Batteries can be as simple as the lemon juice-powered toy car you made in high school, but developing a commercially viable battery from readily available and cheap materials has proven an elusive goal for many a researcher. The latest development on this score comes from Stanford University with the introduction of an aluminum-ion battery that uses an electrolyte made of urea, the main component of urine (after water).

This isn't the first battery to use pee power. Researchers at the University of Bath created a microbial fuel cell powered by human urine, while researchers in West England built a urinal that converts urine directly to electricity.

The battery is especially designed for grid storage of electricity from renewable energy sources, such as wind and solar. It's also an update of a first-of-its-kind aluminum-ion battery introduced in 2015 by Stanford professor Hongjie Dai and his team. That original version used a chemical mixture known as EMIC (1-ethyl-3-methylimidazolium chloride) as its main electrolyte ingredient, which when mixed with aluminum chloride makes a liquid salt, or ionic liquid.

But EMIC is expensive, prompting the researchers to look for an alternative. The new battery is mostly the same, except for the use of urea instead of EMIC, which is 100 times cheaper and produced commercially as a component in fertilizers.

"What you have is a battery made with some of the cheapest and most abundant materials you can find on earth," says Dai. "And it actually has good performance."

It doesn't have the same energy capacity of a lithium-ion battery (it's energy density is less by half), but the charge-discharge rate is higher, it's nonflammable, charges in 45 minutes, and is much cheaper.

"Urea is a good ingredient for a battery to be made on large scales for grid storage because it is so cheap," says Michael Angell, Stanford PhD candidate in chemistry and an author of a paper on the battery. "Also, the Coulombic efficiency of the battery is very high, 99.7 percent, which suggests that the cycle life is very long."

The Coulombic Efficiency is a measure of the amount of charge you get back from the battery, divided by the amount of charge you put in originally. This means the chemistry of the battery reaction is very reversible, which leads to long cycle life.

For commercial viability, a grid storage battery would need to have at least a 10-year lifespan. The urea battery has reached 1,500 charge cycles in lab conditions, though the researchers are pushing to extend that by tinkering with the battery's chemical processes. For now, the most attractive feature is its low cost.

"If produced on a large enough scale, EMIC was estimated to cost about US$50/kg, while urea currently costs $0.50/kg when produced on large scales," Angell tells New Atlas. "So the cost difference in the electrolyte, which is the most expensive part of the battery, is large. The cost of aluminum chloride, the other electrolyte component, if produced on an industrial scale, is something I don't have a number for, but it could also be made very cheaply."

Aluminum and graphite, the electrode materials, are also very cheap and would be negligible in the overall cost of the battery.

The team's findings were published in Proceedings of the National Academy of Sciences.