Lithium-sulfur Batteries Based On Sulfonated Covalent Organic Framework And Its Derivatives

Lithium sulfur battery（LSB）is regarded as one of the potential energy storage devices because of its high theoretical specific capacity(1675 mAh g^-1),abundant raw material,and pollution-free characteristic.However,lithium-sulfur batteries still confront many problems,such as poor conductivity of sulfur and lithium sulfide,huge volume change of the cathode during reaction,and the shuttle effect of polysulfides,which greatly restrict its progress on the road to commercial applications.Covalent organic framework（COF）,as a crystalline porous material with tunable pore size and functionality,has shown great potential in the field of LSB in recent years.We used“Schiff base”COF material with sulfonic acid groups to modify polypropylene separator,and explored the effects of COF material on inhibiting shuttle effect through“limiting diffusion”and“accelerating conversion”.The main contents of this thesis are as follows:In order to restrain the diffusion of polysulfides through the separator,a sulfonated covalent organic framework modified separator（SCOF@PP）was designed,which can hinder the shuttling of polysulfide anions and promote the diffusion of Li⁺.Especially,the sulfonate with negatively charge can restrain the diffusion of polysulfide by electrostatic repulsion effect,improving the stability of the battery.At the same time,the separator promotes the Li ions with positively charge to migrate.Therefore,the cell assembled with SCOF@PP separator has a capacity retention rate of 81.1%after 120 cycles at 0.5C,which is much higher than that of the cell with PP（55.7%）.In addition,after cycling 600 times at 1C,the SCOF@PP battery’s capacity decay rate per cycle is 0.067%,and it provides a high discharge capacity of 576 mAh g^-1 at 2C.Then,in order to accelerate the conversion of polysulfides,the CSCOF-Co catalytic material was prepared by introducing cobalt into the sulfonated covalent organic framework-derived carbon by forming Co-N and Co-S coordination bonds.CSCOF-Co catalyst can promote the rapid redox reaction of polysulfides,so as to reduce the residence time of polysulfides in the electrolyte and restrain the shuttle effect.As a proof of concept,the capacity retention rate of the battery with PP@CSCOF-Co separator is as high as 88%after120 cycles at 0.5C.After 800 cycles at 1C,the PP@CSCOF-Co battery has a low capacity decay rate of 0.059%per cycle,which greatly improves the cycle stability of the lithium-sulfur battery.In addition,the derived carbon as a secondary current collector also improves the conductivity and sulfur utilization of the battery,so that the battery has good rate performance.The PP@CSCOF-Co battery can provide a high discharge specific capacity of 740 mAh g^-1 at a high current density of 2C.