Application And Performance Of Phthalocyanine Complexes In Cathode Catalysis Of Lithium-Sulfur Batteries

With the growing energy density of energy-storage chemistry in the LIBs arriving theory limitation,LIBs are facing huge challenges in the sustainable and renewable energy filed.Nowadays,researchers are paying special attentions to next generation rechargeable lithium batteries with high energy densities.In the past decade,lithium-sulfur(Li-S)batteries are one of the most promising candidates for next-generation secondary batteries owing to their high theoretical energy density,low-cost and environmentally friendly elemental sulfur.Nonetheless,sulfur and its discharge product,lithium sulfide(Li2S),are both electronic and ionic insulators.There is a large density difference between S and Li2S,and the volume change after complete discharge is as high as 79%.In addition,the intermediate product lithium polysulfides(LiPSs)leads to the "shuttle effect",causing the loss of active sulfur,which restricts the commercial application of Li-S batteries.Low-dimensional carbon nanomaterials such as carbon nanotubes and carbon nanofibers can form a conductive network framework with high specific surface areas.When used as sulfur host materials,they not only solve the problem of poor conductivity of sulfur,but also effectively relieves volume expansions.Nonetheless,their catalytic performance is poor,thereby the further conversion of LiPSs is more difficult.Appropriate catalytic materials can further improve the conversion kinetics of LiPSs.The design of a high efficiency cathode catalytic material should take into account the following three principles:First,maximized exposure of the catalytically active sites to enable satisfactory regulation of the sulfur redox at a moderate loading amount.Second,facilitated conversion of LiPSs in the liquid phase as well as during the liquid-solid transitions of active sulfur species.Third,a high intrinsic electrical conductivity that enables fast electron and Li-ion transports to support continuous electrochemical sulfur redox.Transitional metal phthalocyanine complexes possess an 18π-electron conjugated system.The coordination structure composed of the central metal and the surrounding four nitrogen groups exhibits excellent catalytic performance.They have been widely used in photocatalysis and thionyl chloride batteries.Based on the catalytic mechanism of phthalocyanine compounds,this thesis adjusts and designs the molecular composition and structure of phthalocyanine compounds,regulates the combining method of phthalocyanine compounds and different carbon materials,establishes a catalytic system based on phthalocyanine compounds,and develops Li-S battery cathode catalyst materials design method.Cobalt Tetraaminophthalocyanine chemically modified multi-walled carbon nanotubes for high-performance Li-S batteries were studied.Tetraamino-substituted cobalt phthalocyanine was synthesized by a method of organic synthesis,in which the amino group replaces the fluorine atom of the fluorinated carbon nanotube and is covalently connected to the carbon nanotube through C-NH-C(secondary amine).The N atom in the TaPcCo molecular ring and the central Co atom have good affinity with Li atom and N atoms respectively,which can effectively adsorb LiPSs.Elevated kinetics of LiPSs reactions in the liquid phase as well as liquid-solid transitions were revealed by electrochemical measurements and density functional theory calculations.The catalyzed sulfur redox is also significantly facilitated by the fast electron and Li-ion transport to and from the reaction sites through the conductive MWCNT skeletons and the lithiophilic substituent amino groups on TaPcCo.Fully exposed TaPcCo can provide abundant sulfur-philic(S-Co)and lithium-philic(Li-N)binding sites.With 6 wt%addition of TaPcCo-MWCNT in the cathode coatings,high specific capacities of 0.2 C(1327 mAh g-1)to 4 C(574 mAh g-1)were achieved under a sulfur area load of 1 mg cm-2 for TaPcCo-MWCNT cathode.Cobalt phthalocyanine nanorods are physically loaded on carbon nanofibers for high-performance Li-S batteries to promote Li2S nucleation.There is a strong π-πadsorption between the phthalocyanine complex and the carbon matrix,and the complex can be loaded on the surface of the matrix two-dimensionally or self-assembled into a nanostructure in a liquid environment.Through liquid phase mixing and dropwise addition,the cobalt phthalocyanine is recrystallized and precipitated in concentrated sulfuric acid,which is effectively loaded with carbon nanofibers.The composites exhibit a cobalt phthalocyanine nanorod structure,uniformly distributed on the carbon nanofibers.The nanorod CoPc can fully contact with LiPSs,then chemical interaction occurs,which promotes the nucleation and growth of Li2S,and improves the utilization of S.DFT calculations indicate that CoPc nanorods can promote the migration of Li2S4 and the diffusion of lithium ions.When the sulfur area load is 4.73mg cm-2,the prepared pouch cell achieves a capacity of 900 mA h g-1 at 0.2C.which can cycle stably for 50 weeks.This further confirms that CoPc@CNF has a positive effect on the LiPSs reaction of lithium-sulfur batteries.Comparison of phthalocyanine complexes with different central metal atoms w ere performed with respect to their catalytic performance for Li-S batteries.and loaded for high-performance batteries.Phthalocyanine molecules with the central atoms of cobalt,iron,nickel,manganese and copper were loaded on carbon nanofibers in the liquid phase by π-π self-assembly.Through electrochemical tests,the catalytic activity on Li-S reactions of phthalocyanine complexes with different central metal atoms is compared.Furthermore,the adsorption of Li2S6 on different metal phthalocyanine complexes was studied by DFT calculations.The calculated adsorption energy of CoPc and FePc to Li2S6 is relatively large,which can effectively inhibit the shuttle effect.The calculation of the frontier molecular orbital energy shows that the lowest unoccupied molecular orbital(LUMO)of FePc is mainly concentrated on the Fe atom.Among them,the contribution value of Fe 3dz2 orbital is 86.19%,which effectively promotes the exchange of electrons,so that FePc exhibits better catalytic performance.