Volumetric expansion of Lithium-Sulfur cell during operation – Fundamental insight into applicable characteristics

Abstract

During the operation of a Lithium-Sulfur (Li-S) cell, structural changes take place within both positive and negative electrodes. During discharge, the sulfur cathode expands as solid products (mainly Li2S or Li2S/Li2S2) are precipitated on its surface, whereas metallic Li anode contracts due to Li oxidation/stripping. The opposite processes occur during charge, where Li anode tends to expand due to lithium plating and solid precipitates from the cathode side are removed, causing its thickness to decrease. Most research literature describe these processes as they occur within single electrode cell constructions. Since a large format Li-S pouch cell is composed of multiple layers of electrodes stacked together, and antagonistic effects (i.e. expansion and shrinkage) occur simultaneously during both charge and discharge, it is important to investigate the volumetric changes of a complete cell. Herein, we report for the first time the thickness variation of a Li-S pouch cell prototype. In these studies we used a laser gauge for monitoring the cell thickness variation under operation. The effects of different voltage windows as well as discharge regimes are explored. It was found that the thickness evolution of a complete pouch cell is mostly governed by Li anodes volume changes, which mask the response of the sulfur cathodes. Interesting findings on cell swelling when cycled at slow currents and full voltage windows are presented. A correlation between capacity retention and cell thickness variation is demonstrated, which could be potentially incorporated into Battery Management System (BMS) design for Li-S batteries

Electrolyte Decomposition and Electrode Thickness Changes in Li-S Cells with Lithium Metal Anodes, Prelithiated Silicon Anodes and Hard Carbon Anodes

“…This result is in disagreement with reported measurements in the literature. As stated by Waluś et al 5 discharge currents of C/2 enabled a stable cell thickness during cycling of a 21 Ah cell. Lower currents of C/10 and especially C/20 led to a significant and fast increase of the overall cell thickness.…”

“…Mikhaylik et al 12 reported a complete lithium depletion after 60-80 cycles with DME:DOL based electrolyte. Consequently the sulfone based electrolyte applied by Waluś et al 5 seems to be a much better choice for the Li-S system. Fig.…”

“…Post-mortem thickness measurements of the electrodes can provide first insights. 4 The comparison of in-situ thickness measurements for various charge and discharge C-rates 5 in combination with post-mortem SEM images 5,6 proved that the applied current has a major impact on the growth of the lithium metal anode and sulfur cathode.With the Li-S standard electrolyte there are already several reports illustrating that DME:DOL and LiNO 3 are not an ideal choice: Schneider et al illustrated severe gas creation in Li-S pouch cells and documented that initial cycles show especially high relative amounts of hydrogen and nitrogen with the addition of methane and ethane in later cycles.7 Jozwiuk et al examined the impact of LiNO 3 in Li-S cells by applying DEMS and isotope labelled LiNO 3 and proved N 2 and N 2 O as corresponding products next to H 2 and CH 4 . …”

“…Post-mortem thickness measurements of the electrodes can provide first insights. 4 The comparison of in-situ thickness measurements for various charge and discharge C-rates 5 in combination with post-mortem SEM images 5,6 proved that the applied current has a major impact on the growth of the lithium metal anode and sulfur cathode.…”

Perspective—Commercializing Lithium Sulfur Batteries: Are We Doing the Right Research?

“…Li-S batteries also undergo significant volume changes during operation, which poses a particular challenge for battery pack system designers and is being studied only since recently. 22 These observations have only been possible since large form factor pouch cells are available. The effect of precipitation on useable capacity and reversible capacity loss, 23 and the influence of precipitation kinetics on rate capability, 24,25 have also only recently been understood, after being observed in large cells.…”