Synthesis of Water-soluble Binders for Sustainable Electrode Processing of Lithium-Sulfur Batteries

Abstract

Lithium sulfur batteries (LSBs) have emerged as a promising alternative technology to lithium-ion batteries (LIBs) due to their much higher theoretical capacity. However, the intrinsic drawbacks, such as low conductivity, volume expansion, and polysulfide shuttling effect limit the cycling stability and energy density. A very crucial yet often overlooked component with great effect on the battery performance is the binder. However, conventional binders such as PVDF and PAA suffer from irreparable cracking during charge/discharge cycles and insufficient polysulfide adsorption ability. The former is especially problematic due to the undesirable toxicity of the solvent NMP, in addition to the increasingly stricter PFAS regulations limiting its future applicability. Hence, more advanced alternatives are necessary. Herein, a new water-soluble binder structure was synthesized through the free radical polymerization (FRP) of [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (DMAPS) and 2-hydroxyethyl acrylate (HEA), denoted as DMAPS-HEA. The successful synthesis was confirmed by nuclear magnetic resonance (NMR) and Fourier Transform Infrared (FTIR) spectroscopy. The zwitterionic (DMAPS) moiety features a cationic quaternary ammonium and anionic sulfonate, enabling strong polysulfide anchoring capabilities through synergistic interaction with both lithium and sulfur in the polysulfide structure, in addition to reversible bonding between charges, enabling self-healing properties. A superior capacity and cycling stability were demonstrated compared to the reference PAA-binder samples. At the rates 0.1 C, 0.5 C, and 1 C, the initial galvanostatic charge/discharge (GCD) capacities were 1219, 824, and 499 mAhg−1 for the synthesized DMAPS-HEA binder, whereas PAA only achieved capacities of 858, 579 at the rates 0.1 an 0.2 C. The cell with DMAPS-HEA11 at 0.5 C also demonstrated superior capacity and cycling stability over the LSBs with PAA at 0.1 and 0.2 C within 10 cycles, whereas DMAPS-HEA11 at 1 C met the performance of PAA at 0.1 C after only 50 cycles and outperformed it within 70 cycles. CV and EIS measurements suggest improved electron transfer at the solid-electrolyte interphase (SEI) and polysulfide conversion kinetics. A closer inspection in SEM reveals evenly distributed discharge product deposition with higher aspect ratio, suggesting that the DMAPS-HEA11 binder also facilitates a beneficial Li2S precipitate geometry, with the underlying mechanism a possible future research direction.