(190b) Electrochemical Behavior of Positive Electrodes for Calcium-Based Liquid Metal Battery

As a large-scale electrochemical energy storage device, we have developed a liquid metal battery (LMB), which consists of a low-density liquid negative electrode, an intermediate-density molten salt electrolyte, and a high-density liquid positive electrode. The system self-segregated into three distinct layers due to their mutual immiscibility and density difference. During battery discharge, the negative electrode is oxidized to form an itinerant ion which conducts through the electrolyte to the liquid positive electrode, where the ion is electrochemically reduced to neutral metal, which alloys with the positive electrode. This process is reversed on charging. As the negative electrode of such large-scale batteries, calcium is an attractive material due to its low electronegativity (consequent high cell voltage) and earth abundance, and the resulting low electrode material cost. However, the use of metallic calcium as a negative electrode material has historically had a high barrier due to its high melting temperature, high reactivity, and high solubility in its molten salts. In this work, the electrochemical behavior of candidate positive electrodes (e.g. Bi, Sb) was systematically investigated in three-electrode electrochemical cell to evaluate the feasibility of Ca-based LMBs. Due to the low activity of Ca in the positive electrodes (< 10^-10), the solubility of Ca from the positive electrodes into the electrolyte minimized. As a result of this low solubility, self-discharge current was as low as ~1 mAcm^-2 and the alloying-dealloying process exhibited 99 % coulombic efficiency and small capacity loss with cycling (< 0.01%). The kinetics of alloying and dealloying of Ca at the positive electrodes were facile enough to charge-discharge the cell at ~1 Acm^-2 current density without irreparable damage. These data combined with the favorable costs of these metals and salts make the Ca-based liquid metal battery attractive for grid-scale energy storage.