Computers are getting smaller and smaller, just as current cell phones offer computing
power similar to that of a laptop. And the trend toward miniaturization continues.
Smart dust applications (tiny microelectronic devices), such as biocompatible sensor
systems in the body, demand computers and batteries smaller than a dust mote. So far,
this development has been hindered by two main factors: lack of on-chip power sources for
operation anytime and anywhere and difficulties in producing integrable microbatteries.
In the journal Advanced Energy Materials,
researchers present a solution to these challenges. They discuss
how battery-powered smart dust applications can be realized in the sub-millimeter-scale
and present the world's smallest battery by far as an application-oriented prototype.
The researcher says that their results show
encouraging energy storage performance at the sub-square-millimeter scale.
The power to run tiny sub-millimeter-scale computers can be provided by developing
appropriate batteries or "harvesting" methods to generate electricity.
In the area of "harvesting," micro-thermoelectric generators, for example, convert heat to
electricity, but their output power is too low to drive dust-sized chips. Mechanical
vibrations are another source of energy for powering tiny-scale devices. Small photovoltaic
cells that convert light into electrical energy on small chips are also promising.
However, light and vibrations are not available at all times and in all places,
making on demand operation impossible in many environments. This is also the case,
for example, in the human body, where tiny sensors and actuators require a continuous power supply.
Powerful tiny batteries would solve this problem.
However, the production of tiny batteries is very different from their everyday
counterparts. For example, compact batteries with high energy density,
button cells for instance, are manufactured using wet chemistry. Electrode materials and additives
(carbon materials and binders) are processed into a slurry and coated onto a metal foil.
On-chip microbatteries produced using such standard technologies can deliver good
energy and power density but have a footprint of significantly more than one square millimeter.
Stacked thin films, electrode pillars or interdigitated microelectrodes are used
for on-chip battery manufacturing. However, these designs often suffer from inferior energy storage,
and the footprint of these batteries cannot be reduced significantly below
one square millimeter. The goal of the research team was therefore to design
a battery significantly less than one square millimeter across and integrable on a chip,
which still has a minimum energy density of 100 microwatt hours per square centimeter.
To achieve this, the team winded up current
collectors and electrode strips at the microscale – a similar process
also used by Tesla on the large scale to manufacture the batteries for its e-cars.
The researchers use the so-called "Swiss-roll" or "micro origami" process. A layered system
with inherent tension is created by consecutively coating thin layers of polymeric, metallic and
dielectric materials onto a wafer surface. The mechanical tension is released by peeling off
the thin layers which then automatically snap back to roll up into a Swiss-Roll architecture.
Thus, no external forces are needed to create such a self-wound cylinder
micro-battery. The method is compatible with established chip manufacturing technologies
and capable of producing high throughput micro-batteries on a wafer surface.
Using this method, the research team has produced rechargeable microbatteries that
could power the world's smallest computer chips for about ten hours - for example, to measure
the local ambient temperature continuously. A tiny battery with great potential for future micro- and
nanoelectronic sensorics and actuator technologies in areas such as the Internet of Things,
miniaturized medical implants, microrobotic systems and ultra-flexible electronics.