A liquid metal battery (LMB) consists entirely of liquid active components: a low-density liquid metal negative electrode, an intermediate-density molten salt electrolyte and a high-density liquid metal positive electrode. Due to their mutual immiscibility these active components further self-segregate into three distinct layers according to their densities. On discharge, the negative electrode is oxidized to form an itinerant ion, which migrates across the molten salt electrolyte to the positive electrode, where the itinerant ion is electrochemically reduced to neutral metal, alloying with the positive electrode. This process is reversed upon charging. The LMB is well-positioned to satisfy the demands of grid-scale energy storage due to its ability to vitiate capacity fade mechanisms present in other battery chemistries and to do so with earth abundant materials and easily scalable means of construction1,2.

Owing to its high solubility in molten salts calcium is impractical as an electrode1,3,4. Metal solubility renders the molten salt electronically conductive5, which leads to loss of coulombic efficiency in electrolysis and loss of stored energy in a battery, that is, so-called self-discharge. In addition, the strong reducing capability of this electropositive element dispersed in the molten salt makes containment problematic, as most commonly used materials are susceptible to calciothermic reduction6. Herein we have made calcium the negative electrode of the LMB by devising parallel mitigation strategies to dramatically decrease its chemical potential so as to suppress both solubility and reactivity while advantageously lowering the melting temperature of the metal–salt couple.

The detrimental dissolution reaction of calcium metal in calcium halides can be represented by the following4,5,7:

where calcium metal (Ca) reacts with calcium cations (Ca2+) to form subvalent ions (Ca+or ). Using the latter case as an example, the equilibrium constant of the dissolution reaction is therefore given as:

where is the activity of dissolved subvalent calcium, a Ca the activity of calcium metal in negative electrode, and the activity of calcium cation in the electrolyte. Focusing on the contributions of the reactants, we reason that suppressing the activity of calcium metal in the negative electrode, a Ca , by alloying with more electronegative metals acting as diluents and that lowering the activity of Ca2+ in the electrolyte, , by the introduction of other cations, should result in attendant reductions in the concentration of subvalent while simultaneously decreasing the reactivity and melting temperature of the negative electrode.

It has been shown in our previous studies that alloying calcium with various positive electrodes drops the activity of calcium to values as low as 10–9 (for bismuth) and 10–10 (for antimony)8,9, indicative of strong chemical interactions. The low activity of calcium in these electrodes contributes to the high-cell voltage of calcium-based cells as well as to the suppression of calcium metal dissolution from the positive electrodes3. In previous work, we have confirmed the bi-directionality of these positive electrodes with coulombic efficiencies exceeding 99% for both calcium–bismuth (Ca–Bi) and calcium–antimony (Ca–Sb)10,11. Clearly, to obtain high-cell voltage, the activity of calcium in the negative electrode should be as high as possible. The challenge is how to suppress the solubility of calcium metal from the negative electrode without making it denser than the electrolyte or raising the melting point, while minimally reducing cell voltage. The present study shows that magnesium (Mg) is an effective diluent as it lowers the liquidus temperature of calcium–magnesium (Ca–Mg) alloy (T eutectic =443 °C and 517 °C (ref. 12)) while supporting adequate calcium activity over a wide range of composition12. Specifically, an electromotive force study of Ca–Mg alloys suggests that the cell voltage reduction can be minimized to <0.1 V as long as the calcium concentration remains above 30 mol% (ref. 12).