Modified Separators For High Performance Lithium-Sulfur Batteries

Lithium-Sulfur batteries(LSBs)has gained much attraction as the future-generation rechargeable batteries owing to its exceptional specific energy density of 2600 Wh kg-1 and theoretical specific capacity of 1675 mAh g-1,which is remarkably higher than the state-of-art lithium-ion batteries(LIBs).Moreover,sulfur also offers an advantage of high abundance and environmental friendliness.However,there are many drawbacks that are hindering its commercialization.The most critical one is the so called “shuttle effect”,which is due to the dissolution of the long-chain lithium polysulfides(Li2Sx,4 ? x ? 8)into the electrolyte and their free movement between the anode and cathode.The research in this thesis mainly focuses on the development of modified separator to inhibit the shuttling of polysulfides for obtaining high performance LSBs and is summarized as follows.Molybdenum trioxide(MoO3)is prepared by facile hydrothermal method and is used to modify the separator to improve the performance of LSBs.It significantly suppresses the polysulfides shuttling by successfully blocking the polysulfides on the separator.The randomly arranged layers of MoO3 nanobelts provide enough porous networks that provide effective space for electrolyte infiltration and facile pathway for fast ionic transportation.Meanwhile,due to the excellent diffusion of lithium ions and improved reaction kinetics,the enhanced electrochemical performance is achieved.The MoO3-modified separator exhibited a very high initial capacity of 1377 mAh g-1,684.4 mAh g-1 after 200 cycles at 0.5 C,and the capacity fading rate of only 0.251% per cycle.Moreover,the modification with MoO3 provides good surface protection of lithium anode and depresses the lithium anode degradation,thus protracts its lifetime.Based on excellent lithium-ion diffusivity of molybdenum sulfide(MoS2),we have successively applied MoS2 as the separator modified material by facile slurry coating strategy.This layer successfully blocks the dissolution of polysulfides,inhibits their leakage from cathode side,and exhibited a strong electro-catalytic effect,thus enhances the overall performance of lithium-sulfur cell.Furthermore,MoS2 eases the lithium ion transportation because of its excellent conductivity of lithium ions,which ultimately provided excellent cycle life and good rate capacity.The MoS2-modified separator exhibited a very high initial capacity of 1287.3 mAh g-1,623.9 mAh g-1 after 200 cycles at 0.5 C,and the capacity fading rate of only 0.257% per cycle.In addition,lithium metal anode protection is also evident from the SEM results;further indicate the strong polysulfide blocking ability of MoS2.Molybdenum carbide(Mo2C)powder was applied as coating material for separator.It exhibited good results in terms of capacity and active material utilization,when applied in LSBs.However,these results were low as compared to MoO3 and MoS2,which might be due to the low electrocatalytic effect from Mo2 C.Even though,it still inhibits the shuttle phenomena by blocking the polysulfides at the cathode side and showed better results than unmodified separator.The cell with Mo2 C modified separator and 70% sulfur loading exhibited an initial discharge capacity of 942.7 mAh g-1 and a significant discharge capacity of 641 mAh g-1 even after 100 cycles,with capacity decay of 0.34% per cycle.