Lithium Polysulfide Flow Batteries Enabled By Percolating Nanoscale Conductor Networks

The high solubility of lithium polysulfides in nonaqueous electrolytes and associated loss mechanisms have long been a barrier to the development of Li-S batteries.  The same attribute allows lithium polysulfide solutions to be used in a flow battery architecture, where the high cell voltage and redox-active species concentrations (>2M) compared to conventional aqueous flow batteries provides significantly higher energy density and potentially lower cost (<$100/kWh at system level).  Here we demonstrate a new design approach wherein diffusion-limited aggregation of nanoscale conductor particles at ~1 vol% concentration is used to impart mixed electronic-ionic conductivity to redox-active solutions, forming flow electrodes with embedded current collector networks that self-heal after shear.  As illustrated in Fig. 1, flow cells of this architecture can distribute electrochemical activity throughout the volume of flow electrodes rather than being confined to surfaces of stationary current collectors.  Applying this concept to lithium polysulfide semi-flow batteries, we demonstrate that the nanoscale network architecture enables cycling of polysulfide solutions deep into precipitation regimes that otherwise show poor capacity utilization and reversibility.  Figure 2 compares results for identical 2.5 mol S/L solutions (as Li₂S₈ in TEGDME with 1 wt% LiNO₃ and 0.5M LiTFSI) tested in half-cells using a conventional fibrous carbon current collector, and using the nanoscale conductor network  approach without the current collector.  At C/4 rate, the latter exhibits specific capacity of 1200 mAh g^-1 (vs. theoretical capacity of 1460 mAh g^-1 for this solution), about a factor of 5 greater than with the carbon fiber current collector.

Lithium polysulfide half-flow cells are demonstrated for the first time, operating in two modes: 1) Potentiostatic cycling under continuous flow (the typical configuration for flow batteries), in which at C/10-C/15 rates reversible capacity is ~380 mAh g^-1 and catholyte energy density is ~60 Wh/L (with respect to Li/Li⁺), and 2) Galvanostatic cycling with intermittent flow (in which fluid is pumped in discrete volumes with complete charging or discharging of the electroactive region in between), in which reversible capacities are ~650 mAh g^-1  and catholyte energy density is ~100 Wh/L.  An example of potentiostatic cycling results appears in Figure 3; the performance of such devices from fundamental Li-S reaction kinetics to non-Newtonian rheology to electrochemical cell design will be discussed.

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.