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Rechargeable Zn–Air Batteries Enabled By Zn Sponge Anodes and Bi(tri?)Functional Cathodes

Thursday, 5 October 2017: 15:50
Maryland A (Gaylord National Resort and Convention Center)
J. F. Parker (U.S. Naval Research Laboratory), C. N. Chervin (Former Staff Scientist at U.S. Naval Research Laboratory), M. Vila (Undergraduate Research Student), D. R. Rolison, and J. W. Long (U.S. Naval Research Laboratory)
Zinc–air batteries have the potential to overcome many of the disadvantages of current Li-ion batteries: safety concerns associated with toxic and flammable electrolytes, high costs, and specific energy <200 Wh kg–1. Zinc–air provides higher specific energy (up to 450 Wh kg–1), while using safe, low-cost components and aqueous electrolytes. Although commercially successful as a primary battery, broader implementation of Zn–air batteries is hindered by limited rechargeability and modest zinc utilization (typically <60% of theoretical discharge capacity). This long-standing performance roadblock is attributed to the manner in which Zn electrodes are commonly fabricated—by mixing Zn powder with binders and gelling agents to form composite electrodes. During discharge of these ad hoc structures, Zn is oxidized and forms soluble zincate, which diffuses from its point of electrogeneration until it supersaturates and then dehydrates to high-impedance products (e.g., ZnO). Upon electrochemical recharge, ZnO, in equilibrium with soluble zincate, is reduced back to Zn0, often accompanied by a shape change. Discharge–charge cycling leads to pronounced morphological changes, especially at points of high local current density, where short-inducing dendrites can form. We developed a Zn “sponge” form factor comprising interpenetrating, co-continuous networks of solid and void which imposes more uniform current distribution throughout the volume of the electrode structure, minimizes uneven reaction loci, and suppresses formation of dendrites. We have demonstrated that the Zn sponge electrode can be cycled in prototype Ag–Zn and Ni–Zn cells without the formation of dendrites, but translating that to the Zn–air case requires the use of air-cathodes that also support electrochemical discharge and recharge. For that more ambitious cell chemistry, we turn to powder-composite air cathodes that include mesoporous oxides of manganese and/or a solid-solution or mixed-phase oxides of transition metals such as nickel and/or iron prepared via sol–gel synthetic methods. Zinc–air cells were constructed and cycled at high depths-of-discharge relative to the Zn, an achievement that not only validates the ability for the air cathode to be bifunctionally catalyzed, but to operate at capacities and rates that turn all of the Zn in the anode over enough times to generate a technologically relevant specific energy.