A Battery for Electronics That Lasts Twice as Long

An exotic form of silicon, called silicene, could enable a new generation of faster computers.

A new kind of lithium-ion battery could let portable electronics such as smartphones and smart watches last twice as long between charges.+

The battery was developed by SolidEnergy, a company spun out of MIT in 2012. The secret to boosting energy storage lies in swapping the conventional electrode material—graphite—for a thin sheet of lithium-metal foil, which can store more lithium ions.+

A scanning tunneling microscope image of silicene.

Battery makers have been trying to use lithium-metal electrodes in batteries for decades, with only limited success. SolidEnergy seems to have solved a couple of key problems, which have caused such batteries to either stop working after a few charges or burst into flames.+

Lithium metal tends to react with a battery cell’s electrolyte, forming compounds that trap lithium ions and keep them from generating electrical current, steadily decreasing the amount of energy the battery can store. The reaction also creates dendrites, metal spikes that can cause short circuits that generate enough heat to ignite the flammable electrolyte.+

The standard solution has been to replace the liquid electrolyte with a solid one that is less reactive and also acts as a physical barrier to prevent short circuits. But solid electrolytes don’t conduct ions as well as liquid ones, which hurts battery performance.+

SolidEnergy’s solution is to use both a solid electrolyte and a liquid one. The solid electrolyte is applied to the lithium-metal foil—the ions don’t have far to travel through this thin material, so it doesn’t matter that they’re moving relatively slowly.

Once ions move through the solid electrolyte, they reach the liquid electrolyte, which provides a path into the opposite electrode. Unlike conventional liquid electrolytes, this one isn’t flammable. And it has additives that prevent the lithium metal from reacting with it and that prevent dendrites from forming.+

As with most announcements of battery breakthroughs, this one should be viewed with some caution. The transition from making a few high-performing prototypes to making large volumes consistently can be very difficult, and over the past several years, several announcements of breakthroughs have come to nothing (see “The Sad Story of the Battery Breakthrough That Proved Too Good to be True”). Also, while SolidEnergy’s battery may prove a good fit for portable electronics, it can’t be recharged enough times to work well in electric cars. +

In recent months, at least two other startups have made similar claims about using lithium-metal electrodes to double energy storage capacity, though they have used different means of solving these problems (see “A Prototype Battery Could Double the Range of Electric Cars”).+

However, SolidEnergy’s technology stands out because, unlike some competing approaches, it doesn’t require new lithium-ion battery manufacturing equipment. Furthermore, while other prototype batteries can be recharged only a few times, the company says its prototype can be recharged 300 times while retaining 80 percent of its original storage capacity—closer to what you’d need in portable electronics. It also works at room temperature, whereas some other lithium-metal batteries operate at temperatures too hot to be practical.

Atom-Thick Silicon Makes Crazy-Fast Transistors

An exotic but tricky-to-use new form of silicon is being eyed as a way to build much faster computer chips.

And now, those who see its potential can claim a minor victory by making the first transistors out of the stuff.+

SolidEnergy’s prototype stores more energy than the battery in an iPhone 6, even though it is only half the size.

The material in question, called silicene, comes in layers of silicon just one-atom thick. This structure gives the material fantastic electrical properties, but it also means it’s devilishly tricky to produce and work with. Even testing its basic properties in the lab has proved difficult.+

Now Deji Akinwande, a computer engineer at the University of Texas at Austin, has figured out how to work with the stubborn material well enough to make the first silicene transistors. His first-of-their-kind devices are described today in the journal Nature Nanotechnology, and they live up to silicene’s promise by switching with extraordinary speed.+

Another atom-thick material, graphene, which is made from carbon, has gained attention in recent years for its own electrical properties. The appeal of silicene, says Akinwande, is that it’s made from the stuff Silicon Valley was built on. In theory, it should be easier for chipmakers to work with than some new material. “If we can get good properties out of it, it can be translated immediately by the semiconductor industry,” Akinwande says.+

In 2007, Lok Lew Yan Voon, a physicist at Citadel Military College of South Carolina, who published some of the first theoretical work on silicene, calculated that the material’s electrical properties should be similar to those of graphene. In theory, electrons can cruise through both graphene and silicene without encountering as many obstacles, enabling very speedy circuits.+

Unlike graphene, however, silicene doesn’t occur naturally. It has to be grown in the lab on a sheet of silver. Carbon is also more stable in its two-dimensional form, whereas silicon atoms are under strain in this form. To date, only a handful of groups have succeeded in making silicene in the lab. One group, in France, grew a nanoscale ribbon of the stuff in 2010. A few others succeeded in fabricating the material in 2012.+

Once silicene is made, its instability means it must be protected, and that makes it difficult to work with. Akinwande found a way around this problem by growing silicene on a thin film of silver capped with aluminum oxide. The whole thing is then peeled off, and then placed on a silicon dioxide wafer with the silver side up. Finally, the silver is patterned to make the electrical contacts for a transistor. Once the device is finished, it is stable under vacuum conditions.

That might not turn out to be commercially practical, but it’s an important first demonstration, says Lok. The performance of the transistors also lines up with theoretical predictions about silicene’s speedy highway for electrons. “They managed to do what many people have been trying to do,” he says.+

This demonstration is especially important because there has been skepticism about silicene’s potential, says Patrick Vogt, a researcher at the Technische Universität Berlin and one of a handful of researchers who have succeeded in growing the material. Vogt is currently working on new methods for making it.+

Fengnian Xia, an electrical engineer at Yale University who is developing electronics based on graphene, phosphorene, and other two-dimensional materials, might be counted as one of the skeptics. He says the transistor results reported by the Texas group look good and represent a big scientific advance. But as for silicene’s commercial potential, Xia says he’s not convinced that it would be easier to commercialize than graphene, or that it can do anything graphene can’t.+

Vogt says silicene probably won’t replace silicon entirely, but it might add new functionality to today’s chips. “This shows you can actually do something with silicene,” he says.