Pitfalls in Li-S Rate-Capability Evaluation

Rate-capability tests are widely used to highlight advances in Li-S-system development. Here we show, by studying the individual effects of a number of cycling- and electrolyte-related parameters on a simple Li-S cell, that the rate-capability results are sensitive not only to the applied current but also to the cycling prehistory of the cell. On the one hand, the performance is affected by the order in which cycling rate is changed. Slow initial rates aggravate material loss and reduce the achievable capacity. On the other hand, the results of rate-capability tests are significantly different when the rates are varied only on the charge or only on the discharge. The charge rate does not directly affect the measured capacity, whereas the discharge rate does, especially when discharge cutoff voltages are high, due to slow discharge reactions. High charge rates, however, do affect the long-term stability, which is difficult to predict from usual rate-capability tests. Our findings also provide insights into the relative reaction rates and the influence of the general cycling procedures, underlining the importance of understanding the kinetics of Li-S systems, while designing them for high performance batteries.