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.
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.
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.