Magnesium���Antimony Liquid Metal Battery for Stationary Energy Storage David J. Bradwell, Hojong Kim,* Aislinn H. C. Sirk,��� and Donald R. Sadoway* Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, United States * S Supporting Information ABSTRACT: Batteries are an attractive option for grid- scale energy storage applications because of their small footprint and flexible siting. A high-temperature (700 ��C) magnesium���antimony (Mg||Sb) liquid metal battery comprising a negative electrode of Mg, a molten salt electrolyte (MgCl2���KCl���NaCl), and a positive electrode of Sb is proposed and characterized. Because of the immiscibility of the contiguous salt and metal phases, they stratify by density into three distinct layers. Cells were cycled at rates ranging from 50 to 200 mA/cm2 and demonstrated up to 69% DC���DC energy efficiency. The self-segregating nature of the battery components and the use of low-cost materials results in a promising technology for stationary energy storage applications. L arge-scale energy storage is poised to play a critical role in enhancing the stability, security, and reliability of tomorrow���s electrical power grid, including the support of intermittent renewable resources.1 Batteries are appealing because of their small footprint and flexible siting however, conventional battery technologies are unable to meet the demanding low-cost and long-lifespan requirements of this application. A high-temperature (700 ��C) magnesium���antimony (Mg||Sb) liquid metal battery comprising a negative electrode of Mg, a molten salt electrolyte (MgCl2���KCl���NaCl), and a positive electrode of Sb is proposed (Figure 1). Because of density differences and immiscibility, the salt and metal phases stratify into three distinct layers. During discharge, at the negative electrode Mg is oxidized to Mg2+ (Mg ��� Mg2+ + 2e���), which dissolves into the electrolyte while the electrons are released into the external circuit. Simultaneously, at the positive electrode Mg2+ ions in the electro- lyte are reduced to Mg (Mg2+ + 2e��� ��� MgSb), which is deposited into the Sb electrode to form a liquid metal alloy (Mg���Sb) with attendant electron consumption from the external circuit (Figure 2). The reverse reactions occur when the battery is charged. Charging and discharging of the battery are accompanied by volumetric changes in the liquid electrodes. The difference in the chemical potentials of pure Mg (��Mg) and Mg dissolved in Sb [��Mg(in Sb) ] generates a voltage that can be expressed as Ecell RT aMg(in aMg 2F ln��� Sb) = ��� ��� ��� ��� ��� ��� ��� where R is the gas constant, T is temperature in Kelvins, F is the Faraday constant, aMg(in Sb) is the activity of Mg dissolved in Sb, and aMg is the activity of pure Mg. Recent work on self-healing Li���Ga electrodes for lithium ion batteries has demonstrated the appeal of liquid components.2 While solid electrodes are susceptible to mechanical failure by mechanisms such as electrode particle cracking,3 these are inoperative in liquid electrodes, potentially endowing cells with unprecedented lifespans. The self-segregating nature of liquid electrodes and electrolytes could also facilitate inexpensive manufacturing of a battery so constructed. However, there do not appear to be economical materials options that exist as liquids at or near room temperature. Previous work with elevated-temperature liquid batteries demon- strated impressive current density capabilities (1000 mA/cm2 when discharged at 0 V) with a variety of chemistries.4���7 However, that work generally used prohibitively expensive metalloids (such as Bi and Te) as the positive electrode. The resulting cells exhibited self-discharge current densities of 40 mA/cm2, attributed to the solubility of the negative electrode metal (i.e., Na) in the Received: October 17, 2011 Published: January 6, 2012 Figure 1. Sectioned Mg||Sb liquid metal battery operated at 700 ��C showing the three stratified liquid phases upon cooling to room temperature. The cell was filled with epoxy prior to sectioning. Communication pubs.acs.org/JACS �� 2012 American Chemical Society 1895 dx.doi.org/10.1021/ja209759s | J. Am. Chem.Soc. 2012, 134, 1895���1897

