An unexpected discovery by a team from the US Pacific Northwest National Laboratory has led to a rechargeable battery that is said to be as inexpensive as conventional car batteries, but which has a much higher energy density.

The discovery came after the team realised its battery worked in a different way than they had assumed. “The idea of a rechargeable zinc-manganese battery isn’t new,” said laboratory fellow JunLiu. “Researchers have been studying them as an inexpensive, safe alternative to lithium-ion batteries since the late 1990s. But these batteries usually stop working after just a few charges. Our research suggests these failures could have occurred because we failed to control chemical equilibrium in rechargeable zinc-manganese energy storage systems.”

When the scientists, collaborating with the University of Washington, started considering zinc manganese oxide batteries as an alternative to lithium-ion, they assumed zinc ions would also move in and out of that battery’s electrodes like lithium.

After tests, the team found their device was undergoing a different process. Instead of moving zinc ions around, their battery was undergoing a reversible chemical reaction that converted its active materials into entirely new ones.

Liu and his colleagues built a battery with a negative zinc electrode, a positive manganese dioxide electrode and a water based electrolyte. Their tests showed the battery quickly lost its ability to store energy after just a few charging cycles.

Looking to explain the results, the team found that manganese oxide was reacting reversibly with protons from the water based electrolyte, creating zinc hydroxyl sulphate. Knowing the battery underwent chemical conversions, they determined the rate of manganese dissolution could be slowed by increasing the electrolyte’s initial manganese concentration.

When they tested the revised battery, the team saw it could store 285mAhr per gram of manganese oxide over 5000 cycles, while retaining 92% of its initial storage capacity.

Liu said: “Zinc-manganese oxide batteries could be a more viable solution for large-scale energy storage than the lithium-ion and lead-acid batteries used to support the grid today.”