Sulfonated Poly(Ether Ether Ketone) as Enhanced Separators for Lithium-Ion Batteries

Separators in batteries are porous membranes that insulate the positive and negative electrodes from direct contact while accommodating electrolyte to facilitate ion transportation. It is desirable for the separators to have high thermal stability to prevent melting or shrinkage during battery operation. To this end, poly (ether ether ketone) (PEEK) is a potential candidate due to its superior heat resistance compared to commercial polyolefin separators. However, the employment of PEEK as battery separators is hampered by its poor solubility in organic solvents, preventing the use of well-developed separator fabrication techniques, such as electrospinning. To address this, we introduced negatively charged groups (i.e., sulfonic acid) to the PEEK polymer backbone, which is known to improve the solubility of the polymer. Then, we developed a nonwoven porous separator based on the sulfonated PEEK via electrospinning. The advantages of using electrospun fibers include the ultra-high surface area and the nano-porous structure, which can further enhance ionic exchange. While sulfonated PEEK (SPEEK) membranes have been studied for filtration applications such as water treatment, their potential in battery separator applications has yet to be demonstrated, as the ionic exchange process of SPEEK in electrolytes is not fully documented. Hence, in this study, the effect of sulfonation on lithium-ion conductivity and battery performance was explored. Aside from PEEK, other common polymers such as polystyrene (PS) and polyethylene (PE) can also be modified through sulfonation, imparting these typically non-conductive materials with ionic characteristics. Therefore, the findings from this study will accelerate the understanding of how the sulfonation process can be used for tailoring material properties and micro/nanostructure, thereby expanding their applications.<br/><br/>Apart from enhancing solubility, sulfonation of PEEK may also influence other separator properties (e.g., hydrophobicity and ionic conductivity), which strongly depend on the degree of sulfonation (DS) of PEEK. Studying the degree of sulfonation will help pave the way for the modification, stabilization, and selection of PEEK separators towards enhancing the safety and electrochemical performance of lithium-ion batteries. In this study, we synthesized PEEK with different DS by varying the reaction conditions. The sulfonation process was characterized by Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Then, we fabricated the separators by electrospinning the SPEEK. Different separator thicknesses were fabricated via different processing times, and the morphology was characterized by scanning electron microscopy (SEM). We have demonstrated that there is a strong correlation between the degree of sulfonation, which leads to changes in morphology and therefore the battery performance. As a result, the overall battery performance was enhanced. We believe the findings from this work will pave the way for future studies on the degree of sulfonation and its enhancement of battery safety.