LiFePO4-ferri/ferrocyanide redox targeting aqueous posolyte: Set-up, efficiency and kinetics

Abstract

Redox Targeting Redox Flow Batteries (RT-RFB), in which a solid insertion material reacts with soluble flowing active species (the mediators), were recently developed as alternative to the stellar vanadium technology. We have studied and optimized a posolyte having LiFePO4 as solid material and ferri/ferro cyanide complexes as soluble mediator in a DMSO/water solvent mixture. LiFePO4 particles were shaped as dense pellets with controlled total porosity, obtained by Spark Plasma Sintering using NaCl microcrystals as hard template. The redox potential of the mediator couple was tuned by playing with the DMSO/water ratio in order to match the insertion/uptake potential of the solid. High reversibility and fast kinetics of the mediator/solid reaction were chemically assessed ex-situ. Electrochemical results under flow showed perfect reversibility, full reaction of the soluble mediator complex, and a linear increase in capacity of the posolyte with the quantity of added active solid. The best performances are obtained with 40% porous pellets and at a current density of 0.25 mA cm−2. In these conditions, full capacity is reached for both the mediator and the solid material, with a first-cycle reversibility of 99 %, doubling the capacity with only 1 vol% of added LiFePO4 in the electrolyte.