Thermal properties and structural evolution of Na2SO4-MgSO4 eutectic molten salts for large-scale energy storage: Unveiling mechanisms through deep potential molecular dynamics

Abstract

The physical and transport properties of molten salts are critical for optimizing and ensuring the sustained efficient operation of large-scale molten salt energy storage systems. This study presents a deep potential (DP) model based on density functional theory (DFT) to investigate the thermophysical properties and microstructural evolution of Na₂SO₄-MgSO₄ eutectic molten salts. The DP function has been further optimized through supplementary training with DP-GEN on complex microstructures, enabling it to capture the microstructural features with the accuracy of DFT. The findings indicate that the sulfate ion microstructure remains stable and unaffected by temperature, consistently retaining a tetrahedral configuration across the examined temperature range. Analysis of microstructural evolution reveals that increasing temperatures induce greater disorder within the Na₂SO₄-MgSO₄ system, resulting in a more loosely packed microstructure and a reduction in coordination number. Furthermore, Mg ions encounter higher energy barriers compared to Na ions, which leads to more restricted mobility within the system, as evidenced by the significantly lower self-diffusion coefficient of Mg ions in contrast to that of Na ions. The thermophysical properties of the Na₂SO₄-MgSO₄ eutectic molten salt exhibit a characteristic negative temperature dependence, with calculated density and specific heat demonstrating deviations from experimental data of 1.7 % and 3.2 %, respectively. This research aims to provide theoretical insights that will facilitate advancements in the application of sulfate molten salts for large-scale energy storage systems.