Solar cells only work during the day, when the sun shines. While solar cells become better and better, and some can even generate power when it’s cloudy, they still don’t work in an absence of light (we should probably change the name if they did). But the power that is generated during the day has to be stored somewhere. Current batteries, like lithium-ion or lead acid batteries, are costly and have limited lifespans. Researchers at Stanford University have developed a battery from some of the most abundant and cheapest materials found on earth: urea, aluminium, and graphite.

Urea is an ingredient commonly found in plant fertilisers (and mammal urine), making it a cheap and abundant material. Another advantage of urea is that it is non-flammable, as opposed to lithium-based batteries.

In 2015, the team made a first version of the rechargeable aluminium battery. This system charged in less than a minute and lasted thousands of charge-discharge cycles. However, that version of the battery had one major drawback: it involved an expensive electrolyte.

The newest version includes a urea-based electrolyte and is about 100 times cheaper than the 2015 model, with higher efficiency and a charging time of 45 minutes. It’s the first time urea has been used in a battery.

According to the researchers, grid storage is the most realistic goal for the battery, because of its low cost, high efficiency and long cycle life.

To meet the demands of grid storage, a commercial battery will need to last at least ten years. By investigating the chemical processes inside the battery, the team hopes to extend its lifetime. The outlook is promising. In the lab, these urea-based aluminium ion batteries can go through about 1,500 charge cycles with a 45-minute charging time.

Coulombic efficiency is a measurement of how much charge exits the battery per unit of charge that it takes in during charging. The Coulombic efficiency for this battery is high: 99.7 per cent.