The ability to store energy on the electric grid would greatly improve its efficiency and reliability while enabling the integration of intermittent renewable energy technologies (such as wind and solar) into baseload supply. Batteries have long been considered strong candidate solutions owing to their small spatial footprint, mechanical simplicity and flexibility in siting. However, the barrier to widespread adoption of batteries is their high cost. Here we describe a lithium-antimony-lead liquid metal battery that potentially meets the performance specifications for stationary energy storage applications. This Li||Sb-Pb battery comprises a liquid lithium negative electrode, a molten salt electrolyte, and a liquid antimony-lead alloy positive electrode, which self-segregate by density into three distinct layers owing to the immiscibility of the contiguous salt and metal phases. The all-liquid construction confers the advantages of higher current density, longer cycle life and simpler manufacturing of large-scale storage systems (because no membranes or separators are involved) relative to those of conventional batteries. At charge-discharge current densities of 275 milliamperes per square centimetre, the cells cycled at 450 degrees Celsius with 98 per cent Coulombic efficiency and 73 per cent round-trip energy efficiency. To provide evidence of their high power capability, the cells were discharged and charged at current densities as high as 1,000 milliamperes per square centimetre. Measured capacity loss after operation for 1,800 hours (more than 450 charge-discharge cycles at 100 per cent depth of discharge) projects retention of over 85 per cent of initial capacity after ten years of daily cycling. Our results demonstrate that alloying a high-melting-point, high-voltage metal (antimony) with a low-melting-point, low-cost metal (lead) advantageously decreases the operating temperature while maintaining a high cell voltage. Apart from the fact that this finding puts us on a desirable cost trajectory, this approach may well be more broadly applicable to other battery chemistries.
Calcium is an attractive material for the negative electrode in a rechargeable battery due to its low electronegativity (high cell voltage), double valence, earth abundance and low cost; however, the use of calcium has historically eluded researchers due to its high melting temperature, high reactivity and unfavorably high solubility in molten salts. Here we demonstrate a long-cycle-life calcium-metal-based rechargeable battery for grid-scale energy storage. By deploying a multi-cation binary electrolyte in concert with an alloyed negative electrode, calcium solubility in the electrolyte is suppressed and operating temperature is reduced. These chemical mitigation strategies also engage another element in energy storage reactions resulting in a multi-element battery. These initial results demonstrate how the synergistic effects of deploying multiple chemical mitigation strategies coupled with the relaxation of the requirement of a single itinerant ion can unlock calcium-based chemistries and produce a battery with enhanced performance.
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...
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