A high power, low temperature molten sodium battery

Abstract

Low temperature molten sodium batteries promise low-cost, grid-scale energy storage using earth-abundant materials. To be economically viable, they must demonstrate high current (power) at relevant discharge times. Here a low temperature (135 °C) molten sodium battery with a NaI–AlCl₃ molten salt catholyte and NaSICON separator is explored, minimizing cell ohmic resistances and avoiding precipitation reactions in the molten salt catholyte. Operating currents were increased by as much as 100× compared to the energy-dense baseline design (197 Wh kg⁻¹ and 248 mAh cm⁻²). Cells were cycled 100 times at 50 mA cm⁻² (150 mW cm⁻²), averaging 99.8% coulombic efficiency and 79.7% energy efficiency for 30% of the theoretical capacity. Charging currents up to 250 mA cm⁻² (1070 mW cm⁻²) are demonstrated for 10% of the theoretical capacity. Toward extending the accessible capacity in these high current systems, the volume ratio of catholyte molten salt to a graphite felt current collector was varied, altering (1) the local current density in the graphite felt and (2) the discharge time at a fixed current density. Optimizing the catholyte : felt ratio dramatically increased capacity utilization to 60% (102 mAh cm⁻²) at 50 mA cm⁻² (150 mW cm⁻²) charge and 10 mA cm⁻² discharge. This catholyte : felt ratio of 1.4 significantly decreased the cell impedance, minimizing electrode blocking effects seen previously in NaI–AlCl₃ catholytes. Together, these high areal loadings and impressive power outputs demonstrate how low temperature molten sodium batteries can compete with commercialized sodium systems operating at more than twice the temperature.