Long Chain Polysulfides Control via Ferroelectric (Ba0.9Sr0.1TiO3) Nanoparticles Doped Sulfur Cathode for High-Capacity Li-S Batteries
Authors: Castillo Ivan , Tripathi Balram , Barrionuevo Danilo , Morell Gerardo , Katiyar Ram S 
Published: December 2024

Lithium-sulfur (Li-S) batteries show promise in energy storage technology due to their high energy density and cost-effectiveness. However, practical application faces challenges, including low electrical conductivity of sulfur and discharge products, and the generation of insoluble compounds like Li2S during cycling. Soluble polysulfides can migrate, perpetuating a shuttle effect that leads to deposition of solid Li2S2 and Li2S on the anode, reducing efficiency and cycle life. Efforts to enhance cathode conductivity and suppress polysulfide loss during cycling are ongoing to overcome these challenges. In this study we are reporting electrochemical performance of Ba0.9Sr0.1TiO3 (BST) having polarization of 14.58 μC/cm2 doped sulfur/carbon black/polyvinylidene fluoride (S/BST/CB/PVDF) composite as cathode materials for Li-S batteries. The performance of S50BST30CB10PVDF10, and S40BST40CB10PVDF10 fabricated cathodes in terms of structural, electronic, morphological, and electrochemical response have been tested. X-ray diffraction spectra confirm tetragonal symmetry (c/a=1.0073), Raman spectroscopic study confirms Raman modes (A1(TO1), A1(TO2), A1(TO3) and A1(LO3)) of the tetragonal orientation for BST modified composites. All the compositional cations are observed from SEM images confirm homogeneous distribution of BST in the sulfur cathode system having grain sizes (1-1.5 μm) which is based on microscopic analysis. BST coupled C-S composite cathodes showing improved electrochemical performance in comparison to C-S composites. The high-capacity composites cathode in 1st cycle is S50BST30CB10PVDF10, tested @100 mA/g & 200 mA/g with polypropylene (PP) separator showing specific capacities ~820 mAh/g & ~540 mAh/g respectively, with improved capacity retention up to 60%. We observed that the hybrid composite cathode S40BST40CB10PVDF10 which was tested @100 mA/g showing specific capacity ~1080 mAh/g and remained up to 395 mAh/g after 100th cycle with capacity retention of 36%, very stable response till 100th cycles attributes polysulfide migration is effectively reducing due to ferroelectric particles doping in the composite cathode. Two plateaus were observed in between 2.3V to 2.0 V and 2.0 V to 1.5 V in the charge/discharge characteristics and high cyclic stability substantiate the superior performance of the designed ferroelectric nanoparticles doped S/CB composite cathode materials due to the efficient reduction in the polysulfide shuttle effect in these composite cathodes. Oxidation peaks for S50BST30CB10PVDF10 composite are at 2.6 V & 2.7 V and reduction peaks are at 2.0 V & 2.2V suggests an enhanced kinetics of the reduction reaction with the BST doped cathode as expected due to the alteration in the kinetics might be due to ferroelectric nanoparticles coupling & reversible transformation of Li2S to short/long chain LiPSs and finally to S8. In the case of symmetric cells with the positive and blocking electrodes. We have confirmed that the active material, sulfur, does not contribute to the resistance of the positive electrode, however due to inclusion of BST up to 20 & 30 wt% and applying RC(R)W circuit model for interfacial parameters. The observed values for S50BST30CB10PVDF10 are as Rs (2.688) C= 9.1 µF Rct (145 Ώ) and W (0.02312) showing the improved diffusion pathways for Lithium ions. Considering that polar substances have good affinity towards polysulfide and can provide more stable reacting environment in the cathodic site, to trap polysulfide intermediates via induced permanent dielectric polarizability. It is expected that spontaneous polarization induced by asymmetric crystal structure of ferroelectrics provide internal electric fields and increase chemisorption with heteropolar reactive.