A unified theory of electrochemical energy storage: Bridging batteries
and supercapacitors

Date:
March 18, 2022
Source:
Drexel University
Summary:
An international team of researchers suggests that all
electrochemical energy storage mechanisms exist on a spectrum
between physical and chemical retention of ions.



FULL STORY ==========================================================================
For decades researchers and technologists have regarded batteries
and capacitors as two distinct energy storage devices -- batteries,
known for storing more energy but releasing it slowly; capacitors,
for quickly discharging it in smaller spurts. Each new energy storage
device has therefore been categorized as one or the other, or as some
relation to one of the two, depending on the electrochemical mechanism
enabling it. But an international team of researchers, who are leaders
in developing and studying energy storage technology, has now suggested
that these mechanisms actually exist on a smooth spectrum, and trying
to categorize a device as "more than" or "less than" a battery or a
capacitor could be hampering progress in the field.


==========================================================================
In a perspective paper, recently published in the journal Nature Energy, researchers from Drexel University, North Carolina State University,
the University of California, Vanderbilt University, Saarland University
in Germany and Universite' Paul Sabatier in France, suggest that all electrochemical energy storage mechanisms exist somewhere on a continuum between those at work in batteries and those that enable capacitors.

"We propose a unified approach that involves a transition from the
'binary' view of electrochemical charge storage in nanoconfined spaces as either a purely electrostatic phenomenon, or a purely Faradic phenomenon,"
they write.

"It should rather be regarded as a continuous transition between the
two determined by the extent of ion solvation and ion-host interaction."
In simple terms, one end of the spectrum is a chemical bond -- the basic mechanism of connection, a physical link at the atomic level. The other
end is an electrostatic attraction that temporarily entraps ions within
and on the surface of a material.

The former phenomenon, called a Faradic reaction, gives batteries their excellent energy storage capacity and allows them to release charge
gradually.

But it's also the reason it takes them so long to charge. The latter,
more of a fleeting attraction than a true bond, enables the rapid bursts
of energy that power camera flashes and the short-term uptake of energy
from hybrid and electric car braking.

With each new development in energy storage technology, whether it's
a new combination of electrode materials and electrolyte solutions,
or physical or chemical additives to curtail or enable the transfer
of ions, researchers strive to observe and accurately characterize the electrochemical storage mechanism at hand.



==========================================================================
But the authors say that in many cases, these narrow definitions are
neither accurate, nor helpful when it comes to tailoring the devices to
the very specific energy storage needs of new technology.

"What happens in-between classic batteries and supercapacitors has
been a controversial topic for a long time," said Yury Gogotsi, PhD, Distinguished University and Bach professor in Drexel's College
of Engineering, who was a co- author of the paper. "So-called 'pseudocapacitors' and hybrid energy storage devices have been studied
for at least 30 years, but some scientists have attempted to reject pseudocapacitance completely, claiming that there are only these two
extreme cases and everything else is a superposition of two mechanisms
acting in parallel." The authors point out that in many of these
hybrid devices, ions are nearly absorbed between the layers of electrode materials. In others, where porous nanomaterials in electrodes have been designed to maximize the full chemical intake, or adsorption, of ions, researchers have seen much faster energy discharges, likely due to the persistence of the electrolyte substance preventing the ions from fully intercalating.

Both instances fall outside of the ideal, but their properties are proving
to be a valuable combination when it comes to powering new technology.

"We expect that understanding the ion desolvation (stripping ions of
solvent molecules) and its role in determining the energy storage
mechanism will allow us to reach the point when we combine high
energy and high power in a single energy storage device," Gogotsi
said. "Think of batteries charging within a few minutes -- you plug
your cell phone in, unplug it a few minutes later, and can use it at
least for a few hours. In case of 2D materials, like MXene or graphene,
we can make flexible batteries for flexible and wearable electronics."
The researchers recognize the importance of the standard-bearers for electrochemical energy storage, both for their role as the pillars of
our theoretical understanding of the field and as the enablers of modern technology. But they argue that moving forward means operating somewhere
in the middle, with the understanding that a right-fit energy storage
device could be more effective than a better battery or a supercapacitor.

"We acknowledge that there are two 'ideal situations' -- batteries and supercapacitors. There are equations derived for those cases. And there
are commercial devices with billion-dollar industries producing them. But
now we also know how to predict, design and manufacture devices that have properties between conventional extreme cases," said Volker Presser, PhD a co-author from Saarland University in Germany, and former research fellow
in Gogotsi's group at Drexel. "New industries that require flexible, transparent, conformal, wearable energy storage, devices combined with
energy harvesting, and other unconventional electrical energy supplies
will benefit greatly from the new agile energy storage. And we are
moving toward an electrical energy-driven economy, Internet of Things
and other new, advanced technologies for sustainable applications. So,
it will be very important to acknowledge and work to characterize
these new devices as existing within a spectrum, rather than falling
somewhere short of either end of it." In addition to Gogotsi, Simon Fleischmann and Veronica Augustyn, from North Carolina State University;
Yuan Zhang, from Saarland University; Xuepeng Wang, from the University of California; Peter T. Cummings, from Vanderbilt University; Jianzhong Wu,
from the University of California; Patrice Simon, from the Universite'
Paul Sabatier; and Volker Presser, from Saarland University, contributed
to this research.


========================================================================== Story Source: Materials provided by Drexel_University. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Simon Fleischmann, Yuan Zhang, Xuepeng Wang, Peter T. Cummings,
Jianzhong
Wu, Patrice Simon, Yury Gogotsi, Volker Presser, Veronica Augustyn.

Continuous transition from double-layer to Faradaic charge
storage in confined electrolytes. Nature Energy, 2022; DOI:
10.1038/s41560-022- 00993-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220318104910.htm

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