Formulation of flowable anolyte for redox flow batteries: Rheo-electrical study

Abstract

In an attempt to optimize a suspension electrode for redox flow batteries, this work demonstrates the effect of solid content and additive material on the electrical and rheological behavior of an anolyte made up of lithium titanium oxide (Li4Ti5O12 (LTO), as active material), carbon black (Ketjen black (KB), as a conductive material) suspended in organic medium (1 M lithium bis(trifluoromethane)sulfonimide; LiTFSI in propylene carbonate). The rheo-electrical properties of the anolyte are very sensitive to the Li4Ti5O12 content. The 20 wt% LTO is the maximum loading the percolated KB network can sustain without significant loss of the electronic conductivity and flowability of the electrode. Interestingly, this critical concentration increases to 25 wt% by addition of trace amount of conductive carbon nanofibers (CNFs) which electronically wire the conductive pathways and even reduce viscosity of the suspension electrode. Under shear flow, the suspension electrodes show three-regime flow curves with intermediate shear-thickening regions in accordance with minima in the conductivity. These minima are sharper at higher KB content, but nearly disappear in suspension electrodes with CNFs additive implying its role in wiring the ruptured conductive pathways under flow.