Electrochemical Performance of Fe Doped TiO2 Nanopowder Coatings on Separators for Li-S Batteries

Abstract

Li-S batteries are increasingly being investigated due to their high theoretical and practical energy density. During charge/discharge processes, reaction of S8 with Li+ ions leads to formation of soluble lithium polysulfides (LiPS) in the electrolyte, which diffuse toward the anode and cathode, known as the shuttle effect, which is the most serious problem about capacity degradation. Diffusion of these active spieces of insulating nature subsequently causes decrease in ion mobility and loss of active material. [1, 2] One rational strategy to improve electrochemical performance of the cell relies on controlling the reactions on the coated separator surface so as to reduce LiPS shuttle between electrodes by means of either trapping them on the surface or reactivating the sulfur ions in LiPS due to reactions between coating material and LiPS [3, 4]. Among several additives to the coatings on separator, metal oxides with high LiPS chemical adsorption ability have been used to improve polysulfides trapping and to enhance the electrical conductivity. [5,6] In this work, Fe-doped (1 and 5 wt. %) TiO2 synthesized via sol-jel method using deionized water, ethanol, hydrochloric acid and titanium tetra isopropoxide (TTIP) in situ. First, 5 g TTIP was dissolved in 120 ml ethanol for 30 min. and 5 ml HCI was added into solution, then stirred at 60o C for 2 hrs. in magnetic stirrer. Solution was then doped with anhydrous FeCl3 at 1 wt.% TTIP. Finally, 20 ml deionized water was added into solution and ultrasonicated for 30 min. to accelerate hydrolysis. Finally, the gel was dried at 800 C for 12 h and calcinated at 5000 C for 2 hrs in air to obtain Fe-doped TiO2 anatase phase. Fe doped TiO2 nanopowders was placed in an agate mortar and ground, subsequently 12 wt. % polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidone (NMP) solvent was prepared. A Fe doped TiO2 nanopowders / PVDF binder in NMP solvent slurry was prepared by mixing 90 wt.% Fe doped TiO2 with 10 wt.% PVDF binder in NMP solvent, which was coated onto the cathode-facing side of the glass fiber separator with a thickness of 51 µm. A baseline separator was also prepared with TiO2 without any Fe doping coated separator for comparison. Electrohemical performance tests on baseline sample showed an initial capacity of 1748 mAh g-1, but after 100 cycles capacity retention was only 504 mAh g-1. The cells with Fe doped TiO2 cathode-facing side coated glass fiber separator maintained a high Li-ion transport and improved electrolyte uptake as well as hindered polysulfides diffusion by physical obstruction. Due to the high electrical conductivity and strong LiPS capture ability of the modified separators, the cell with 1 wt. % Fe doped TiO2 separator exhibited a high initial capacity of 1940 mAh g-1 and capacity retained at 998 mAh g-1 after 100 cycles at 1 C. When weight ratio is increased to 5 wt. % Fe, the initial capacity was 1928 mAh g-1 and capacity retained at 1009 mAh g-1 after 100 cycles at 1 C. After 100 cycles at 1 C, there was notable difference for the cells with 1 or 5 wt. % Fe doped coatings. Long cycling stability performance at current rate of 1 C also were also investigated. It was found that the cell with 5 wt. % Fe doped separator showed superior long term capacity retention over 1 wt. % Fe doped case. After 500 cycles, the cell with 5 wt. % Fe doped separator showed low capacity decay rate of 0.08% per cycle while the one with 1 wt. % Fe doped showed high capacity decay rate of 0.27% per cycle. The electrochemical impedance tests were performed and found that the cell with 5 wt. % Fe doped TiO2 displayed smaller interfacial resistance (Rct) than the one with 1 wt. % Fe doped TiO2.