Layered Cathode Materials:Interface Modifications,Structure Analysis And 4.5V Battery Performances

Layered LiCoO₂ and LiNi_0.8Co_0.15Al_0.05O₂（NCA）are cathode materials for lithium-ion batteries.They have stable electrochemical properties,great commercial values,and wide application prospects in the fields of 3C or electric vehicles.In order to meet the ever-growing demands of higher energy density and high power density,the development of cathode materials which can still work stably in wider operation voltage windows has become an urgent issue in the entire battery industry.In this thesis,three independent research topics were carried out around the interface structure modification of layered LiCoO₂ and NCA and the performance optimization of 3.0-4.5 V semi-battery,which involved the synthesis of nano-LiCoO₂ by hydrothermal method and interfacial aluminum doping,and the exploration of coating modification of NCA based on MOF and oxalate materials.The main research contents are as follows:1.Hydrothermal-assisted synthesis of surface aluminum-doped LiCoO₂ nanobricks.LiCoO₂ nano-particles precursor was synthesized through a mixed-alkalis（LiOH-NaOH）hydrothermal reaction,and finally sintered into LiCoO₂ nanobricks with a sickness of³00nm.This LiCoO₂ nanobrick cathode delivered a specific capacity of131.8 mAh g^-1 at 1 C between 3.0 and 4.2 V and 90%capacity retention after 100cycles.Those synthesized LiCoO₂ nanobricks were further treated by surface Al³⁺doping to achieve much enhanced 4.5 V lithium storage capability and cycling stability.EIS results showed the surface Al³⁺doping operation can signification decrease the charge-transfer resistances of the LiCoO₂ cathodes for both before and after cyclings,and its SEI is also more stable during cyclings.2.Surface modification of LiNi_0.8Co_0.15Al_0.05O₂ cathode by using meltable metal-organic framework micro-materials.The synthesized Mil-53lt（Al）was dispersed on the surface of NCA through wet method,and then turned in to LiAlO₂ coatings via further8 hours calcination（600°C）in oxygen.X-ray photoelectron spectroscopy（XPS）,Transmission Electron Microscope（TEM）,results prove that a LiAlO₂ coating layer（7nm）is closely coated on the NCA surface.The modified NCA displayed a discharge capacity of 178.32 mAh g^-1 at 5C between 3.0 and 4.5 V and exhibited a capacity retention of 68.05%（the uncoated sample was 48.65%）after 200 cycles.Cyclic voltammetry（CV）and electrochemical impedance spectroscopy（EIS）results indicate that the coated sample exhibits lower polarization and better lithium diffusion.Thus,interface stability and capacity retention rate of the electrode materials were improved.3.Based on oxalate precursor,Al-doped Li_0.73CoO₂ coating layer was grown on the NCA surface.The aluminum doped oxalate nano-powder were firstly designed andattached to the NCA surface;then,Al-doped Li_0.73CoO₂ thin layer was coated on NCA surface by calcining.The presence of this lithium colbat oxidecoating layer was confirmed by scanning electron microscope（SEM）,Transmission electron microscope（TEM）and X-ray photoelectron spectroscopy（XPS）.In the voltage range of 3.0-4.5 V,the initial discharge capacity of the coated material at 5C is 172.24 mAh g^-1,and the capacity retention rate after 200 cycles is 64%.TheLi_0.73CoO₂ coating layer generated at the interface consumed the residual lithium impurities on pristine NCA surface.Moreover,the Al-doped Li_0.73CoO₂ coating could ensure the coated NCA with better lithium ion and electron conductivities because of the particularity of the Co-O layer and the co-exsitence of Co⁴⁺and Al³⁺.Finally,the electrochemical activity and stability of the coated NCA material were obviously improved.