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

Sammanfattning

Lithium sulfur batteries (LSBs) have emerged as a promising alternative technologyto lithium-ion batteries (LIBs) due to their much higher theoretical capacity.However, the intrinsic drawbacks, such as low conductivity, volume expansion, andpolysulfide shuttling effect limit the cycling stability and energy density. A very crucialyet often overlooked component with great effect on the battery performanceis the binder. However, conventional binders such as PVDF and PAA suffer fromirreparable cracking during charge/discharge cycles and insufficient polysulfide adsorptionability. The former is especially problematic due to the undesirable toxicityof the solvent NMP, in addition to the increasingly stricter PFAS regulations limitingits future applicability. Hence, more advanced alternatives are necessary. Herein, anew water-soluble binder structure was synthesized through the free radical polymerization(FRP) of [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammoniumhydroxide (DMAPS) and 2-hydroxyethyl acrylate (HEA), denoted as DMAPS-HEA.The successful synthesis was confirmed by nuclear magnetic resonance (NMR) andFourier Transform Infrared (FTIR) spectroscopy. The zwitterionic (DMAPS) moietyfeatures a cationic quaternary ammonium and anionic sulfonate, enabling strongpolysulfide anchoring capabilities through synergistic interaction with both lithiumand sulfur in the polysulfide structure, in addition to reversible bonding betweencharges, enabling self-healing properties. A superior capacity and cycling stabilitywere demonstrated compared to the reference PAA-binder samples. At the rates0.1 C, 0.5 C, and 1 C, the initial galvanostatic charge/discharge (GCD) capacitieswere 1219, 824, and 499 mAhg−1 for the synthesized DMAPS-HEA binder, whereasPAA only achieved capacities of 858, 579 at the rates 0.1 an 0.2 C. The cell withDMAPS-HEA11 at 0.5 C also demonstrated superior capacity and cycling stabilityover the LSBs with PAA at 0.1 and 0.2 C within 10 cycles, whereas DMAPS-HEA11at 1 C met the performance of PAA at 0.1 C after only 50 cycles and outperformedit within 70 cycles. CV and EIS measurements suggest improved electron transferat the solid-electrolyte interphase (SEI) and polysulfide conversion kinetics. Acloser inspection in SEM reveals evenly distributed discharge product depositionwith higher aspect ratio, suggesting that the DMAPS-HEA11 binder also facilitatesa beneficial Li2S precipitate geometry, with the underlying mechanism a possiblefuture research direction.