L
ook in almost anything that’s electronic but doesn’t need a plug to run, and you will find a rechargeable lithium battery providing the energy. Tere are alter- natives, sure. But instead of being a few ounces, your
smartphone would weigh a few pounds. So far, lithium-ion bat- teries are the very best way we know of to cram the most energy into a small, fixed space. No other type of battery can hold as much energy as lithium-ion. But if you’ve ever seen a video of a hoverboard going up in smoke, the probable cause is the battery. Lithium-ion batteries can be dangerous when not used properly. But scientists at the ARL are getting WiSE and trying to change that. WiSE stands for water-in-salt-electrolyte, but more about that later.
To put batteries in perspective, you have to look at the amount of energy (watt-hours, or the amount of energy the battery stores) and the weight of the battery (in this case, kilograms). Te kind of battery in most cars, lead-acid, offers about 50 watt-hours per kilogram. Nickel metal-hydride is a little better at 120 watt- hours per kilogram. But lithium-ion is the undisputed leader of rechargeable batteries, soaring to 200 watt-hours per kilogram with the potential to reach as many as 350.
It is because of their high energy density that Soldiers carry lithium-ion batteries into the field. And for a good reason— lithium-ion batteries’ huge energy advantage leaves room for ammunition and extra equipment. As important as batteries are for civilians, they’re even more so for Soldiers. And they’re rapidly approaching the point where they have the potential to power hybrid and electric vehicles on the battlefield.
INSIDE A BATTERY Modern lithium-ion batteries have three main parts. First is the anode, which is made of something like pencil lead. Second is the cathode, which is typically a metal oxide. In between anode and cathode is a special liquid: the electrolyte. Te electrolyte is a solution of lithium salt in a liquid solvent, and it allows the flow of lithium ions back and forth between the anode and cathode as the battery is used and recharged.
Lithium batteries that are given hefty tasks, like powering an electric car, can overheat. Battery fires are fueled by the electro- lyte, which is readily flammable. Battery fires are particularly dangerous because the burning electrolyte releases toxic fumes, and the fire can’t be extinguished with water.
Tat's because if you throw lithium metal into water, it reacts violently to form lithium hydroxide. Worse, throw that lithium
{[Li(H2 O)4 ](H2 O)4
Primary solvation sheath Free TFSI
Free H2
O molecule
Secondary solvation sheath }+nH2 On≥1 Salt-in-Water BIG BOOM
When used in a battery, a normal water electrolyte could explode above 1.5 volts because of the generation of hydrogen and oxygen gas. As a result, most lithium-ion batteries are engineered to use only a fraction of their maximum potential energy.
salt from the electrolyte into water and it forms hydrofluoric acid, which can dissolve glass and decalcify bone.
Te danger of lithium-ion battery fires sparked a need within the industry to engineer protective packs that regulate temperature and manage the health of the battery. As an extra precaution, most lithium-ion batteries are engineered to use only a fraction of their maximum potential energy to minimize the stress— and risk of fire.
CRACKING WISE Researchers want to change the model and allow the lithium- ion battery to use its full capacity. Te unlikely hero of that effort? Water.
Not all lithium salts react so dangerously with water. Tere is one that forms no such spontaneous reaction. In fact, dissolving it in water is entirely safe. Remarkably, it is possible to dissolve 6 kilograms of this special lithium salt (called LiTFSI) in one kilogram of water. Tat’s roughly the equivalent of dissolving two-thirds of a pound of common table salt in one cup of water.
ASC.ARMY.MIL 75
SCIENCE & TECHNOLOGY
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