Unlike conventional zinc powder anodes (left), spongelike zinc anodes resist dendrite formation (right).
The relatively low cost of zinc coupled with its wide availability and favorable electrochemical properties should give zinc-based batteries a competitive advantage over other battery chemistries. In particular, Zn batteries could be safer than lithium-ion batteries because Zn ones use aqueous electrolytes instead of the flammable organic kinds standard in Li-ion batteries.
But aqueous Zn-based batteries fail quickly. Upon recharging, the metal forms wiry dendrites that can grow uncontrollably and pierce the separator between the electrodes. The dendrites can then connect the positive and negative electrodes and short-circuit the battery.
Scientists at the Naval Research Laboratory (NRL) in Washington, D.C., have shown that those problems can be bypassed by using zinc electrodes with a spongelike structure instead of conventional pressed powder electrodes.
The team, which includes Joseph F. Parker, Debra R. Rolison, and Jeffrey W. Long, prepared the zinc sponges by adding zinc powder to an emulsion of oil and water and then allowing the mixture to dry overnight.
The sponge structure leads to more uniform oxidation of the zinc metal during discharge and, consequently, a more uniform coating of the discharge product, zinc oxide, on the sponge anode. Likewise, the structure makes the reverse reaction during charging—ZnO reduction to metallic Zn—more uniform.
Even when 90% of the zinc is oxidized during discharge, Parker notes, the sponge retains a metallic zinc core. The core causes electric currents to be distributed uniformly throughout the sponge, making it physically difficult to form dendrites, he adds.
The team found that the sponge electrodes protected a Ni-Zn battery when it cycled repeatedly between charging and discharging under demanding current conditions that induce dendrite formation in reference batteries. It also enabled the battery to withstand tens of thousands of cycles required for “start-stop” microhybrid vehicles.
“For quite some time, this team and others have been attempting to use 3-D structured electrodes to enhance rechargeable battery performance,” says Paul V. Braun, professor of materials science and chemistry at the University of Illinois, Urbana-Champaign.
Braun notes that the NRL team “has found a particularly compelling system, where the 3-D electrode structure provides high power, as expected, but perhaps surprisingly, results in dendrite suppression and thus very good long term cycling.”
He adds “this discovery is particularly useful because it is accomplished with an intrinsically safe, earth abundant, and relatively high-energy-density nickel-zinc chemistry.”