College Station, Texas, Februry 1, 2018 -- A multi-institution team of
scientists led by Texas A&M University chemist Sarbajit Banerjee has
discovered an exceptional metal-oxide magnesium battery cathode material,
moving researchers one step closer to delivering batteries that promise higher
density of energy storage on top of transformative advances in safety, cost and
performance in comparison to their ubiquitous lithium-ion (Li-ion)
counterparts.
"The
worldwide push to advance renewable energy is limited by the availability of
energy storage vectors," says Banerjee in the team's paper, published today
(Feb. 1) in the journal Chem,
a new chemistry-focused journal by Cell
Press. "Currently, lithium-ion technology dominates; however, the
safety and long-term supply of lithium remain serious concerns. By contrast,
magnesium is much more abundant than lithium, has a higher melting point, forms
smooth surfaces when recharging, and has the potential to deliver more than a
five-fold increase in energy density if an appropriate cathode can be
identified."
Ironically,
the team's futuristic solution hinges on a redesigned form of an old Li-ion
cathode material, vanadium pentoxide, which they proved is capable of reversibly
inserting magnesium ions.
"We've
essentially reconfigured the atoms to provide a different pathway for magnesium
ions to travel along, thereby obtaining a viable cathode material in which they
can readily be inserted and extracted during discharging and charging of the
battery," Banerjee says.
This
rare phenomenon is achieved by limiting the location of the magnesium ions to
relatively uncomfortable atomic positions by design, based on the way the
vanadium pentoxide is made -- a property known as metastability. This
metastability helps prevent the magnesium ions from getting trapped within the
material and promotes complete harvesting of their charge-storing capacity with
negligible degradation of the material after many charge-recharge cycles.
The Ins and Outs of Intercalation
Banerjee,
a Davidson Professor of Science in the Texas
A&M Department of Chemistry and an affiliated faculty member in the Department of Materials Science
and Engineering, has been working for a number of years to better
understand ion intercalation -- the critical process by which ions like lithium
and magnesium move in and out of other materials within intercalation
batteries.
Using
one of the world's most powerful soft X-ray microscopes -- the Scanning
Transmission X-ray Microscope (STXM) and X-ray Emission beamlines -- at the Canadian Light Source in tandem with one
of the world's highest resolution aberration-corrected transmission electron
microscopes housed at the University of Illinois
at Chicago (UIC), Banerjee and collaborators from the Lawrence Berkeley National Laboratory, the UIC
and Argonne National Laboratory were able to
observe the unique electronic properties of their novel vanadium pentoxide and
directly prove magnesium-ion intercalation into the material. Collectively, the
team applied decades of combined experience in materials science to explain the
fundamental reasons why this new type of vanadium pentoxide is superior to the
old version as well as to Li-ion batteries.
Laptops
and cell phones are two examples of the many technologies enabled by the rapid
development of the lithium-ion battery, which revolutionized energy storage
capacity and rechargeability in comparison to its lead-acid and nickel-metal
hydride predecessors. However, given the widespread use of lithium not only in
portable electronic devices but increasingly in the much larger batteries
required for electric vehicles and grid energy storage, lithium is expected to
be in increasingly short supply in the long-term. Furthermore, Li-ion batteries
are a risky game, as highlighted by recent widely publicized reports detailed
in Scientific
American, Reuters
and Forbes,
for example, in which Li-ion-powered devices have either caught fire or
exploded as a result of the fundamental flammability and reactivity of lithium.
"Apart
from being much safer for consumer applications, magnesium-ion technology is
appealing fundamentally because each magnesium ion gives up two electrons per
ion -- twice the charge, whereas each lithium ion gives up only one," says
Texas A&M chemistry graduate student and NASA Space Technology Research
Fellow Justin
Andrews, first author on the team's paper. "This means that, all other
considerations aside, if you can store as much magnesium in a material as you
can store lithium, you immediately almost double the capacity of the
battery."
Double the Capacity, Double the Trouble
But
for all their perceived advantages, magnesium batteries have proven too good to
be true since they were first proposed in the 1990s and essentially sidelined
by a variety of problems; primarily, the lack of a suitable cathode, or
positive electrode -- otherwise known as the part of a battery where the
magnesium ions enter during discharge of the battery to power an electronic
device and then exit during charging.
"Indeed,
the most exciting thing about magnesium ions -- namely, that they store twice
the charge in battery applications -- also forms the basis for the biggest
challenge," says collaborating UIC chemist Jordi Cabana. "The
higher charge of the magnesium ions make them 'stick' much more strongly with
surrounding atoms."
In
other words, Banerjee says, the magnesium ions get waylaid as they are
traversing through the paths within the cathode material. Their sluggish
movement is what makes it so difficult to make viable magnesium batteries.
"In
many structures, some of these interactions are very favorable, meaning that
the magnesium is quite happy to sit and stay a while in those specific
sites," Andrews explains. "In our material, the magnesium is
'frustrated' as it moves through the lattice, because it encounters many
less-than-optimal environments. In this sense, it is more than happy to just
keep moving right along, leading to an improvement in capacity and
diffusion."
The
team's National Science Foundation-funded
research features two additional current and former Texas A&M graduate
students, Abhishek Parija and Peter M. Marley, respectively. David Prendergast,
a Facility Director at Berkeley Lab's Molecular
Foundry, a U.S. Department of Energy
National User Facility for Nanoscale Science Research, helped the Texas A&M
team design and interpret their calculations, which were experimentally
verified in part by Fakra using Berkeley Lab's Advanced Light Source along with
structural data collected at Argonne National Lab's Advanced Photon Source.
Atomic resolution images of the new form of vanadium pentoxide were collected
in collaboration with UIC physicist Robert
F. Klie and physics graduate student Arijita Mukherjee and show direct
evidence of magnesium intercalated within the material. Battery
measurements that show reversibility and confirm the robustness of the cathode
material complete the story and were conducted in collaboration with Cabana and
former Cabana group member Hyun Deog Yoo.
"On
paper, magnesium batteries are highly desirable because they promise greater
energy density on top of the ability to solve several of the key issues
researchers -- and unfortunately consumers -- are discovering with lithium-ion
batteries, including cost, safety, and performance at the most fundamental
levels," Andrews says. "But the shift from lithium- to magnesium-ion
technologies is not straightforward, and the many problems encountered when
designing magnesium-ion cathodes have stymied the development of these more
sustainable and safer batteries."
Working Toward a Safer Energy Future
Andrews
says the team's research marks an important turning point in the field because
it represents a significant advance toward solving the cathode problem while
also highlighting the inherent advantages of using much more imaginative,
metastable materials like this new form of vanadium pentoxide. But even he
admits there's much more work to do before this particular '90s trend comes
back in vogue.
"While
this research has provided a great deal of insight, there are still several
other fundamental problems to overcome before magnesium batteries become a
reality," Andrews adds. "Nevertheless, this work moves magnesium
batteries one step closer to reality -- namely, a reality where batteries would
be less-expensive, lighter and safer for allowing for easier adoption to
large-area formats necessary for electric vehicles and to store energy
generated by solar and wind sources."
The
team's Chem paper, "Reversible Mg-Ion Insertion in a Metastable
One-Dimensional Polymorph of V2O5," can be viewed online
along with related figures and captions.
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