Preparation Of Lithium-sulfur Battery Separator By Modification Of Aramid Nanofiber And Its Performance Study

The urgent need for large-scale energy storage and green transportation has stimulated a research boom in long-life and high-energy density batteries.Lithium-sulfur(Li-S)batteries with high theoretical capacity(1675 m Ah g^-1)and high theoretical capacity density(2600 Wh kg^-1)are among the most promising energy storage systems.However,the weak conductivity of sulfur and sulfur reduction products,shuttle effect caused by soluble lithium polysulfide and volume expansion(80%)hinder the practical application of Li-S batteries.In addition,the separator plays a critical role as a key component between the positive and negative electrodes,providing a channel for lithium ion transfer and preventing short circuiting of the cell.The polyolefin separators currently used are widely used in Li-S batteries because of their chemical stability,desirable porosity and acceptable cost.Due to its poor thermal stability and low mechanical strength,it is difficult to cope with thermal runaway under extreme conditions,leading to fire or explosion.Moreover,its large pore size and low specific surface area cannot effectively inhibit the growth of lithium dendrites,which may threaten the safety of the battery.Therefore,the above problems seriously affect the development and application of Li-S batteries.In order to solve the above problems,the study on the modification of cathode and para-aramid(PPTA)-based separator was carried out to improve the battery performance.The specific studies are as follows:First,under the co-regulation of dicyandiamide and sodium chloride,n-doped ordered porous layered carbon embedded with Mo₂C(Mo₂C@NC-Na Cl)was prepared from carbon materials.In the carbon materials,the biomass-derived porous layered carbon skeleton served as an advanced sulfur host,which accelerated electron transport and buffered volume changes.The synergistic effect of Mo₂C nanoparticles and pyridine N could anchor polysulfides and enable fast sulfur electrochemistry.Benefiting from these advantages,Mo₂C@NC-Na Cl with sulfur loading of 2.5 mg cm^-2 exhibited a high initial discharge specific capacity of 1226.2 m Ah g^-1 at 0.2 C.Even at a high rate of 1 C,the initial capacity of the battery was still 853.7 m Ah g^-1,and the average capacity decay rate per cycle after500 cycles was 0.065%.Secondly,PPTA was dissolved with DMSO/KOH/H₂O system,and cetyl tertiary methyl ammonium bromide(CTAB)added as a porogenic agent,which were mixed to obtain the cast film solution,and the ANFs film was prepared by scraping coating and phase conversion method.1 wt%CTAB addition resulted in high porosity(73%)of ANFs separator(1-ANFs separator)and excellent electrolyte wettability,which was favorable for the migration of lithium ions.Thermogravimetric analysis shows that the decomposition temperature of the separator was up to 447?,which was more than 175?higher than that of the PP separator.Meanwhile,the tensile performance of 1-ANFs separator reached 77.5MPa,which was 387.5%higher than PP separator.The results show that although the cells prepared by 1-ANFs separators released a high initial specific capacity of 550.3 m Ah g^-1,which was lower than that of cells prepared by PP separators,the decay rate per turn was only 0.27%and the Coulomb efficiency was stable at 99%,which was significantly higher than that of PP separators.Considering the overall performance of the separator,the 1-ANFs separator showed a good balance of comprehensive properties such as high modulus,flame retardancy,thermal stability and electrochemical stability,which may be suitable for further use in extreme environments.Finally,based on the unsatisfactory electrochemical performance of the cells assembled by CTAB-modified ANFs separator in Chapter 3,Ti O₂ nanoparticles were further selected as the porogenic agent and 1-Ti O₂/ANFs separator was prepared by simple blending and phase conversion.The electrochemical performance and ionic conductivity of the cell were optimal when the addition of Ti O₂ nanoparticles reached 0.4 wt%.It was found that the 1-Ti O₂/ANFs separator possessed about 85.3%porosity and 266.1%electrolyte uptake.In addition,the stretching rate of the separator was significantly improved.At room temperature,the 1-Ti O₂/ANFs separator has a high ionic conductivity of 0.61 m S cm^-1 and a low interfacial impedance,which leaded to better cycling stability and multiplicative performance.As a result,cells with 2-Ti O₂/ANFs separators released a high discharge specific capacity of about 906.1 m Ah g-1 at 0.2 C,which was maintained at625.7 m Ah g^-1 after 200 cycles.In addition,cells with 2-Ti O₂/ANFs separators exhibited a higher average coulombic efficiency(99.3%)than cells using PP(98.4%)separators,which corresponds to a cycle with a slow capacity decay rate of 0.155%.