Case Studies on Non-Aqueous Zn Ion Systems to Develop Multivalent Ion Batteries
Authors: Han Sang-Don , Senguttuvan Premkumar , Kim Soojeong , Lipson Albert L. , Tepavcevic Sanja , Ingram Brian J. , Fister Timothy T , Johnson Christopher , Vaughey John T. 
Published: February 2020

For applications involving transportation and the electricity grid, future energy storage systems will require high energy density, fast charge/discharge times, increased safety, and low cost compared to current Li-ion batteries. Non-aqueous multivalent metal (Mg, Zn, Ca, Al) based cells are a promising advanced energy storage technology due to their higher theoretical volumetric capacity, limited dendrite formation, and low cost. A major need for these systems is the development of compatible electrolytes for both electrodes that show reversible multivalent intercalation cathodes.1,2 In the case of non-aqueous Mg or Ca ion systems the electrolyte compatibility issues (e.g., low Coulombic efficiency, a high overpotential, and corrosion) hold back the development of Mg or Ca metal batteries.3 However, non-aqueous Zn2+ ion chemistry in Zn metal cells with a reversible intercalation cathode is an exception among multivalent metals with a number of promising features including high volumetric capacity,1 similar ionic radius compared with Li+ and Mg2+ ions,4 relatively lower activation barrier energy for diffusion in cathode materials (e.g., FePO4, CoO2 and V2O5)5 and highly-efficient reversible Zn deposition behavior on a Zn metal anode with wide electrochemical window.3 Considering these advantages, a non-aqueous Zn system provides an opportunity to delve into the mechanisms in multivalent-ion cell chemistry and solve the present issues in multivalent cell design and prototyping.3

In this study, the intercalation chemistry on a variety of cathodes materials (e.g., V2O5 and Mn2O4) and reversible deposition/dendritic growth issues on a Zn metal anode have been investigated in various non-aqueous Zn electrolytes. The electrochemical and transport properties―reversible Zn deposition behavior, Coulombic efficiency, anodic stability, ionic conductivity and diffusion coefficient―were characterized utilizing the experimental and computational analysis. Among various Zn metal cells, a hydrated Zn/nanostructured bilayered V2O5 cell with an acetonitrile(AN)-Zn(TFSI)2 electrolyte demonstrates good reversibility and stability for 120+ cycles with nearly 100% Coulombic efficiency and ~170 mAhg-1 of gravimetric capacity, albeit operating at a cell voltage of 0.7 V.6 A low crystalline Zn/Nanostructured δ-MnO2 cell with an AN-Zn(TFSI)2 electrolyte also shows good reversibility (~100% Coulombic efficiency) and stability for 50+ cycles with ~100 mAhg-1 capacity and relatively higher operating voltage of 1.2 V. On the other hand, Zn dendrite growth studies on a Zn metal anode in non-aqueous Zn electrolytes have been performed under various conditions, including various current densities (0.1, 1.0, and 10 mA cm-2) and time (0.2 and 2.0 h cycle-1). The cycled Zn metal anodes were characterized using SEM-EDX and X-ray tomography to analyze morphological changes and dendritic growth in both selected regions and overall samples.

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