i3 - January/February 2017 - 25

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CREDIT

Super-fast 5G
mobile devices
will be here
around 2020.
The first wave
of autonomous
cars could be
deployed by
that same year.
By 2022, according to the research firm
Bloomberg New Energy Finance, the
unsubsidized total cost of ownership of
battery electric vehicles (EVs) will fall
below that of an internal combustion
engine vehicle.
But what of the batteries that will
power these new phones and cars?
Will new technology be available to meet
the demands of 5G mobile broadband
without overheating concerns, and
provide the power and range needed
by EVs?
Breakthroughs in a battery technology
known as lithium-air, sometimes called
Lithium O2, has provided optimism.
Lithium-air (Li-air) chemistry has been
a Holy Grail quest for battery makers
due to its extremely high theoretical
energy density, which far exceeds
existing energy storage technology.
Li-air batteries can store 10 times the
energy of commercial lithium-ion
(Li-ion) cells which power cell phones
and tablets. This translates into powering a mobile phone for an entire
week without recharging.
C TA . t e c h / i 3

Looked at from an automotive perspective, Li-air cells could power electric
cars for more than 400 miles
on a single charge using a battery pack
that's a fifth of the weight of today's
EV batteries. This would thrust electric
cars into the same ballpark as gasolinepowered vehicles in terms of the amount
of energy derived from a given weight of
fuel, erasing range concerns and permitting practical, widespread use of fully
electric vehicles.
Today batteries in electric cars can't
compare with gasoline in the amount
of energy derived from a given weight
of fuel. The Li-air battery has a theoretical specific energy (energy per unit
mass) of 3.5 kWh/kg (kilowatt hours per
kilogram). By comparison, Li-ion batteries have only 105 Wh/kg (watt hours
per kilogram) at the pack level, limiting
fully electric cars to about 150 km of
driving range. The energy density of
gasoline is roughly 13 kWh/kg, of which
1.7 kWh/kg of energy is provided to the
wheels after losses.
Accounting for the weight of a full
Li-air battery pack (casing, materials,
etc.) the energy density will be considerably lower but estimates range up to
1.7kWh/kg to the wheels - right there
with gasoline. Given the high efficiency of
electric motors that should be sufficient
to deliver much longer driving on a single
charge than is now possible.
Add to that the issue of lithium-ion
batteries overheating and possibly igniting
if they're charged too quickly or charged
past their designed capacity. These reasons
alone have spurred worldwide efforts to
find a better solution.

Though different, Li-air isn't a full starboard tack away from Li-ion batteries.
A Li-ion battery has three basic parts: a
positive terminal (cathode), a negative
terminal (anode) and an electrolyte. In
the batteries we use in our smartphones
and laptops, a metal oxide such as lithium
cobalt oxide is used to form the cathode.
The anode can be formed from materials
such as a graphite mixture and the electrolyte is usually a lithium salt dissolved
in an organic solvent. A thin separator
keeps the electrodes from touching and
allows lithium-ions to pass though during
charging or discharging. The energy of
the battery depends on the movement
of these ions between terminals, which
generates a current.
While Li-ion batteries can store energy
in a small container, their capacity deteriorates with age, and their low energy
density means they need to be recharged
frequently. Like Li-ion, a Li-air battery
also has an anode and a cathode and
uses oxidation of lithium at the anode
and reduction of oxygen at the cathode
to induce a current flow. Here, however,
the anode is made of lithium metal but
the cathode is oxygen pulled from the air.
As with the Li-ion battery, the electric
current flows between the anode and the
cathode via a liquid electrolyte.

Overcoming Hurdles
Realizing the enormous potential of
Li-air, however, is a very challenging
scientific problem, mostly centered
on achieving a high percentage of the
theoretical energy density, improving
the electrical efficiency of recharging,
increasing the number of times the
JANUARY/FEBRUARY 2017

25
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