Abstract
The recently launched concept of Semi-Solid Flow Batteries (SSFBs) shows a strong potential for flexible energy storage, but the liquid-dispersed state of the electrode materials introduces several aspects of which a scientific understanding is lacking. We studied the effect of electrochemical cycling on the rheological and electrical properties of a SSFB anolyte containing Li4Ti5O12 (LTO) and Ketjen Black (KB) particles in EC:DMC solvent with 1M LiPF6, using an adapted rheometer that allows in situ electrochemical cycling and electrical impedance spectroscopy. Charging (lithiation) caused a reduction in the electronic conductivity, yield stress and high shear viscosity of the fluid electrode. For mildly reducing voltages (1.4 V), these changes were partially reversed on discharging. For more reducing voltages these changes were stronger and persistent. The finding of comparable trends for a fluid electrode without the LTO, lends support to a simplistic interpretation, in which all trends are ascribed to the formation of a surface layer around the conductive KB nanoparticles. This Solid Electrolyte Interphase (SEI) insulates particles and reduces the van der Waals attractions between them. SEI layers formed at less reducing voltages, partially dissolve during the subsequent discharge. Those formed at more reducing voltages, are thicker and permanent. As these layers increase the electronic resistance of the fluid electrode by (more than) an order of magnitude, our findings highlight significant challenges due to SEI formation that still need to be overcome to realize SSFBs.
Highlights
Semi-Solid Flow Batteries (SSFBs), as recently introduced by Duduta et al [1], comprise a promising addition to the spectrum of rechargeable battery systems
Experimental timescales do not always allow access to this limit, and the low frequency real impedance (LFRI see Fig. 3 panel B) at 0.01 Hz was taken as practical measure of the electronic resistance
Two key properties are very sensitive to the electrochemical state and history: lithiation causes the electronic resistance (LFRI) to increase and the yield stress to decrease, and vice versa for delithiation
Summary
Semi-Solid Flow Batteries (SSFBs), as recently introduced by Duduta et al [1], comprise a promising addition to the spectrum of rechargeable battery systems. The advantages of SSFBs over conventional batteries lie in the decoupling of power (cell size) and energy (tank size), and the potential for adjusting the chemistry of the system during operation. In particular nonaqueous SSFB systems are interesting, since they offer much higher energy densities as compared to more conventional aqueous redox flow systems [1]. The ability of non-aqueous SSFBs to provide and store energy in a flexible way makes them promising for grid applications. A. Narayanan et al / Electrochimica Acta 251 (2017) 388–395 and/or shear flow, the microstructure may adapt to electrochemical changes
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