Design Of Polypyrrole-based Electrode Materials And Its Application In Lithium Battery

Lithium batteries have experienced many challenges since developed.Expecially from 1991,rechargeable lithium-ion batteries?LIBs?have gradually occupied the main energy storage market for portable electronic devices?such as mobile phones and laptops?,owing to their high energy density,excellent cyclic performance,good rate capability,low self-discharge rate,and no memory effect.However,some other key markets such as the electric vehicle industry cannot be satisfied as much higher energy density is needed.Many efforts have been devoted to exploring high capacity materials and new battery system to fulfill the requirements for HEVs and EVs applications.Developing new high-capacity anodes as substitutes for commercial graphite anodes is of great significance to improve the overall electrochemical performance of LIBs.Specifically,SnO₂ has a twice higher theoretical specific capacity?782 mAh/g?than graphite?372 mAh/g?,which is one of the most promising anode materials.But during the charge and discharge process,SnO₂ anode experiences large volumetric variations,resulting in the mechanical failure of the electrode,tremendously attenuated the capacity of the materials.Furthermore,rechargeable lithium-sulfur batteries are being intensively pursued due to their much higher specific energy density which can reach2600 Wh/kg based on a Li anode and an S cathode.However,the insulating nature of the sulfur,the electrode pulverization due to the large volumetric expansion and the shuttle effect caused by soluble polysulfides intermediates lead to low utilization of active materials,poor cycling life,and low rate capability,which seriously hamper the practical applications of Li-S batteries.The typical conductive polymer polypyrrole?PPy?with a conjugated structure has many attractive features,such as excellent charge transfer capability,reversible electrochemical ability,ease in preparation,environmental friendliness,and flexible mechanical property.Introducing PPy as a matrix for the utilization of SnO₂ anode materials and sulfur cathode materials has been proved to be a very effective method to optimize the electrochemical performance of LIBs and Li-S batteries.In this thesis,a series of PPy based SnO₂ anodes and sulfur cathodes were designed and prepared,and their electrochemical properties were systematically examined.Additionally,the structure-activity relationship of as-prepared electrodes was also explored,the main results are shown as follows:?1?First,a facile one-step method was explored to synthesize SnO₂ nanomaterials.The nanostructures of SnO₂ were controlled by increasing PVP dosages and the electrochemical properties of SnO₂ materials were studied.Compared with SnO₂microcrystals and nanoflowers,SnO₂ nanosheets exhibit superior discharge capacity and rate capability due to its porous nanosheet structure.SnO₂ nanosheet anodes display an extraordinary initial specific capacity of 2231 mAh/g at a current density of 0.2 A/g,a high retained capacity of 668 mAh/g after 60 cycles and an excellent rate performance of 387 mAh/g discharge capacity at a high current density of 4 A/g.These results confirm the nanostructure of SnO₂ directly affects the electrochemical performance of LIBs.Afterward,PPy was introduced into the anode design to prepare PPy coated SnO₂hollow nanospheres.As a conductive substrate,PPy can not only increase the conductivity of the electrode material but also buffer the volumetric changes of the electrode during the charge and discharge.After introducing the PPy matrix,the cycling stability,rate capability and coulombic efficiency were all improved.?2?Hollow PPy nanospheres were fabricated using polystyrene nanospheres as the template.After sublimating sulfur on the PPy nanospheres,the obtained PPy based sulfur materials were tested as Li-S batteries cathodes.Hollow PPy nanospheres matrix could enhance the conductivity of the electrode,mitigate the huge volumetric changes during the charge/discharge process,and inhibit the shuttle effect by limiting polysulfides diffusion,thereby improving cathode performance of Li-S batteries.The as-obtained composites exhibit an initial discharge capacity of 1563.3 mAh/g at a 0.2C current density and the discharge capacity retention rate was 89%after 300 cycles.The introduction of the PPy backbone greatly improves the electrochemical properties of the sulfur cathode.?3?PPy nanotubes were synthesized by a self-degraded method using methyl orange as a chemical template.Porous carbon nanotubes with micropores and mesopores on the surface have been fabricated using tubular PPy precursors as sulfur hosts for lithium-sulfur batteries.The sulfur content and electrochemical performance are examined and compared to figure out the optimal porous structure.The most favorable sulfur cathodes using carbon nanotubes with 4-5 nm mesopores on the PCNT surface exhibit a much higher sulfur content,cycling stability,and rate capability.Because these mesopores could promote the sulfur melting into the inner space of nanotubes,increasing the sulfur mass content in the materials.Furthermore,the mesoporous structure could provide an efficient electrolyte diffusion pathway,accelerating the response for fast charge transfer as well as avoid soluble polysulfides directly exposure outside the nanotubes,suppressing the“shuttle effect”.Afterward,coaxial sulfur-PPy tubular nanocomposites were fabricated via a facile one-pot method.In the designed structure,the PPy backbone can facilitate the charge transport,alleviate volumetric changes,and restrain the soluble polysulfide diffusion,improving the cycling performance,rate capability and coulombic efficiency of Li-S batteries.And the uniform active sulfur layer can efficiently react with Li⁺assisted by the PPy nanotubes,increasing the sulfur utilization.The as-prepared coaxial sulfur/polypyrrole tubular nanocomposites exhibited a high initial discharge capacity of 1117 mAh/g at0.05 C rate,retained discharge capacities of 692 mAh/g and 525 mAh/g at current densities of 0.2 C and 1 C after 200 cycles,and a discharge capacity of 470 mAh/g at a high 2 C current density.?4?SnO₂ nanoparticles were uniformLy anrched on PPy nanotubes to fabricate PPy based SnO₂ anodes for LIBs.In this design,the PPy backbone can promote charge transfer and moderate volume expansion during the charge and discharge process.And the ultrafine SnO₂ nanoparticle and the porous structure introduced itself can facilitate the electrolyte diffusion and improve the active material utilization.The as-synthesized PPy based SnO₂ anode exhibited excellent cyclic stability and rate capability.It maintained a discharge capacity of 605.0 mAh/g after 300 cycles at a 1 C rate and delivered 422.2 mAh/g discharge capaciry at 2 C rate.Furthermore,this PPy based SnO₂ materials was used as the substrate to load sulfur to prepare a novel ternary sulfur cathode for Li-S batteries.Due to this unique nanostructure,the as-prepared cathodes show an outstanding electrochemical performance.PPy nanotubes as a conductive matrix can enhance the electrode conductivity,buffer the sulfur volumetric expansion,and relatively limit the polysulfide diffusion,while the SnO₂ nanoparticles as high-efficient polysulfides trap can greatly mitigate the shuttle effect due to the chemical bond between SnO₂ and polysulfides.Moreover the large surface area and porous structure in the cathode are beneficial to the sulfur and lithium sulfides accommodation.As a result,the as-prepared cathode with 64.7%sulfur mass content exhibit excellent cycling stability for a decay rate of 0.05%per cycle along with 500 cycles at 1 C,rate capability of 383.7 mAh/g at 5 C and coulombic efficiency above 90%.