Issue Date: May 6, 2013
Batteries Get Flexible
Imagine how versatile a rechargeable battery would be if it were lightweight and thin and could be flexed, stretched, and rolled up. Or if it were long-lasting, powerful, and free from the safety concerns that have plagued segments of the battery industry lately.
Highly flexible batteries would free product designers from the constraints of rigid and predetermined forms. A smartphone might fold up in your pocket. Electronically controlled drug delivery patches could stretch around your arm. Products could take on the form best suited to the application, not the form required by a bulky power source. And powerful, lightweight batteries made from inherently safe materials could quickly advance development of electric vehicles and other transportation applications.
Organic polymers endowed with unusual combinations of properties may hold the key to these kinds of advanced batteries. They may also be safer than the battery materials that recently smoldered in passenger airplanes and cars. That potential payoff is driving researchers in academia and industry to explore and develop electrically conducting polymeric materials to serve as electrodes and electrolytes in rechargeable lithium batteries.
But for such batteries to be implemented on a large scale and in a variety of applications, scientists will need to come up with polymers with improved ionic conductivity relative to the few examples known today.
The potential and the problems motivated a number of researchers working on lithium-polymer batteries and related devices to gather early this spring at the American Physical Society national meeting in Baltimore. Those scientists, along with others, predict that new types of high-performance lithium-polymer batteries will soon join ranks with the small number already commercialized.
As a result of lithium’s low density, electrochemical properties, and other features, batteries based on this lightweight element can pack more energy into smaller and lighter packages than other common batteries. Those qualities have driven lithium-ion batteries to the number one spot for cell phones and nearly all portable consumer electronic devices. Li-ion batteries are also used in equipment demanding more muscle, such as power tools and some lines of hybrid-electric city buses and passenger cars.
In today’s ubiquitous Li-ion batteries, polymers serve as inert or passive materials in two limited roles. One is as a glue, the other is as a separator, explains Nitash P. Balsara, a battery specialist and chemical engineering professor at the University of California, Berkeley.
One polymer, typically polyvinylidene fluoride, functions as a glue that binds tiny conductive carbon particles to the particles of metal oxide and graphite that normally compose the battery’s cathode and anode, respectively. The other polymer, often polyethylene or polypropylene, serves as a barrier between the electrodes. It physically separates the battery’s positive and negative terminals to prevent the battery from short-circuiting but allows lithium ions to shuttle back and forth between the electrodes as the battery is discharged to provide power and then recharged.
However, by judiciously choosing polymers to replace some battery components, researchers and a handful of manufacturers are now designing batteries with atypical properties such as mechanical flexibility. Lakeland, Fla.-based Solicore, for example, makes a line of thin-film, flexible, lithium batteries that feature a polyimide-based electrolyte that functions like a solid.
In contrast, conventional Li-ion batteries depend on a liquid electrolyte as the medium through which lithium ions swim from one electrode to the other. Liquid electrolytes are generally composed of lithium salts dissolved in organic solvents such as ethylene carbonate and ethyl methyl carbonate. Typically, lithium ions diffuse more slowly through solid electrolytes than through liquid ones, but solid electrolytes offer key advantages.
As Solicore Chief Operating Officer Daniel J. Tillwick explains, the company’s batteries can be flexed and bent, and they tolerate high-temperature manufacturing processes such as laminating because the polymer matrix electrolyte at the core is solidlike. For those reasons, Solicore batteries are used to power so-called smart cards—electronic credit-card-sized antifraud devices that include a miniature display and keypad. Solicore batteries also power radio-frequency identification devices such as tags used for remote inventory control and tracking. And soon the batteries will be used to power a number of wearable medical products, including flexible patient monitors and electronically controlled drug delivery patches.
The quest for flexible power drives Jodie L. Lutkenhaus to explore conjugated polymers for use in electrodes. The Texas A&M University chemical engineering professor points out that the theoretical charge capacity of polyaniline and other conjugated polymers stacks up well against, and in some cases exceeds, that of conventional Li-ion battery cathode materials such as LiCoO2 and LiFePO4.
In addition, conjugated polymers are mechanically versatile, and their properties can be tuned readily via synthesis to enhance their utility in electrochemical energy storage. Lutkenhaus adds that these materials are compatible with printing technology and capable of being prepared inexpensively.
Yet despite this collection of promising properties, polyaniline suffers from irreversible oxidation reactions. The oxidation reduces the electrode’s cycle life and charge capacity. Another strike against polyaniline electrodes comes from the material’s slow mass transport, which results in sluggish batteries.
By applying a combination of experimental and computational techniques, Lutkenhaus and coworkers found that a highly oxidized and normally unstable form of polyaniline can be stabilized by polymerizing the monomer in the presence of a propanesulfonic polyacid. In a just-published study, the team reports that electrochemistry tests show that the new polyaniline-polyacid electrodes retain a charge capacity near the maximum theoretical value for more than 800 charge-discharge cycles (Phys. Chem. Chem. Phys., DOI: 10.1039/c3cp51620b).
To address the slow mass transport, the Texas group prepared highly porous versions of polyaniline-based cathodes. Specifically, they synthesized long, thin polyaniline nanofibers that form a porous mesh, and they hybridized the material with V2O5, a capacity-enhancing compound. According to Lutkenhaus, the group’s initial tests show that the flexible, porous nanofiber form of the electrode outperforms the nonporous version in terms of energy ratings, power ratings, and other key parameters.
