| As a traditional method to prepare polymer membranes,phase inversion has the advantages of simple-preparation process,tunable structure and large-scale preparation.It is often used in gas-separation membranes or ultrafiltration and nanofiltration process.But its application in Li-S sulfur was rarely concerned.The membrane prepared by phase inversion usually exhibit porous-asymmetrical morphology,which contains dense layer and porous support layer.This porous structure with high specific surface area is suitable for the active sulfur storage as the cathode in lithium-sulfur batteries.Polar metal oxides have strong chemical adsorption capacity toward lithium polysulfide,which is an intermediate product generated during discharge/charge process in lithium-sulfur battery,owing to their polar oxygen-containing functional groups.They are often used as additives in composite electrodes to enhance the cycle stability of the battery.Based on the phase inversion method,different carbon composite materials decorated with metal-compounds were prepared as electrodes or barriers for Li-S batteries with high electrochemical properties.Firstly,the casting containing 20 wt.%nano-SiO2 was prepared by mixing SiO2 and polyacrylonitrile to N,N-dimethylformamide.Deionized water was used as the coagulation bath in the phase transformation process.The asymmetric porous composite membrane was formed by solvent-nonsolvent exchange and phase separation process.Secondly,after following carbonization and sulfur impregnation,the C/SiO2-S electrodes were formed.Compared with pure carbon membrane without nano-SiO2 additive and Al-S electrode scraped by traditional C-S slurry,the electrode has superior electrochemical stability.The C/SiO2-S electrode with sulfur loading of 1.5 mg cm-2 delivered initial discharge specific capacity of 1229 mA h g-1,and it can remain at 717 mA h g-1 with the high coulomb efficiency?98.7%?and slowly capacity decay rate?0.21%each cycle?after 200 cycling.Moreover,the C/SiO2-S electrode with 2.8 mg cm-2 sulfur content,exhibited the initial capacity of 1089 mA h g-1 and retained about 699 mA h g-1 after 200 cycles at 0.2 C current density,respectively.The strong interaction between nano-SiO2 and lithium polysulfide via chemisorption was demonstrated by means of DFT simulation and XPS analysis.Secondly,we have added soluble metal salts in the casting to tailor the structure of pores in membrane.The metal salts ions will enhance the viscosity of the casting and generate osmotic pressure difference between casting and coagulation during phase inversion process.Thus,the pore structure of the membrane will change obviously during solvent-nonsolvent exchange process due to the variation of thermodynamic and dynamic properties of the casting.Furthermore,FeCl3·6H2O can also act as the precursor of Fe3C,the C/Fe3C-S electrode was formed after carbonization and sulfur loading.At the current density of 0.5 C,The C/Fe3C-S electrode with the sulfur loading of 1.6 and 3.0 mg cm-2 delivered the specific capacity of 719.7 and 615.1 mA h g-1 after 100 cycles,with the Coulomb efficiency of 98.9%and 96.2%,respectively.Finally,the application of the membrane prepared by phase inversion method in battery separator was also explored.Polyacrylonitrile acts the function of binder and pore-forming material by means of increasing the CNT mass ratio to PAN in the casting during the phase inversion process.Thus,the conductive C/CNT porous interlayer with high specific surface area and high porosity was prepared,which was employed as a physical barrier to alleviate polysulfides shuttling as inserting it between positive sulfur and separator.At a high current density of 1 C,a high initial discharge capacity can reach 1381.7 mA h g-1 slowly decreases to742 mA h g-1 after 200 cycles.Furthermore,the electrode exhibits excellent rate capacity,which achieved to 1310,1036,969.8,902.6 and 853.1 mA h g-1 at the current density of 0.1,0.2,0.5,1 and 2 C,respectively. |
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