Self-charging battery both generates and
stores energy
(a)
In the self-charging power cell, the piezoelectric material PVDF replaces the
conventional separator material and acts as a nanogenerator inside a Li-ion
battery. (b) On the bottom of a shoe, the power cell converts the compressive
energy generated by walking into chemical energy and stores it without
converting it to electricity. Image credit: Xue, et al. ©2012 American Chemical
Society
(Phys.org)
-- Renewable energy technologies generally consist of two distinct processes:
energy generation (using sources such as coal, solar, wind, etc.) and energy
storage (such as batteries). These two processes are always accomplished
through two separate units, with the first process converting the original form
of energy to electricity, and the second process converting electricity to
chemical energy. Now for the first time, engineers have demonstrated that
energy can be generated and stored in a single device that converts mechanical
energy directly to chemical energy, bypassing the intermediate step of
electricity generation. The device basically acts as a hybrid generator-battery
unit, or in other words, a self-charging power cell.
The
researchers, Xinyu Xue, Sihong Wang, Wenxi Guo, Yan Zhang, and Zhong Lin Wang,
from the Georgia Institute of Technology in Atlanta, Georgia, have published
their study on combining energy
generation and storage in a single unit in a recent issue of Nano Letters.
“This is a
project that introduces a new approach in battery technology that is
fundamentally new in science,” 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.”
To
fabricate the self-charging power cell, the researchers started with a
coin-type Li-ion battery and replaced the polyethylene separator that normally
separates the two electrodes with PVDF film. As a piezoelectric material, PVDF
film generates a charge when under an applied stress. Because of its position
between the battery electrodes, the PVDF film causes positive Li ions to
migrate from the cathode to the anode in order to maintain a charge equilibrium
across the battery. This ion migration process charges the battery without the
need for any external voltage source; since the PVDF separator provides the
voltage, or potential difference between electrodes, the battery is essentially
self-charging.
In order to apply a stress to the separator, the researchers attached the coin-sized battery to the bottom of a shoe, and found that walking could generate enough compressive energy to charge the battery. 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
new equilibrium between electrodes is reached, the self-charging process
ceases. The cell can begin supplying power after the applied stress is
released, since the piezoelectric field disappears and the Li ions can diffuse
back from the anode to the cathode to reach a new equilibrium. As in a
conventional Li-ion battery, ion diffusion involves electrochemical
reduction-oxidation reactions, which here generate a current of about 1 μA that
can be used to power a small electronic device.
“The Li
ions will not flow back immediately after the applied stress is removed because
it forms a new compound with the anode material (LiTiO),” Zhong Lin Wang said. “The
charges are preserved as in a conventional battery. They are released at a later
time when power is required.”
Although
these voltages and currents are low, the researchers showed that the power cell
can also self-charge with higher voltages of around 1.5 V, which could make it
useful for a broader range of applications. The researchers predict they can
further improve the power cell’s performance by making several modifications,
such as by using flexible casing to allow for greater deformation of the
piezoelectric material.
Explore
further: First
Synchronous, Switch-Mode Battery Charge Integrated Circuit (IC) with Internal
Power FETs
More
information: Xinyu Xue, et al. “Hybridizing Energy Conversion and Storage
in a Mechanical-to-Electrochemical Process for Self-Charging Power Cell.” Nano
Letters. DOI:
10.1021/nl302879t
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