A Hexacyanomanganate Negolyte for Aqueous Redox Flow Batteries

Aqueous redox flow batteries (RFBs) have emerged as promisinglarge-scaleenergy storage devices due to their high scalability, safety, andflexibility. Manganese-based redox materials are promising sourcesfor use in RFBs owing to their earth abundance, affordability, andvariety of oxidation states. However, the instability of Mn redoxcouples, attributed to the unstable d-orbital configuration of Mn3+(d(4)) known to involve strong Jahn-Tellereffects, has hindered their practical use. Here, we discover thatthe [Mn(CN)(6)](5-/4-/3-) negolyteoffers advantages in terms of reversibility, stability, and reactionkinetics owing to the addition of NaCN supporting electrolyte, whichinhibits ligand exchange reactions, resulting in high performance.[Mn(CN)(6)](5-/4-/3-) negolytepossesses stable multielectron reactions from Mn(I) to Mn(III), leadingto a high capacity of 133.7 mAh after 100 cycles. We provide chemicalevidence obtained from in situ Raman analysis for unprecedented Mn(I) stability during electrochemical cycling, openingup new avenues for the design of low-cost Mn-based redox systems.