Coupling Hierarchical Sulfur Composites with in-situ Cross-linked Binder to Build Stable High-areal-capacity Sulfur Cathodes
Authors: Pang Quan , Nazar Linda F 
Published: February 2020

Lithium-sulfur battery is considered one of the most promising electrochemical system for energy storage, owing to the high theoretical energy density (2600 kWh/kg) and the natural abundance of sulfur. Their practical challenges facing commercial viability lie in the insulating feature of the end-products (S and Li2S), lithium polysulfide dissolution and shuttling between electrodes and random precipitation of Li2S from the dissolved polysulfides. This leads to rapid capacity fading over long-term cycling and relatively low sulfur utilization. This becomes even more problematic for high sulfur loading cells, since the large concentration of dissolved polysulfides can not only lead to uncontrolled precipitation of Li2S that eventually clog the cathodes, but also to increased side-reactions with lithium anodes. Great efforts have been made to address these issues with a focus on the cathode materials, but mostly thin electrodes with sulfur loadings below 2 mg/cm2.[[1]] In order to fabricate high-loading Li-S cells, recently reported approaches are to construct 3D conductive framework composed of graphene or carbon nanofibers. However, one intrinsic problem is the high porosity of the 3D architecture usually requires large volume of electrolyte, leading to high electrolyte/sulfur ratio and thus low volumetric energy density. Therefore, developing high loading electrodes using traditional slurry-based process is critical for practical applications.[[2]]
Here we will present a multifaceted strategy to construct stable and high sulfur loading cathodes based on the traditional slurry processing, by coupling a hierarchical sulfur composite with in-situ cross-linked polymer. We incorporated our previously reported highly nitrided material with conductive framework. A light-weight, electrically conductive and highly porous hybrid was thus obtained. The hybrid material exhibit a micrometer sized structure, which is beneficial to fabricate crack-free thick electrodes. This is due to the enhanced connection between individual particles even with lower percentage of binder compared to nanoparticles with low tap density. The XPS survey spectrum shows the presence of a high concentration of nitrogen at 19.9 at%. Our first-principles calculations show that our hybrid has a much higher polysulfide binding energy towards than functionalized polymer or doped carbons.
We will further present an integrated cathode enabled by in-situ cross-linked binder and a combination of CNTs and Super P carbon additives. The inter-particle electronic connection is greatly enhanced. Fourier transform infrared spectroscopy (FTIR) confirms the successful cross-linking. The low-magnification SEM images clearly shows that on contrary to the large cracks in the PVDF based electrodes (formed upon evaporation of the slurry solvent), the cross-linking binder based electrode surface is flat and compact, without any cracks. It is worth noting that such an integrated structure was achieved with as low as 5 wt% of binder, which greatly increases the gravimetric energy density. We demonstrate that cathodes with sulfur loading up to 15.0 mg/cm2 have been fabricated with stable cycling performance. Insights into the evolution of the impedance will also be presented.
References

[1] Liang, X.; Hart, C.; Pang, Q.; Garsuch, A.; Weiss, T.; Nazar, L. F. A Highly Efficient Polysulfide Mediator for Lithium−Sulfur Batteries .Nat. Commun.2015, 6, 5682.
[2] Rosenman, A.; Markevich, E.; Salitra, G.; Aurbach, D.; Garsuch, A.; Chesneau, F. Review on Li-Sulfur Battery Systems: an Integral Perspective.Adv. Energy Mater.2015, 10.1002/aenm.201500212