Preparation Of Transition Metal Phthalocyanine/Carbon Composites And Catalytic Performance Of Lithium-Sulfur Batteries Cathode

Lithium sulfur battery is considered to be one of the most promising secondary batteries to replace lithium-ion battery because it has 3～4 times higher theoretical energy density and environmental friendliness than lithium-ion battery.However,due to the inherent problems such as the insulation between sulfur and Li2S,the volume expansion of cathode after discharge,the shuttle effect of lithium polysulfide and the growth of lithium dendrite on anode,the practical application of lithium sulfur battery is seriously hindered.In order to solve these problems,researchers have carried out research from many directions,such as cathode catalytic materials,modified separator and binder.Among them,the research on cathode materials is more mature and systematic.In the cathode catalytic materials,transition metal phthalocyanine complexes can not only adsorb soluble lithium polysulfide,but also promote the conversion of lithium polysulfide,which shows great application potential.Therefore,this thesis will systematically study the effects of different transition metal phthalocyanine complexes on the electrochemical performance of lithium sulfur battery.This research has a positive effect on improving the service life of lithium sulfur battery and realizing its commercial application.Firstly,four transition metal phthalocyanines,CoPc,FePc,NiPc and CuPc were selected and mixed with carbon nanotubes by manual grinding in the best grinding proportion.The results of electrochemical performance test show that CoPc,FePc,NiPc and CuPc can improve the performance of lithium sulfur battery.Among them,CoPc has the best catalytic effect,followed by FePc,NiPc and CuPc.The specific discharge capacity of the full cell with CoPc/MWCNT cathode after 200 cycles at 0.2 C is 752.4 mAh g-1.Based on the above research,CoPc and MWCNT with the best catalytic effect were calcined at different proportions at high temperature,and the effect of pyrolysis ratio on the electrochemical performance of lithium sulfur battery was studied.At high temperature,the coordination structure formed by the central metal and four nitrogen-containing groups in the transition metal phthalocyanine structure is destroyed,which could form uniform and dense Co-Nx active sites on carbon nanotubes.The introduced Co-Nx catalytic active sites can adsorb and catalyze the conversion of lithium polysulfide,effectively immobilize S8/LiPSs,inhibit the shuttle effect and finally improve the utilization of sulfur.When the material prepared with the best pyrolysis ratio is used as the sulfur carrier,the specific capacity retention rate of the battery is as high as 82.9%after 200 cycles at 0.5 C.It can show good electrochemical stability even under high current density and high surface sulfur loading.Finally,CoPc,FePc,NiPC,CuPc and MnPc were compounded with carbon nanofibers by π-π self-assembly.When the phase,load and surface morphology are consistent,the electrochemical performance test shows that CoPc is conducive to the charging process and FePc is conducive to the discharge process.CoPc and FePc were combined into Co,FePc@CNF cathode.The full cell with this cathode has a high specific discharge capacity of 1143.0 mAh g-1 at 1 C.After 350 cycles,the specific discharge capacity is 779.7 mAh g-1.The specific capacity attenuation rate of each cycle is only 0.091%,which is better than CoPc@CNF and FePc@CNF full cells.