Researchers at the US Department of Energy’s Pacific Northwest National Laboratory believe that they may have stumbled on a way to make zinc-manganese batteries a viable option for energy storage. Whereas previous studies consistently showed that zinc-manganese batteries have a short life span, quickly losing their ability to recharge, the new study shows that these batteries can be modified to have much better recharging rates.
Publishing their findings in the journal ’Nature Energy‘ PNNL Laboratory Fellow Jun Liu said that, “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.”
Battery Breakthrough
There has always been interest around rechargeable zinc-manganese batteries because both components (zinc and manganese) are relatively cheap and yet the battery can have an energy density similar to lead-acid batteries.
By building their own battery with “…a negative zinc electrode, a positive manganese dioxide electrode and a water-based electrolyte in between the two” the research team saw how the battery quickly failed to recharge to its original energy capacity. They then began to analyse the battery to discover why.
As the online journal ‘Science Daily‘ explains, “To find out, they first performed a detailed chemical and structural analysis of the electrolyte and electrode materials. They were surprised not to find evidence of zinc interacting with manganese oxide during the battery’s charge and discharge processes, as they had initially expected would happen. The unexpected finding led them to wonder if the battery didn’t undergo a simple intercalation process as they had previously thought. Perhaps the zinc-manganese battery is less like a lithium-ion battery and more like the traditional lead-acid battery, which also relies on chemical conversion reactions.”
By examining the electrodes in greater detail they found that, “…[the] manganese oxide was reversibly reacting with protons from the water-based electrolyte, which created a new material, zinc hydroxyl sulfate.
Typically, zinc-manganese oxide batteries significantly lose storage capacity after just a few cycles. This happens because manganese from the battery’s positive electrode begins to sluff off, making the battery’s active material inaccessible for energy storage. But after some manganese dissolves into the electrolyte, the battery gradually stabilizes and the storage capacity levels out, though at a much lower level.”
A New Battery Product
With a greater understanding of the process inside zinc-manganese batteries the team looked at ways to limit the rate of manganese sluff off.
As a report by Pacific Northwest National Laboratory acknowledges, “Knowing the battery underwent chemical conversions, they determined the rate of manganese dissolution could be slowed down by increasing the electrolyte’s initial manganese concentration.
So 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 of285 milliAmpere-hours per gram of manganese oxide over 5,000 cycles, while retaining 92 percent of its initial storage capacity.”
Whilst it is still too early to say in exactly what fields zinc-manganese batteries would be most suitable (the research was published on 18th April 2016), it has already caught the attention of battery manufacturers. Chemical traders have also shown an interest for the added value it may provide to suppliers of zinc and manganese, especially given the growing need for large-scale energy storage in a power hungry world.
As Liu himself noted when he said, “As a result [of our team’s discovery], 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.”