Physical and Electrochemical Properties of Transition Metal Oxides with Hollow Structure As Sulfur Host Material for Lithium-Sulfur Batteries

Abstract

INTRODUCTION Because of its high theoretical energy density, possibly low cost, and environmental friendliness, lithium-sulfur (Li-S) batteries have received a lot of interest. However, the principal disadvantage impeding the success of Li–S batteries lies in the severe leakage and migration of soluble lithium polysulfide intermediates out of cathodes upon cycling caused “shuttle effect”. The loss of active sulfur species incurs significant capacity decay and poor battery lifespans which has prevented it from commercial realization.Several researches have been conducted with an attempt to improve capacity of batteries while maintaining life of the Li-S cellใ including an introduction of nanostructured cathode host materials. Two main approaches have currently been explored a. chemical confinement and b. physical confinement to anchor the lithium poly-sulfide (LiPSs) reaction products durin the charging process. Transition metal oxides such as TiO2 and SnO2 has been studied with a potential material to anchor polysulfides via Lewis acid–base interactions and thus maybe able to chemically confine LiPSs within or on the transition metal oxide. Physical confinement may also be possible by trapping LiPSs within a transition metal oxide host structure e.g. hollow structure and thus help impedes polysulfide dissolution into the electrolyte. In this study, TiO2 and SnO2 are focused to understand the physical and electrochemical properties of sulfur host material with hollow structures. The in situ XAS is also conducted to understand the behavior of the dissolution and migration of long-chain LiPSs intermediates (Li2S x , 4 ≤ x ≤ 8) in the sulfur cathode upon discharge. EXPERIMENTAL The transition metal oxide, TiO2 and SnO2, were successfully synthesized by a templated method. The synthesis process involved four steps, preparation of SiO2 spheres, synthesis of SiO2@transition metal oxide core-shell structure by coating transition metal oxide shell on SiO2 and crystallization by calcination and etching the SiO2@transition metal oxide to form hollow transition metal oxide spheres. Structural properties of the synthesized samples were studied by X-ray diffraction (XRD) using Cu-Kα (λ=1.5418 Å) radiation. Particle morphologies and size of the resulting compounds were observed using a field emission scanning electron microscope. Composite electrodes for electrochemical studies were prepared by doctor blade coating of a slurry compound onto carbon-coated aluminium foil current-collectors. The transition metal oxide and sulfur composite, conductive carbon and PVDF binder were mixed in the ratio of 80:10:10 by weight. Electrochemical characterizations were performed by in 2032 coin cells and Galvanostatic charge-discharge cycling was done between 1.8-3.0 V at 0.1C using a MACCOR Series 4000 at room temperature. RESULTS AND DISCUSSIONS Based on the XRD results, single phase anatase TiO2 and tetragonal SnO2 were both successfully synthesized by the templated method. SEM images (Fig. 1a and 1b) demonstrate that TiO2 and SnO2 are generally spherical structures with nanometer scale dimensions ranging from 250-350 nm and 450-500 nm, respectively. The interior hollow region can be seen clearly in the images of broken spheres. The shelled surface of the synthesized powders is hierarchically made up of interconnected nanocrystals providing a possibly porous structure. Hence, the hierarchically porous and hollow structure has a large surface area for adsorbing the LiPSs distributed in the electrolyte.Fig. 1c shows the charge and discharge voltage profiles of SnO2 and sulfur composite electrode cathode in the first five cycles. Two discharge plateaus were clearly observed, which might be attributed to the two-step sulfur-lithium reaction process. The initial plateau, centered about 2.30 V, was commonly attributed to the reduction of the S8 ring and the formation of S8 2−. The second discharge plateau at 2.10 V was attributed to further reduction of the higher polysulfides (Li2S n , 4 ≤ n ≤ 8) to the lower polysulfides (Li2S n , n ≤ 3). There exhibited two plateaus in the charge process at about 2.25 and 2.35 V, respectively. Electrochemical performance in relations to capacity retention and the in situ XAS to understand the behavior of the dissolution and migration of LiPSs will be discussed in the meeting. Figure 1