| Since the 1990s,lithium-ion batteries have become the main power source on the market.With the continuous consumption of fossil energy and the continuous development of society,the requirements for electronic products have gradually increased,especially in the field of electric vehicles.The required high energy density battery system is particularly important.Researchers mostly conduct battery research and development from the aspect of battery structure.The positive electrode,negative electrode,and electrolyte of the battery have become research targets.The separator,as an important part of the battery,has also become a research hotspot in improving battery performance in recent years.Lithium-sulfur batteries with high energy density（2600 Wh/kg）have gradually become research hotspots and are expected to become a new generation of high-specific-capacity battery systems,but their commercialization is greatly limited due to their own shortcomings.Severe volume expansion occurs during the charging and discharging of the sulfur electrode,and the final discharge product generated by the redox reaction has low conductivity.The generated soluble lithium polysulfide diffusion causes a serious shuttle effect and reduces the utilization of the active material.In addition to improving the performance of lithium-ion batteries,it is also possible to improve the electrochemical performance of lithium-sulfur batteries by adding functional coatings to improve the electrochemical performance of lithium-sulfur batteries.In this thesis,the study of modified separators is mainly carried out from the perspective of improving the performance of lithium-sulfur batteries.The main research contents and research results of this thesis include:（1）Research on tubular nitrogen-doped carbon fiber（TNCF）modified separator.A hollow tubular polypyrrole material was synthesized by a self-degraded template method,and then a hollow structured TNCF material was obtained by pyrolysis.The TNCF material was applied to the original diaphragm surface by vacuum suction filtration.The TNCF coating can be used as an additional current collector.The doping of N atoms in the carbon matrix can enhance the chemical adsorption of lithium polysulfide.These advantages have given lithium-sulfur batteries higher rate performance and long-term cycling.The rate performance and cycle performance of lithium-sulfur batteries using TNCF modified separators have been greatly improved compared to lithium-sulfur batteries using original separators.At 0.5 C,the corresponding capacity attenuation rates of 200 cycles are 0.13%and 0.33%,respectively.The results show that the TNCF coating modified separator is a promising high-performance lithium-sulfur battery high-performance separator.（2）Study on Al2O3@PDA-C modified separator.The inorganic ceramic material Al2O3is used to modify the separator to improve its performance by utilizing its heat resistance,lyophilicity and mechanical properties.By using dopamine as a pyrolytic carbon source,a nitrogen-doped carbon layer is coated on the surface of Al2O3to improve the conductivity of the coating layer and avoid the introduction of high-resistance particles to greatly affect the original battery resistance.The coating material was coated uniformly on the surface of the separator by vacuum filtration,and an Al2O3@PDA-C modified separator was prepared.The research results show that the modified separator has improved stability such as stability and wettability to the electrolyte.We used a sulfur electrode sheet with a surface load of 1.2-2.0 mg/cm2 as the positive electrode,and circulated 500 times at a rate of 0.5 C.The specific discharge capacity decreased from 1109.5 m Ah/g to 576.8 m Ah/g,and coulomb efficiency has been stable at around 99.3%.（3）Study on NiFe2O4/KB Modified Separator.NiFe2O4nanoparticles were prepared by a simple hydrothermal method.NiFe2O4 nanoparticles and Ketjen Black（KB）mixed materials were uniformly coated on the surface of the separator by a doctor blade coating method.Good lyophilic properties of NiFe2O4 nanoparticles were prepared.And the ability to bind to polysulfides,inhibit the shuttle effect through chemisorption.The introduction of porous carbon KB to construct a porous transmission channel,improve the conductivity of the interface between the separator and the positive electrode,and balance the relationship between the diffusion balance of polysulfide and the shuttle movement of lithium ions to avoid increasing the internal resistance of the battery.The capacity of NiFe2O4/KB modified separator battery can reach 1079.6 m Ah/g at 0.5 C.After 100 cycles,the capacity remains at 753.1 m Ah/g,the capacity retention rate is 69.7%,and the coulomb efficiency is stable at 98%.At high rate 1 C,the initial discharge capacity is 887.16 m Ah/g.At high current density,the decay rate is faster than low density.After 100 cycles,the battery capacity decays to 573.1 m Ah/g,and the capacity retention rate is 64.4%.Attempts to use modified separators to assemble lithium iron phosphate batteries have proved that the modified separators also have a certain effect on the improvement of lithium ion battery performance,and provide new ideas for future research on modified separators. |
