Chemistry Evolution of Low-Temperature Divalent Vanadium V(II) Precipitation Enables Wide Temperature Range Vanadium...

Vanadium flow batteries (VFBs) are promising technologies for large-scale energy storage. However, the precipitation of divalent vanadium [V(II)] species in the negative electrolyte at low temperatures hinders their further application. At present, the microscopic structure and formation mechanism of this precipitate remain unclear. Herein, single-crystal X-ray diffraction (SCXRD) reveals the crystal structure of the low-temperature precipitate as VSO₄·6H₂O. The precipitation mechanism was clarified by combining in situ variable-temperature Raman spectroscopy with density functional theory (DFT) calculations. The results indicate that the precipitation originates from the enhanced deprotonation of HSO₄ ^- (HSO₄ ^- + H₂O ⇌ H₃O⁺ + SO₄ ^2-) at low temperatures. The SO₄ ^2- acts as an anionic bridge, directly inducing the dimerization of two V(H₂O)₆ ²⁺ units via hydrogen bonding, which in turn triggers precipitation. Furthermore, we propose a dual-site solvation engineering strategy, where the co-introduction of acetonitrile (ACN) and HCl precisely modulates both the primary solvation shell of V(II) (forming [V(H₂O)₅ACN]²⁺) and its secondary solvation environment (reducing SO₄ ^2-). The designed electrolyte enables stable operation of a VFB for over 500 cycles (∼30 days) at -10 °C and 40 mA cm^-2, with energy efficiency (EE) > 80%, demonstrating its potential in freezing regions.