Unique flake-shaped sulfur morphology favored by the binder-free electrophoretically deposited TiO2 layer as a promising cathode structure for Li-S batteries

Metal oxides have been extensively studied as catalyst additives in Li-S batteries due to their high affinity for soluble lithium polysulfides (LiPSs). The present study developed a novel carbon nanomaterials- and binder-free 3D sulfur host by electrophoretic deposition (EPD) of TiO2 nanoparticles on carbon fiber paper (CFP). Benefiting from the unique well-connected TiO2 porous layer formed by EPD, the initial discharge capacity of Li-S battery was increased from 1160 mAh g- 1 in conventional cast TiO2/binder/S cathode (Cast:CFP/PVDF/TiO2/S) to 1310 mAh g- 1 in the EPD:CFP/TiO2/S cathode, along with an overpotential drop of 16 %. Electron microscopic studies at 100% state-of-charge (SoC) revealed that while notorious sulfur particulates were formed on Cast:CFP/PVDF/ TiO2/S, the surface of EPD:CFP/TiO2/S was covered with flake-shaped sulfur precipitates. The formation of sulfur flakes effectively suppressed pulverization in EPD:CFP/TiO2/S, leading to superior electrochemical performance and cycle life. Microstructural studies and real-time EIS investigations revealed that sulfur flakes nucleated across the Li2S matrix at the initial stages of charge and grew vertically from the sulfur/TiO2 interface. The preferential growth mechanism of sulfur flakes was attributed to the superior surface conductance of the EPD-TiO2 film enabled by the ordered clusters formed during EPD, enhancing the kinetics of electrocatalytic reactions of polysulfides at the TiO2/sulfur interface. Our results show that developing novel methods to modify the morphology of secondary sulfur upon cycling is critical for enhancing charge-discharge performance in Li-S batteries.