Solid-state batteries

Article from The Economist

The power of the press

A new process will make solid-state rechargeable batteries that should greatly outperform existing ones

ELECTRONICS made a huge leap forward when the delicate and temperamental vacuum tube was replaced by the robust, reliable transistor. That change led to the now ubiquitous silicon chip. As a consequence, electronic devices have become vastly more powerful and, at the same time, have shrunk in both size and cost. Some people believe that a similar change would happen if rechargeable batteries could likewise be made into thin, solid devices. Researchers are working on various ways to do this and now one of these efforts is coming to fruition. That promises smaller, cheaper, more powerful batteries for consumer electronics and, eventually, for electric cars.

The new development is the work of Planar Energy of Orlando, Florida—a company spun out of America’s National Renewable Energy Laboratory in 2007. The firm is about to complete a pilot production line that will print lithium-ion batteries onto sheets of metal or plastic, like printing a newspaper.

“Thin-film” printing methods of this sort are already used to make solar cells and display screens, but no one has yet been able to pull off the trick on anything like an industrial scale with batteries. Paradoxically, though thin-film printing needs liquid precursor chemicals to act as the “ink” which is sprayed onto the metal or plastic substrate, it works well only when those precursors react to form a solid final product. Most batteries include liquid or semi-liquid electrolytes—so printing them has been thought to be out of the question. Planar, however, has discovered a solid electrolyte it believes is suitable for thin-film printing.

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Charge!

A battery’s electrolyte is the material through which ions (in this case lithium ions) pass from one electrode (the cathode) to another (the anode) inside a battery cell. Electrons prised from those ions make a similar journey, but do so in an external circuit, usually through a wire. That means the energy they carry can be employed for some useful purpose. Push electrons through the wire in the opposite direction and the ions will return to their original home, recharging the battery.

Many sorts of ion can be used in batteries, but lithium has become popular in recent years because it is light. Rechargeable batteries based on lithium chemistry store more energy, weight for weight, than any other sort. In the case of a lithium-ion battery the electrolyte is usually in the form of a gel. It is possible to make such a battery with a solid electrolyte, but until now that has been done by a process called vacuum deposition. This uses complex and expensive machinery to build up atomic layers of material on a substrate. Batteries made this way tend to be small and costly, suited for specialist devices like sensors. To be any use in consumer electronics, and especially electric cars, solid-state batteries would need to be bigger and capable of being cranked out in greater numbers.

What Planar has come up with is a ceramic electrolyte which it says works as well as a gel. It can print this electrolyte (along with the battery’s electrodes) onto a sheet of metal or plastic that passes from one reel to another in a process similar to that used in a traditional printing press. Nor does it have to be done in a vacuum. Once printed, the reels can be cut up into individual cells and wired together to make battery packs.

For the cathode, Planar uses lithium manganese dioxide; for the anode, doped tin oxides and lithium alloys. For the crucial solid electrolyte it turns to materials called thio-LISICONs—shorthand for lithium superionic conductors. Exactly which thio-LISICON is best needs further investigation, but the principle certainly works.

The crucial trick is that although both the electrodes and the electrolyte appear solid, they are actually finely structured at the nanometre scale (a nanometre is a billionth of a metre). This is to allow the lithium ions free passage. Getting the materials in question to settle down in an appropriate arrangement has taken blood, sweat and tears but Planar’s scientists think they have cracked the problem.

The “inks” they use to print their battery cells are waterborne precursor chemicals that, when mixed and sprayed onto the substrate in appropriate (and proprietary) concentrations and conditions, react to form suitably nanostructured films. Once that has happened, the water simply evaporates and the desired electronic sandwich is left behind in a thousandth of the time that it would take to make it using vacuum deposition.

Printing batteries this way also offers the possibility of incorporating other thin-film devices, such as ultracapacitors, directly into the cells. An ultracapacitor is an electricity-storage device that can be charged and discharged rapidly. In electric cars, ultracapacitors can capture energy from regenerative braking and use it for fast acceleration.

