Not one,
but two!
Techno Talk
Mark Nelson
Types of battery, that is. Confused? You won’t be, if you’ve enough charge to reach the end of this article.
G
o to your favourite online
electronics emporium’s website,
drill down to batteries and you’ll
find a vast array of varieties, capacities and sizes. With types suitable for
every imaginable application, why on
earth would any company contemplate
adding to this variety? The answer is
obvious – because they can. And who
knows, the innovations might well fulfil
a vital need.
Not a gimmick?
The two new batteries presented here
are certainly attention-grabbers – no
gimmicks, according to their entirely
serious proponents. For starters, why
should batteries be opaque? Might it not
be better to make them invisible or at
least as inconspicuous as possible? That
was the challenge facing researchers at
Stanford University, who succeeded in
2011 in creating a small, see-through
and flexible lithium-ion battery. It was
not totally transparent, however, and
appears not to have progressed beyond
the proof-of-concept stage.
Fast forward to autumn 2019, the
R&D Labs of Japanese telecomms giant
NTT (Nippon Telephone and Telegraph)
revealed a new type of transparent battery that was clearly more practical.
On display in Tokyo were a number of
transparent batteries, one of which was
a thin plastic ‘window pane’, the same
size as an A4 notepad and as see-through
as window glass.
OK, so what does it do and why might
you need it? To quote NTT, ‘Today, we
are surrounded by all kinds of devices.
In the future, the presence of these devices could become an eyesore if the
number continues to grow. Our goal is
to develop devices that adapt to the surroundings.’ By selecting a material that
easily suppresses light absorption for the
electrode and fabricating an electrode
that easily suppresses the absorption
and reflection of light, they have made
a transparent battery with 23% light
transmittance, comparable to that of
ordinary sunglasses. Another version
lets through 69% of light, equivalent to
a less-than-ideal piece of window glass.
10
Scope for development
Future research at NTT will examine the
balance between battery performance
and transparency according to application. At this stage the performance
of these plastic batteries is somewhat
underwhelming: a voltage of 1.7V and
a discharge capacity of 0.03mAh. You
would need a battery of approximately
4.5m2 to achieve capacity equivalent
to a CR1025 coin cell. However, the
transparent battery can also operate as a
rechargeable secondary battery that can
light an LED even after being discharged
and charged 100 times. What’s more, it
is also flexible, and its electrodes can be
formed on conductive films with gelatine
electrolytes. A wide range of potential
applications is envisaged, including
wearable devices, information displays
and integration into construction materials such as the windows of buildings.
Edible capacitors?
Capacitors are increasingly employed as
effective batteries. That’s certainly the
intention for the type described next –
but ‘edible’? Well, not really, although
this new kind of energy invention was
inspired by croissants.
Dr Emiliano Bilotti and his team of researchers at Queen Mary University of
London (Queen Mary College, as was)
developed it. In itself it’s nothing more
than a polymer film capacitor, which
uses an insulating plastic film as the
dielectric. They first came to most people’s attention as the colourful and stripy
‘tropical fish’ capacitors introduced by
Mullard (see http://bit.ly/pe-feb20-fish
for a memory jerker). You can buy their
modern equivalents for pence from
your favourite component supplier and
many of you have certainly used them
in your projects. However, despite their
numerous advantages over other types
of capacitor (http://bit.ly/pe-feb20-cap),
they tend to be physically larger than
alternative varieties and this is an obvious drawback that Dr Bilotti’s team
have now countered.
Bakers make croissants by layering and
pressing dough, a technique the Queen
Mary team have mimicked by pressing
and folding their capacitor’s polymer
film. They were able to store 30-times
more energy than the best-performing
commercially available dielectric capacitor, achieving the highest energy density
ever reported in a polymer film capacitor.
Storing intermittent energy
The significance of this is most timely. It is widely agreed that we need to
substitute power generated using fossil
fuels (to combat climate change) with
renewable energy (for example, solar
and wind). Consequently, we need to
develop affordable, efficient, low-cost
and environmentally friendly systems
for storing electric energy. As Dr Bilotti
explains, ‘storing energy can be surprisingly tricky and expensive, and this
is problematic with renewable energy
sources which are not constant and rely
on nature. With this technique we can
store large amounts of renewable energy
to be used when the sun is not shining
and it is not windy.’
These ‘pastry power’ capacitors, with
their ultra-high power density, should
be ideal for the function just described.
Other electrochemical energy storage
technologies exhibit disappointingly low
power density or use exotic materials.
Yet more applications
The ability of croissant capacitors to accumulate energy over a period of time
and then release it almost instantly means
they could find numerous industrial applications; for example, motor drives,
mobile power systems, space vehicle
power systems and electrochemical guns.
Professor Mike Reece, one of Dr Bilotti’s
colleagues, clarifies: ‘This finding promises to have a significant impact on the
field of pulse-power applications and
could produce a step change in the field
of dielectric capacitors, so far limited by
their low energy-storage density. Even
better, although achieving high energy
density in polymer film capacitors normally involves complex and expensive
production processes, the new pressing
and folding technique is unique for its
simplicity, record high energy density
and potential to be adopted by industry.’
Practical Electronics | February | 2020
