Surfactant for Enhanced Rheological, Electrical and Electrochemical Performance of Suspensions for Semi-Solid Redox Flow Batteries and Supercapacitors

Abstract

Redox-flow batteries (RFBs) store electrochemical energy in two fluids contained in external tanks, so called anolyte and catholyte which are pumped and flow through an electrochemical reactor in which electro-active species are oxidized and reduced. This feature provides to RFBs a unique ability in decoupling the energy and the power, therefore providing a significant design freedom for stationary applications. An interesting concept to deal with the energy density of RFBs, proposed by Chiang et al. [1], consists to use solid electro-active particles suspended in a Li+ containing electrolyte. The interest of such Semi-Solid Flow Cell (SSFC) system has recently been demonstrated with energy density ten times higher compared to classical RFBs [2,3]. Subsequently, Gogotsi et al. proposed an analogous to supercapacitors, the so called electrochemical flow capacitor (EFC) in which the energy is stored in the electric double layer of charged carbon particles [4]. A flowable carbon-electrolyte mixture is employed as the active material for capacitive energy storage, and is handled in a similar fashion to flow as for semi-solid batteries.Performance of SSFC (and EFC) system strongly depends on the flow ability of the anolyte and catholyte suspensions combined with their electronic conductivity which is governed by the extent of the carbon black percolation. In this regards, we have recently investigated the rheological and electrical behaviours of two carbon blacks [5], namely, Ketjen black EC-300 (KB) and C-NERGY Super C45, which differ by their primary particle size, and blends of Li4Ti5O12 (LTO) and KB, suspended in an organic electrolyte, a solution of 1M of Lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) in propylene carbonate (PC). The electrochemical performance of the LTO/KB/PC-LiTFSI suspension was investigated vs. lithium metal electrode as function of the cycling rate in static mode (i.e. no flow) and using a home-made cell that allows studying the influence of the thickness of the suspension [6].Recently, we demonstrated the very beneficial influence of the addition of a nonionic surfactant in the composition of the LTO/KB/PC-LiTFSI anolyte suspension. All practical properties are improved with respect to semi-solid redox flow application. As a matter of fact, the viscosity is decreased, the electronic conductivity is increased and is less affected by the shear flow, and the electrochemical performance is increased. Moreover, a much better stability with time of the suspension is obtained, which means the preservation of its performance with time.