Flexible and Structural Power Storage September 2012
Quantifiable performance improvements tend to be the most common measure of progress in portable-power technologies. However, characteristics beyond specifications like energy density, charge time, or operational temperature range can be equally important to the success or failure of new types of power storage. Advances in micromanufacturing and nanomaterials, as well as new approaches to old technologies, are creating opportunities for batteries and other power sources to assume new form factors and characteristics. Advances in flexibility, versatility, and structural integration could be as critical to commercialization as improvement on traditional parameters has been. Although flexible and structural batteries already exist, experiments with a wider range of materials and construction techniques may help lower costs and increase the commercial availability of these nontraditional power systems and perhaps create specific advantages for portable applications.
New Approaches to Flexible Power Storage
Even as researchers continue to explore the impressive energy-storage potential of various carbon allotropes, many researchers believe that silicon could eventually replace carbon-based materials as the preferred lithium-ion (Li-ion) battery anode material, assuming researchers are able to resolve structural challenges. Silicon can absorb up to ten times more lithium than graphitic carbon can, but it swells significantly as it absorbs charge-carrying ions, creating significant structural issues. Multiple techniques may be able to mitigate this swelling. Most techniques focus on containing or immobilizing expanding silicon in a rigid anode assembly. Researchers at Rice University think instead that they can manufacture a low-cost, bendable Li-ion battery using a flexible silicon anode. According to project lead Pulickel Ajayan, the researchers developed a scalable technique to etch arrays of nanowires out of discarded silicon and then cover them with copper conductors and a polymer electrolyte. Together, the nanowires, copper, and electrolyte form a flexible matrix strong enough to survive separation from the parent wafer (previous efforts found that bare silicon nanowires were too weak for removal). Dr. Ajayan hopes the process can lead to low-cost, Li-ion batteries with better performance characteristics and greater flexibility than those of any existing batteries.
LG Chem, a major Li-ion battery manufacturer, is also working on a flexible battery but is focusing on a novel form factor rather than on novel materials. By coating a copper wire with nickel-tin alloy and then wrapping it in a tight spiral helix, LG created a spring-like anode that bends and loops and can even tie in a knot. A separator, a lithium-cobalt oxide cathode, and an insulator surround the helical anode, and a liquid electrolyte fills the hollow-cable assembly. According to LG, the cable battery operates similarly to a traditional Li-ion cell but will be able to coil into tight spaces, provide external power to electronics, or even form woven and wearable textiles. The company thinks it could begin selling commercial models in 2017, and unlike many research groups, it already has the necessary manufacturing capabilities for initial mass production. Whether a cable battery is actually viable, however, will depend on a number of currently unknown factors, such as cost, durability, and target applications.
Although batteries attract a significant portion of interest in flexible power storage, work on ultracapacitors is also producing new flexible-power technologies. In the August 2012 Advanced Materials, researchers from Peking University described using pen ink to coat a thin carbon electrode and form a powerful, bendable ultracapacitor. Typically, ultracapacitors require some type of highly porous material such as activated carbon to form a large surface area for charge accumulation, but the Peking University team discovered that simple pen ink could serve as a high-capacity charge-accumulation surface. Remarkably, standard ink actually contains a multitude of carbon nanoparticles—so many that 1 gram of ink can provide an astounding 27 square meters for charge accumulation. The Peking researchers formed a cable-like capacitor—similar in size and structure to LG's battery—by coating the pen ink onto parallel carbon-fiber strands. According to the researchers, the ink provides up to ten times the charge storage of similar carbon-fiber-based ultracapacitors. The use of pen ink also points to the technology's potential low cost and highlights the fact that nanomaterials need not be cutting-edge or exotic technologies. Indeed, nanoparticles exist in a host of common substances. Many such substances may hold unnoticed or underutilized properties that could benefit future portable-power devices.
Everything is a Battery
One major benefit of flexible power storage is the ability to conform to a variety of shapes and occupy unused space inside or outside electronic devices. Another solution is to give existing components a secondary purpose as power storage or to build batteries onto existing surfaces. Since 2010, Emile Greenhalgh has led an Imperial College of London project in developing a structural-composite carbon-fiber-and-resin material that doubles as power storage. The material could potentially replace structural metals in vehicles, reducing weight by as much as 15% while providing extra energy storage—Volvo has taken an interest and is reportedly testing the material. Portable electronic devices is another category that could logically benefit, and many manufacturers would likely be interested in charge-holding casings if the technology ever reached the market.
Related developments include a similar structural battery from British conglomerate BAE Systems for vehicular, military, and consumer applications, as well as new research into spray-on batteries from Rice University that could potentially turn a variety of surfaces into battery substrates. Unlike Dr. Greenhalgh's material, which operates more like a capacitor than a battery, BAE's current focus is on nickel-based batteries, and the company hopes soon to adapt Li-ion cells for the same purpose. BAE thinks it can also use the carbon-fiber-based technology to create flexible, fabric-like batteries for folding equipment like tents and bags. Meanwhile, Rice University researchers continue to work on their "paintable" Li-ion batteries. The batteries still hold relatively little energy and require carefully controlled environmental conditions during the spray-painting manufacturing process but could potentially turn any surface into a battery. So far, the Rice team has successfully stored charge on tile, steel, glass, flexible polymers, and even a beer stein.
Implications of Development
New forms of malleable and structural power storage are not going to upend the portable-power industry. Existing prismatic and cylindrical batteries and capacitors still have performance advantages and benefit from existing mass-production manufacturing capabilities that keep them lower in cost. However, flexible, structural, and other unique forms of power storage do open up new applications and increase the versatility of integrated power sources. Smaller electronics—including phones, media players, RFID tags, and wireless sensors—could benefit from custom power-supply forms. These electronics would allow manufacturers to use any empty space within the devices for power storage, rather than designing internal electronics around rigid dimensional constraints.
If such power systems actually reach commercialization, their unique properties may open a range of unprecedented opportunities for wearable, pervasive, or covert electronics. However, because successful sales depend on the existence of a compelling application, the same unique properties—such as flexibility or structural utility—that permit new product designs may also slow the pace of commercialization. Most manufacturers are comfortable with existing rigid power sources and may be hesitant to repurpose their existing products around unproven or limited-availability power systems. Commercialization depends on product designers' coming up with a compelling application, but designers need reassurance of the technology's availability, manufacturability, and affordability before creating products that depend on it.
One way to bring new types of power storage out of the lab and into the marketplace is for someone to develop a compelling product that capitalizes on their unique characteristics. Another option may be for power-storage manufacturers to develop initial commercial products themselves. Either way, coordination between technology developers and product manufacturers will be essential if radically new types of power storage are to become more than a novelty.