The electrochemical behavior of a ternary eutectic mixture of sodium amide (NaNH₂), sodium hydroxide (NaOH) and sodium iodide (NaI) was investigated to elucidate its validity as an electrolyte for sodium-based liquid metal batteries. The liquid metal battery consists of two metals with different electronegativity separated by molten salt. The self-segregating nature of the three layers allows for easy assembly and scalability. Moreover, the all-liquid design allows the batteries to operate at high current densities with negligible losses because of the high conductivity of molten salts and fast transport properties in the liquid state.

Sodium-based liquid metal batteries are particularly attractive due to the low cost of sodium metal (Na) and its low melting temperature (98°C). However, due to the high melting temperature of sodium halide salt (T_m > 500°C), the early efforts to build a sodium-based liquid metal battery faced the obstacle of high self-discharge and short life cycle.¹ In our previous work, using a eutectic NaOH-NaI salt (T_m: 223^°C), we demonstrated a Na||Bi-Pb cell operated at lower temperature ~250°C with small self-discharge. In order to further decrease operating temperature to less than 200 °C, in this work, we investigated a ternary eutectic 52%NaNH₂,38%NaOH and 10%NaI (T_m: 127^°C) ².

By using a three-electrode setup consisting of a liquid Na in β"-Al₂O₃ as a reference electrode, and tungsten wires as counter and working electrodes, cyclic voltammetry was performed at different scan rates (50, 100, and 200 mVs^-1) and at 180 °C operating temperature. The electrochemical window of the ternary electrolyte was identified as 1.3 V with the sodium deposition as the cathodic limit and the oxidation of NH₂^- anions to form N₂H₄gas as the anodic limit.

Bismuth-lead (Bi-Pb) alloy is a candidate for positive electrode due to its low eutectic point (125 °C). Known thermodynamic properties of Na-Bi-Pb³ and previous work⁴ indicate that the operating voltage of the Na||Bi-Pb cell is expected to be within the electrochemical window of NaNH₂-NaOH-NaI electrolyte. In this work, we focused on the discharge behavior of positive electrode, corresponding to the alloying process of Na in Bi-Pb alloy, in the ternary electrolyte. Using a similar three electrode setup utilizing liquid Na in β"-Al₂O₃ reference electrode, eutectic Bi-Pb working electrodes and a Na-Bi-Pb counter electrode, we obtained the voltage-time trace during alloying process and prepared the samples at varying Na concentration and current densities at 180°C operating temperature. The samples were quenched at target concentration of Na and cross-sectioned samples were analyzed using energy dispersive x-ray spectroscopy and scanning electron microscopy. We found that the Na preferentially alloys with Bi due to its stronger interaction than that with Pb (Pb works as diluent) and forms intermetallic, such as Na₃Bi, similar to Li-Sb-Pb system ⁵ and Ca-Sb-Pb system.⁶ A smooth voltage profile on discharging to 10 at% of Na in the Bi-Pb electrode at 100 mAcm^-2suggests potential feasibility of the ternary electrolyte for the sodium-based liquid metal batteries.

References

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