Solubility of Pvdf in Dioxolane-Based Li-S Electrolytes and Improving Cycling Using a Double Cross-Linked Starch Binder

Abstract

Lithium-Sulfur (Li-S) is a coveted battery chemistry for low-cost lightweight cell production. Polyvinylidene fluoride (PVDF) is the most popular binder used for S-cathode fabrication in R&D. However, unbeknown to the broad research community, the dioxolane used in the similarly popular Li-S electrolyte formulation falls within the Hansen solubility sphere of PVDF, leading to slow dissolution of the PVDF-binder. The mechanically compromised S-cathode increases electronic impedance and unnecessarily leaches polysulfides during cycling, contributing to capacity fading. In this study, a double cross-linked starch binder (DCS) is developed to contrastively verify the hypothesis and attempt to address the issue. Compared to PVDF, DCS remains insoluble in the electrolyte, thus maintaining the structural integrity of the S-cathode and retaining 2.5 times lower impedance after 200 cycles. Its superior polysulfide retention was confirmed through operando Raman and ex situ UV-vis. DCS-bound cathodes maintain a discharge capacity of 522 mAh·gs-1 at 0.1 C rate after 200 cycles and a capacity fade of 0.14% per cycle after stabilization (beyond the initial 20 cycles). In contrast, the PVDF-bound electrode displays a discharge capacity of 434 mAh·gs-1 after 200 cycles, with a higher decay rate of 0.24% per cycle.

Keywords: cross-linked binder, lithium-sulfur battery, sulfur cathode, polysulfide, operando cell