Design Of Triazine-Based Porous Organic Framework Material And Its Application In Lithium-Sulfur Batteries

Lithium-sulfur battery is considered to be one of the most promising secondary battery systems because of its high energy density and theoretical specific capacity.However,there are some problems that seriously affect the performance of lithium-sulfur batteries,such as the discharge intermediate polysulfides soluble in electrolyte,the low conductivity and volume expansion of sulfur.Therefore,there is an urgent need to develop a high-performance carrier material to solve the above problems.Porous organic framework materials（POFs）have the characteristics of low density,high specific surface area,rich pore structure and good stability,and their chemical structure is highly controllable and easy to modify.The above advantages make the POFs as the sulfur carrier material of lithium-sulfur battery can obtain excellent electrochemical performance.Based on this,two kinds of triazine-based POFs were prepared and the functional sulfur carrier materials were prepared through further structural design and post-modification.The main research contents are as follows:（1）A triazine-based covalent organic framework（COF）with high specific surface area and regular pore structure was synthesized,and then a layer of graphene oxide（GO）was coated on the surface of COF aggregates by spray drying process,and the composite COF@GO with net pocket morphology was prepared.The special structure of COF@GO not only helps to achieve high sulfur loading,but also protects the discrete sulfur in the system,avoids the loss of active substances and improves its utilization.The triazine functional groups in the composites can improve the electrochemical stability of the cells by adsorbing polysulfides through strong chemical interactions.COF@GO/S cathode shows excellent high loading performance and reached an areal capacity of 4.0 m Ah cm^-2,with the sulfur loading of 5.3 mg cm^-2 and the electrolyte sulfur ratio is controlled at 8 m L g^-1.（2）A kind of porous organic framework material（HUT-8）with hollow rod shape was synthesized,and then Co S₂ nanoparticles were grown in situ on the skeleton to prepared Co S₂modified porous organic framework Co S₂-HUT-8.Co S₂-HUT-8 has hollow structure and rigid skeleton,so it can solve the problem of sulfur volume expansion during charging and discharging process.The uniformly distributed metal particles in the system can accelerate the transmission of electrons and improve the conductivity of cathode materials.With the introduction of metal compounds,Co S₂-HUT-8/S cathode has a strong electrocatalytic function,which can efficiently adsorb long-chain polysulfides and reduce the overpotential of polysulfides,thus realizing the rapid conversion between polysulfides.Co S₂-HUT-8/S cathode has a fast redox reaction kinetics and it exhibits high specific capacity and excellent cycling stability.The initial discharge specific capacity of the Co S₂-HUT-8/S cathode is 761m Ah g^-1 at a current density of 1 C,and it still retains a specific discharge capacity of 583m Ah g^-1 after 500 cycles,with a decay rate of 0.046%per cycle.