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A rate-dependent nucleation theory is developed to analyze the dependence of the capacity of lithium-sulfur (Li-S) batteries on discharge rates. The theory is based on expressing the nucleation rate of solid products in terms of the surface oversaturation of the precipitating species and rewriting the classical Kolmogorov-Avrami model in the form of a differential system of equations. By coupling this system of equations with the multicomponent transport equations we are able to explain qualitatively well the decrease of the specific capacity with increasing discharge rates and the characteristics of the discharge curves under variable discharge currents in Li-S batteries. Special attention is given to accurately determine the values of model parameters from experimental data, including the solubility constants of the solid products and the mobilities and diffusivities of the ions. The anodes of the batteries used in our analysis were made using Li foils and the cathodes were fabricated using freestanding carbon nanotube foams, on which solid sulfur was uniformly distributed to maximize the dissolution rate. The experimental data combined with the theoretical analysis show that the capacity limitation in our batteries is due to surface passivation by the final reaction products. In order to increase the specific capacity of Li-S batteries under normal discharge rates we conclude that it is better to avoid the solid precipitation of intermediate lithium polysulfides (LiPS) by using electrolytes with a high solubility of LiPS. In addition, it is better to use catalyst particles in the cathode that would promote the nucleation rate at specified locations. The number of catalyst particles should not be too small to avoid electrolyte oversaturation or too large to avoid the rapid passivation of the carbon surface with solid products. [ABSTRACT FROM AUTHOR]

Lithium-sulfur (Li–S) batteries suffer from poor utilization of active material and short cycle life due to the complicated multi-step reaction mechanisms. Herein, three conditional cycling methods, i.e. asymmetrical cycling, constant voltage (CV) discharge cycling, and partial cycling are designed in order to increase the cyclability of Li–S batteries. It is found that the solid deposition process that takes place during the lower plateau of discharge is the major limiting step for achieving high discharge capacity and cycle retention, and the cathode surface coverage can be deferred by applying an optimal discharge/charge rate and CV discharge cycling. The asymmetrical cycling renders a specific capacity of ca. 700 mAh g−1 after 200 cycles, 30% higher than that under symmetrical cycling, while applying a CV discharge cycling enables a full retention of target specific capacity of ca. 800 mAh g−1 over 50 cycles. The partial cycling with a low number of phase transformation steps and reduced surface coverage at the end of discharge/charge also enhances cyclability. This work paves the way for understanding and improving the cycling performance of Li–S batteries without increasing the cost of electrode design or changing the configuration of the cell. [ABSTRACT FROM AUTHOR]

Abstract Lithium sulfur (Li-S) batteries are promising candidates to replace lithium ion batteries as the next-generation energy storage devices. However, the operation of conventional Li-S batteries at low electrolyte/sulfur ratios is rather challenging with many obstacles to be addressed. In this work we use solid-state Li 2 S 4 as the cathode active material to construct a new type of Li-Li 2 S 4 battery. With the aid of electrochemical impedance spectrum, we find that the use of Li 2 S 4(S) in the cathode alleviates the issue of surface coverage encountered under the lean electrolyte condition. We also find that the discharge capacity is not determined by the solubility of Li 2 S 4 on the lower plateau of discharge. Consequently, the Li-Li 2 S 4 cell with a high sulfur content of 84% and a low E/S ratio of 4.4 ml g−1 delivered a much higher discharge capacity compared to a conventional Li-S cell. In addition, improved cycle retention was achieved when controlling the Li-Li 2 S 4 cell cycling between Li 2 S 4 and Li 2 S. We believe that the reported results open a new direction of cathode development for high energy density Li-S batteries. Highlights • Lithium polysulfide electrochemical reactions are restricted by its solubility. • Li 2 S 4 as cathode is not limited by the electrolyte. • Theoretical specific energy of Li 2 S 4 batteries is given. [ABSTRACT FROM AUTHOR]

Although the cathode of lithium-sulfur (Li-S) batteries has a theoretical specific capacity of 1,672 mAh g −1 , its practical capacity is much smaller than this value and depends on the electrolyte/sulfur ratio. The operation of Li-S batteries under lean electrolyte conditions can be challenging, especially in the case when the solubility of lithium polysulfide (LiPS) sets an upper bound for polysulfide dissolution. In this work, specially designed cathode structures and electrolyte configurations were built in order to analyze the effects of LiPS solubility on cell capacity. Two reaction pathways involving the reduction of LiPS in liquid and solid phase are proposed and analyzed. We show that at discharge rates above 0.4 mA cm −2 the reaction in the liquid phase dominates the discharge process. Once the electrolyte becomes saturated, the solid phase LiPS cannot be further reduced and does not contribute to the capacity of the cells. This phenomenon prevents Li-S batteries from achieving their high theoretical specific capacity. Finally, the specific energy of the Li-S cell is reevaluated and discussed considering the limitation imposed by LiPS solubility. [ABSTRACT FROM AUTHOR]