Planar says its cells will be more reliable than conventional lithium-ion cells, will be able to store two to three times more energy in the same weight and will last for tens of thousands of recharging cycles. They could also be made for a third of the cost.

Material benefits

These are bold claims, but as Scott Faris, Planar’s boss, points out, a lot of the benefits come from savings in materials. About half of a typical lithium-ion battery is made of stuff that plays no direct part in the battery’s chemistry. This includes a stout casing and what is known as a permeable polymer separator, which stops the electrodes in the cell touching each other and causing a short circuit. Thin-film technology eliminates the need for so much casing, and Planar’s solid-state electrolyte doubles as a separator. The result, says Mr Faris, is that 97% of the materials used to construct a Planar cell are actively engaged in storing electricity.

If the pilot production line is successful, the company hopes to begin operations in earnest in about 18 months. To start with it will make small cells for portable devices. It will then scale up to larger cells and, in around six years’ time, it hopes to be producing batteries powerful enough for carmakers. If, by then, anyone needs a replacement battery for a Chevy Volt, such technology may offer a solid-state alternative that could increase that car’s all-electric range from about 65km (40 miles) to some 200km. Lack of range is reckoned one of the main obstacles to the widespread use of electric cars. If solid-state batteries could overcome such range anxiety that would, indeed, be a revolution on a par with the silicon chip.

A Battery-Ultracapacitor Hybrid

A device for power tools may also help regenerative braking.
BY PRACHI PATEL

Battercapacitor: A new energy storage device blends the chemistry of an ultracapacitor with that of a lithium-ion battery.
Credit: Ioxus

By combining the chemistries of ultracapacitors and lithium-ion batteries, a company calledIoxus has created a hybrid energy-storage device that could recharge power tools in minutes and might never need to be replaced. The company says future incarnations could perhaps be used to capture energy from braking vehicles.

Ultracapacitors capture and release energy in seconds and can do so millions of times, but they store only about 5 percent as much energy as lithium-ion batteries. The hybrid can store more than twice the energy by volume of standard ultracapacitors. That's still much less than a lithium-ion battery, but the hybrid can be recharged quickly over 20,000 times as against a few hundred cycles for a typical battery.

A power tool using the lithium-ion ultracapacitor would run for only a 15th as long as it would on a battery but would recharge in just a minute. "Our product is for weekend warriors who don't use the power tool much every day" but want very fast charging, says Mark McGough, CEO of Ioxus. The company, which is based in Oneont, New York, already makes conventional ultracapacitors for hybrid-electric buses and for engine start-stop systems that are used to increase fuel economy in cars.

The hybrid energy-storage device consists of an etched aluminum film coated on one side with carbon slurry, which is similar to the electrode found in an ultracapacitor. The other electrode, on the other side of the film, is coated not with carbon but with a lithium-ion material, providing more energy-storage capacity. The film is wound into a cylinder to make the finished device.

Ultracapacitors are being tested in some city buses as a way to capture the energy generated by braking and quickly release it for reacceleration, an approach that promises to improve fuel efficiency. If the hybrid lithium-ion ultracapacitor can be scaled up, it could improve fuel efficiency further by storing more energy. But its cycle life will need to be improved, as vehicle breaking systems need to be recharged hundreds of thousands of times.

The concept of hybrid lithium-ion ultracapacitors has been around for 20 years, but there is more demand for other types of energy-storage devices, says Theodore Bohn, an engineer at Argonne National Laboratory's Advanced Powertrain Research Facility.

Bohn says the hybrid technology could nonetheless be ideal for small, lightweight applications that would benefit from having some of the power advantage of ultracapacitors and some the energy advantage of a battery. "The hybrid is good for the power pulse and OK for the energy," he says.

Only one other company—JSR Micro, in Tokyo—makes hybrid devices of this type, having brought them to market in 2009. The company says its device has three times the energy density of a conventional ultracapacitor and a cycle life of 100,000 recharges. Jeff Myron, a program manager at JSR Micro, says the device is mainly intended as a backup power supply in medical-imaging equipment.