below those of Li-ion types. The low performance of solid-state cells, Laslau said, arises because solid electrolytes tend to impede the flow of ions, hurting output.

And although manufacturing processes to make these products is well-established (using fabrication techniques such as chemical vapor deposition), they are complex and so have proven difficult to scale up. Various attempts to make solid-state batteries using low-cost roll-to-roll and other printing-type processes have yet to bear fruit.

Among the notable participants in the solid-state battery segment are Seeo, a start-up that is pursuing electrolytes made of block copolymers incorporating a component that improves ion conduction and another that provides structural integrity. Other contenders include Imprint Energy, Excellatron, and Sakti3. Since 2008, Toyota has worked with U.K.-based Ilika to develop solid-state electrolytes.

Laslau predicted that military applications will provide the entry point for next-generation batteries around 2020, mainly because defense uses are cost-insensitive. Consumer electronics should follow a little later with significant adoption of solid-state batteries. However, he concluded, most next-generation batteries will face cost and technology hurdles in finding use in transport.

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