(316d) Enhanced Cycling Performance of Iron Metal Batteries for Low-Cost Energy Storage

Renewable energy sources, like solar and wind, can decarbonize our energy generation and help to address the climate change grand challenge. However, their intermittent nature makes integrating them into the grid challenging. Battery energy storage can buffer the mismatch between renewable electricity generation and grid demand, but its high cost is a hurdle to wide deployment. Fe is the second most abundant metal in the earth’s crust and is the most-produced metal commodity, outperforming other metals like zinc and lithium. In addition, the Fe metal also has a high capacity, so the material-level energy storage cost is only $0.06/kWh, making it extremely promising for achieving the U.S. Department of Energy’s cost target for large-scale grid energy storage

However, the cycle life of Fe metal batteries is limited by the low Coulombic efficiency of the Fe deposition/stripping reaction. Aqueous electrolytes based on Fe chloride or sulfate salts are commonly used in Fe metal batteries, but they suffer from low Coulombic efficiency (<91%) under mild operating conditions due to the undesirable hydrogen evolution reaction (HER).

To address this issue, a series of novel Fe electrolytes reinforced with Mg ions (FERMI) and Ca ions (FERCI) have been developed in this study. The FERMI and FERCI electrolytes exhibit significantly improved Coulombic efficiency, higher conductivity, and better deposition morphology compared to the baseline FeCl₂ electrolyte. Adding 4.5 M MgCl₂ or CaCl2 into the FeCl₂ electrolyte enhances the Fe deposition/stripping efficiency to 99.1%, significantly improving the cycling performance of Fe metal batteries in both half-cells and full-cells.

Experiments and computational studies reveal that the enhanced Fe deposition/stripping efficiency can be attributed to two reasons (Figure): (1) The presence of Mg²⁺/Ca²⁺ leads to Fe deposition with a larger particle size and a smaller surface area, which tends to form less dead Fe during stripping. (2) Suppression of the HER through reduced free water, stabilization of the water structure toward reduction resistance, and modification of the Fe solvation structure.

These novel electrolytes not only enable a highly reversible Fe metal anode for low-cost energy storage technologies but also have the potential to address the HER side reaction problem in other aqueous electrochemical technologies, especially acidic systems, such as CO₂ reduction, NH₃ synthesis, and iron electrochemical making.

The findings presented in this study are significant for the development of low-cost and high-performance Fe metal batteries for grid-scale energy storage applications. The FERMI and FERCI electrolytes hold great potential for the commercialization of Fe metal batteries and have the potential to pave the way for more efficient and sustainable energy storage technologies.