| With the fast development of science,technology and society,portable electronic devices such as smart phones,cameras and notebook computers have become widely available,as well as the rise of globe's emerging markets of new energy vehicles.Energy storage technologies and facilities have been in unprecedented demand.Rechargeable lithium-ion batteries have become an irreplaceable energy storage system for a long time due to their advantages such as overlong cycle life,relatively high energy density as well as environmental friendliness.However,the traditional lithium-ion battery cathode materials with relative low actual specific capacity cannot satisfy people's growing energy demands.Consequently,it is urgent to find other alternatives with high-density and high-capacity to meet the ever-increasing energy requirements.Lithium-sulfur batteries?LSBs?have received extensive attention in recent years due to their high energy density of 2600 Wh kg-1 and high theoretical specific capacity of 1675 mAh g-1.At the same time,sulfur is earth-abundant,economical,non-toxic and ease to synthesize.However,LSBs are plagued with three serious defects,thus restrict its commercial application as follows,?i?the intrinsic insulating nature of sulfur(5×10-30 S·cm-1 at 25?)and Li2S;?ii?the flagrant“shuttle effect”caused by the dissolution of lithium polysulfide intermediates?Li2Sx,3?x?8?in the electrolyte;?iii?the obvious volume expansion?80%?during cycle process because the sulfur has a much greater density than Li2S.To overcome the above mentioned drawbacks,we focus on the modification of LSBs cathode material.The principal research results in this paper are as follows:?1?In this paper,we prepared S/PPy composite with different content of sulfur by means of solution drop method and studied the optimum synthesis condition.Firstly,the granular PPy was synthesized by chemical oxidation polymerization method.Then the CS2 solvent with sulfur solution was added dropwise.For comparison,we studied the morphology and structure and the electrochemical performance of S/PPy with different mass ratios of 6:4,7:3 and 8:2.The experimental results show that the composite have optimal performance when the mass ratio of sulfur and PPy is 7:3.And it displays high initial discharge capacity of 1151 mAh g-1and maintains 623 mAh g-1 after 120 cycles at a current density of 200 mA g-1.?2?We designed the S/Al2O3/PPy ternary hybrid material structure by making use of synergistic effect between polar metallic oxide and conducting polymer.First,we adopt a simple and amicable process including ball milling and melt diffusion method to synthesis S/Al2O3 composite materials,and then in-situ chemical polymerization method was used to prepare S/Al2O3/PPy ternary composite.The experimental results show that S/Al2O3/PPy composite exhibits optimal performance.At a current density of 200 mA g-1,the discharge capacity of S/Al2O3/PPy delivers more outstanding initial discharge specific capacity of 1088 mAh g-1 and maintains730 mAh g-1 after 100 cycles,and the average coulomb efficiency reaches about 99%.Even at high current density of 1 C(1 C=1675 mA g-1),the ternary hybrid material has an initial capacity of 683 mAh g-1 while maintaining 567 mAh g-1 after 100cycles with 82.7%capacity retention.?3?In addition,we studied the effect of cut-off voltage on the cycle performance of LSBs by the test of cycle performance of the ternary composites under different voltage window.S/Al2O3/PPy manifests high initial discharge specific capacity of1208 mAh g-1,and still remains a capacity of 749 mAh g-1 after 200 cycles within the potential range of 1.7-2.8 V.This discharge specific capacity and cycled stability are significantly better than the results obtained at same current rate for galvanostatic charge/discharge in the potential window of 1.5-3 V.We analyzed the results and concluded that deep discharge must be avoided when LiNO3 is used as an additive of the electrolyte,which is conductive to increasing the cycle stability of LSBs. |
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