Bromine–Graphite Intercalation Enabled Two-Electron Transfer for a Bromine-Based Flow Battery

Abstract

Br2/Br− is a promising redox couple in flow batteries because of its high potential, solubility, and low cost. However, the reaction between Br− and Br2 only involves a single-electron transfer process, which limits its energy density. Herein, a novel two-electron transfer reaction based on Br−/Br+ was studied and realized through Br+ intercalation into graphite to form a bromine–graphite intercalation compound (Br–GIC). Compared with the pristine Br−/Br2 redox pair, the redox potential of Br intercalation/deintercalation in graphite is 0.5 V higher, which has the potential to substantially increase the energy density. Different from Br2/Br− in the electrolyte, the diffusion rate of Br intercalation in graphite decreases with increasing charge state because of the decreasing intercalation sites in graphite, and the integrity of the graphite structure is important for the intercalation reaction. As a result, the battery can continuously run for more than 300 cycles with a Coulombic efficiency exceeding 97% and an energy efficiency of approximately 80% at 30 mA/cm2, and the energy density increases by 65% compared with Br−/Br2. Combined with double-electron transfer and a highly reversible electrochemical process, the Br intercalation redox couple demonstrates very promising prospects for stationary energy storage.