Next-Generation Energy Storage: Batteries and Supercapacitors Look to Micromanufacturing Solutions to Improve Performance
By Paula Doe, SEMI Emerging Markets
Fast developing supercapacitor technology may make batteries last longer in consumer gear as well as hybrid buses. Thin-film battery technology, pressed ceramic solid state batteries, and silicon anodes also all make progress.
Solid state batteries are finally seeing real revenues in some specialty niches, but scaling the technology to more demanding mainstream applications will still require significant investment. Battery makers are starting to look at silicon electrodes to improve workhorse lithium-based battery technology. But the technology most likely to significantly impact energy storage markets in the near term is supercapacitors, where there are plenty of opportunities for better processes and materials.
“Supercapacitors are now at the tipping point,” says Peter Harrop, chairman of IDTechEx. “They may be a way to leapfrog battery technology.” He notes that it is already a real market, revenues are growing at 30 percent a year, and technology is improving rapidly. Typically the electrostatic charge storage in supercapacitors charges and discharges fast, but stores much less energy than electrochemical batteries. “It used to be true that energy density was lower, but with supercapacitors, whatever you knew last week has changed this week,” says Harrop. “Five companies now say they can make a supercapacitor with the energy density of a battery, up to 20-40Wh/kg typical of a lead-acid battery, and some are targeting 130Wh/kg, approaching the 160Wh/kg of an exceptional lithium-ion battery.” The supercapacitors remain more expensive, but since the electrostatic device life is much longer than electrochemical batteries, they often have lower total cost long term.
Main markets so far are to supplement batteries when extra power is needed, to make the batteries last longer, or for short-term backup when other power fails. Electric or hybrid buses use them for extra power to go up hills, and for the instant starts in “stop-start” technology to make the batteries last longer. Buses also use them to open the doors in an emergency if the power fails. Trucks use them for sure starting in cold weather. Wind turbines use them to set the blades to protect from damage in big winds if the power fails. Mobile phones will use them for more powerful flash for lighting at greater distance, or for supplementing demands for power for wi-fi, to save the battery or allow use of a smaller one or cheaper one.
These growing markets are spurring demand for better performance from the relatively simple devices of porous carbon coating on aluminum foil, with the two sides separated by a membrane. And there’s plenty of room to improve the processes and materials. Thinner foil of a few microns thickness could potentially reduce cost and increase overall energy density. There’s potential for better processes to handle the thinner foil, and to roughen its surface. Better ionic liquids may improve the electrolyte. And the carbon coating, now made largely from burnt coconut shells, will likely move towards nanostructured films and ultimately to graphene for higher surface area, but while controlling pore shape to avoid sharp edges. More environmentally friendly alternatives to acetonitrile could be possible. “Everything’s up for grabs,” says Harrop.
Dry Process Reduces Supercapacitor Costs; Consumer Applications Moving to Thin Film?
Leading supercapacitor maker Maxwell Technologies says government regulation to limit carbon emissions from vehicles has been a big driver of demand, particularly for the stop-start function for the hybrid bus market. “Demand is also starting to grow for megapixel cameras,” says Earl Wiggins, Maxwell VP of operations, noting that battery makers are looking at ways to improve battery performance in consumer electronics by combining the battery with a thin supercapacitor.
Cost has come down dramatically in recent years with higher volumes and improved equipment. While most suppliers make the carbon electrodes by wet coating, Maxwell uses a dry process, mixing the carbon powder with a binder, then pressing the mix into a <100µm film on foil with heat and pressure. “The dry process is cheaper, more controllable, and solvent free,” says Wiggins, noting the major cost of running the usual drying furnace for the wet process. Like the rest of the industry, the company has found that carbon from coconut shells happens to provide fine soot of ~10µm particles with many pores, which increase surface area to 10-20x the particles’ diameter. “You want a waste product as the carbon feedstock for the best cost structure,” he adds.
