Design And Synthesis Of Triazine-based Covalent Organic Frameworks And Their Performance As Main Materials For Lithium-sulfur Battery

The development of advanced energy storage system is of great significance to social progress and environmental protection.Lithium-sulfur（Li-S）batteries have attracted much attention due to their high theoretical energy density of 2600 Wh kg^-1,abundant raw material resources and good environmental benefits.However,so far,most studies have proved that lithium-sulfur batteries have insufficient reversible capacity and poor cycle stability,which seriously hinders the practical development and application of lithium-sulfur batteries.As an organic porous material with adjustable pore size and functional framework,covalent organic framework material（COF）can effectively inhibit the shuttle effect.In this paper,two kinds of triazine-based covalent organic framework materials with different structures were designed,and they were used as cathode materials for lithium-sulfur batteries to suppress polysulfide shuttle and enhance the performance of lithium-sulfur batteries.The specific research is as follows:（1）A new two-dimensional covalent organic framework is designed at the molecular level by using synergistic effect.The allyl-rich 5-（allyloxy）m-phthalaldehyde was prepared by three-step organic reaction,and the two-dimensional covalent organic framework material（ART-COF）was prepared by further reaction with 4,4’,4’’-（1,3,5-triazine-2,4,6-triphenyl）triphenylamine.With the C-S covalent interaction between allyl and elemental sulfur,and the N-Li electrostatic adsorption between triazine and lithium ion,ART-COF showed excellent performance in charge-discharge and rate test.The initial discharge capacity of the battery is 1270 m Ah g^-1at 0.2 C,and the capacity can be maintained at 1220 m Ah g^-1after 100 cycles.The capacity retention rate is more than95%and the coulombic efficiency is close to 100%.At the same time,at a high current density of 1 C,the ART-COF/S cathode still maintained a high discharge specific capacity of 818 m Ah g^-1after 500 cycles（the decay rate per cycle was only 0.035%）.Even at the current density of 5 C,the battery still has 420 m Ah g^-1discharge capacity.When the current density is adjusted from 5 C to 0.2 C,the specific capacity of the battery can be restored to 95%of the initial discharge capacity.（2）A new three-dimensional COF material was constructed by two-dimensional COF interlayer crosslinking strategy.The two-dimensional ART-COF interlayer crosslinking was realized by using olefin metathesis reaction to construct a new three-dimensional material CAGE-COF.The material has the characteristics of two-dimensional COF crystallization in the layer.The regular channel ensures the rapid conduction of lithium ions,and has a three-dimensional structure between the layers,which effectively inhibits the diffusion of polysulfides.The cathode sulfur loading of CAGE-COF/S was 1.3～1.5 mg cm^-2.At 0.2 C,the first discharge specific capacity of CAGE-COF/S was 1107 m Ah g^-1,and after 100 cycles,the discharge capacity decreased to 965 m Ah g^-1,with good capacity retention（0.128%attenuation per cycle）and coulombic efficiency close to 100%.At 1.0 C,the discharge specific capacity of 573m Ah g^-1can still be maintained after 500 cycles,and the attenuation rate of each cycle is only 0.081%.