Section européenne
Terminale S
Baccalauréat général
Epreuve de
Sciences Physiques en Anglais
Solid-state batteries
The power of the press
Source: The Economist, January 29th 2011
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.
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 pro cess 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.
Section européenne
Terminale S
Baccalauréat général
Epreuve de
Sciences Physiques en Anglais
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 pro cess 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 thioLISICONS - 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 ceils 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 electricitystorage 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 ceils, 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 ceil 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 ceils 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 allelectric 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.
Section européenne
Terminale S
Baccalauréat général
Epreuve de
Sciences Physiques en Anglais
Question
Introduce the document, bring out its key ideas, and enlarge upon the topics it deals with.