When Joel Schindall ‘63, SM ‘64, PhD ‘67, came back to MIT in 2002 after 35 years in industry, he knew, as all electrical engineers knew, that there was no way the wimpy charge-storage devices called capacitors would ever be able to power a car. But when a colleague handed him a canister the size of a soda can and told him it could store more than a million times as much charge per volt as common capacitors on the market, he rapidly became a convert. Ultracapacitors, as these next-generation capacitors are called, became a major focus of his research; he was determined to come up with a way to make them store even more energy.
Finding better technologies for energy storage will be crucial as we work to switch from fossil fuels to more environmentally friendly power sources, says Schindall, a professor of the practice in electrical engineering and computer science. “When we reach the point where renewables are the main forms of energy, we’re stuck if we don’t have a place to store it for when it’s needed,” he says. Unlike coal and gasoline, which can be burned whenever they’re needed, renewable energy sources are intermittent. Solar power, for example, can be captured only when the sun is shining. Wind power peaks at night in most locations, whereas energy demand typically peaks in the afternoon. If energy from these sources can’t be stored, it will be lost. It must be integrated into the electrical grid, where it can help even out discrepancies between supply and demand. And better energy storage is also needed to make clean electricity a more widely useful power source for transportation.
Today’s energy-storage devices won’t work for these purposes, because they are too expensive, too cumbersome, or too limited in capacity. Take batteries, the best-known storage technology. Sodium-sulfur batteries have the capacity to store wind power that can’t be used immediately, but adding them to a wind farm would quintuple the price of electricity per kilowatt-hour, according to one estimate. State-of-the-art rechargeable lithium-ion batteries, which are used in laptop computers and plug-in hybrid cars, are likewise too expensive to be incorporated into the grid in bulk. For that matter, since they wear out and need to be replaced every few years, they’re too expensive for widespread use in cars. Lead-acid batteries–the kind used in ordinary gas-fueled cars–are cheaper but too heavy, and too short-lived, to serve as a vehicle’s power source. As for ultracapacitors, they can improve efficiency when used in tandem with batteries, but they don’t yet have the capacity to solve the storage problem.
At MIT, materials scientists and engineers have been working on better, cheaper energy storage since long before the current vogue for all things green. The Institute has been behind some of the most significant developments of the past several years, including the batteries that will power Chrysler’s line of electric vehicles: made by A123 Systems, a Massachusetts company cofounded by Professor Yet-Ming Chiang, they’re 10 times as powerful as conventional lithium-ion batteries. People at MIT have also developed new designs for ultracapacitors as well as some more unusual technologies, including battery materials made by viruses. Here’s a look at five ongoing projects that could make green energy viable by improving the performance of energy-storage devices while significantly reducing their price.
Click here to see Angela Belcher discuss her nanostructure engineering work at a “Lunch with a Luminary” session at the MIT Museum in May 2009.
Click here to see Angela Belcher discuss her work to engineer battery materials from organisms in this 2006 lecture.
Click here to see Yet-Ming Chiang discuss the need to improve lithium ion batteries in this 2006 forum.
Researchers measure the performance of an energy-storage device according to two main criteria: energy density and power. Think of the device as a bucket. The energy density tells you how much energy the bucket can hold, and the power tells you how fast it can be filled and emptied. In general, batteries have higher energy density than ultracapacitors–they can store more total energy. And capacitors tend to be more powerful than batteries–they can take in and release energy more quickly. That’s because the two technologies operate in different ways.
In batteries, positive and negative electrodes are immersed in a conductive medium called an electrolyte; chemical reactions at the electrodes cause ions to circulate between them. The process frees electrons that flow through an external circuit to power, say, a car. Batteries can store a lot of energy for release through these reactions (which in some cases can be reversed to recharge the device). The lithium-ion battery in a Macbook Pro, for example, can power the laptop for about five hours. By contrast, an ultracapacitor the size of a D battery couldn’t power a flashlight for more than two minutes. But batteries wear out: the reactions change the structure of the electrodes, causing them to degrade over time. And they have historically not been good at releasing or absorbing big bursts of power in a hurry, since the speed at which the charged particles can move is limited by the rate of the reaction.
An ultracapacitor, meanwhile, is made up of two porous electrodes immersed in an electrolyte and divided by a separator. But these electrodes never react with the electrolyte to generate electrons. Instead, applying an electrical current separates positive and negative ions in the electrolyte, causing the positive ions to accumulate on the surface of the negative electrode while the negative ions accumulate on the positive electrode. This physical separation of charges stores the energy in the current. Reversing the polarity of the electrodes–so that the positive side becomes negative–causes the ions to rapidly switch sides, allowing the energy to be released again. Today’s ultracapacitors can store only about 5 percent as much energy as a conventional lithium-ion battery. But they can take up and release that energy in a matter of seconds, because the process doesn’t rely on a chemical reaction. And they can do it hundreds of thousands of times without degrading.
