The result is a rechargeable battery that cost only as much as a car battery but store more energy, according to the US Pacific Northwest National Laboratory, which is grooming the technology for matching renewable energy sources to power grid loads.
Aqueous zinc-manganese oxide is not new battery chemistry, but “these batteries usually stop working after just a few charges,” said PNNL scientist Jun Liu. “Our research suggests these failures could have occurred because we failed to control chemical equilibrium in rechargeable zinc-manganese energy storage systems.”
Working with the University of Washington, PNNL researchers were working on a zinc-ion analogue to lithium-ion batteries, expecting zinc ions to shuttle between anode and cathode during charge and discharge.
“After tests, the team was surprised to realise their device was undergoing an entirely different process. Instead of simply moving the zinc ions around, their zinc-manganese oxide battery was undergoing a reversible chemical reaction that converted its active materials into entirely new ones,” said PNNL.
The battery has a negative zinc electrode, a positive manganese dioxide electrode and a water-based electrolyte in between the two.
Investigation showed no evidence of zinc interacting with manganese oxide during the battery’s charge and discharge processes, as they had initially expected – as it would in a zinc-ion cell.
Work with electron microscopes, nuclear magnetic resonance and x-ray diffraction – all available at the lab – showed manganese oxide was reversibly reacting with protons from the electrolyte creating zinc hydroxyl sulfate – a previously undiscovered material.
Short cycle life
Typically, according to PMML, zinc-manganese oxide batteries last only a few cycles because manganese leaves the positive. However, after some manganese dissolves into the electrolyte, the battery gradually stabilises and the storage capacity levels out at a much lower level.
The team used its new understanding of the electrochemistry knowledge to prevent manganese leaving by increasing the electrolyte’s initial manganese concentration.
Resulting capacity was 285mAh/g of manganese oxide over 5,000 cycles, while retaining 92%of initial storage capacity.
“This research shows equilibrium needs to be controlled during a chemical conversion reaction to improve zinc-manganese oxide battery performance,” Liu said. “As a result, zinc-manganese oxide batteries could be a more viable solution for large-scale energy storage than lithium-ion or lead-acid batteries.”
The team is now identifying in-between steps in the chemistry and working on the electrolyte.
For more, see ‘Highly reversible aqueous zinc/manganese oxide energy storage from conversion reactions‘ in Nature Energy.