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US researchers have shown that replacing the lithium planar anode in a lithium sulfur mediated redox flow battery with a high surface area scaffold enables 10 times faster cycling, up to 10 mA cm-2, without short circuits or voltage instability .
Redox flow batteries incorporating solid energy storage materials are attractive for high capacity grid-scale energy storage due to their high theoretical energy densities. However, its practical application is hindered by its low rate capacity.
Now, a group of researchers led by Albuquerque-based Sandia National Laboratories has demonstrated a lithium-sulfur redox-mediated flow battery that uses a large-area lithium scaffold to enable cycling even 10 times faster.
In their previous research, the group had designed a redox-mediated lithium-sulfur hybrid flow battery containing a lithium-sulfur metal anode in the catholyte reservoir. However, they had observed that the charging rate was limited by the growth of dendrites on the lithium anode.
This problem, common to many lithium metal-based redox flow batteries, imposes considerable design constraints. Specifically, the limited charging rate increases the minimum required size of the electrochemical cell for a given power and increases the costs of the system.
To address these limitations, the researchers replaced the planar lithium anode in lithium sulfur-mediated redox flow batteries with a high surface area scaffold, which enabled 10 times faster cycling, up to 10 mA cm-2, compared to the same systems with planar anodes, without short circuits or voltage instability.
In their latest study, the researchers first tested the high-surface nickel foam in symmetrical lithium/lithium cells and then in a full prelithiated redox flow battery.
In addition, they improved the cells performance by adding zinc oxide to the nickel foam, which promotes better wetting of the lithium, which improves the Coulombic efficiency of the cells.
Finally, they demonstrated that the use of zinc oxide and nickel foam scaffolding also allows redox flow batteries to be built in an “anodeless” configuration, which improves safety and reduces the cost of assembling and shipping a battery.
More importantly, these improved cells have also proven their scalability. Specifically, when the sulfur load is increased from 2.4 to 5 mg cm-2, the capacity also increases. In fact, at 5 mgS cm-2 charges, the power density of its redox flow battery exceeded 20 Wh L-1, making it comparable to vanadium redox flow batteries.
“Having addressed the limitations of the Li anode, there is now further scope to improve the system by investigating the kinetic limitations of the Li-S reaction and the capacitance fade caused by polysulfide displacement,” the researchers write. “The rapidity of the cycles and the scalability of the system demonstrate that it is viable for future grid-scale energy storage.” |
