Synthesis And Modification Research Of Layered Ni-rich LiNi_xCo_yMn_1-x-yO₂?x?0.5? Cathode Materials For Lithium-ion Batteries

Layered LiNixCoyMn1-x-yO2 cathode materials have been extensively studied because of its relatively high specific capacity,high work voltage,low cost and environmental benignancy.In this thesis,layered Ni-rich LiNixCoyMn1-x-yO2(x>0.5)cathode materials were used as the research object.LiNi0.5Co0.2Mn0.3O2 cathode material was synthesized by a sol-gel method with citric acid as chelating agent.The thermal analysis of the citrate polymerized dry-gel was carried out.The organics were completely decomposed at 450? and obtained a good high-temperature sintering precursor.Quantitative elemental analysis found that the initial amount of lithium salt added in excess of 5%(n(Li)/n(Ni+co+Mn)=1.05)can be better to supplement the loss of Li+ caused by the high-temperature sintering and obtained stoichiometric ratio of LiNi0.5Co0.2Mn0.3O2 cathode material.The crystal structure analysis of the sintered materials under different temperature found that the material sintered at 900? can obtain ideal crystal structure and the lowest Li/Ni mixing.Based on the LiNi0.5Co0.2Mn0.3O2 cathode material,the content of nickel ions was further increased to prepare LiNi0.6Co0.2Mn0.2O2 cathode material.The structure and electrochemical performance of LiNi0.5Co0.2Mn0.3O2 and LiNi0.6Co0.2Mn0.2O2 cathode materials were compared and analyzed.It was found that the LiNi0.6Co0.2Mn0.2O2 had a higher initial discharge specific capacity,however,the LiNi0.5Co0.2Mn0.3O2 had more stable electrochemical performance.For the layered nickel-rich cathode materials usually have a poor electrical conductivity,poor high magnification performance and the side reaction with electrolyte,we chose LiNi0.5Co0.2Mn0.3O2 as the modified precursor and researched on the material surface modified with graphene oxide.After the surface modification,the cycle performance and rate performance of the material were obviously improved.The capacity retention increased from 64.1%to 72.4%after 100 cycles at 55? with current densities of 54 mA·g-1.The capacity was increased from 55.5 mAh·g-1 to 65.4 mAh·g-1 at a high current density of 540 mA·g-1.This is due to the graphene oxide has a good conductivity and the ability to inhibit the side effects of the active material and the electrolyte.Based on the surface modification with single-layer graphene,we prepared double-layer coating LiNi0.5Co0.2Mn0.3O2 cathode material with an inner V2O5 layer and an outer conductive graphene oxide layer.After the surface modification,the capacity retention increased from 67.8%to 74.2%after 50 cycles at 55? with current densities of 54 mA·g-1.The specific capacity increased from 105.7 mA·g-1 to 116.6 mA·g-1 at a high current density of 270 mA·g-1.This can be ascribed to the inner coating raw material,NH4VO3,can react with the impure lithium compound(Li2CO3,LiOH etc.)attached to the surface of the active material.And V2O5 is favorable for lithium ion diffusion,while the outer graphene oxide contributes to electron transfer and improves the conductivity of the material.