Fast kinetics for sulfur reduction reactions (SRR) is important for fast charge-discharge in Li||S batteries. Reported SRR activity for electrocatalysts is widely described via thermodynamic trends including the volcano plot. However, there is not a kinetic trend to describe how fast SRR is. A consequence is that charge-discharge rate for Li−S batteries remain slow, especially with high sulfur loadings and lean electrolyte. This significantly limits practical use of Li−S batteries for high-power output.

The questions therefore are, 1) which parameter is directly linked to SRR kinetics, and; 2) how can SRR kinetics be boosted to increase charge-discharge of Li||S batteries? Le Chatelier’s principle is widely used to establish reaction trends. This posits that, for an equilibrium reaction under specific external pressure and temperature, the reaction kinetics accelerate with greater concentration of reactants. With Li||S batteries (Fig. 1) it is reasonable to assume that a greater polysulfide concentration yields faster polysulfide reduction. Overall conversion from S₈ to Li₂S determines cell capacity, and therefore the polysulfide concentrations at each speciation step is important in SRR kinetics.

Figure 1. Schematic for comparison in SRR reaction pathways with 80 % conversion efficiency for each step, “on” and “off” imply, respectively, Li||S batteries with and without, external applied current.

This work reports establishing kinetic correlation via in situ monitoring of polysulfide concentration in the non-aqueous liquid electrolyte with kinetic current applied on the battery positive electrode that contains 3d transition metal clusters as model heterogeneous catalyst. Based on mathematical relationships between polysulfide concentration (C), kinetic current (J) and charge-discharge (V), it is shown that SRR rate increases with increment in polysulfide concentration (Fig. 2a). Importantly, the first derivative of the logarithmic ratio between specific current and polysulfide concentration increases linearly with applied charge-discharge rates (d log J / d log C ~ V) (Fig. 2b). Based on these findings a nanocomposite CoZn/carbon catalyst is designed and is used in formulation of sulfur-based positive electrodes. Compared with reported Li||S battery catalysts (Fig. 2c), specific discharge capacities at high current rate with a sulfur loading of 5 mg cm^-2 and electrolyte-to-sulfur mass ratio of 4.8 is confirmed. Operability of the Li||S cell with the CoZn/carbon catalyst at high current rates establishes the practical design for Li||S batteries with fast charge and discharge (e.g. < 5 min).

Figure 2. (a) Targeted goal to boost SRR kinetics via increasing polysulfide concentration; (b) Relationship between logarithmic value of Li₂S₄ concentration ratio (log C) and logarithmic value of specific current (log J); (c) Comparison of charge-discharge current between Li||S coin cell with CoZn/carbon catalyst reported in this work and selected Li||S cells reported in the literature.

Findings confirm that fundamental investigations on SRR kinetics are important to design for a wide range of nanocomposite catalysts to boost high-power performance in Li||S batteries. For further details, please see the article, “Developing high-power Li||S batteries via transition metal/carbon nanocomposite electrocatalysts engineering” in Nature Nanotechnology (https://www.nature.com/articles/s41565-024-01614-4).