Research Of Controllable Synthesis And Doping Modification Of Nano LiMnPO₄/C Positive Electrode Materials

Olivine structure Li MnPO4 with affluent raw materials,low cost,good safety and high energy density,ect,is a very promising cathode material.However,the electronic conductivity and ionic conductivity of LiMnPO₄ are very low,leading to poor electrochemical performance,which limits its commercial application.This work fabricated nano flake LiMnPO₄ via a solvothermal method,and used doping and surface coating modification to improve its electrical conductivity,ionic conductivity and cycle performance.LiMn_1-xFe_xPO₄/C?0 ? x ? 0.5?nanocomposites were synthesized by solvothermal method,and its physical properties and electrochemical properties were studied.The results show that all nanocomposites have good olivine type structure.Moreover,Fe²⁺doping not only does not destroy the crystal structure of LiMnPO₄,but also reduces its lattice volume.When the doping amount of Fe²⁺ is less than 0.5,the morphology of samples is nanoplate with a length of about 200 nm,while the morphology of the LiMn_0.5Fe_0.5PO₄ is nanorod with a length of 100²00 nm.The first discharge capacities of Li Mn1-xFex PO4/C?x=0,0.1,0.2,0.3,0.4,0.5?composites are 109.5,132.7,144.3,143.1,141.7 and 156.4 mAh·g-1 at 0.05 C rate,respectively.LiMn_0.5Fe_0.5PO₄/C cathode material exhibits the best electrochemical performance among all materials.LiAlO₂ micro/nano-particle was synthesized by high temperature solid state method and used for surface coating of LiMn_0.5Fe_0.5PO₄ cathode material.The results show that synthesized LiAlO₂-LiMn_0.5Fe_0.5PO₄/C composites have good olivine structure,and the morphology of all composites is nanorod with a length of about200³00 nm and a width of about 100 nm.The first specific capacity of LiAlO₂-Li Mn0.5Fe0.5PO4/C?wt?LiAlO₂?=0,3%,5%,10%?composites are 119.8,130.9,137.6 and 105.1 mAh·g-1 at 0.05 C rate,respectively.The first specific capacity of materials are 104,106.3,107 and 91.1 mAh·g-1 under 5C rate,respectively,and the capacity retention of materials are 72.3%,88.9%,86.4% and 87.8% after 100 cycles,respectively.The proper amount of Li Al O₂ coating can obviously improve the rate performance and cycle performance of LiMn_0.5Fe_0.5PO₄ cathode material,but the excessive amount of LiAlO₂ coating will reduce the discharge capacity of the material.Li_0.3La_0.56TiO₃?LLTO?-LiMn_0.5Fe_0.5PO₄/C nanocomposites were synthesized byhydrothermal method and high temperature calcination carbon coating method.The results show that the strongest diffraction peaks of all samples can be attributed to the characteristic diffraction peaks of the olivine structure,and LLTO coating does not change the original structure of the materials.The morphology of all samples is nanorod with a length of about 100²00 nm and a width of about 50¹00 nm,and the surface of the nanorods is coated with a layer of carbon conductive material with a thickness of about 3 nm.LLTO coating can improve the rate performance and cycle performance of Li Mn0.5Fe0.5PO4.When the amount of LLTO coating is 3 and 5wt.%,the cycling performance and rate performance of the two materials is the best.The discharge specific capacity of the materials are 78.2,106.4,106.2 and 97.7 mAh·g-1under 5C rate,respectively,and the capacity retention of the materials are 67.2%,81%,82.5%,and 78.3% after 100 cycles,respectively.EIS spectra shows that LLTO coating can reduce the charge transfer resistance Rct,which is beneficial to the diffusion of Li+,and improves the conductivity and diffusion coefficient of the materials.