"If the charge carrier is in solution, it doesn't have the problem other batteries have when [the electrode] crystallizes," which happens with some lithium-ion batteries, Cook said.
The second step was finding a substance that could dissolve in liquid and carry electrons. The researchers found that BODIPY was a very effective electron carrier; it both gives up and takes in electrons easily, Cook said. This means the glow-in-the-dark substance is more efficient at delivering energy.
This redox battery may be a safer option than lithium ion batteries, which sometimes catch fire. This happens because the lithium in them is ionized, meaning it has given up an electron. That makes the element very reactive with the oxygen in water, including the moisture in the air, forming lithium oxide and releasing hydrogen.
"What you're left with is two ionized hydrogens that were attached to water, and we have two lithiums that gave up electrons to join with the water, and that reaction is kicking off a lot of heat too," Cook said.
The hydrogen that can burn if a spark hits it, or the chemicals used for the electrolyte, can react with air. (Lithium itself is so reactive that a popular demonstration in chemistry classes is dropping lithium in water to watch it bubble and generate hydrogen.)
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When lithium ion batteries catch fire it's usually because the casing of the battery cracks, exposing the insides to the air, or because the membranes that separate the chemical species inside the battery get damaged, allowing reactions to occur inside the battery. Those reactions generate gases, heat, and sometimes fire.
"A [lithium]-based battery has a lot of energy, and if there are mechanical failures, like the membrane separator is ruptured, the battery will discharge all at once, creating a tremendous amount of heat," Cook said.
Recently, Samsung issued a recall of its Galaxy Note 7 smartphones because of faulty batteries that, in some cases, were exploding or catching fire. These battery problems could potentially happen in any lithium-ion-dependent battery system, the researchers said.
The liquid in a redox battery is instead stored in tanks, and can be recycled through the volume of the battery. Eventually, even redox batteries degrade, but a fresh liquid supply can allow them to be used again, according to the study.
Here on Earth
Cook said the technology was originally developed at NASA for space probes, but the agency eventually found better battery solutions for spacecraft. However, redox batteries could be useful for more Earthbound applications, the researchers said. And while redox batteries' liquid can oxidize, the acetonitrile that Cook's team used won't burn, the scientists said.
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Cook added that it's important to note that his battery tests were done only on tabletop systems, producing just a few volts. But it took only small concentrations of BODIPY to get results, he said. The downside is that redox batteries usually have to be larger because their energy density is smaller, so odds are they will be more useful for storing large amounts of power in homes and cars rather than phones, Cook said.
About 1,320 gallons (5,000 liters) using Cook's current chemical mixture could power a house. That would mean a battery the size of a septic tank for a four-bedroom house. Cook thinks with some work on the concentration of BODIPY, he can get the volume needed down to about 175 gallons (660 liters), about the size of a refrigerator or large fish tank, he said.
Meanwhile the need for efficient batteries in homes may not be so far in the future, if renewable energy keeps making gains. "We didn't really need medium- to large-scale energy storage before," Cook said. "Now, in the last decade, flow batteries [are] becoming [a] popular field of study."
The research was published Nov. 16 issue in the journal ChemSusChem.
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