Consumer electronics applications will need higher power density, Wiggins notes, and key to that is very thin films. Maxwell is working on improving pressure and gap control of its deposition process, aiming to reduce the thickness of its dry film by half. But wafer-fab type sputtered thin films could also be a solution, although the films would need to be of softer carbon than the usual sputtered diamond type, and would need better pore structure. Best paths to increase energy density are to increase the voltage by doping the carbon or changing the material, and by improving the ionic liquid electrolytes. Getting voltage up from the current ~2.7V to around 4.0V would also allow most consumer electronics products to use only one supercapacitor cell instead of two, helping to bring down costs. Packaging for consumer electronics will be another key challenge, to protect the device for volume wave or reflow soldering, as the electrolyte tends to break down in heat above about 85°C. And there’s room for improvement from closer integration of the supercapacitor with the battery as well, instead of just putting two separate devices into a bigger package.
Low-Power Solid State Batteries Find Some Niches
On the battery side, products using disruptive thin-film or printed battery technology will come to a $6.5 billion market by 2016, largely from sensors, medical implants and powered smart cards, projects NanoMarkets. “These batteries have found some high value niches,” says Lawrence Gasman, principal analyst of NanoMarkets. “But they all have to compete with coin cell batteries that cost basically nothing. The sector is kind of in limbo.”
Source: NanoMarkets, “Thin-Film and Printed Battery Markets – 2012"
Real markets have developed for wireless sensor networks, thanks to significant economic benefit from savings on copper wiring and energy usage in buildings, and from improving asset management and process monitoring in factories. But unit volumes for thin-film batteries perpetually recharged by PV or other energy harvesting remain very small. Automakers, for example, are all using active (powered) tags to track assets, says Raghu Das, CEO of IDTechEx, but the total market remains a only a few million units, and many of those units remain powered by conventional coin cell batteries. There’s a compelling case for using rechargeable thin-film batteries with energy harvesting instead, since it’s considerable trouble to locate the asset and replace the battery when it dies. And for places where replacement is difficult the total cost of using a longer-lasting thin-film battery may well be lower. But the higher upfront costs still make it a hard sell. “It’s been difficult to educate users about replacement costs in 5 to 7 years — it’s not their problem,” notes Das. “The successful companies in the wireless sensor market are largely those making the systems. Thin-film batteries are seeing slow uptake.”
Powered smart cards — where coin cells are too thick to use — are another likely market, which NanoMarkets now expects will reach $960 million by 2016. Gasman notes that battery suppliers, like market leader Solicore, with its hybrid lithium polymer battery with coated electrolyte, are getting real revenues from this market. The powered cards so far are mostly used to generate one-time passwords for added security for credit and bank card transactions overseas, where the banks can save by preventing fraud. Bank of America, eBay and Mastercard in the U.S. all now issue these cards, but both banks and consumers remain slow to take them up. Das says sales are likely still only in the tens of thousands of units. But with the increase in Internet banking, and increasing security requirements from European regulators now requiring one-time passwords for online transactions, the smart cards start to look increasingly attractive. Das notes he now has to use a clunky calculator-like device to generate a passcode to sign in online to his bank in the U.K., making a card he could carry in his wallet look very attractive.
Higher powered cards, however, may have more applications. Infinite Power Solutions says powered cards are among its early design wins using its sputtered thin film batteries, with takeup particularly in secure identification, which they expect markets of at least 10,000 units. “Powered cards have been held back from wide industry adoption by cost and the limitations of the short-lived non-rechargeable battery used as a power supply,” says Tim Bradow, VP of marketing at Infinite Power Solutions (IPS). “They didn’t have enough power or lifetime capacity to run a fingerprint sensor, and now people want a display, a microprocessor with reasonable speed and RF connectivity too. But with all that functionality added, the card becomes too costly to throw out after only two years when its primary battery dies. Our higher power density, rechargeable thin-film batteries can solve these problems.”
Other applications seeing traction for the thin-film batteries, says Bradow, are powering health and fitness wireless sensors worn on the body to monitor heart rate and the like, where the thin, flexible battery fits more comfortably and invisibly on the body under the clothes than a bulky coin cell. There’s also some demand for use with PV energy harvesting for indoor wireless sensor networks in commercial buildings for energy savings, and for backup power for real-time clocks. Improvements in the production process and yields over time have brought volume prices down to the $4-6 dollar range for the “postage stamp”-sized batteries.
