A new approach to flow battery design is demonstrated wherein diffusion-limited aggregation of nanoscale conductor particles at ∼1 vol % concentration is used to impart mixed electronic-ionic conductivity to redox solutions, forming flow electrodes with embedded current collector networks that self-heal after shear. Lithium polysulfide flow cathodes of this architecture exhibit electrochemical activity that is distributed throughout the volume of flow electrodes rather than being confined to surfaces of stationary current collectors. The nanoscale network architecture enables cycling of polysulfide solutions deep into precipitation regimes that historically have shown poor capacity utilization and reversibility and may thereby enable new flow battery designs of higher energy density and lower system cost. Lithium polysulfide half-flow cells operating in both continuous and intermittent flow mode are demonstrated for the first time.

Metal-sulfur couples are of interest due to the high energy density and potentially low cost of these chemistries. However, the existence of soluble polysulfide intermediates—which exist in all known polar electrolyte systems—has challenged the cycle life and efficiency of these systems. Recently, interest in catholyte cells based on dissolved polysulfide solutions has renewed, in part due to the possible use of these catholytes as flow electrodes with low cost.1-3To realize the potential advantages of dissolved polysulfide catholytes, highly concentrated solutions are needed to obtain high charge storage capacities, which simultaneously retaining adequate rate capability. To achieve this, the rate limiting mechanisms in polysulfide solutions must be understood as a function of concentration and solution composition. Here, we present experimental measurements of the exchange current density and ionic conductivity and of lithium polysulfide solutions of varying concentration in non-aqueous solvents. Exchange current densities are measured by three different methods: steady-state voltammetry at a C-fiber microelectrode, electrochemical impedance spectroscopy at a glassy carbon macroelectrode, and galvanostatic polarization measurements at a glassy carbon macroelectrode. Solvents of interest include tetraethylene glycol dimethyl ether (TEGDME), 1,2-dimethoxyethane (DME), 1,3-dioxolane (DOL), and mixtures thereof. Ionic conductivies are measured with and without supporting LiTFSI salt, as shown for several selected solvent systems in Figure 1(A). The measured ionic conductivities are correlated with measured solution viscosities, as shown for the TEDGME system in Figure 1(B). Lithium polysulfide solutions with sulfur concentrations of 1-8 M in these solvents have measured exchange current densities on the order of 0.01 – 1 mA cm-2 and ionic conductivities are on the order of 1 – 10 mS cm-1. This work represents the first report of systematic measurements of these fundamental charge transfer and transport properties for polysulfide solutions. These results enable rational interpretation of the limiting processes in conventional solid sulfur cells as well as the catholyte systems and provide a quantitative basis for selection of electrolyte solvents. References 1. Demir-Cakan, R. et al. Li–S batteries: simple approaches for superior performance. Energy & Environmental Science 6,176 (2012). 2. Yang, Y., Zheng, G. & Cui, Y. A membrane-free lithium/polysulfide semi-liquid battery for large-scale energy storage. Energy & Environmental Science 6,1552 (2013). 3. Manthiram, A., Fu, Y. & Su, Y.-S. Challenges and Prospects of Lithium–Sulfur Batteries. Acc. Chem. Res. 46,1125–1134 (2013). Acknowledgements This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

An aqueous Li-ion flow cell using suspension-based flow electrodes based on the LiTi2(PO4)3-LiFePO4 couple is demonstrated. Unlike conventional flow batteries, the semi-solid approach utilizes fluid electrodes that are electronically conductive. A model of simultaneous advection and electrochemical transport is developed and used to separate flow-induced losses from those due to underlying side reactions. The importance of plug flow to achieving high energy efficiency in flow batteries utilizing highly non-Newtonian flow electrodes is emphasized.