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.
Drs. N BALASUBRAMANIAN & MRIDULA BHARADWAJ