A Facile, Lithium Salt in Polymer Interfacial Layer for Lithium Anode Stability in Lithium-Sulfur Batteries

Abstract

Lithium-sulfur batteries (LSBs) have garnered interest recently due to their 8-fold increase in theoretical capacity compared to state-of-the-art Li-ion batteries (LIBs), the affordability of sulfur at $100/ton, and the low environmental impact of sulfur. However, just like LIBs, LSBs suffer from anode instability due to dendrite formation and an unstable solid electrolyte interface (SEI). In this work, we address anode stability via a facile, polymer and lithium salt interfacial layer. Stable SEI formation was achieved by using a fluorinated polymer and lithium salt. A conventional Celgard separator was used for mechanical support and the polymer:salt ratio was tuned. To confirm the effectiveness of the interfacial layer for anode stability, it was used as a standalone gel polymer electrolyte in a Li-Li symmetric cell. It showed remarkable stability beyond 700 hours at an energy density of 1 mA cm-2 and capacity of 1 mAh cm-2, with a steady polarization voltage of 16mV. By comparison, a Li-Li symmetric cell with a Celgard separator began to show increasing polarization voltage after just 100 hours, with a polarization voltage that gradually increased to beyond 500mV (Figure 1). This stability was achieved by a robust SEI layer that contained LiF and Li2O, hindering the formation of the dead lithium layer. The presence of these compounds was confirmed by post-mortem X-ray photoelectron spectroscopy. Dendrite formation was also physically inhibited by the presence of the polymer matrix that had a uniform morphology and pore diameter around 1 μm. Additionally, using this interfacial layer in a lithium-sulfur coin cell provided a capacity of 866 mAh g-1 at 200 cycles, a 26% improvement over lithium-sulfur cells without the interfacial layer. Figure 1