The intense press coverage of Li-ion battery fires and recalls in recent years highlights another key aspect of current battery research—a focus on battery safety. In the most recent episodes in January, Li-ion batteries burned and smoldered aboard Boeing 787 Dreamliner airplanes operated by two airlines based in Japan (C&EN, Jan. 28, page 7). No one was hurt in the incidents.
Safety problems with Li-ion batteries “start with the flammability and electrochemical instability of the electrolyte,” Balsara says. Unlike other common battery electrolytes, which consist of aqueous solutions of acid or base, the ethylene carbonate and ethyl methyl carbonate electrolyte in Li-ion batteries are flammable organic solvents. The flammability coupled with the battery’s high current and the micrometer-scale spacing between its electrodes means that if a Li-ion battery fails it may lead to an explosion or fire. Battery safety experts estimate that failure rates are on the order of one in 10 million cells. That would be comforting—except the number of batteries far, far exceeds 10 million. Some 4 billion Li-ion cells were manufactured just in 2012 (C&EN, Feb. 11, page 33).
Unlike standard liquid organic electrolyte solutions, solid polymer electrolytes are nonflammable and nonvolatile, critical safety advantages for lithium-polymer batteries. According to Karim Zaghib, director of energy storage and conversion at Hydro-Québec’s research institute, in Montreal, one of the common solid electrolyte materials, poly(ethylene oxide) (PEO), doped with a lithium salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), is stable up to 350 °C.
As Zaghib explains, the inherent safety of the solid electrolyte makes it possible to use lightweight lithium metal, a highly reactive material, as an anode without compromising battery safety. In contrast, standard Li-ion batteries, which use reactive liquid electrolytes, rely on graphite doped with lithium ions, a heavier anode. The upshot is that lithium batteries that use a metal anode can store more energy per unit weight than ones with a conventional graphite anode.
Studies led by Hydro-Québec since the late 1970s eventually resulted in a lithium-metal solid-polymer electrolyte battery design that was commercialized for telecommunications backup power. Today’s version of that technology rests in the hands of Bathium Canada, in Montreal, which is part of Bolloré, a French industrial group. According Bathium General Manager Jean-Luc Monfort, Bathium makes lithium batteries that power Bluecar, a small electric car available through Autolib, an electric-car-sharing service launched in Paris by Bolloré. Bluecar’s battery includes a lithium anode, a solid PEO-LiTFSI electrolyte, and an FePO4 cathode.
Monfort is quick to list a half-dozen of the battery’s safety and performance attributes. But the results of an impromptu “road test” conducted in April 2011 by Parisian vandals also provide a convincing summary. For reasons that aren’t entirely clear, the hoodlums doused six Bluecars with gasoline and set them ablaze, according to newspaper accounts of the incident. Three of the cars were totally destroyed, yet none of the batteries caught fire and none exploded. Monfort adds that electronic systems that monitor and regulate battery performance and store the data show that the batteries essentially continued operating normally.
One challenge in using solid polymer electrolytes is that these materials must satisfy requirements that tend to be at odds with one another. On the one hand, the polymer must be conductive enough to shuttle ions between electrodes, a property that generally calls for a soft material. On the other hand, it must be sufficiently rugged—hard enough—to keep the electrodes apart and prevent the battery from shorting. But hard polymers tend to be nonconductive.
Balsara’s approach to resolving this problem is to form copolymers that contain a hard block such as polystyrene and a soft block such as PEO. Depending on experimental conditions, the salt-doped version of the copolymer can spontaneously segregate into domains of the soft component surrounded by the hard component and thereby satisfy the requisite conductivity and hardness requirements simultaneously.
The idea of combining mechanical stability with ionic conductivity via block copolymers was the impetus for launching Berkeley spin-off Seeo in 2007. Based in Hayward, Calif., the company was founded by Balsara, Hany Eitouni, and Mohit Singh, Seeo’s vice president of R&D and engineering. Balsara is no longer with the company.
According to Singh, Seeo aims to commercialize its energy-dense Li-metal solid-polymer electrolyte batteries for use in electric cars and buses, telecommunications backup, and electric grid applications. Earlier this year, Seeo conducted a large-scale demonstration of the use of its batteries to store and supply electricity generated by solar panel systems developed by SunEdison of Belmont, Calif.
In a twist on the block-copolymer approach to fine-tuning charge-transport properties, Balsara’s group recently showed that a single diblock copolymer composed of poly(3-hexylthiophene) and poly(ethylene oxide) (P3HT-PEO) can simultaneously conduct ions and electrons. In so doing, P3HT-PEO serves the roles of three materials in conventional Li-ion batteries: the polymer glue that binds the cathode particles, the carbon particles that facilitate electron conduction, and the liquid electrolyte that mediates Li-ion conduction (Angew. Chem. Int. Ed., DOI:10.1002/anie.201102953).
Because P3HT is a semiconductor, it may be possible to devise more advanced electrodes than is possible with ordinary conductors. Such electrodes might be able to protect themselves from over-discharging, Balsara suggests.
But efficient movement of ions within these polymer-based batteries is still a hurdle. For the batteries to be produced in large scale and used in a variety of products, “we need to increase the efficacy of Li-ion transport in solid-polymer electrolytes,” Balsara asserts. That improvement will likely follow as researchers develop a better understanding of the fundamental factors that govern ion transport through polymers.
From the tiny power needs of small irregularly shaped devices to the hefty demand for electricity by large buses and power grids, already ubiquitous lithium batteries may be poised to become even more commonplace. It may be hard to predict what the battery of the future will look like. But Li-metal polymer battery aficionados say it has already arrived.
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