Synthesis,Characterization And Electrochemical Performance Of High-voltage Micro/nanostructured Spinel Lithium Nickel Manganese Oxides

This thesis mainly provides the study on the preparation, characterization and performance evaluation of high-voltage spinel lithium nickel manganese oxides with optimized micro/nanostructured morphologies and crystal structures. The morphologically controlled synthesis of lithium nickel manganese oxides is typically two-stepped:the preparation of MnO2or Mn3/4Ni1/4CO3precursors with certain morphologies followed by solid-state reactions with other reactants. Hence, several types of products with different morphologies and crystal structures are able to be obtained through the adjustment of precursors, reactants, heat treatment conditions, etc. Finally, the samples are subjected to rigorous characterization and performance evaluation in order to determine the structure—performance and morphology-performance correlations. The main contents are as follows:1. Detailing the research background of a wide range of lithium-ion battery materials, especially cathode materials, thereby clarifying the significance of high-voltage spinel LiNi0.5Mn1.5O4as well as the issues to be resolved for such material.2. LiNio.5Mn1.5O4micro/nanostructures (nanostructured microparticles) are prepared from urchin-like γ-MnO2. The products calcined at700℃preserve the urchin-like architecture, while the samples obtained under800℃are microspheres assembled by nanoparticles. Besides, both of them are subjected to an annealing treatment in order to gain resultants with different crystal structures. The samples subjected to rigorous characterization, especially in terms of micro-region analyses through high-resolution transmission electron microscope and associated energy dispersive X-ray line-scanning spectroscopy. As cathodes in lithium-ion batteries, the samples show different electrochemical properties related with their morphologies and structural characteristics. Among them, the sample calcined at800℃exhibits the best overall performance, which delivers91mAh·g-1at50C, and100.5mAh·g-1after300cycles at5C. 3. Two types of LiNi0.5Mn1.5O4(LNMO) microspheres with different pore conditions are prepared through a facile two-step method. Initially, nickel manganese carbonate microspheres are obtained through a solvothermal reaction, and then they are heated with different lithium sources to obtain the two products. Scanning electron microscopy images clearly disclose that the two types of microspheres are respectively covered with dense tinier pores and sparse larger pores while both of their interiors are constituted by nanoparticles in similar size. Nitrogen adsorption/desorption analyses indicate that their maximum pore diameters are2.2nm and3.5nm. As cathodes of lithium ion batteries, the LNMO microspheres equipped with larger pores exhibit much more excellent electrochemical performance especially in terms of rate performance, achieving a discharge capacity of101.7mAh·g-1even at50C, while their counterparts only receive14mAh·g-1coupled with severe polarization. And the capacities of them respectively maintain at102.9and67mAh·g-1after100cycles at20C. Their distinct performance is suggested due to both the pore parameter and its related surface area. Besides, LNMO microspheres with larger pores are assembled in a full battery coupled with spinel Li4Ti5O12, this battery exhibits rational capacity and favorable rate performance, thereby showing the possibility of application.