Electrochemical investigation of the ageing of a 400 Wh/kg Li-S pouch-cell

Lithium-Sulfur (Li-S) technology has emerged as an alternative for the current market dominating lithium-technology for certain applications. In this work, pouch cells were produced using a straightforward protocol with PEDOT-coated sulfur particles as active material in the positive electrode. 1.45 Ah Li–S pouch cells consisting of five double-side coated cathodes and six lithium metal foil anodes, were designed and assembled. The obtained gravimetric energy density was 400 Wh/kg at a current density of 0.167 mA/mgsulfur i.e., C/10. The cell showed a limited loss of capacity until C-rate of C/2.5 with an energy retention superior to 80%, but dropped to 26% at 1C rate. In addition, the cell retained 70% of the initial capacity after 50 cycles at C/10. It was observed an initial activation period during which contact resistance drops due to the enhancement in the contact area for the sulfur/electrolyte and sulfur/carbon interfaces. The cell aging dynamics followed a two-step scenario with an initial limited capacity fading of -6.98 mAh/cycle (-0.5%/cycle) until the 30th cycle that was later doubled to -15.57 mAh/cycle (-1.1%/cycle). Electrochemical investigations suggest a transitory regime ruled by Fick's second law at the surface of the electrodes with the appearance and growing of a third plateau during the successive galvanostatic discharges. This phenomenon was attributed to an insufficient Li-ion transport inside the electrolyte, becoming continuously more limiting with extended cycling, and caused by the reactivity of a standard ether-based electrolyte with the Li-metal electrode. Electrochemical impedance spectroscopy (EIS) of the fully charged cells reveals a non-monotonic evolution of the cell impedance during the extended cycling tests. The ohmic resistance increases during the first twenty cycles suggest an increase of the polysulfide concentration in the electrolyte, which are thereafter stabilized for the remaining cycles.