| Lithium-sulfur batteries are a promising new energy storage technology because of their high theoretical specific capacity(1675 m Ah g-1)and energy density(2600 Wh kg-1).However,the practical application process of lithium-sulfur batteries still has some problems.On the one hand,the intermediate products of lithium polysulfides（Li PSs）generated during the charging and discharging process of sulfur cathode can cross the separator and cause a"shuttle effect",resulting in low coulombic efficiency and rapid capacity decay.On the other hand,the uneven diffusion and deposition of lithium ions on the lithium anode leads to uncontrolled growth of lithium dendrites,resulting in the formation of"dead"lithium,unstable solid electrolyte interfaces（SEI）,and even safety hazards.Separators as an important part of lithium-sulfur batteries can interact with both positive and negative electrodes,so the design of separator materials is a simple way to solve both of these problems.Cerium-based metal-organic frameworks（Ce-MOF）have abundant microporous structures and catalytic active sites,which can effectively adsorb Li PSs and catalyze their conversion;meanwhile,the abundant oxygen-containing groups in Ce-MOF can induce uniform deposition of lithium ions and avoid dendrite formation.Therefore,in this thesis,two kinds of Ce-MOF-based separator materials were designed and prepared with the starting point of commercial separator modification and construction of new separators,and their electrochemical performance in lithium-sulfur batteries was investigated in depth.The research work in this paper is as follows:（1）The bifunctional Ce-MOF-808/PP symmetric separator was obtained by in situ growth of continuous Ce-MOF-808 membranes on both sides of polypropylene（PP）separator with regular pore channels and a large number of polar groups,which can play a triple role of adsorption,sieving and catalysis for Li PSs and thus inhibit the shuttle effect.At the same time,the homogeneous pore channels and abundant oxygen-containing groups of the Ce-MOF-808 membrane can promote rapid and uniform diffusion and deposition of lithium ions,which in turn can inhibit the growth of lithium dendrites.Therefore,the lithium-sulfur battery using this bifunctional separator has excellent electrochemical performance with an initial discharge specific capacity of 955 m Ah g-1at a current density of 1 C.After 500 cycles,the specific capacity decay rate is only 0.025%per cycle;even if the current increases to 5 C,it has a specific capacity of 681 m Ah g-1.（2）Ce-UiO-67 grains were synthesized by solvothermal method,and the new UiO-67-GF separator was constructed by vacuum filtration modified glass fiber membrane（GF）.Ce-UiO-67 has high porosity,polar functional groups and active sites,which can effectively adsorb and catalyze the conversion of Li PSs and suppress their shuttle effect;meanwhile,the lipophilic oxygen-containing functional groups of MOF helps to accelerate the lithium ion transfer and uniform diffusion,thus ensuring a stable lithium deposition process.Based on the above advantages,the lithium-sulfur battery with UiO-67-GF separator exhibits excellent cycling performance with an initial specific capacity of 919m Ah g-1at a current density of 1 C.The average decay rate per cycle is only 0.04%in 500cycles;a specific capacity of 651 m Ah g-1can still be obtained at a high current density of 5C. |
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