Scientists from the U.S. Army Research Laboratory and the University of Maryland have proposed a number of strategies to help rechargeable zinc metal batteries reach commercial viability and compete with other storage technologies.
The researchers presented their findings in “Realizing high zinc reversibility in rechargeable batteries,” recently published in Nature Energy. They said that the technology needs to achieve extremely reversible zinc plating/stripping, with Coulombic efficiencies (CEs) approaching 100%. This measures the charge efficiency by which electrons are transferred in batteries. It is given by the ratio of the total charge extracted from the battery to the total charge put into the battery over a full cycle.
The main problem with the technology is the interaction between the zinc metal and the electrode, which is responsible for the devices’ poor Coulombic efficiency, the researchers said. They also pointed to large voltage polarization and a propensity for dendritic failure, “each of which has plagued Zn reversibility for centuries dating back to Alessandro Volta’s Cu/Zn piles.”
They said that a lack of standard CE determination protocols under uniform conditions – which precludes effective comparisons across electrolyte systems – is what is currently hindering the commercialization of similar batteries.
“This not only creates public confusion in accurately assessing the technology status but also, more importantly, it prevents researchers from developing and evaluating new materials and obscures funding managers from allocating resources to the most promising systems,” they explained.
In order to address the issue, the academics analyzed all research on the reversibility of electrochemical plating and the stripping of zinc metal electrodes. The four key parameters for their analysis were areal current density, areal capacity per cycle, cumulative capacity of zinc plated, and the associated Coulombic efficiencies. They said their research revealed a significant gap between the reported CE results and their relevance for the manufacturing of zinc batteries.
“For Zn, or perhaps all metal-based battery chemistries, cyclic voltammetry (CV) should be excluded as a method for CE determination in future studies besides its role as a preliminary screening technique,” they wrote. Cyclic voltammetry is a potentiodynamic electrochemical measurement performed by cycling the potential of a working electrode and measuring the resulting current.
As an alternative, they suggested the adoption of a galvanostatic reservoir CE protocol with standard parameters for current density, areal capacity, and upper cut-off voltage. They defined this solution as a “powerful screening tool for gauging commercial viability of Zn systems and establishing a better understanding of the links between plating/stripping reversibility and Zn morphology, dendrite formation and cycle life.”
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