Chemistry Evolution of Low‐Temperature Divalent Vanadium V(II) Precipitation Enables Wide Temperature Range Vanadium Flow Batteries

Abstract

ABSTRACT 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 VSO 4 ·6H 2 O. 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 HSO 4 − (HSO 4 − + H 2 O ⇌ H 3 O + + SO 4 2− ) at low temperatures. The SO 4 2− acts as an anionic bridge, directly inducing the dimerization of two V(H 2 O) 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(H 2 O) 5 ACN] 2+ ) and its secondary solvation environment (reducing SO 4 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.