electrolyte.5 These systems failed to achieve commercial success, possibly because of a lack of interest in grid-scale storage at that time or the use of high-cost metalloids. Sb is less costly ($7/kg average commodity price over the past 5 years) and more earth-abundant than Bi ($24/kg) and Te ($150/kg).8 When costs are compared on a per-mole basis (which is more relevant when considering the cost per unit of energy storage capacity), Sb ($0.74/mol) appears even more appealing than Bi ($4.40/mol) and Te ($19.19/mol). Interestingly, the use of Sb had not, until now, been demonstrated in a liquid metal battery. Mg was selected as the negative electrode material on the basis of its low cost ($5.15/kg, $0.125/mol), high earth abundance, low electronegativity, and overlapping liquid range with both Sb and candidate electrolytes. The electrolyte was MgCl2:NaCl:KCl (50:30:20 mol %), which was selected on the basis of its sufficiently low melting point (396 ��C9) and the greater electrochemical stability of NaCl and KCl in comparison with MgCl2.10 Mg||Sb single cell batteries were assembled in the fully charged state in an Ar-filled glovebox, placed inside a sealed test vessel, and heated in a vertical tube furnace to 700 ��C. When the cell was heated above the melting point of the molten salt, cell open-circuit voltages were found to stabilize at ���0.44 V, consistent with thermodynamic data.11 The cells were electrochemically characterized by cyclic voltammetry (CV) and electrochemical impedance spectrosco- py (EIS) using a two-electrode electrochemical setup with the negative electrode (Mg) as the counter electrode/reference electrode and the positive electrode (Sb) as the working electrode. The cells exhibited negligible charge-transfer over- potentials, as demonstrated by the linearity of the current���voltage relationship in the CV scans and the absence of an obvious semicircle in the EIS scans. The slope of the CV was consistent with the area-normalized solution resistance as measured through EIS (typically 1.1 �� cm2), further demonstrating IR voltage loss to be the dominant overpotential. There were, however, indications of mass-transport limi- tations under certain conditions. The cells exhibited increased cell impedance at lower EIS scan frequencies, suggesting that at long time periods the reaction rates might be limited by diffusion.12 Mass-transport limitations could arise from local depletion of Mg2+ ions in the electrolyte at either of the electrode���electrolyte interfaces or Mg mass-transport limi- tations in the Mg���Sb electrode at the Mg���Sb electrode| electrolyte interface. Further electrochemical characterization was performed. Stepped-potential experiments indicated low leakage current densities of 1 mA/cm2, well below those of previously studied systems. This was attributed to the complexation of Mg2+ by ligand donors from the supporting electrolyte (NaCl, KCl)13 and the attendant suppression of metal solubility in its halide salts.14 Cells cycled at 50 mA/cm2 for a predefined discharge period of 10 h to a cutoff charging voltage limit of 0.85 V achieved a round-trip Coulombic efficiency of 97% and a voltage efficiency of 71%, resulting in an overall energy efficiency of 69% (Figure 3a). At full discharge, the composition of the positive (bottom) liquid electrode was estimated to be 12 mol % Mg and 88 mol % Sb. Cells were fully discharged at various rates ranging from 50 to 200 mA/cm2 with 0.05 V as the discharge cutoff limit (Figure 3b). Operation at higher current density resulted in increased IR voltage loss and decreased capacity, consistent Figure 2. Schematic of a Mg||Sb liquid metal battery comprising three liquid layers that operates at 700 ��C. During charging, Mg is electrochemically extracted from the Mg���Sb alloy electrode and deposited as liquid Mg on the top (negative) electrode. During discharging, the Mg electrode is consumed, and Mg is deposited into the Mg���Sb liquid bottom (positive) electrode. During charging, the battery consumes energy upon discharge, the battery supplies energy. Figure 3. Electrochemical performance of a Mg||Sb liquid metal battery operated at 700 ��C. (a) Variation of the cell voltage with the state of charge over one cycle. The current was set at 50 mA/cm2. (b) Deep discharge results at different current rates. The theoretical cell EMF was calculated from data in the literature.11 Journal of the American Chemical Society Communication dx.doi.org/10.1021/ja209759s | J. Am. Chem.Soc. 2012, 134, 1895���1897 1896