Synergistic Effect of Fe2O3/NC-Gdl Interlayer and Partially Exfoliated Carbon Nanotubes/Sulfur Cathode for Efficient Polysulphide Confinement in High Performance Lithium–Sulfur Batteries

Abstract

Lithium sulfur batteries have gained considerable research interest for electric vehicles and portable electronic devices due to their high theoretical energy density (2600 Wh/Kg) compared to existing metal ion batteries. In addition, the low cost, high abundance and high theoretical specific capacity (1675 mAh/g) of sulfur have made Li-S batteries to be the potential replacement for lithium ion batteries. However, the low electrical conductivity of sulfur (5 × 10−30 S/cm), Li2S, low sulfur loading (<70%) at the composite cathode, large volume expansion during S to Li2S conversion and multistep reduction of sulfur to lithium polysulphides (LPS) act as the prohibitor for the commercialization. Elemental sulfur undergoes a various structural variation during the charge–discharge and forms soluble higher order lithium polysulfides Li2S n (8 ≤ n ≤ 3) and lower order insoluble sulfides Li2S2/Li2S in liquid. The poor conductivity of the intermediates formed and their structural changes result in high polarization. Also, the consecutive shuttling of the soluble polysulfides between the anode and cathode results in reoxidation at the cathode and further leads to capacity fading of the Li-S battery. To mitigate these, researchers have developed several types of conducting carbon matrix i.e., acetylene black, super P, carbon nanotubes, graphene, carbon paper etc. However, due to poor interaction of polar polysulfides with non-polar carbon matrix, polysulfide migrates into the electrolyte and increases the shuttle effect. Metal oxides (TiO2, Fe2O3 , MgO, Co3O4, SiO2) based interlayer though offer abundant polar active sites for polysulfides adsorption, but suffers from low electrical conductivity and high binding energy between oxides and sulphides, which results degradation in the electrochemical performance. Hence optimization of oxide interlayers are necessary to increase the performance of Li-S batteries.So, in our work, further to minimize the LPS confinement and conversion, we have proposed an efficient cathode where partially exfoliated carbon nanotubes are used as sulfur host. Due to the high surface area, and high electrical conductivity, it serves as an efficient cathode material with high sulfur loading (>75%) and results in high sulfur utilization. In addition, we have also considered the effect of polar sites and designed Fe2O3/NC nanoparticles coated on the polytetrafluoroethylene ethylene coated carbon paper (GDL paper) as interlayer between separator and cathode, which can efficiently adsorb the polysulphides and effectively mitigates the shuttling effect due to the active polar sites and oxygen vacancy. From our electrochemical studies, we have observed with PECNT/S cathode without interlayer initial discharge capacity of 900 mAh/g can be obtained at 0.1 C rate. However, with only GDL interlayer this capacity increases to ~1400 mAh/g. Further, on coating of Fe2O3/NC nanoparticles on GDL paper the capacity can further be enhanced and reached to 1610 mAh/g. For GDL-Fe2O3/NC interlayer better reversibility and faster charge transfer kinetics are confirmed from the CV curves. Cyclic stability of 500 cycles is carried out for PECNT/S, PECNT/S/GDL and PECNT/S/GDL- Fe2O3/NC at 0.5 C rate. ~80% of initial capacity retention can be obtained even after 500 cycles. This substantiate that with PECNT/S and GDL-Fe2O3/NC as cathode and interlayer respectively can synergistically result in a high capacity and stable lithium-sulfur batteries.