The Structure Design And Performance Study Of Cathodes And Anodes Based On Hierarchical Three-Dimensional Composite Conductive Networks For Lithium Sulfur Batteries

Lithium sulfur（Li-S）batteries have been widely studied for their advantages of cheap,mineral-rich and high theoretical specific capacity,etc.However,the cathodes of lithium-sulfur battery have high internal resistance,severe polysulfide shuttle effects,and large volume expansion problems.Simultanetously,the anodes have the problems of lithium dendrite growth and huge volume expansion.Constructing positive and negative electrodes with unique structures is one of the effective ways to solve the above bottlenecks.This is also the key and challenge of the study of high-performance lithium-sulfur batteries.In this thesis,the cathodes and anodes of lithium-sulfur battery are evolved,and the hierarchical three-dimensional composite conductive network structures are constructed,which effectively improve the conductivity of the cathodes,alleviate the shuttle effect of polysulfide,and inhibit the volume expansion of cathodes/anodes and the growth of lithium dendrite.The main results are shown as the following:（1）A high-performance carbon foam fiber cathodes based on tube/nanosheet（HPCF）coaxial hierarchical structure has been designed and fabricated via high temperature carbonization and etching methods for lithium-sulfur batteries.The all-carbon material solves the low conductivity of the cathodes,and the three-dimensional multi-porous carbon nanosheet structures effectively inhibit the shuttle effect of polysulfide,alleviate the volume expansion problem,and provide quick electron/ion transfer channels.The as-prepared HPCF-1000 possesses a high sulfur loading of 72wt%and has a high initial discharge capacity of 1097.3 m Ah/g.After 500 cycles,it possesses 68.7%retention capacity at a current density of 0.2 C.（2）The wetting etching combined with thermal reduction expansion is proposed to fabricate the anodes of three-dimensional holey reduced graphene oxide（Hr GO）network based on defect oxygen modulation for lithium-sulfur batteries.The defect oxygen of the prepared Hr GO films provides abundant lithiophilic sites.The large specific surface area reduces the voltage distribution potential,inhibits the growth of lithium dendrites,increases the wettability of electrolyte,and alleviates the volume expansion of lithium metal anodes.The symmetrical cells of Li-Hr GO-4 composite film anodes have the stable overpotential of 11 m V at current density of 0.2 m A/cm² after1900 h.First-principles calculations indicate that C-O and O-C=O groups on defects of Hr GO surfaces lead to excellent lithium affinity and uniform deposition.The as-assembled Li Fe PO₄/Li-Hr GO-4 full cells show 86.1%retain capacity after 300 cycles at the rate of 1 C.The as-assembled HPCF-1000/S-Li-Hr GO-4 full cells show 75.5%retain capacity after 500 cycles at the rate of 0.2 C.（3）A wet chemical reaction combined with high temperature annealing method is proposed to construct free-standing films based on three-dimensional nickel/nickel oxide（Ni@Ni O）multi-layer core/sheath structure,which can realize the long cycling stability of lithium-sulfur anodes.The conductive skeleton of Ni@Ni O films accelerate the electron/ion transmission,and the three-dimensional multi-layer core/sheath structure becomes an effective deposition sites for lithium metal and alleviates the large volume expansion.The symmetrical cells of Li-Ni@Ni O-400 composite film anodes possess the stable overpotential of 13 m V at current density of 1.0 m A/cm² after 2020h.The as-assembled Li Fe PO₄/Li-Ni@Ni O-400 full cells exhibit 93.8%retain capacity after 500 cycles at the rate of 1 C.The as-assembled HPCF-1000/S-Li-Ni@Ni O-400full cells show 83.3%retain capacity after 500 cycles at the rate of 0.2 C.In conclusion,this thesis resolves the problems of low conductivity,polysulfide shuttle effect of cathodes,and volume expansion,lithium dendrite growth of anodes via the design and construction of hierarchical three-dimensional composite conductive network structure,which lays a foundation for the design and application of positive and negative electrode structure of high-performance lithium-sulfur battery.