The Charge of the Ultracapacitors
There’s simply no contest for dominance between batteries and ul-tracapacitors. Batteries are everywhere: in your cell phone and laptop, in your car, in the giant storage banks that back up data centers. Ultracapacitors like the ones to which Schindall was introduced shortly after he returned to MIT are used only for limited applications, often supplementing batteries by providing or absorbing quick bursts of power. In digital cameras, for example, they provide fast zooming. Ultracapacitors power gantry cranes to lift shipping crates in Japanese ports, and then reabsorb the energy generated when this cargo is set down. They perform a similar function in city buses, capturing energy released during braking. But they lack the energy density to run a cell phone or a car on their own.
The more surface area in an ultracapacitor’s electrodes, however, the higher the energy density it can achieve, because charged particles have more places to lodge. The reason the first ultracapacitor Schindall encountered at MIT was able to store so much more energy than conventional capacitors was that its electrodes were fashioned from activated carbon, graphite that’s been treated so that it’s full of holes and can suck up charge the way a sponge takes in water.
On an airplane one day, catching up on some scientific literature, Schindall read a paper about carbon nanotubes and had a brainstorm. These atom-thick sheets of carbon, rolled up into straws only about five nanometers in diameter, have the same chemical makeup as the graphite used to make the ultracapacitors he knew about–but they pack much more surface area into a given volume. Back on the ground, he got to work on a new kind of ultracapacitor with the help of some colleagues: his graduate student and now postdoc Ricardo Signorelli; John Kassakian, at that time director of the Laboratory for Electromagnetic and Electronic Systems; Institute Professor Mildred Dresselhaus, who was one of the first people at MIT to work with carbon nanotubes; and Donald Sadoway, a professor of materials chemistry and renowned battery expert. The group grew a forest of nanotubes using a process called chemical-vapor deposition. They put an iron-coated sliver of silica inside a vacuum chamber, cranked up the heat to 650 °C, and pumped in hydrocarbon gases. The heat caused the iron to ball up into tiny droplets, each of which began snatching carbon from the air. The captured carbon atoms self-assembled into strawlike nanotubes that shot up rapidly, resulting in a dense nanotube carpet.
These nanotube carpets can indeed store a lot of charge. But, Schindall points out, “there’s one small but fatal problem.” The silica substrate is an electrical insulator, and to deliver energy effectively, an ultracapacitor must be based on a conductor. “It’s harder to grow them at the same density on a conductive substrate,” Schindall says of the nanotubes; they grow so sparsely that the result looks “like a shag carpet” instead of an oriental rug. The researchers have made progress toward solving the problem, Schindall says. In fact, they’ve built, and filed a patent application for, a prototype ultracapacitor made on a conductive substrate. This has the potential to increase the energy-storage density of ultracapacitors by a factor of at least five.
The Laboratory for Electromagnetic and Electronic Systems (now part of the Research Laboratory of Electronics), where Schindall is associate director, has long worked with manufacturers such as Toyota and Honda to improve automotive electronics, and Schindall believes that ultracapacitors could eventually play a starring role in vehicles. But like many researchers at MIT, he also hopes for broader applications in storing renewable energy. “The question is, what will be the sweet spot?” he says. “If we, or anyone else, can achieve better electrical storage capacity without the chemical reactions that drive batteries, it would be a game changer.”
Like Schindall, chemical-engineering professor Paula Hammond ‘84, PhD ‘93, and materials-science professor Yang Shao-Horn are working with carbon nanotubes. What they hope to achieve is a hybrid device with the power of a capacitor and the energy density of a battery.
Hammond is a leading practitioner of a technique called layer-by-layer assembly, a way of building high-performance devices by repeatedly dipping a thin substrate, often a flexible polymer sheet, into water solutions containing a mix of positively and negatively charged materials–in this case nanotubes. Using this technique, Hammond and Shao-Horn can make layers of nanotube carpets at room temperature in the open air, a process that’s more manufacturing-friendly than vapor deposition. Shao-Horn, who worked at Eveready before coming to MIT and is a leading researcher in electrochemical energy conversion and storage, is now collaborating with Hammond to make these nanotube carpets into energy-storage devices. Last winter, they demonstrated layered nanotube films that can store an order of magnitude more electrical charge per gram of material than the electrode materials used in ultracapacitors on the market today. Now they’re trying to incorporate lithium ions into the films. They hope the resulting device will hold as much energy as a lithium-ion battery but charge and discharge as rapidly as a carbon-nanotube capacitor.
Bringing Batteries to Life
Layer-by-layer assembly and chemical-vapor deposition are not the only ways to make nanostructured materials that could improve energy storage. Angela Belcher, a professor of materials science and engineering, has come up with another one: programming viruses to make electrodes for more powerful batteries.
Belcher first thought of using viruses for materials science in the late 1990s. Viral genomes, after all, are essentially blueprints for building highly ordered, reproducible molecular structures from the bottom up–the same thing materials scientists try to do in the lab. And viruses are made up of proteins, which bind their targets very selectively, so they are reliable building blocks. Now Belcher has successfully hijacked viral replication to build battery materials. In her years at MIT, she has collaborated on this work with many other Institute researchers, including Hammond, Chiang, professor of materials science and engineering Gerbrand Ceder, and associate professor of chemical engineering Michael Strano.
