Highly Thermally Stable Composite Separators And Electrodes Based On SiO₂ Nanofibers For Lithium-Ion Batteries:Designed Fabrication And Electrochemical Properties

Lithium-ion batteries are widely used in mobile phones,laptops and electric vehicles because of their high energy density,long life,and no memory effect.With the continuous expansion of the application field,the safety of lithium-ion batteries has received increasing attention.As two important components of lithium-ion batteries,separators and electrodes play a key role in the safety of lithium-ion batteries.For the separator,the commercial polyolefin separator is easy to shrink or even melt at high temperature due to its low melting point and poor thermal stability,causing a short circuit inside the battery.For the electrode,the traditional lithium-ion battery electrode includes an active material,a binder,a conductive agent,and a metal current collector,wherein the binder has poor thermal stability and is likely to fail and decompose when heated,causing the active material to separate from the current collector.In this thesis,we have synthesized new thermal stable separator and electrode by rational material selection and structural design,to improve the safety of the entire lithium-ion batteries.High-temperature resistant and electrolyte-friendly SiO₂ was selected as the main material of the separator and the electrode.A new type of separator with both high thermal stability and mechanical strength was prepared based on the SiO₂ nanofibers.Also,a flexible self-supported electrode was designed by using SiO₂ nanofibers as the framework.The main contents and results are listed as below:(1)A SiO₂/PVA nanofiber membrane was prepaed by electrospinning combained with sol-gel chemistry and SiO₂ nanofibers with uniform structure and morphology were obtained by high-temperature calcination.The effects of PVA concentration,electrospinning voltage and the distance from the needle to the collector on the morphology and diameter of the nanofibers were systematically studied.The crystal structure,morphology,thermal stability,hydrophilicity and mechanical properties of the samples were characterized.The results show that the SiO₂/PVA nanofibers with uniform morphology can be obtained when the PVA concentration is between 12%and 16%,and the SiO₂ nanofiber membrane obtained after removing the PVA at 700? has excellent hydrophilicity,good flexibility and mechanical strength.(2)Bacterial cellulose nanowires were used to prepare a high-temperature resistant and high-strength SiO₂-based composite separator by the nano-assembly strategy.The tensile strength of the SiO₂/BC composite separator was greatly improved through the formation of hydrogen bonds between SiO₂ and BC.Moreover,we have investigated the effect of the BC content on the physical and chemical properties of the SiO₂/BC composite separator and characterized the key parameters of the SiO₂/BC separator such as porosity,contact angle and ionic conductivity.The results show that the optimal amount of BC is 20%and the 20%-BC/SiO₂ composite separator exhibits a porosity of 75%,a liquid absorption rate of 251%,an ion conductivity of 1.28 mS cm^-1,a tensile strength of 12.3 MPa,negligible thermal shrinkage at 180? for 1 hour,flame retardant property,and a wide electrochemical stability window.Moreover,the Li||LiFePO₄ half-cell assembled with the 20%-BC/SiO₂ separator shows good cycle performance and rate performance.A discharge specific capacity of 152mA h g^-1 and 135 mA h g^-1 after 100 cycles at 0.5C and 200 cycles at 4C are obtained,respectively.The performance is better than that of half-cells assembled with conventional polypropylene(PP)separators under the same conditions.Especially at 120?,the lithium-ion half-cell assembled with the 20%-BC/SiO₂ composite separator can work normally,while the half-cell assembled with the conventional PP separator quickly loses the voltage due to short circuit caused by thermal shrinkage of the PP separator.The prepared BC/SiO₂composite separator is promising for the field of lithium-ion power batteries,and provides possiblities for new lithium-ion battery separator materials with extreme high-temperature resistance.(3)A SiO₂-based flexible self-supported LiFePO₄ composite electrode was designed and constructed by using SiO₂nanofibers as the framework,ketjen black as the conductive agent and LiFePO₄ as active material through electrostatic interaction.The morphology,structure,conductivity,thermal stability and electrochemical properties of self-supported LiFePO₄composite electrodes were studied.The results show that the SiO₂-based flexible self-supported LiFePO₄ composite electrode delivers excellent electrolyte wettability and thermal stability,and is able to work after thermal treatment at 200? for 1 hour.Using the as-prepared 20%-BC/SiO₂ composite membrane as the separator and the SiO₂-based self-supported LiFePO₄ composite electrode as the working electrode,the Li||LiFePO₄ coin cell and soft-pack battery were assembled.It is found that the self-supported electrode with a LiFePO₄ mass loading of 5 mg cm^-2 has a specific discharge capacity of about 154 mA h g^-1at a current density of 0.5C and 131 mA h g^-1 at a current density of 8C.Since the SiO₂-based self-supported LiFePO₄ composite electrode does not use a current collector,the active material accounts for up to 64%of the electrode and the electrode still has good electrochemical performance even when the mass loading of active materials is up to 20 mg cm^-2.This work provides an experimental basis for the research and development of next-generation high-temperature-resistant,high-energy-density lithium-ion batteries.