Calcium–bismuth electrodes for large-scale energy storage (liquid metal batteries)

Calcium is an attractive electrode material for use in grid-scale electrochemical energy storage due to its low electronegativity, earth abundance, and low cost. The feasibility of combining a liquid Ca–Bi positive electrode with a molten salt electrolyte for use in liquid metal batteries at 500–700 °C was investigated. Exhibiting excellent reversibility up to current densities of 200 mA cm−2, the calcium–bismuth liquid alloy system is a promising positive electrode candidate for liquid metal batteries. The measurement of low self-discharge current suggests that the solubility of calcium metal in molten salt electrolytes can be sufficiently suppressed to yield high coulombic efficiencies >98%. The mechanisms giving rise to Ca–Bi electrode overpotentials were investigated in terms of associated charge transfer and mass transport resistances. The formation of low density Ca11Bi10 intermetallics at the electrode–electrolyte interface limited the calcium deposition rate capability of the electrodes; however, the co-deposition of barium into bismuth from barium-containing molten salts suppressed Ca–Bi intermetallic formation thereby improving the discharge capacity. •Electrochemical behavior of Ca–Bi alloys is investigated for liquid metal batteries.•Electrochemical set-up is developed for calcium as the electro-active species (itinerant ion).•Ca–Bi electrodes exhibit chemically reversible reactions and high coulombic efficiency.•Electrode overpotential of Ca–Bi alloys is investigated in terms of charge transfer and mass transport.•Co-deposition of Ba from the electrolyte improved the electrode performance.