Modification Of Para-aramid And Application In Separators For Lithium-sufur Batteries

The"shuttle effect"caused by the continuous shuttling of the soluble intermediate lithium polysulfide（LiPS）（Li₂S_n,4≤n≤8）between the cathode and anode leads to irreversible loss of the sulfur cathode and lithium metal corrosion,resulting in rapid capacity loss of lithium-sulfur batteries.The separator is a key component for lithium-ion transport and preventing short circuits in lithium-sulfur battery.with unique designs and structures also help prevent polysulfide shuttling and inhibit lithium dendrite growth.However,commercial polyolefin separators such as polyethylene（PE）and polypropylene（PP）and their composites have poor thermal stability,low mechanical strength,and limited electrolyte wettability;therefore,at high temperatures,the above diaphragms are prone to shrinkage and puncture with serious consequences.In order to solve the above problems,in this paper,ANF separator was prepared and modified by a simple sol-gel method and phase transition method,and new diaphragms were designed.The main research is as follows:（1）To prepare ANF by deprotonation:p-Phenylene terephthalamide（PPTA）was added to KOH/DMSO solution for deprotonation and its chemical cleavage to prepare aramid nanofibers.Subsequently,ANF separators were prepared using plate scraping and phase transformation,and the effects of different PPTA additions on ANF separators were investigated to characterize the microscopic morphology,mechanical properties,high temperature stability and electrochemical properties of ANF diaphragms.Compared with commercial PP separators（48.8 MPa）,ANF separators have excellent mechanical property,with a tensile strength of about 100 MPa.ANF separators have good thermal stability and maintain dimensional integrity at high temperature.ANF separators have excellent electrolyte wettability due to their fibrous mesh structure.（2）The ZnO/ANF separator was prepared by incorporating ZnO into aramid nanofibers.A series of tests and characterizations were performed,and the mechanical properties of ZnO/ANF separator were somewhat degraded compared to ANF,but still stronger than commercial PP separator.The high-temperature stability and self-extinguishing properties can be maintained at a high level,and the electrolyte wettability of the ZnO/ANF separator is improved.For the assembled batteries tested,the batteries exhibited higher capacity and cycling performance than the PP separator,and the ZnO nanoparticles provide abundant active adsorption sites,which provide good inhibition of polysulfide shuttle and strong synergistic catalytic effect on polysulfide to drive the sulfur redox reaction process.The battery assembled with ZnO/ANF separator exhibit higher capacity and cycling performance than PP separator,reaching a specific capacity of 960 m Ah g^-1 on initial discharge at a current density of 0.1 C,compared to 835 m Ah g^-1 for PP separator.575 m Ah g^-1 of specific capacity is retained after 100 cycles,with a single cycle decay rate of 0.4%and the average coulombic efficiency is maintained at99.7%.（3）The ZIF-8/ANF separator was obtained by simple physical blending of ZIF-8with aramid nanofibers.ZIF-8 interferes with the regeneration of hydrogen bonds between aramid nanofibers to produce a porous membrane,while the larger internal space in ZIF-8 can be used to load more singlet sulfur and the more pores provide an effective barrier to confine the singlet sulfur and its intermediate polysulfides.Compared with commercial PP separator,ZIF-8/ANF composite separator has excellent thermal stability and flame retardancy,which contribute to improved battery safety,and high electrolyte uptake that promotes ion diffusion and transfer.The lithium-sulfur battery with ZIF-8/ANF composite separator has a reversible capacity of up to 1178 m Ah g^-1first-turn discharge specific capacity and 902.4 m Ah g^-1 capacity after 100 cycles,and after 400 cycles,the single-turn cycle coulombic efficiency is maintained at 99.6%,and the single-turn decay rate can be maintained at about 0.13%,showing excellent electrochemical performance.