A team of scientists working on analyzing energy flows in prototype zinc-manganese batteries have stumbled upon a new way to make these power cells much more reliable, with many more recharge cycles than the humble lead-acid car battery, but costing around the same to produce. The creators claim that the new battery could become an inexpensive, ecologically-sound alternative for storing energy from renewable sources and a high-density solution for storing excess energy from the power grid.
Working at the Department of Energy's Pacific Northwest National Laboratory (PNNL), the researchers discovered a new way to approach the reliability problems of zinc-manganese batteries, that were cheap and easy to make from abundant materials, but which would fail after only a few charge cycles.
"The idea of a rechargeable zinc-manganese battery isn't new; researchers have been studying them as an inexpensive, safe alternative to lithium-ion batteries since the late 1990s," said PNNL Laboratory Fellow Jun Liu. "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."
Collaborating with researchers at the University of Washington, Liu and his team had begun by investigating rechargeable zinc-manganese batteries as inexpensive alternatives to lead-acid batteries because of the plentiful and cheap supplies of zinc and manganese. Whilst not expecting to produce any ground-breaking discoveries, the PNNL researchers had hoped to at least produce a better-performing, longer-lasting Zn-Mn battery by seeing if they could overcome earlier failures by other scientists.
Years of study on lithium-ion (Li-ion) batteries and their electrical characteristics had blinkered many researchers into believing that the behavior of lithium ions in those batteries would be replicated in the Zn-Mn cells. To store and release energy in Li-ion cells, a process known as intercalation (where lithium ions moving in and out of microscopic spaces in between the atoms of the cell's two electrodes) occurs.
Much to the surprise of the PNNL team, however, a range of tests actually showed that the device being analyzed was undergoing a completely different process. Where a Li-ion battery would move its ions around in the charging process, the Zn-Mn version was actually being subject to a (hitherto unknown) reversible chemical reaction that transformed the active materials in the electrodes into a completely different substance known as zinc hydroxyl sulfate.
Once the team realized that something different may be going on in the Zn-Mn unit they built, and that something may be that the Zn-Mn battery acted more like a lead-acid one, they decided to bring out the big guns in the form of X-ray diffraction, nuclear magnetic resonance imaging (MRI), and transmission electron microscopy.
What they found was a complete surprise to them all. Tests showed that the battery's manganese oxide positive anode was reversibly reacting with protons from the water-based electrolyte in which it was immersed, to create the new zinc hydroxyl sulfate material. As a result, the new material soon coated the electrode, and the power flow and cycle capabilities were reduced considerably.
Using their new-found knowledge, the team then went about finding ways to reduce (or even stop) this process. Realizing that chemical conversions were the culprit, they simply figured out that the pace at which the manganese was being transformed could be reduced by upping the manganese concentration in the electrolyte before applying power. (Interestingly, this is not too dissimilar to the research on Lithium-air batteries that sees great improvements when their electrolyte mixes are altered to reduce electrode disintegration.)
And it worked. The researchers claim that the tiny test battery achieved a storage capacity of 285 mAh per gram of manganese oxide over an extraordinary 5,000 cycles, with 92 percent of its initial storage capacity retained.
"This research shows equilibrium needs to be controlled during a chemical conversion reaction to improve zinc-manganese oxide battery performance," said Liu. "As a result, 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."
The researchers plan to carry on their analysis of how the zinc-manganese oxide battery operates, in the hope of further increasing their knowledge of the reactions and to fiddle with the electrolyte concentrations to try and wring out as much efficiency as possible.
The results of this research were published in the journal Nature Energy.
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