Preparetion Of LiNi_1/3Co_1/3Mn_1/3O₂and Surface Coated LiMn₂O₄Materials For Li Ion Battery Cathode

Recently, lithium ion batteries have gained enormous interest for high power andenergy density applications, such as electric vehicles (EVs), hybrid electric vehicles (HEVs)and stationary energy storage. To meet the large-scale requirements of these applications,many researchers are focusing on the cathode materials with high capacity and ratecapability. In this thesis, the purpose is to obtain the cathodes with high performancethrough reviewing the latest development of cathode materials for lithium ion batteries. Theternary layered LiNi1/3Co1/3Mn1/3O2and spinel LiMn2O4cathodes were selected as theobjective in this thesis. And the synthetic strategy of high performanceLiNi1/3Co1/3Mn1/3O2and surface coated LiMn2O4were systematic studied. The contents ofthis thesis are listed below: First, LiNi1/3Co1/3Mn1/3O2nano/micro hierarchical hollowmicrospheres with enhanced performances are synthesized through a simple impregnationmethod. Second, LiNi1/3Co1/3Mn1/3O2nanoplates with exposed {010} active facets aresynthesized in an ethylene glycol medium. Third, TiO2is coated on the surface of spinelLiMn2O4cathode to improve the electrochemical performance through a solvothermalprocessing. Last, Li4Ti5O12is applied as an epitaxial coating layer on the surface of spinelLiMn2O4cathode with excellent electrochemical performance.LiNi1/3Co1/3Mn1/3O2nano/micro hierarchical hollow microspheres with enhancedperformances are prepared through a simple impregnation method. In this synthetic process,MnCO3microspheres are both as self-template and Mn source for the productLiNi1/3Co1/3Mn1/3O2hollow microspheres. The hollow microspheres with diameters ofabout1μm are composed of approximately100nm primary nanoparticles. The walls of thehollow microspheres are about250nm in thick and the interior void is about500nm.LiNi1/3Co1/3Mn1/3O2hollow microspheres displays high discharge capacities, stablecycling life and superior rate capabilities for lithium ion battery cathode. At0.1C, thedischarge capacity of LiNi1/3Co1/3Mn1/3O2hollow microspheres is as high as212mAh g–1.After40charge and discharge cycles, the discharge capacity still remains180.3mAh g–1.When the current density increases to10C, LiNi1/3Co1/3Mn1/3O2hollow microspheres show the discharge capacity of135.9mAh g–1which is higher than most reportedLiNi1/3Co1/3Mn1/3O2cathode.LiNi1/3Co1/3Mn1/3O2nanoplates with {010} active facets exposing are successfullysynthesized in an ethylene glycol medium. Ethylene glycol plays a crucial role which actsnot only as a solvent but also as a chelating reagent in the formation ofLiNi1/3Co1/3Mn1/3O2nanoplates with {010} active facets exposure. In addition, the effectsof different sintering temperatures and time on the morphologies, structures andelectrochemical performances are also evaluated. The results show that theLiNi1/3Co1/3Mn1/3O2nanoplates obtained from850oC for12h expose the most {010}active facets and have the best electrochemical performances. At0.1C, the dischargecapacity of LiNi1/3Co1/3Mn1/3O2nanoplates with the most {010} active facets exposure is207.6mAh g–1and the capacity retention is89.4%during50charge and discharge cycles.In addition, the discharge capacity is up to149.3mAh g–1at7C. The synthetic method hasthe advantages of simple operation, mild condition, easily realizing large-scale commercialproduction.TiO2is coated on the surface of spinel LiMn2O4hollow spheres through asolvothermal processing. The LiMn2O4hollow spheres are composed of approximately100nm primary nanoparticles and diameters are about1μm. The TiO2coating layer isestimated to be about5nm. The XRD, SEM, TEM, XPS, CV, EIS and charge/dischargeperformances of the materials before and after TiO2coated are compared. The resultsdisplay that the morphology and structure of LiMn2O4hollow spheres have no varietybefore and after TiO2coated. Furthermore, the TiO2-coated LiMn2O4hollow spheresimprove the cycleability, high temperature performance and rate capability.A nanoscale Li4Ti5O12epitaxial coating layer on LiMn2O4hollow microspheres arefirstly prepared through a solvothermal-assisted processing. The product is characterized byXRD、SEM、TEM、XPS、HR-TEM、HAADF-STEM and EDS line scan. The results showthat Li4Ti5O12coating layer directly grows on the surface of LiMn2O4hollowmicrospheres with the same lattice orientation. After Li4Ti5O12epitaxially coated, thecycleability, rate capability and high temperature performance are greatly enhanced. At theroom temperature (25oC), the capacity retention is improved to97%during100charge and discharge cycles at1C after Li4Ti5O12epitaxially coated. Under the highertemperature (60oC), the discharge capacity is as high as110.4mAh g–1at12C afterLi4Ti5O12epitaxially coated, which is increased by313%compared with the uncoatedLiMn2O4hollow microspheres.