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.