Modification Research Of Spinel Cathode Materials For Lithium-Ion Batteries

Lithium-ion batteries are of great interest in both life and research because of their high energy density,long cycle life,safety and other advantages.The spinel Li Ni0.5Mn1.5O4（LNMO）cathode material is considered an attractive cathode material due to its high operating voltage,high stability and low cost.However,spinel LNMO cathode materials are subjected to cycling at high potentials and therefore cause severe degradation of the electrolyte and dissolution of transition metal ions,leading to increased surface interface side reactions and causing rapid decay in their capacity.Therefore,it is essential to investigate the mechanism of spinel capacity decay and explore strategies to reduce side reactions.In this thesis,both surface modification and metal cation doping are used to stabilize the electrode/electrolyte interface to optimize cell performance:1.A simple molten salt method was used to prepare the mixed electron-ion conductor NSCO and modify it on the surface of spinel-type Li Ni_0.5Mn_1.5O₄（LNMO）produced by the sol-gel method.To investigate the effects of modifying different levels of NSCO on the structure,morphology,surface defects and electrochemical properties of LNMO.The modifications were found to have a small effect on the phase structure as well as the morphology of LNMO,while the Mn³⁺content gradually increased,as did the concentration of oxygen vacancies on the surface.The electrochemical performance was greatly improved,with LNSCO5 modified with 5 wt.%NSCO achieving a capacity retention of 94.32%after 200cycles at 0.2 C and still achieving 92.06%at a multiplicity of 2 C.The improved performance can be attributed to the physical protection of the LNMO surface by NSCO,which forms a surface modification layer that inhibits interfacial side reactions,while its high electronic and ionic conductivity increases the diffusion coefficient of electrons and Li ions at the interface.2.Lithium-rich spinel Li₄Mn₅O₁₂ has a similar structure to LNMO and is an ideal modification material for LNMO due to its electrochemical inertness of Mn⁴⁺,which makes it highly electrochemically stable.This chapter modifies 0-6 wt.%of onto the LNMO surface using the sol-gel method.The effects of Li₄Mn₅O₁₂ modification on phase structure,chemical composition,morphology and electrochemical performance of LNMO were studied.It was found that the modification had little effect on the phase structure and morphology of LNMO,and with the increase of Li₄Mn₅O₁₂ content,the content of Mn³⁺in LNMO increased.LNMO@Li₄Mn₅O₁₂-2%achieved the highest capacity retention rate of 85.16%after 200 cycles at 0.2 C and 93.26%at 2 C.The improvement of electrochemical performance is due to the good compatibility of Li₄Mn₅O₁₂with LNMO,good cycle stability and three-dimensional Li⁺diffusion channel,which is conducive to the diffusion of Li ions.At the same time,the increase in Mn³⁺content in LNMO increases the conductivity of the material itself and promotes the diffusion of lithium ions/electrons.3.Pr was doped into LNMO by sol-gel method to study the effects of Pr doping on the composition,crystal structure,surface defects,electrical conductivity and electrochemical properties of the samples.It was found that unlike surface modification,Pr doping had a greater influence on the morphology of LNMO,and the undoped truncated octahedron gradually transformed into octahedron.Second,doping leads to an increase in the Mn³⁺content and an increase in the oxygen vacancy concentration.Li Ni_0.5Mn_1.49Pr_0.01O₄ has a discharge specific capacity of 121.2 m Ah g^-1 after 200 cycles at 0.2 C,and a capacity retention rate of 82.42%,which is much higher than the original LNMO.At a magnification of 2 C,Li Ni_0.5Mn_1.49Pr_0.01O₄still achieves a capacity retention rate of 86.81%after 500 cycles.AC impedance spectroscopy showed that Pr doping can improve the bulk phase conductivity of the material,but the high doping amount reduces the electrochemical performance of LNMO,which may be due to the fact that some Pr is not doped into the crystal lattice but in the surface layer,and this Pr-rich surface layer may hinder lithium-ion migration,and the impedance increase reduces the electrochemical performance of LNMO.