Direct Measurement of Polysulfide Shuttle Current: A Window into Understanding the Performance of Lithium-Sulfur Cells

Abstract

The shuttling of polysulfide ions between the electrodes in a lithium-sulfur battery is a major technical issue limiting the self-discharge and cycle life of this high-energy rechargeable battery. Although there have been attempts to suppress the shuttling process, there has not been a direct measurement of the rate of shuttling. We report here a simple and direct measurement of the rate of the shuttling (that we term "shuttle current"), applicable to the study of any type of lithium-sulfur cell. We demonstrate the effectiveness of this measurement technique using cells with and without lithium nitrate (a widely-used shuttle suppressor additive). We present a phenomenological analysis of the shuttling process and simulate the shuttle currents as a function of the state-of-charge of a cell. We also demonstrate how the rate of decay of the shuttle current can be used to predict the capacity fade in a lithium-sulfur cell due to the shuttle process. We expect that this new ability to directly measure shuttle currents will provide greater insight into the performance differences observed with various additives and electrode modifications that are aimed at suppressing the rate of shuttling of polysulfide ions and increasing the cycle life of lithium-sulfur cells. The rechargeable lithium-sulfur battery is of great interest due to its high theoretical specific energy of 2600 Wh/kg and also the relatively low cost of sulfur. However, large-scale deployment of lithium-sulfur batteries has been limited by several performance issues relating to power density and cycle life. [1][2][3][4][5][6][7][8][9] With sulfur at the positive electrode and lithium metal at the negative electrode, the lithium-sulfur battery operates over a voltage range of 1.5 to 2.8 V. The reactions at the positive and negative electrode during charge and discharge are shown schematically in Figure 1. During discharge, sulfur at the positive electrode is reduced progressively to various polysulfides and eventually to the sulfide, while lithium metal at the negative electrode is oxidized to lithium ions. During charge, the lithium ions are reduced to lithium at the negative electrode, and the sulfide is re-oxidized at the positive electrode to the higher-order polysulfides. The organic electrolyte is a 0.5 M solution of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in a 1:1 mixture by volume of dioxolane (DOL) and dimethoxyethane (DME). A "solid electrolyte interphase" (SEI) protects the lithium anode from reacting freely with the organic electrolyte. The polysulfides produced during discharge are usually soluble in the battery electrolyte. Summary of technical challenges with lithium-sulfur batteries.-The principal challenge for the realization of a practical lithium-sulfur battery with long cycle life arises from the solubility of the higherorder polysulfides (S 8 2− , S 6 2− , S 4 2− ) in the electrolyte. These polysulfides, generated at the positive electrode during discharge, diffuse to the negative lithium electrode where they are reduced to...

A Microstructurally Resolved Model for Li-S Batteries Assessing the Impact of the Cathode Design on the Discharge Performance

“…24 There is a general consensus that the reasons behind the theoretical capacity of Li-S batteries not being achieved are short-comings: low sulfur utilization due to poor wiring and soluble polysulfide intermediates giving rise to a parasitic shuttle effect. 25,26 One breakthrough in the last decade was the finding that a host of mesoporous carbon greatly enhanced the active material utilization as compared to a ground C/S mixture. 5 The mesoporous architecture of the cathode primarily assisted in retaining polysulfide intermediates in the cathode.…”

Mixed Conduction Membranes Suppress the Polysulfide Shuttle in Lithium-Sulfur Batteries

“…As mentioned earlier, we have developed a method to measure this type of current to quantify the effect of various cell modifications on the rate of polysulfide shuttling rate. 45 We have used this shuttle current value as a direct measure of the transport rates of the polysulfide shuttle across the cell. In the present study, we have compared the shuttle current in three types of cells: (1) a cell with no membrane modification (2) a cell containing 0.25 M lithium nitrate additive and (3) a cell with the new MCM layer.…”

A simple, experiment-based model of the initial self-discharge of lithium-sulphur batteries

“…This assumption was also used in previous modelling studies of lithium-sulphur cells [24][25][26][27][28][29][30][31], but it will only hold if the reaction of sulphur at the lithium surface is faster than the rate of diffusion. In future work, we plan to improve the model in order to incorporate experiment-based data on the reaction of sulphur and polysulphide on the lithium electrode.…”

“…Several polysulphide species could be incorporated in the model, and specific equations for the rates of comproportionation and disproportionation reactions could be proposed, as well as Butler-Volmer equations to describe the kinetics of electron transfer. This approach has been used in previous work [24][25][26][27][28][29][30][31], and while it is closer to the complexity of the real system, it has the disadvantage that many of the required parameters (equilibrium and rate constants, etc.) are not known.…”

“…More recently, Hofmann et al [27] developed a detailed mechanistic model of the lithium-sulphur cell that was validated with experimental data. Moy et al [28] developed an experimental method to measure the shuttle current by chronoamperometry and they simulated the experimental data with a 1-D diffusion model. Zhang et al included a concentration dependent electrolyte resistance in the model, which provided a better agreement with the experimental data [32].…”