Friday, January 16, 2015

The new type of battery made of glass

For some time now, energy experts have been adamant that we
will need much more clean energy in the future if we are to
replace fossil fuel sources and reduce CO2 emissions. For
example, electric cars will need to replace the petrol-powered
cars driving on our roads. Yet in order for electric cars to travel
greater distances or mobile phones to stay charged for longer,
we will need better batteries and more of them. In the transition
to renewable energy sources, accumulators also play a key role
in storing excess power from wind turbines or solar power plants
and compensating for fluctuations in the energy supply.
In this regard, researchers are diligently looking for new
materials that exhibit a greater energy density and charging
capacity, but which are no heavier or larger than those used in
today’s lithium-ion batteries. Today’s batteries provide a reliable
power supply for our smartphones, electric cars and laptops, but
are unable to keep up with the growing demands placed on
them. Dr Semih Afyon, a scientist at the Electrochemical
Materials Institute, sums up the fundamental idea that is driving
battery research: “What we need is new chemistry and novel
compounds to obtain safe, better and longer-lasting batteries.”
Glass particles instead of crystals
ETH researchers led by Afyon and Reinhard Nesper, professor
emeritus of chemistry, have now made a discovery. Over the
course of their several years of research, they discovered a
material that may have the potential to double battery capacity:
vanadate-borate glass. Researchers are using the glass as a
cathode material, as recently reported in Scientific Reports, a
journal from the publishers of Nature.
The material is made of vanadium oxide (V2O5) and lithium-
borate (LiBO2) precursors, and was coated with reduced graphite
oxide (RGO) to enhance the electrode properties of the material.
The researchers used a vanadium-based compound because
vanadium is a transition metal with various oxidation states,
which can be exploited to reach higher capacities. In crystalline
form, vanadium pentoxide can take three positively charged
lithium ions – three times more than materials presently used in
cathodes, such as lithium iron phosphate.
However, crystalline vanadium pentoxide cannot release all of
the inserted Li-ions and only allows a few stable charge/
discharge cycles. This is because once the lithium ions penetrate
the crystalline lattice during the loading process, the lattice
expands. As a result, an electrode particle swells as a whole, i.e.
it increases in volume only to shrink again once the charges
leave the particle. This process may lead to instabilities in the
electrode material in terms of structural changes and contact
losses.
Researchers therefore had to find a way to retain the structure of
the initial material while maximizing the capacity and also
maintaining its ability to “take” the charges, which is how they
devised the idea of using vanadium as a glass rather than in its
crystalline form. In glass, a so-called “amorphous” material,
atoms do not arrange themselves in a regular lattice as they do
when they are in a crystalline state. Instead, the atoms exist in a
state of wild disarray.
Inexpensive and simple production
To produce the cathode material, Afyon and his colleagues
blended powdered vanadium pentoxide with borate compounds.
“Borate is a glass former; that’s why the borate compounds
were used, and the resulting glass compound is a new kind of
material, neither V2O5 nor LiBO2 at the end”, the researcher
says. The materials scientists melted the powder at 900°C and
cooled the melt as quickly as possible to form glass. The
resulting paper-thin sheets were then crushed into a powder
before use, as this increases their surface area and creates pore
space. “One major advantage of vanadate-borate glass is that it
is simple and inexpensive to manufacture”, states Afyon. This is
expected to increase the chance of finding an industrial
application.
To produce an efficient electrode, the researcher coated the
vanadate-borate powder with reduced graphite oxide (RGO). This
increases conductivity while at the same time protecting the
electrode particles. However, it does not impede electrons and
lithium ions as they are transported through the electrodes.
Afyon used this vanadate-borate glass powder for the battery
cathodes, which he then placed in prototypes for coin cell
batteries to undergo numerous charge/discharge cycles.
As much as twice the power
During initial trials with vanadate-borate electrodes, which were
not made with material coated in RGO, the discharge capacity
dropped drastically after 30 charge/discharge cycles, when the
current rate was increased to 400 milliamp per gram. In contrast,
when the RGO coating was used, the capacity was quite stable at
high rates and it remained at a consistently high level after more
than 100 charge/discharge cycles.
One battery with an RGO-coated vanadate-borate glass electrode
exhibited an energy density of around 1000 watt-hours per
kilogram. It achieved a discharge capacity that far exceeded 300
mAh/g. Initially, this figure even reached 400 mAh/g, but
dropped over the course of the charge/discharge cycles.
“This would be enough energy to power a mobile phone between
1.5 and two times longer than today’s lithium-ion batteries”,
Afyon estimates. This may also increase the range of electric
cars by one and a half times the standard amount. These figures
are still theoretical.
Patent and further development
The researchers have already applied for a patent for their new
material. They also worked with industry partners on the
development. “Our focus was on practical applications, as well
as on fundamental research”, says the researcher. A new
concept usually takes around 10 to 20 years to gain a foothold
in the market.
The researcher’s positive results with the vanadate-borate glass
have encouraged them to continue their research in this area.
Afyon currently works as a project leader in a research
consortium led by Jennifer Rupp, professor of electrochemical
materials, focused on developing an innovative solid-state
battery. They are already using and testing the vanadate-borate
electrode in this system, and their next step is to optimise the
system. In particular, they have to increase the number of
charge/discharge cycles significantly, which could be achieved
by improving battery and electrode designs as well as by using
coatings other than reduced graphite oxide.

No comments:

Post a Comment