Design And Performance Of Composite Separators For Lithium-Sulfur Batteries Based On Thermally Conductive Alumina

Environmental and energy issues are related to human survival and development.Lithium-sulfur batteries can alleviate current environmental and energy crisis due to the advantages of high theoretical energy density,environmental friendliness,and low equipment cost.However,the commercialization of lithium-sulfur batteries is still limited by many problems:the capacity degradation of battery caused by the shuttle effect of the electrochemical intermediate product lithium polysulfides（Li PSs）,and the poor thermal conductivity of separator affecting the uniform dissipation of heat and resulting in the uneven temperature inside the battery,which will cause battery polarization and lithium dendrite growth.This dissertation focuses on the design of separators with thermal conductive function for the lithium-sulfur batteries.Four multi-functional composite separators with Li PSs barrier effect and heat conduction are designed.The main research contents are as follows:（1）In order to improve the thermal conductivity and enhance the Li PSs barrier effect of commercial polyethylene（abbreviated as PE）separators,the asymmetric coating type PE composite separator（abbreviated as Al₂O₃/PE/V₂O₅）was fabricated by coating Al₂O₃nanosheets thermally conductive layer and V₂O₅ microspheres catalytic layer on both sides of PE separator through the double-sided suction filtration process.The results showed that the thermal conductivity of the Al₂O₃/PE/V₂O₅ composite separator was 0.59 W m^-1 K^-1,which was 210%higher than PE separator(0.2 W m^-1 K^-1).The battery with Al₂O₃/PE/V₂O₅composite separator maintained the specific discharge capacity of 360 m Ah g^-1 after 1000cycles at the current density of 1.0 C,higher than battery with PE(135 m Ah g^-1).（2）In order to further improve the thermal conductivity of the PE composite separator,the carbon-coated Al₂O₃（abbreviated as Al₂O₃@C）was prepared through polymerization of dopamine monomers and subsequent high-temperature calcination,and Mo S₂ was in situ grown on the surface of Al₂O₃@C to fabricate the carbon/Mo S₂ bilayer-coated Al₂O₃nanosheets（abbreviated as Al₂O₃@C-Mo S₂）.The Al₂O₃@C-Mo S₂ single-layer coated PE composite separator（abbreviated as PE/A@C-M）was prepared through the blade coating process.The results showed that the composite separator integrated the thermal conductivity of Al₂O₃,the conductivity of carbon,and the catalytic property of Mo S₂.The thermal conductivity of PE/A@C-M was 0.69 W m^-1 K^-1,which was 263%higher than that of the PE separator.The battery with PE/A@C-M composite separator maintained the specific discharge capacity of 380 m Ah g^-1 after 1000 cycles at the current density of 1.0 C.（3）In order to reduce the interfacial thermal resistance between thermally conductive material Al₂O₃ of the coating of the composite separator,the carbon/Al₂O₃ hybrid fiber membrane（abbreviated as Al₂O₃-C）with the complete thermal network was fabricated by electrospinning and pyrolysis.Then,the organosilicon-modified carbon/Al₂O₃ hybrid fiber membrane（abbreviated as Al₂O₃-C@OSi）was obtained by in-situ growth of organosilicon（abbreviated as OSi）in the fiber voids.The Al₂O₃-C@OSi was used as a self-supporting interlayer and composited with a PE separator to form the interlayer-type PE composite separator（abbreviated as PE/Al₂O₃-C@OSi）.The results showed that the thermal conductivity of the PE/Al₂O₃-C@OSi composite separator was 0.75 W m^-1 K^-1,which was295%higher than that of the PE separator.The battery with PE/Al₂O₃-C@OSi maintained the specific discharge capacity of 360 m Ah g^-1 after 1000 cycles at 1.0 C.（4）In order to solve the poor thermal conductivity and thermal stability of the PE separator,single-layer aramid nanofibers/Zn O-coated Al₂O₃ microspheres composite separator（abbreviated as ANF/Al₂O₃@Zn O）with high thermal conductivity and thermal stability was obtained by blending,suction filtration and hot pressing of aramid nanofiber（abbreviated as ANF）and Zn O-coated Al₂O₃ microspheres（abbreviated as Al₂O₃@Zn O）,which was used in batteries as an alternative for PE separator.The results showed that the heat-resistant ANF guaranteed the morphological integrity of the composite separator at220°C.The Zn O coating layer bridged the adjacent Al₂O₃ microspheres to build the continuous thermal conduction network,featuring thermal conductivity as high as 1.04 W m^-1 K^-1,which was 447%higher than that of the PE separator.Meanwhile,Zn O coating layer promoted the conversion of Li PSs and inhibited the shuttle of Li PSs.The battery with Al₂O₃@Zn O composite separator exhibited the specific discharge capacity of 400 m Ah g^-1after 1000 cycles at the current density of 1.0 C.