Synthesis Of Cathode Material Nanostructures For Lithium Ion Batteries

In the new century,the global demand for clean,sustainable energy grow rapidly with the development of society.And there is an outburst in the new technologies and products,such as smart phones and electric vehicles.Under such an era background,governments,industries and academia show strong interest in advanced electrical energy storage systems.Today,lithium ion batteries are found in nearly all consumer electronics due to the high energy density.However lithium ion batteries still need to be improved further in some aspects,such as energy density,power density,cycle life,and safety.Nanotechnology has been considered as one of the most important strategies to improve the performance of lithium ion batteries.In this dissertation,we realize the nanoscale structure engineering of lithium ion battery cathode materials by designing appropriate synthesis routes,and reveal the impact of the nanostructure to the battery performance.1.By using the two-dimensional manganese dioxides?MnO₂?nanosheets as precursor and manganese source,we have synthesized a series of nanostructural manganese-based lithium ion battery cathode materials.Benefited from the unique properties of the nanostructures,these materials exhibit better electrochemical performance.Firstly,we use the Polystyrene sphere as template to synthesize the hierarchical ringlike LiMn₂O₄.Lithium ion batteries based on this material delivers a high discharge capacity of 106.3 mAh g^-11 at 1 C rate,and can retain almost 100%discharge capacity after 100 cycles.And LiMnPO₄ nanobelts are successfully synthesized via a facile one-pot solvothermal process,and we can adjust the length-width ratio of the nanobelt by change the volume ratio of acetone and water in the solvent.The LiMnPO₄ nanobelts yield high discharge capacities of 139.7 mAh g^-1at 0.1 C rate and 127.1 mAh g^-11 at 1 C rate.We also coated the surface of multiwall carbon nanotubes?MWCNTs?with the MnO₂ nanosheet to synthesize the Li₂MnSiO₄/MWNCTs nanocomposite.the Li₂MnSiO₄/MWNCTs nanocomposite process a high discharge capacity of 237.8 mAh g^-11 at the first cycle,however the capacity fade rapidly in the subsequent cycles.This material needs further improvement in cycling stability.2.We propose an approach to synthesize lithium-rich layered oxides?LLO?materials using a manganese dioxide?MnO₂?template strategy,which could control the structure and particle size of final products via choosing different MnO2 templates.Through precisely optimizing,we successfully prepare cross-linked nanorods?CLN?and agglomerate microrods?AM?Li_1.2Ni_0.15Co_0.1Mn_0.55O₂ cathode materials by using carbon-decorated MnO₂ nanowires and MnO₂ nanorods as templates,respectively.The lithium ion battery based on the CLN exhibits excellent performance,delivering a high capacity of 286.2 mAh g^-11 at 0.1 C and 237.5 mAh g^-11 at 1 C.In addition,the device remains 98%and 89%of its initial capacity after 50 cycles at 0.1 C and 100cycles at 1 C,respectively.The enhanced electrochemical performance can be mainly attributed to the cross-linked nanorods structure which can shorten lithium ion diffusion length,enlarge reaction surface to fully active the material and accommodate the volume change in the internal cavity during the cycling.3.We present a facile route to synthesize hierarchical porous Li₂MnSiO₄ and LLO nanoparticles via using polymethyl methacrylate?PMMA?hierarchical porous film.Withtheassistanceofthermopolymerizationandcarbonizationofthe phenol-formaldehyde,the Li₂MnSiO₄/C exhibit a replica hierarchical porous structure of PMMA film.As for LLO nanoparticles,when compared to the LLO materials synthesized without the PMMA films,the lithium ion batteries based on the LLO nanoparticles deliver larger discharge capacity of 284.7 mAh g^-11 at 0.1 C and 233.6mAh g^-11 at 1 C,respectively.The better battery performance of LLO nanoparticles is mainly ascribed to the small particle size,large surface area and loose structure.