Lithium-ion technology dominates today’s energy storage market, but new alternatives are beginning to bubble up from research laboratories. In the latest development, a team of researchers at Pacific Northwest National Laboratories has unlocked the secret behind a reversible formula that could be used for low cost grid scale energy storage.
Re-Thinking An Energy Storage Formula
The new energy storage breakthrough is based on the familiar zinc-manganese formula. Researchers have been tinkering around with rechargeable zinc-manganese batteries for many years but they have gotten stuck on one problem: getting the manganese to stay on the positive electrode.
After only a few cycles manganese drifts off the electrode and settles into the electrolyte. Once that happens, the battery can hold only a fraction of its intended charge.
That may seem like a lost cause, but the allure of storing energy with relatively cheap, abundant, non-toxic materials is irresistible, so the PNNL team decided to give it another try.
The researchers started from scratch and built their own battery with the aim of finding out exactly why the manganese sloughs off. After subjecting it to chemical and structural analysis, they decided to rethink their entire approach.
The PNNL realized that they had been coming to the problem just like other researchers, with the idea that the zinc-manganese energy storage formula works like lithium-ion.
In a lithium-ion battery, lithium ions are simply passed back and forth between two electrodes. The new study revealed that when a zinc-manganese battery goes through charging cycles, it does something entirely different. The active materials in the battery undergo a reversible chemical reaction and create a new material, zinc hydroxyl sulfate.
A (Really) Rechargeable Non-Toxic Energy Storage Alternative
With that knowledge in hand, the PNNL team developed a strategy for slowing down the rate at which manganese could detach from the electrode:
…they added manganese ions to the electrolyte in a new test battery and put the revised battery through another round of tests. This time around, the test battery was able to reach a storage capacity of 285 milliAmpere-hours per gram of manganese oxide over 5,000 cycles, while retaining 92 percent of its initial storage capacity.
Sweet. Here’s the lowdown from the study, which you can find in the journal Nature Energy under the title “Reversible aqueous zinc/manganese oxide energy storage from conversion reactions:”
Here we demonstrate a highly reversible zinc/manganese oxide system in which optimal mild aqueous ZnSO4-based solution is used as the electrolyte, and nanofibres of a manganese oxide phase, α-MnO2, are used as the cathode. We show that a chemical conversion reaction mechanism between α-MnO2 and H+ is mainly responsible for the good performance of the system.
The team, which includes the University of Washington, also found that the zinc anode was highly stable.
However, the new PNNL energy storage solution could easily win on cost as well as energy density.
For now, the research team is focusing on grid scale energy storage for the new battery. It’s also possible that some variation of zinc-manganese could be used in EVs, or in gasmobiles as a replacement for conventional lead-acid batteries (as much as we love EV tech, the fact is that millions of gasmobiles — fossil and/or biofuel — will be with us for the foreseeable future).
The next steps for PNNL include digging deeper into the chain of reactions leading to the formation of zinc hydroxyl sulfate.
Meanwhile, work is rapidly progressing on systems for integrating renewable energy into the grid with the help of new energy storage solutions. That old trope about the unreliability of wind and solar power may have worked just a few years ago, but not today.
Image (cropped): via PNNL.
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