Development of a Resilient Binder for Stable Cycling of High Sulfur Loading Lithium-Sulfur Batteries
Authors: Raulo Avinash , Lateef Saheed Adewale , Jalilvand Golareh 
Published: December 2024

Lithium-sulfur battery (LSB) is considered one of the most promising alternatives to modern lithium-ion batteries (LIBs) due to its high theoretical energy density (2600 Wh kg–1) – five times higher than that of LIBs. Despite its high theoretical energy density, the path to deployment of LSBs has not been paved due to its poor cycle life. One of the root causes of capacity decay in LSBs is the significant volume change of sulfur during lithiation and delithiation, which results in severe structural damage, thus cathode degradation and capacity loss. Such an effect is amplified in electrodes with high sulfur loadings, which typically involve thicker electrodes.
Over the years, a substantial number of studies have reported various approaches to address the challenges in LSBs. Novel structural sulfur hosts are predominantly used to accommodate sulfur volume expansion. However, the excessive hollow spaces in those host structures decrease the practical energy density of sulfur cathode. Therefore, other approaches to improve mechanical integrity of sulfur cathode during lithiation-delithiation cycles, without compromising the LSB energy density is imperative.
Binder plays a critical role in sulfur cathode by glueing the active material and conductive carbon together and to the current collector and thereby, maintaining the mechanical framework of the electrode.1 Accordingly, modifying binders to provide desired structural and mechanical characteristics is a promising approach to enhance longevity of LSBs, while preserving the high energy density of the battery. PVDF is a widely used binder in battery electrodes, due to its excellent chemical, electrochemical, and thermal stability in wide ranges of voltage and temperature. However, its adhesive properties are insufficient to withstand the significant stresses that highly loaded sulfur cathodes experience during both the electrode preparation (due to shrinkage of thick electrode film after slurry deposition), and battery cycling (due to volume variation of sulfur). These stresses further result in the formation of cracks and subsequently an increase in electrode impedance and ultimately, cell failure.2

Among various approaches to alleviate the insufficient adhesion and mechanical properties of PVDF, crosslinking is considered a potential solution. Cross-linked PVDF can provide a more stable framework for the active material and conductive agents through a covalently bonded 3D nanonet structure. To date, electromagnetic beam radiation has been the most common technique to crosslink PVDF.3,4 However, crosslinking through radiation is impractical for batteries on an industrial scale due to its complicated, expensive, and safety-demanding process. On the contrary, chemical methods can be simple and scalable, thus are highly preferred to cross-link PVDF for battery electrodes. However, cross-linking PVDF through chemical methods is known to be challenging due to its high thermal stability, which requires temperatures above 250°C for its effective cross-linkage. This is while most chemical crosslinking agents are unstable at such high temperatures. To the best of our knowledge, there has been no report on cross-linking PVDF through simple and scalable chemical methods for battery applications.
In this work we present a simple chemical process in solution phase at room temperature for rapid crosslinking of PVDF, making it a promising method to produce crosslinked PVDF binder for high sulfur loading cathodes. Through this method, in addition to the adhesive properties of PVDF, the ionic conductivity of PVDF is also improved. The process involves the controlled addition of organically dissolved PVDF to an aqueous basic solution containing Li ions. Chemical evaluations are conducted using electron paramagnetic resonance (EPR), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) to confirm the cross-linkage of PVDF. The formation of a resilient structure of the crosslinked PVDF is ascertained by tensile measurement. Moreover, electrochemical measurements are carried out to study the ionic conductivity, charge-discharge process, and cycle performance of the cross-linked PVDF. The cathode with cross-linked PVDF binder at an areal sulfur loading of 4mg cm-2 remained stable for 200 cycles at 0.1 C-rate. Further post-cycling characterizations show improved structural uniformity of the electrodes using cross-linked PVDF. The obtained results suggest cross-linked PVDF as a promising binder for LSBs with high energy density and longevity.
References
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