MOFs-assisted Synthesis And Modification Of LiNi_0.5Mn_1.5O₄ Cathode Materials For Lithium Ion Batteries

Spinel LiNi_0.5Mn_1.5O₄materials with high working voltage and high energy density are regarded as promising candidates for the cathode materials of new generation lithium-ion batteries.However,there are still some shortcomings in LiNi_0.5Mn_1.5O₄such as poor cycle performance and serious voltage decay.In this dissertation,LiNi_0.5Mn_1.5O₄was synthesized by MOFs-assisted hydrothermal method,which based on the advantages of metal-organic frameworks（MOFs）in metal ions adsorption and molecular tailoring.Moreover,the in-situ coating was simultaneously achieved on the LiNi_0.5Mn_1.5O₄surface.This dissertation focuses on the preparation of LiNi_0.5Mn_1.5O₄materials and regulation of the in-situ coating layer to improve their electrochemical performance:（1）Comparison of structure and properties for LiNi_0.5Mn_1.5O₄prepared by traditional hydrothermal method,organic co-precipitation method and MOFs-assisted hydrothermal method.Among them,the MOFs-assisted hydrothermally synthesized LiNi_0.5Mn_1.5O₄material delivered the specific discharge capacity of 131.8 m A h g^-1at 0.1 C(1C=147 m A g^-1),which was higher than that of hydrothermal(124 m A h g^-1)and organic coprecipitation(119.3 m A h g^-1).Furthermore,the MOFs-assisted hydrothermally synthesized LiNi_0.5Mn_1.5O₄showed a capacity retention rate of 93.45%after 200 cycles at 2 C.Even at 10 C,the specific discharge capacities of as-prepared LiNi_0.5Mn_1.5O₄could still achieve 113 m A h g^-1,which has the most excellent electrochemical performance.During the synthesis process,MOFs can not only serve as metal ion complexing agents,Li⁺adsorbents,and molecular tailoring templates,but also can use the pyrolysis of organic components in MOFs to form Li₂CO₃in-situ coating layers on the LiNi_0.5Mn_1.5O₄surface.（2）Based on the MOFs-assisted hydrothermal method,the thickness and uniformity of the Li₂CO₃in-situ coating layer on the LiNi_0.5Mn_1.5O₄surface were regulated by the carbon content of organic precursors,calcination atmosphere and calcination time.Thereby it could be find that their effects on the electrochemical performance of the LiNi_0.5Mn_1.5O₄materials.As the results showed that the formation of the in-situ Li₂CO₃layer was related to the pyrolysis of organic components in the MOFs precursor and the escape of lattice oxygen from the LiNi_0.5Mn_1.5O₄materials during calcination.The thickness of Li₂CO₃layer increaseed with the the amount of CO₂released during the pyrolysis of organic precursors,and was proportional to the active oxygen content under different calcination atmospheres and calcination times.When calcined in oxygen atmosphere,the thickness and uniformity of the Li₂CO₃layer on LiNi_0.5Mn_1.5O₄surface was reduced.A suitable thickness and continuous uniform in-situ Li₂CO₃coating is beneficial to improve the long-cycle performance of the LiNi_0.5Mn_1.5O₄materials.The LiNi_0.5Mn_1.5O₄material obtained after calcination in air for 10 h with PTCDA as the ligand still had a capacity retention rate of 93.86%after cycling at 1 C.（3）Study on in-situ conversion coating of LiNi_0.5Mn_1.5O₄and process optimization of coating content.A thin and uniform Li₃PO₄layer was successfully converted on the surface of LiNi_0.5Mn_1.5O₄via H₃PO₄pretreatment.The in-situ conversion coating did not need to add additional lithium source,which not only improved the tight bonding between the coating layer and the host structure,but also effectively enhanced the structural stability and acid corrosion resistance of LiNi_0.5Mn_1.5O₄.Through coating amounts comparison and optimization,the 1 wt%Li₃PO₄-coated LNMO delivered the discharge specific capacities of 107 m A h g^-1at 10 C and exhibited a capacity retention ratio of 92.86%after 200 cycles at 2 C.