Promise of lithium batteries
Lithium (Li), the lightest metal
known, is what dreams are made of. It is part of the batteries in cell phones,
sensors in medical devices, laptops, automobiles, defence equipment and
aircrafts. Bolivia dreams of building its economy on its Li deposits. But is
lithium-ion battery a ‘dream deferred’?
Li-ion battery is in the news
because of two incidents in the Boeing 787 Dreamliner aircraft. There was a
fire in one aircraft parked in the airport and in the other smoke was detected
after take-off. The US Federal Aviation Administration issued a directive to
ground these planes and the National Transportation Safety Board is doing a
complete investigation. Other countries, including India followed suit and
grounded these planes at a considerable financial loss.
Li-ion batteries are reliable and
their failure rate is 1 in 10 million cells. Yet fires and recall of equipment
do occur due to battery failure. A battery contains several cells. Each cell
consists of a cathode, an anode and a separator between the two, electrolyte
and current collectors. The cell generates power due to the motion of Li-ions.
The anode is graphite containing Li. A typical cathode (and the one used in
dreamliner battery) is made up of lithium cobalt oxide. The electrolyte
contains lithium salts in an organic solvent, which is flammable. This is
because Li reacts with water violently.
Li-ion battery delivers high power
per volume or weight to start a jet engine fast. It delivers 250- 340 Watts/ kg
compared to 150 W/kg of nickel-cadmium (Ni-Cd) battery. This also occupies less
space because its energy density is 250-620 Watt hour per litre compared to
50-150 Watt hour per litre of Ni-Cd.
But how do we make it safer? Let us
analyse the role of each constituent in a cell and the way cells are put
together. Energy is generated by a cell due to the motion of Li-ions from anode
to cathode through the electrolyte and a separator which is a polymer of
micrometer thickness. There are safer cathodes like lithium manganese oxide and
lithium iron phosphate, but the voltage and energy density are lower. We at the
Center for Study of Science, Technology and Policy combine quantum mechanics
theory and data mining to predict cathode compounds that have high voltage
(>4.5V). In general cathodes with high voltage have high oxidation strength
which means poor safety. The trick is to find compounds that buck the trend.
There are many factors that lead to a short circuit with a consequent increase
in temperature in the cell and ignition of electrolyte. Contaminants from
manufacturing processes can form short circuit between electrodes. Overcharging
can cause reaction between cathode and electrolyte resulting in gases leading to
thermal runaway. It can drive more ions from the cathode to anode leading to
lithium metal plating. Excess lithium can grow needle-like dendrites to short
the electrodes. Recently amorphous nano-silicon is being considered for anode
to overcome this problem. There are many methods available to switch off the
battery when it gets heated. In one approach polyethylene with a melting point
of 135 degree C is inserted between two layers of separator. When the
temperature rises to this value, polyethylene melts and closes the pores in the
separator which prevents motion of Li ions and battery stops. Non-flammable
ionic liquid electrolytes can be used.
Dreamliner is the most electrified
aircraft using 2 batteries of 28.5 kg each. In each battery eight cells are packed
next to each other in a sealed metal box. Some experts suggest using a large
number of small size batteries with spaces in between so that fire does not
spread to others in a domino effect.
Other steps contemplated are
insulating each cell using ceramic materials and using special steel for
casing. Airbus has found it prudent to meet its delivery schedules with Ni-Cd
while continuing with Li-ion in trial runs.
Approaches involving nanomaterials
may lead to entirely new systems. Batteries have a role beyond transportation.
The sun does not always shine and wind does not always blow and energy storage
is critical to renewable energy and a low carbon world. With continuing
improvements in batteries, lithium may contribute to economies rather than dreams.
Dr. N BALASUBRAMANIAN Dr. MRIDULA BHARADWAJ TH130328
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