Understanding the Performance of Lithium-Sulfur Batteries through Analysis of Shuttle Currents

The development of a rechargeable lithium-sulfur battery has been of considerable interest due to its high theoretical specific capacity of 1675 mAh/g and high energy density of 2600 Wh/kg. These properties coupled with the low cost and non-toxicity of sulfur makes lithium-sulfur batteries very attractive for multiple battery applications. One of the major problems of lithium-sulfur batteries, however, is their poor cycle life. Typically 20-50% of the capacity is lost in around 100 cycles. This poor cycleability results from the parasitic reaction known as the polysulfide shuttle.  

The polysulfide shuttle exists because the polysulfide discharge products of the sulfur electrode, S₈^2- and S₄^2-, are soluble in the electrolyte. These soluble polysulfide species diffuse towards the lithium anode where they are reduced. Afterwards, the reduced polysulfides shuttle back towards the sulfur cathode where they have to be re-oxidized.^1-2During cycling, this phenomenon causes incomplete charging. When the cell is idle, this phenomenon results in self-discharge.

The polysulfide shuttle also impacts the cycle life because some of the reduced polysulfide discharge products formed at the anode, S₂^2- and S^2-, are insoluble. Normally during cycling, these insoluble discharge products are produced at the sulfur cathode where they can later be re-oxidized. However, when these insoluble discharge products are produced at the lithium anode, they will precipitate out and cause an irreversible capacity loss. We herein present a technique of directly measuring the currents associated with the polysulfide shuttle and an analysis of how these shuttle currents impact the performance of the cell.

The technique of measuring shuttle currents is based on reversing the inherent self-discharge caused by the shuttle process. The current measured by holding the cell voltage constant results from the shuttle process. In this case, as soon as the soluble polysulfide species diffuse from the cathode, the concentration gradient of sulfur species at the cathode will change, affecting the potential of the cell. In order for the cell to maintain its potential, the cell must oxidize polysulfide species at the cathode as they shuttle back.

The shuttle currents measured at various cell voltages are shown in Figure 1. We present analysis of these shuttle currents as a function of state-of-charge, cell design, and additives. We provide insight into how the magnitude of the shuttle currents relate to the reduced cycle life and discharge capacity of the cell.