Electrolyte-rock interactions in salt cavern flow batteries

The salt cavern flow batteries (SCFBs) system utilizes a saturated NaCl solution as supporting electrolyte, with salt cavern serving as storage location for electrolytes. Unlike conventional flow batteries, where the electrolytes are stored in above-ground tanks made of plastic or metal, the electrolytes in SCFBs directly contact with surrounding rocks and thus face challenges with high temperatures, high pressures, and complex multi-ionic environments. Therefore, understanding the physical and chemical mechanisms of interactions between the surrounding rocks and electrolytes is of prime importance for the electrochemical performance of SCFBs. In this study, the brine and rock cores from a salt mine in Huai'an of China, were used to investigate the interactions. The SCFBs were assembled with N,N,N,2,2,6,6-heptamethylpiperidinyloxy-4-ammonium chloride (TMA-TEMPO) as the catholyte and N,N′-dimethyl-4,4′-bipyridinium dichloride (MV) as the anolyte. The results indicate that the permeability of interlayer samples only increases by one order of magnitude, while the permeability of salt rock samples consistently maintains an order of magnitude of 10⁻²⁰ m². Changes in permeability of salt rock and interlayer samples do not affect the tightness of salt caverns. Complex multi-ionic measurements show almost no influence on the redox behaviors of TMA-TEMPO and MV electrolytes, except for a shielding effect of CO₃²⁻ on TMA-TEMPO. The 0.1–1.5 M TMA-TEMPO/MV SCFBs exhibits a high energy efficiency of 82 % and a low capacity-fade rate of 0.046 %/cycle (0.40 %/day) over 23 days at 40 mA cm⁻². This study provides valuable insights for future site selection and system design of SCFBs systems in China.