Ultra-low cost rechargeable batteries could store sporadic wind and other energy sources until needed. A zinc-based battery seemed promising, but it quickly failed. Scientists discovered a reversible chemical conversion reaction mechanism, similar to what powers lead-acid car batteries. They used the reaction to manipulate the chemical balance inside the battery. The modified battery could be charged and discharged over 5,000 cycles, while retaining 92 percent of its initial storage capacity.
Large-scale energy storage for wind and other intermittent sources could make renewable energy easier to use. The research showed that rechargeable zinc-manganese oxide could be a more viable solution than today’s lithium-ion and lead-acid batteries. In addition, the zinc-based batteries use abundant, inexpensive, and environmentally friendly materials.
Zinc-manganese oxide rechargeable batteries have been studied for more than 20 years, but the batteries usually stop working after a limited number of charge-discharge cycles. Researchers at Pacific Northwest National Laboratory asked “why?” Most researchers had assumed that rechargeable zinc-based batteries work via a mechanism similar to rechargeable lithium-ion batteries. They assumed that during use the charged atoms (ions) move in and out of microscopic regions within the battery. However, they found that zinc-manganese oxide batteries have a reversible chemical reaction. In this reaction, one material is transformed into another, similar to the chemistry in lead-acid batteries. They found the manganese oxide on the positive electrode reversibly reacted with protons from the water-based electrolyte, zinc sulfate, to form manganese oxyhydroxide and zinc hydroxyl sulfate. The team made these findings using instruments at the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy scientific user facility. By understanding the inner workings of the batteries, they found that an optimal concentration of manganese sulfate added to the electrolyte suppressed the manganese ion dissolution reaction and stabilized the electrode. The addition slowed the degradation of the battery during use. The result was a better battery. It has a capacity of 285 mAhg-1, a much higher energy density than what lead-acid batteries offer. It retains 92 percent of this capacity after 5,000 charge cycles. This finding opens new opportunities for the development of low-cost, high-performance rechargeable aqueous batteries to store renewable energy for later use in the electrical grid.