Lithium/sulphur batteries: an electrochemical study

Abstract

The lithium/sulfur battery has been investigated as an attractive candidate for the rechargeable energy storage system, since it can potentially deliver a much higher energy than a typical lithium ion battery of the same weight. In this work, sulfur/acetylene black composite electrodes were prepared by the ball milling method and studied cells with various electrolyte systems. A significant impact of the electrolyte viscosity on the electrochemical performance of the cells was explained in terms of the electrolyte penetration into the sulfur electrode structure, the diffusion rate of lithium salts and the dissolution rate of the solid active materials. Sulfur/acetylene black (AB) composites were also prepared using a direct precipitation method. The smaller particles of sulfur, as well as the well-distributed AB on the surface of sulfur particles resulting from the precipitation method was found to provide a more uniform and conductive S/AB composite with a larger surface area. The resulting electrodes showed less degradation on cycling than those prepared by milling method.

A simple quantitative one-dimensional model of the initial self-discharge has been developed in terms of diffusion of a polysulfide shuttle species. Despite the simplicity of the model, it reproduces very well the decrease in the open circuit potential of the cells under a range of experimental conditions (varying the number of separators between the electrodes, the amount of AB in the sulfur electrode and the pre-saturation of the electrolyte with sulfur). The model provides a detailed understanding of the mechanism of self-discharge, quantifying the two main causes of sulfur loss from the positive electrode: dissolution followed by diffusion down a concentration gradient and direct reaction with polysulfides arriving from the lithium electrode.

Galvanostatic Intermittent Transient Technique (GITT) measurements were conducted to study the diffusion behaviour in Li/S cells. Analysis of the transient voltage change during and after current pulses was performed at different states of discharge/charge. The relaxation time was optimised to avoid errors due to poor equilibration at short times and self-discharge during longer periods.

Finally, the effect of the shuttle reaction on the electrochemical performance of Li/S cells was investigated using a lithium ion conducting glass ceramic (LICGC) separator in an effort to eliminate the self-discharge. This resulted in a higher discharge capacity and more accurate GITT results, showing that better controlled diffusion conditions can be achieved in a Li/S cell containing LICGC separator.

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Identifiers

ORCID for John Owen: orcid.org/0000-0002-4938-3693

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