PNNL's New Rechargeable Battery Offers Inexpensive, Eco-Friendly Alternative for Storing Renewable Energy

A research team has unexpectedly discovered a rechargeable battery, which is as cheap as the standard car batteries, but with very high energy density. This new battery could be used as an alternative to support the power grid and store renewable energy in an environmentally friendly and cost-effective manner.

PNNL's improved aqueous zinc-manganese oxide battery offers a cost-effective, environmentally friendly alternative for storing renewable energy and supporting the power grid. (CREDIT: PACIFIC NORTHWEST NATIONAL LABORATORY)

Researchers from the Department of Energy's Pacific Northwest National Laboratory (PNNL) discovered this energy storing device after they realized that the new battery operates in a totally different manner than what was assumed by them.

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, 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.

Jun Liu, Laboratory Fellow, PNNL

Chemically inclined

Researchers have been focusing for a very long time on rechargeable lithium-ion batteries, and this has resulted in them exploring the back-and-forth movement of lithium ions. A process known as intercalation is used by the lithium-ion batteries to store and discharge energy. In this process, lithium ions enter and leave microscopic spaces existing between the atoms of two electrodes of a battery.

This concept has been extensively established in energy storage research. When PNNL scientists collaborated with the University of Washington, they started to consider a cost-effective, safe alternative to lithium-ion batteries. The scientists considered a zinc-manganese oxide battery capable of being recharged, as they believed that zinc will be capable of similarly entering and exiting the battery's electrodes.

The team carried out a number of tests and was surprised to discover that the device was experiencing a totally different process. Instead of just moving around the zinc ions, the team’s zinc-manganese oxide battery was going through a reversible chemical reaction that transformed its active materials into completely new ones.

Attractive alternative

Liu and his team started to examine rechargeable zinc-manganese batteries because they are found to be attractive on paper. These batteries can be extremely cheap, like the lead-acid batteries, because they utilize inexpensive and abundant materials like manganese and zinc. The battery can also have higher energy density than lead-acid batteries. The PNNL scientists assumed that by investigating more on the inner performance of the zinc-manganese oxide battery, they would be able to develop a battery with enhanced performance.

Based on this assumption, the scientists went ahead and developed a battery with a positive manganese dioxide electrode, a negative zinc electrode, and a water-based electrolyte in between the two. The team placed tiny, button-sized test batteries through the wringer, and then constantly charged and discharged them. As had been discovered previously, their test battery rapidly lost it's ability to store energy after just a few charging cycles.

Detailed investigation

To investigate the cause for this issue, the scientists carried out an exhaustive structural and chemical analysis of the electrode materials and the electrolyte. To their surprise, the scientists failed to identify evidence of zinc mingling with manganese oxide during the charging and discharging processes of the battery. This unexpected finding led the scientists to wonder if the battery failed to go through a simple intercalation process as they had earlier thought. There is the possibility of the zinc-manganese battery to be unlike a lithium-ion battery and more similar to the standard lead-acid battery, which also depends on chemical conversion reactions.

To investigate at a deeper level, the scientist analyzed the electrodes with a number of enhanced instruments along with a wide range of scientific techniques, such as nuclear magnetic resonance, X-ray diffraction, and transmission electron microscopy. The instruments used for this analysis were obtained from the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science user facility located at PNNL. By incorporating all of these techniques, the scientists discovered that manganese oxide was reacting in a reversible manner with protons from water-based electrolyte. This resulted in developing a new material called zinc hydroxyl sulfate.

Generally, zinc-manganese oxide batteries lose a considerable amount of storage capacity immediately after a few cycles. This occurs when manganese from the positive electrode of the battery starts to sluff-off, making the active material of the battery to be inaccessible for storing energy. The battery begins to stabilize in a gradual manner and the storage capacity of the battery is leveled out when a small amount of manganese dissolves into the electrolyte.

A simple fix

The scientists used this new information to avoid further occurrence of the manganese sluff-off. Being aware that the battery went through chemical conversions, the team estimated that the rate of manganese dissolution can be reduced by increasing the initial manganese concentration of the electrolyte.

The team introduced manganese ions into the electrolyte in a new test battery, and then allowed the updated battery to pass through another set of test rounds. This time, the battery reached a storage capacity of 28 mA-hours per gram of manganese oxide over 5,000 cycles, while preserving 92% of its initial storage capacity.

This research shows equilibrium needs to be controlled during a chemical conversion reaction to improve zinc-manganese oxide battery performance. 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.

Jun Liu, Laboratory Fellow, PNNL

The scientists plan to continue with their research on the basic operations of the zinc-manganese oxide battery. Having understood the products of the chemical conversion reactions of the battery, the team now plans to determine different in-between processes that help to develop those products. The scientists will also focus on the battery's electrolyte to learn how the battery’s performance will be affected by additional changes.

This research was supported by DOE's Office of Science and used resources at the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science user facility located at PNNL.

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