Scaling up Solid-State Battery Technology Remains a Challenge
“The real challenge in making solid-state thin-film batteries — and in potentially extending the technology for more demanding applications like mobile phones — is the capital equipment,” says Bradow. “It doesn’t exist.” IPS has modified LCD sputtering equipment to deposit on strips of metal foil, by laying out the strips on a pedestal in the chamber and using shadow masks to define the regions for deposition of the LiCoO2 electrode and LiPON electrolyte, then depositing the Li by thermal evaporation. This requires specialty material targets, where there’s been little motivation to date for suppliers to invest in improved technology for the relatively small market. Post deposition the strips are laminated together with metallized flexible circuit material for hermetic packaging, and then laser singulated.
Bradow says the company has now improved the deposition process to produce a thicker cathode to store more energy and maintain sufficient stability for charging cycles, so it believes the technology could be scaled up for more demanding applications such as mobile electronics. It figures these thick cathodes could be sputtered on both sides of thin metal foil in a roll-to-roll process, and then stacked together, for roughly 25 percent better energy density and much higher power capability than the prismatic lithium ion batteries now used in cell phones. “But scaling up the process and building the fab for volume production would cost millions,” says Bradow.
IPS suggests a new and more economical solution to scale up solid-state batteries may be a powder-formed ceramic battery, developed by its CTO Bernd Neudecker and director of Engineering Shawn Snyder. Neudecker was also one of the co-inventors of the solid-state thin film battery technology at Oak Ridge National Laboratory in mid 1990s. The new ceramic approach improves energy density, in part by eliminating the substrate and minimizing the packaging. Researchers have taken known materials with good performance in other battery types, done some additional post-processing of them into nanoscale forms, then compressed them into stratified layers using off-the-shelf ceramics pressing equipment. The current lab product reportedly has twice the energy density of Li-ion batteries, or 1000Wh/l in a 1mm thick coin-type battery. “We think this could scale to larger cells, but it would need custom equipment to prototype it to see what’s possible,” says Bradow. “We’re in the final months of optimizing the chemistry for a proprietary product at a customer’s request, but are also announcing a more generic commercial version, and hope that companies interested in working on this will contact us for more details.”
In other solid-state battery developments, auto battery startup Sakti3 appears to be moving towards starting thin-film production, as it’s been advertising to hire an assortment of vacuum process engineers for sourcing and maintaining thin-film coating equipment, and for process development. Meanwhile, however, local Florida newspaper reports say the other larger-scale US thin-film battery maker Planar Energy has run short of funding and largely stopped operations and is looking to license its solution-processed, high-energy density thin-film technology.
Li-Ion Battery Makers Look Seriously at Silicon to Significantly Boost Energy Density
One of the best options for major improvement in lithium-ion batteries may be silicon, which theoretically absorbs up to 10x more lithium than the carbon commonly used in battery anodes now. Unfortunately, the silicon absorbs so many lithium ions in discharge of the battery that it swells to several times its original size and then contracts again during charging, so it degrades too quickly to be of any use.
University researchers and startups are working on plenty of exotic solutions using silicon nanowires or nanotubes with various coatings to improve silicon’s stability, but 3M has a more evolutionary approach that’s actually moving into production and sampling product to battery makers. Chris Milker, business manager for battery materials in 3M electronics markets materials division, says 3M’s silicon alloy powder combines silicon with carbon in an anode composition that can potentially double the energy density compared to graphite, and control swelling for improved cycle performance. But battery makers will limit risk by adding low loadings of silicon at first to their fairly standard graphite and binder coatings, to gain about a 6 percent improvement in capacity in consumer electronics batteries, although they’ll also need to make some adjustments in their battery cell designs. Higher loading of silicon alloy can reportedly could bring a ~12 percent improvement in battery capacity.
The silicon alloy powder uses micron-sized particles for optimizing packing and energy density, and a low surface area to maximize thermal stability and minimize irreversible capacity and parasitic reactions. “We’ve found that nanomaterials don’t perform that well — they’re too reactive,” says Milker. The company is now working on higher capacity cathode materials to match the silicon to improve performance further, focusing on nickel-manganese-cobalt with a core-in-shell approach, combining high-energy-density core materials with a high-voltage, stable shell for improved cycling. An optimized pairing has been demonstrated to give a 44 percent energy improvement in automotive batteries and 27 percent for consumer products, says Milker.
November 6, 2012