A very simple virus called M13 was Belcher’s starting material. M13 was easy to work with because it has only a few genes, all of which had been sequenced by the time her research began. Engineering it to do what she wanted was a matter of both chance and design. “You take its DNA, cut certain genes, and put in random DNA,” says Belcher. “If you do this a few billion times, you get viruses whose proteins differ by a few amino acids.” In effect, she has induced fast-forward artificial evolution. To make her battery electrode, which she described in the journal Science this past spring, she screened for viruses with two specific proteins. The first makes up M13’s outer coat; Belcher was looking for a version capable of accumulating iron phosphate. The second is a single protein, on the tip of each virus, that she selected for its ability to bind carbon nanotubes.
M13 was also a good virus for Belcher’s purposes because it is long, thin, and uniform in size: “We lucked out with the shape,” she says. Coating the engineered viruses with iron phosphate turns them into nanowires. When researchers add these M13 nanowires to a water solution of carbon nanotubes, they bind tightly to the nanotubes, which are also long and thin. The networks of viral nanowires and carbon nanotubes look like a jumble of pickup sticks but are orderly. They form what Belcher calls “highways” for electrons, speeding them along so that a battery can charge and discharge faster. Other researchers have tried to achieve this kind of nanoscale wiring to improve the conductivity of battery electrodes, but the connections between the wires were random. By using identical viruses as the basis for the wires, Belcher is able to control those connections precisely.
The batteries based on the virus electrodes are as good as lithium-ion batteries currently on the market. “Now we want to make them 1.5 to two times better,” says Belcher.
The Virtual Battery
One way to improve the performance of a lithium battery is to try combining the lithium with other elements, as Yet-Ming Chiang did to create A123’s batteries: he doped nanoparticles of the electrode material lithium iron phosphate with metal to create a material that charged and discharged 10 times as fast as lithium-ion batteries then on the market. But such phenomenal success with that approach is rare, says Gerbrand Ceder. “Cumulatively, tens of thousands of lithium compounds have been made,” he says. “Out of that, there have been three or four interesting ones.”
To improve the odds, Ceder tests out battery materials in a computer model before turning to the lab bench. “The value of virtual experimenting is that it tells you what is possible and what is not possible,” he says. “Some argue it is too idealized, but that is its strength as well.” A scientist using trial and error might keep trying something that is actually impossible, mistakenly assuming that faulty technique is the problem. Conversely, a shortcoming in an experimental setup might cause a potentially useful material to be rejected as hopeless. Computer modeling prevents such scenarios from holding up experimental progress.
This spring, after 14 years of systematically designing and applying methods for predicting properties such as the voltage of a battery material and the speed at which lithium ions will move through it, Ceder’s group demonstrated on the bench a battery material that charges and discharges very fast. The process used to produce it is unusual. Ceder overlaid lithium phosphate on a nanostructured layer of lithium iron phosphate to create an electrode whose surface conducts lithium ions rapidly. Although he’s still working on improving the material’s energy density, the batteries he made can discharge in 10 seconds, putting them on a par with ultracapacitors. “We set out for excessive charge rates as proof of concept,” he says. “Computation shows us that some of the materials people didn’t think would work will work. It allows us to see the potential of things.”
Big Energy in Small Packages
Chiang, meanwhile, is moving on from his success with A123’s lithium batteries. “The high-power lithium-ion battery is a reality and is maturing at a fast rate,” he says. “Now we understand power. Going forward, the bigger challenge is getting higher energy density.”
Higher energy density is important for tiny batteries like the ones that power pacemakers and other implantable medical devices; the more total energy they can hold, the longer they last and the less frequently they need to be replaced. But achieving it is challenging, because the smaller a battery gets, the less energy per unit volume it can store. “Extrapolation of the existing technology will never get you there,” says Chiang.
Seeking energy-dense batteries for microdevices, the U.S. Defense Advanced Research Projects Agency recently put out a call for a battery 10 cubic millimeters in volume with an energy density comparable to that of early lithium-ion batteries: 200 watt-hours per liter. “Getting a density comparable to that of today’s lithium-ion technology in a microbattery is extremely difficult,” says Chiang. Part of the problem is that as size decreases, the packaging and electrical connections account for an increasingly large portion of the battery. By redesigning the electrodes and packaging, however, he has made a five-cubic-millimeter battery that holds 650 watt-hours per liter–half the size and more than three times the energy density that the military sought.
Chiang is now applying those designs to bigger batteries. “We’re using microbatteries as a way to test high-energy-density design concepts that apply to larger-scale storage,” he says. One application for this research would be in plug-in or hybrid electric vehicles. The more energy their batteries can store, the farther they can travel without needing to recharge or dip into the gas tank.
That’s not the only environmental benefit that might flow from developing batteries with greater energy density. The smaller a battery that stores a given amount of energy, the cheaper it will be. The cheaper the technology, the more of the batteries utility companies will be able to buy and use to incorporate renewable energy into the electrical grid. “If you need less battery for a given application, you can drive down the cost, which means wider adoption of these technologies,” Chiang says. And that means we can at last start relying on green power sources to meet a significant fraction of our energy needs.
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