© Xue, et al / American Chemical Society

Two things that are becoming increasingly important parts of our clean technology future are improved batteries and mechanical energy harvesting devices, also known as piezoelectric devices, that can generate electricity from our everyday movements. Typically in renewable energy set up, there is the energy generator (whether using mechanical, solar, wind or other sources) and then, ideally, there is the energy storage component, very often a lithium-ion battery. In that scenario the generator turns the renewable energy into electricity and then the battery turns the electricity into chemical energy for storage.

In a new technology breakthrough, researchers at Georgia Tech have developed the first self-charging power cell that is both a mechanical energy harvester and a battery at the same time. Essentially, the device skips the step of generating electricity and converts the mechanical energy directly into chemical energy.

“This is a project that introduces a new approach in battery technology that is fundamentally new in science,” one of the researchers, Zhong Lin Wang, told Phys.org. “This has a general and broad application because it is a unit that not only harvests energy but also stores it. It does not need a constant wall jet DC source to charge the battery. It is mostly to be used for driving small, portable electronics.”

The breakthrough was accomplished by converting a coin-type lithium-ion battery. The team replaced the polyethylene that normally separates the two electrodes with PVDF film. The PVDF acts as a piezoelectric generator when pressure is applied and, because of its position between the two electrodes, the voltage it creates charges the battery.

To test the performance, the researchers put the battery on the heel of a shoe. The pressure of walking provided the compressive energy needed to charge the battery.

Phys.org reports, "A compressive force with a frequency of 2.3 Hz could increase the voltage of the device from 327 to 395 mV in 4 minutes. This 65 mV increase is significantly higher than the 10 mV increase it took when the power cell was separated into a PVDF piezoelectric generator and Li-ion battery with the conventional polyethylene separator. The improvement shows that achieving a mechanical-to-chemical energy conversion in one step is much more efficient than the mechanical-to-electric and electric-to-chemical two-step process used for charging a traditional battery."

Once the stress on the battery ceases, the cell can begin supplying power to a device, like our many gadgets or medical devices.

The researchers are now working on increasing the voltage it can charge with and upping the performance by using a flexible material for the external casing of the cell, which would allow it to bend and compress more easily.