Synthesis And Modification Of LiNi_0.5Mn_1.5O₄as Cathode Material For Lithium-ion Batteries

As cathode material for lithium-ion batteries, spinel LiNi0.5Mn1.5O4has drawn considerable attention. In this thesis, we synthesized LiNi0.5Mn1.5O4for industrial production by solid state method. According to the analysis on the crystallization, morphology and electrochemical performances of the products, the better conditions of synthesizing LiNi0.5Mn1.5O4were optimized by changing sintering temperature and anneal treatment. The first discharge capacity of the as-prepared LiNi0.5Mn1.5O4was129.8mAh/g and then128.7mAh/g after100charge and discharge cycles at room temperature. At55℃, the first discharge capacity of the as-prepared LiNi0.5Mn1.5O4dropped to124.5mAh/g, and its capacity retention rate was88.6%after100cycles.By liquid method, Al2O3particles were coated on the surface of LiNi0.5Mn1.5O4to improve its electrochemical performances. According to the analysis on crystallization, morphology and electrochemical performances of the Al2O3-coated products, the better process conditions of coating Al2O3on the surface of LiNi0.5Mn1.5O4were optimized by changing the type and concentration of precipitants, heat treatment time and amount of Al2O3. The first discharge capacity of the Al2O3-coated LiNi0.5Mn1.5O4was133.3mAh/g and then131.2mAh/g after100cycles at room temperature. At55℃, the first discharge capacity of the Al2O3-coated LiNi0.5Mn1.5O4dropped to117.4mAh/g, and its capacity retention rate was96.7%after100cycles. The room-temperature rate performance was also greatly improved.For Al2O3-coated LiNi0.5Mn1.5O4sample, Al2O3coating layer on the surface of LiNi0.5Mn1.5O4not only hinders the direct contact between active material and electrolyte, but also has the HF scavenging ability due to the reaction of Al2O3and HF, which brings about the reduction in surface impedance of active material, and effectively suppresses the spinel structural destruction due to the dissolution of Mn. Thereby, Al2O3-coated LiNi0.5Mn1.5O4can enhance the cycle performance and room-temperature rate performance of LiNi0.5Mn1.5O4as5V cathode material for lithium-ion batteries.In this thesis we also synthesized garnet-type Li7La3Zr2O12by glycol assisted sol-gel method, which is expected to replace the commercial liquid electrolyte system. The process conditions of synthesizing Li7La3Zr2O12were optimized by changing sintering temperature and sintering time. And the ionic conductivity of Li7La3Zr2O12was tested. The results showed that a single cubic phase of Li7La3Zr2O12could be obtained at medium-temperature of700℃, but its ionic conductivity was low.Ultra-fine nickel powders were prepared by electrochemical reduction of NiO powder using lithium-ion battery as a micro-reactor. X-ray measurement showed that the synthesized powder consisted of pure nickel phase with grain size of about5nm. Laser particle size determination showed the nickel powder had a narrow particle size distribution with D50in the range of200-600nm.Transmission electron microscopy showed the nickel powder was80-200nm.The particle size increased when increasing the discharge current density and discharge temperature. The dispersion performance of nickel can be optimized by surfactants.