Application Of PAN-based Asymmetric Carbon Membranes For High-Sulfur-Loaded Lithium-Sulfur Batteries

The rapid development of electronic equipment and electric vehicles has accelerated the demand for high-energy storage battery systems.However,the current commercial graphite-based lithium-ion battery has a low energy density,and lithium-sulfur batteries are considered to be the most promising next-generation secondary batteries due to their high energy density,environmentally friendly sulfur cathode active materials,and abundant reserves.However,the following obstacles still hinder the further industrial application of lithium sulfur batteries:?1?the electronic and ionic insulation of sulfur and the discharge product Li₂S directly reduces the utilization rate of sulfur,?2?the large volume change caused by the density difference between sulfur and Li₂S is as high as 80%,leading to the damage to the electrode structure after multiple charge/discharge processes,?3?the shuttle effect induced by the high solubility and diffusion of the intermediate polysulfides leads to the loss of active material sulfur,?4?the low sulfur load on the electrode results in a low battery energy density.To solve the above problems and improve the energy density of lithium-sulfur batteries,PAN based asymmetric carbon film?C/Fe₃C film and CeO₂-CNT/C membrane?was prepared based on the structural design of positive sulfur carrier material in this paper.It has good ion/electron transfer channel and polysulfide-trapping adsorption pore structure,and can be directly used as the positive sulfur-loading material of lithium sulfur battery to prepare high-sulfur battery or as the barrier material an interlayer material to inhibit the shuttle effect through the dual effects of physical barrier and chemical adsorption to improve the energy density and cycle stability of lithium-sulfur batteries.A stackable asymmetric porous C/Fe₃C membrane was prepared by phase inversion method.The one-dimensional ordered large pore channels in the membrane can increase the loading of active material sulfur and promote the transmission of lithium ions and electrons.The results show that polysulfide is adsorbed in the mesoporous network structure of the macropore wall after circulation,and the one-dimensional macropore structure remains intact,which is conducive to the diffusion transfer of ions and electrolyte.The simulation of adsorption energy shows that the Fe₃C nanoparticles and N dispersed in the membrane provide adsorption sites for polysulfides,which can effectively inhibit the shuttle effect.The layer-by-layer stacking of C/Fe₃C membranes can prepare lithium-sulfur batteries with sulfur loading from1.5-7.1 mg cm^-2.The five-layer membrane electrode with a sulfur loading of 7.1 mg cm^-2 can provide a high capacity of 726 mA h g^-1 after 100 cycles at a current density of 0.2 C,which corresponds to the ultra-high of 5.15 mA h cm^-2.The metal oxide CeO₂ and conductive carbon nanotubes were introduced into the film casting liquid system to prepare the asymmetric porous CeO₂-CNT/C.The addition of CNTs can enhance the conductivity of the membrane electrode material;form a membrane skeleton with a cross-linked porous structure to promote the transmission of lithium ions and electrolyte in the material;increase the specific surface area of the material and increase the adsorption site of polysulfide.The simulation results prove that the doped-CeO₂ is capable of adsorbing polysulfides through strong chemical interaction and catalyzing the conversion of the sulfur and thus inhibiting the shuttle effect.Relying on the above characteristics,this as-prepared CeO₂-CNT/C membrane was used as interlayer between the positive electrode and the separator to construct a sandwich-structured membrane electrode.The rationally designed sandwich-structured membrane electrode,with a sulfur loading of 6.2 mg cm^-2,delivers a high areal capacity of 5.4 mA h cm^-2 at 0.2 C after 100 cycles,This simple and scalable anode material and structure design method is of great significance for promoting the practical application of lithium-sulfur batteries.