Calcium-Antimony Alloys as Electrodes for Liquid Metal Batteries

Abstract

The performance of a calcium-antimony (Ca-Sb) alloy serving as the positive electrode in a Ca Sb liquid metal battery was investigated in an electrochemical cell, Ca(in Bi) | LiCl-NaCl-CaCl 2 | Ca(in Sb). The equilibrium potential of the Ca-Sb electrode was found to lie on the interval, 1.2-0.95 V versus Ca, in good agreement with electromotive force (emf) measurements in the literature. During both alloying and dealloying of Ca at the Sb electrode, the charge transfer and mass transport at the interface are facile enough that the electrode potential varies linearly from 0.95 to 0.75 V vs Ca(s) as current density varies from 50 to 500 mA cm −2 . The discharge capacity of the Ca Sb cells increases as the operating temperature increases due to the higher solubility and diffusivity of Ca in Sb. The cell was successfully cycled with high coulombic efficiency (∼100%) and small fade rate (<0.01% cycle −1 ). These data combined with the favorable costs of these metals and salts make the Ca Sb liquid metal battery attractive for grid-scale energy storage. The liquid metal battery (LMB) has been shown to be an attractive potential solution to the problem of grid-level storage.1,2 The LMB comprises two liquid metal electrodes separated by a molten salt electrolyte that self-segregate into three liquid layers according to density and immiscibility. In the search for even lower-cost chemistries based on this formula, the Ca-Sb system became the focus of attention because Ca, thanks to its ubiquitous abundance, 3 offers high performance at a low price point. Previous measurements in this laboratory of the thermodynamics of liquid Ca-Sb alloys 4 revealed this system to be high-voltage (0.94-1.04 V) and low-cost (69 $ kWh −1 ). However, Ca is highly soluble in its salts 5 conferring such a high level of electronic conductivity that a battery fitted with a liquid Ca electrode would exhibit an unacceptably high self-discharge current. In this study we show how to suppress the Ca metal solubility in the molten salt electrolyte so as to make practical a Ca-Sb LMB.The literature indicates that the solubility of calcium metal is significantly reduced by alloying calcium with other metals to decrease its activity (a Ca ).5 Following this, one can envisage a Ca-Sb liquid metal battery as:where A is the negative electrode host and the electrolyte can be a solution of molten halide salts with calcium cation as the itinerant. The alloying of calcium with a more noble A metal such as magnesium desirably decreases the melting temperature of the negative electrode, reduces the reactivity of pure calcium metal, and decreases the solubility of calcium in the molten salt electrolyte while undesirably decreasing the cell voltage, 6 albeit an acceptably small amount. For the cell with Mg as the negative electrode host and Sb as the positive electrode, the half-cell reactions are negative : Ca(in Mg) = Ca 2+ + 2epositive : Ca 2+ + 2e − = Ca(in Sb) [3] and the resulting overall cell reaction is cell : Ca(in Mg) = Ca(in Sb).[4]On discharge, calcium...

Electrolytes for electrochemical energy storage

“…Recently, there are several types of molten salts based liquid metal batteries being developed, including Mg│MgCl2-KClNaCl│Sb, 332 Ca│LiCl-NaCl-CaCl2│Bi, 334 CaLiCl-NaCl-CaCl2│ Sb, 334 Na │ NaOH-NaI │ Pb-Bi 335 and Li │ LiCl-LiF│ Bi 336 . In these liquid metal batteries, Mg, Ca, Na, and Li have been used as the reactive elements on the negtrode, whereas Sb, Bi, and Pb-Bi have been utilised as the positrode materials due to their relatively low melting temperatures.…”

Liquid Metal Electrodes for Energy Storage Batteries

“…However, during the charging process, the solid phase was turned back to pure liquid Bi, avoiding any dendrite formation and solid phase accumulation. More importantly, the [18,19] Li-Te, [20] Li-Bi, [21] Li-Sb, [22] Li-Sb-Pb [23] Na 92.3 1165 0.057 -2.71 1.6-3.0 [18,24,25] Na-Sn, [26] Na-Bi, [27] Na-S, [28] Zebra [29] K 64 687.2 5.1 -2.93 4.9-19 [18,30] K-Hg [31] Ca 842 1340 0.14 -2.87 2.3-9.6 [18,32] Ca-Bi, [33] Ca-Sb [34] Mg 650 2233 0.069 -2.37 0.2-1.2 [18,35] Mg-Sb [14] wileyonlinelibrary.com formation of the solid phase did not cause high concentration polarization under 1000 mA cm -2 , implying that the formed Li-Bi solid phase had high Li + conductivity and ensuring the high utilization of Bi. Table 3 shows the properties of positive electrode candidates corresponding to Li negative electrode.…”

“…Reproduced with permission. [34] Copyright 2014, The Electrochemical Society (ECS). [98] Na 1+x Zr 2 Si x P 3-x O 12 (0 ≤ x ≤ 3) was a typical NASICON produced by Ceramatec, Inc (Salt Lake City, Utah), and has been proposed to be used as a solid electrolyte for Na-S batteries.…”

Cathode candidates for zinc-based thermal-electrochemical energy storage

“…There is a negligible background current of <3 mA cm −2 , which could be attributed to charging currents or a leakage current due to Zn dissolution into the molten salt. Previous reports have deemed 1–2.5 mA cm −2 acceptable for LMBs containing Mg or Ca, whereas Na‐based systems have unacceptably high leakage currents around 50 mA cm −2 . The KCl component of the electrolyte plays three important roles: (i) it is a known suppressant of metal dissolution when added to that metal's molten chloride salt ; (ii) it forms a eutectic whose melting point lies below that of pure ZnCl 2 , thus permitting lower operating temperatures; and (iii) its viscosity is four orders of magnitude lower than ZnCl 2 , which, at 50 wt.% composition, greatly enhances ion diffusivity in the electrolyte while still maintaining a very high content of active Zn 2+ species.…”