PhD Dissertation: Büşra Çetiner, EFFICIENT CATALYTIC CONVERSION OF POLYSULFIDES BY MXENE-BASED HETEROSTRUCTURES TOWARDS LONG-LIFE LI-S BATTERIES,

EFFICIENT CATALYTIC CONVERSION OF POLYSULFIDES BY

MXENE-BASED HETEROSTRUCTURES TOWARDS LONG-LIFE

LI-S BATTERIES

Büşra Çetiner
Materials Science and Nanoengineering, PhD Dissertation, 2025

Thesis Jury

Prof. Selmiye Alkan Gürsel (Dissertation Supervisor)
Prof. Mehmet Ali Gülgün

Prof. Fevzi Çakmak Cebeci
Prof. Özgül Keleş

Assoc. Prof. Damla Eroğlu Pala

Date & Time: December 18th, 2025 -  4:45 PM

Place: FENS G032

Zoom link: https://sabanciuniv.zoom.us/
Meeting ID: 757 160 5835

Keywords: Lithium–sulfur batteries, Electrospun interlayer, MXene/TMO hybrid,

Hydrogen-treated TiO₂, Polysulfide shuttle, Catalytic conversion

Abstract

Lithium–sulfur (Li–S) batteries suffer from polysulfide dissolution, sluggish conversion kinetics, insulating Li₂S, and large volume changes, all of which intensify at practical sulfur loadings. This dissertation develops a sequential, mechanism-driven design strategy—from electrospun PVDF scaffolds to catalytic MXene–TMO interlayers, high-loading cathodes, and finally a defect-engineered H–TiO₂ host—to systematically address these limitations. Electrospun PVDF membranes were first developed as porous, mechanically robust, and highly wettable interlayers. Their limited polarity was then overcome by integrating MXene nanosheets with transition-metal oxides (TiO₂ or SnO₂), forming MXene–TMO heterostructures that unite metallic conductivity, strong polar adsorption, and catalytic charge-transfer sites. In medium-loading cells (2–3 mg cm⁻²), these interlayers reduced charge-transfer resistance from 13.4 to 5.6 Ω, sharpened CV peaks, increased Li⁺ diffusion coefficients by nearly an order of magnitude, and delivered initial capacities exceeding 930–1030 mAh g⁻¹ with minimal fading. The design was next translated to high-loading electrodes (5–6 mg cm⁻²), where diffusion constraints and LiPS accumulation dominate. Here, the MXene–TMO interlayer decreased R_ct from 150 to 8 Ω (94% reduction), suppressed polysulfide shuttling, and maintained high capacities (>800 mAh g⁻¹ at 0.1 C), confirming catalytic acceleration of LiPS conversion even in thick electrodes. Finally, a hydrogen-treated hollow TiO₂ host (H–TiO₂) was engineered to introduce Ti³⁺ centers and oxygen vacancies. These defects enhance binding to long-chain LiPS, improve electronic transport, and guide uniform Li₂S precipitation within the hollow interior. The resulting R_ct dropped from 4.5 to 0.31 Ω, and the battery exhibited >80% capacity retention. DFT calculations validate the strengthened adsorption energetics and defect-mediated catalytic pathways. By integrating catalytic interlayers with a defect-rich host, this work demonstrates a synergistic system achieving fast kinetics, high capacities, and stable operation under realistic sulfur loadings, offering a scalable blueprint for next-generation Li–S batteries.