POM Anolyte for All‐Anion Redox Flow Batteries with High Capacity Retention and Coulombic Efficiency at Mild pH

Abstract A highly soluble Li 5 BW 12 O 40 cluster delivers 2 e − redox reaction with fast electron transfer rates (2.5 × 10 −2 cm s −1 ) and high diffusion coefficients (≈2.08 × 10 −6 cm 2 s −1 ) at mild pH ranging from 3 to 8. In‐operando aqueous‐flowing Raman spectroscopy and density functional theory calculations reveal that Raman shift changing of {BW12} clusters is due to the bond length changing between W‐O b ‐W and W‐O c ‐W at different redox states. The structure changing and redox chemistry of Li 5 BW 12 O 40 are highly reversible, which makes the Li 5 BW 12 O 40 cluster versatile to construct all‐anion aqueous redox flow batteries (RFBs). The cation‐exchange Nafion membrane will also repel the cross permeability of the anion redox couples. Consequently, by coupling with Li 3 K[Fe(CN) 6 ] catholyte, the aqueous RFB can be operated at pH 8 with a capacity retention up to 95% and an average Coulombic efficiency more than 99.79% over 300 cycles within 0 to 1.2 V. Meanwhile, Li 5 BW 12 O 40 cluster can also be paired with LiI catholyte to form aqueous RFBs at pH 7 and pH 3, the capacity retention of 94% and 90% can be realized over 300 cycles within 0 to 1.3 V.