Communication—Molten Amide-Hydroxide-Iodide Electrolyte for a Low-Temperature Sodium-Based Liquid Metal Battery

Abstract

The low cost and high abundance of sodium make it an attractive choice for the negative electrode in a liquid metal battery. However, sodium has not found use in this application owing to the high solubility of the metal in its molten halides which results in poor coulombic efficiency and an unacceptably high rate of self discharge. In this work, we investigated the electrochemical behavior of the ternary eutectic of NaNH 2 , NaOH and NaI (m.p. 127 • C) and evaluated its usefulness as an electrolyte for sodium-based liquid metal batteries. Cyclic voltammetry revealed an electrochemical window of 1.3 V at 180 • C. The anodic limit is set by the oxidation of amide anions to form hydrazine gas. Liquid metal batteries consist of two metals of different electronegativity separated by molten salt. During discharge, the battery operates through alloying an electropositive liquid metal (e.g. Mg, Na, Li, or Ca) in an electronegative metal (e.g. Bi, Pb, Sb, or Zn). The all-liquid design allows the battery to operate at high current densities (1 A/cm 2 ) 1 with low overvoltage due to fast mass transport in the liquid state and high ionic conductivity of molten salts. Moreover, the absence of solid electrodes endows the battery with an extended service lifetime (>10,000 cycles). 2,3The negative and positive electrodes are selected based on melting point and density as well as the voltage associated with alloy formation, whereas the electrolyte is chosen based on melting point, metal solubility, density, ionic conductivity, and electrochemical window. 2Metal solubility in the electrolyte varies exponentially with operating temperature. High metal solubility in the electrolyte can influence the performance of liquid metal battery through increasing electronic conductivity, which increases the rate of self-discharge leading to lower coulombic efficiency. A low-melting electrolyte (<200• C) can substantially reduce metal solubility thereby allowing batteries to be built with low-cost negative electrodes (e.g., Na, Ca) that are otherwise inaccessible at higher temperatures (>500• C). Moreover, reducing the operating temperature below 200• C can also reduce the total battery cost through savings associated with cell container, thermal insulation, and wiring.Early work on low-melting sodium-bearing salts focused on mixtures of NaOH, NaI, and NaBr and was driven by their utility for sodium electrowinning. [4][5][6] The thought was that electrolysis at a lower temperature would reduce operating costs and improve process efficiency. More recently, in connection with work on sodium-based liquid metal batteries, Spatocco et al.7 investigated a binary eutectic of NaOH and NaI as an electrolyte operable at intermediate temperature (<280• C). An electrochemical window of 2.4 volts was measured at 250• C. In contact with liquid sodium, significantly lower values of self-discharge current were observed in this melt compared to those in early Na||Bi cells operating at much higher temperatures (necessitated by their higher-melting halide elec...

Competing forces in liquid metal electrodes and batteries

“…A third thermocouple is connected to a proportional-integral-derivative controller, which allows for the temperature to be maintained at 160 • C (at the bottom of the cell). The temperature is chosen to match the operating conditions of a Na||PbBi battery [87]. The apparatus minimizes heat exchange by using ceramic insulation, which allows for the operating temperature to be maintained for the duration of the experiment [see 49,88].…”

Fluid Mechanics of Liquid Metal Batteries

“…Much lower temperatures would allow less expensive container materials, like plastics instead of stainless steel. Thus substantial effort is being dedicated to the search for viable low-temperature liquid metal battery chemistries [79,23,204,205,206]. If found, they would have significantly different material properties than today's liquid metal batteries and therefore give rise to significantly different fluid mechanics.…”

Magneto-hydrodynamic stability of a liquid metal battery in discharge

“…-With the advancement of renewable intermittent energy sources the idea of Liquid Metal Battery (LMB) as a device for stationary energy storage is gaining attention [1]. The development of small-scale (about few cm) prototypes demonstrates technical feasibility of the battery and its advantages to existing solid batteries [1,2]. The liquid state of the main components necessitates consideration of the fluid dynamics in the LMBs.…”

“…The vertical component = 3 of the equation (1) gives the hydrostatic pressure distribution in the liquid layers accounting for the variable interface shapes 1 ( , , ) and 2 ( , , ) [12]. The depth averaged continuity equation (2) is accurate to all orders of approximation:…”