Study On Synthesis And Modification Of Li_1.2Fe_0.2Mn_0.6O₂ Cathode Materials By Self-propagating Combustion

In order to cope with the rapid development of portable devices,electric vehicles and energy storage,lithium-ion batteries as high-performance energy storage devices have received more and more attention from scientific researchers,and improving energy density has always been the focus of research.The lithium-rich layered Li_1.2Fe_0.2Mn_0.6O₂ material is an ideal low-cost and high-capacity cathode material because of its high specific capacity（>200 m Ah/g）,low cost（rich Fe and Mn resources）,and environmental friendliness.In this paper,the self-propagating combustion method was used to synthesize Li_1.2Fe_0.2Mn_0.6O₂ nanoparticles.First,it is optimized the material’s calcination temperature,Li content in the component and the ratio of CH₃COO^-/NO₃^-in the raw materials during self-propagating combustion;then,it is modified by polyacrylate lithium coating and doped with Ni ions.XRD,TEM and SEM are used to characterize the crystal structure and surface morphology of the material,and studied on its electrochemical performance through constant current charge and discharge,cyclic voltammetry（CV）and electrochemical impedance（EIS）tests.The main research contents of this paper are as follows:（1）First,by adjusting the calcination temperature to 400℃,450℃and 500℃,the self-propagating combustion method was used to successfully synthesize Li_1.2Fe_0.2Mn_0.6O₂ nanoparticles.Constant current charge and discharge tests analysis show that the Li_1.2Fe_0.2Mn_0.6O₂ material（LFMO-450）synthesized at a calcination temperature of 450℃has the largest reversible capacity（the first discharge specific capacity is 237.5 m Ah/g at 0.1C rate）and the best good cycle performance（at 1C rate,the capacity retention rate is 82.5%after 60 cycles）.Then,after determining the optimal calcination temperature,it is explored the effect of Li content in materials（taking x as0.25,0.2,0.15）on the material properties.Li_1+x(Fe_0.25Mn_0.75)_1-xO₂ materials with different Li content were successfully prepared at the optimal calcination temperature of 450℃.It was confirmed by XRD that the cation order ofα-Na Fe O₂ structure decreased with the increase of Li content.When x=0.2,the Li_1.2Fe_0.2Mn_0.6O₂ material crystal contains mixed cations and has the best electrochemical performance.Finally,explore the effect of CH₃COO^-/NO₃^-ratio on the surface morphology and electrochemical performance of the material.According to SEM characterization and analysis,the material（LFMO-2A1N）synthesized from the raw materials with a CH₃COO^-/NO₃^-ratio of 2:1 is composed of primary particles with a particle size distribution of 36-79 nm.Constant current charge and discharge test analysis shows that LFMO-2A1N has the better rate performance,but the cycle performance is not good（the discharge specific capacity is 80.5 m Ah/g after 100 cycles at 1C,and the capacity retention rate is 65.3%）.It may be because the smaller nanoparticles increase the corrosion of the electrolyte to the active particles,causing the capacity to decay rapidly.（2）In order to improve the cycle performance of the synthesized nanoparticles when the CH₃COO^-/NO₃^-ratio is 2:1,lithium polyacrylate（PAALi）was used to coat and modify the Li_1.2Fe_0.2Mn_0.6O₂ material,and the PAALi coating amount was studied.Impact on material properties.Constant current test analysis shows that when the coating amount is 4%,the sample LFMO-4%P has the best rate and cycle performance.The first discharge specific capacity is 231.9 m Ah/g at 0.1C,and the capacity is still101.3 m Ah/g at 2C;the reversible capacity is still 102.3 m Ah/g after 100 cycles at 1C.The impedance test results show that LFMO-4%P has the smallestσvalue(101.73Ω·cm²·s^-1/2)and the largest D⁺_Li(5.938?10^-16 cm²·s^-1),which proves that the low frequency AC impedance test Under the conditions,when PAALi is 4%,the material has the best ion diffusion characteristics.This may be because the PAALi coating weakens the damage of the electrolyte to the material structure,thereby improving the rate and cycle performance of the material.（3）First,the influence of the Ni amount doped y on the overall performance of the Li_1.2Fe_0.2Ni_yMn_0.6-yO₂ material is explored.Constant current charging and discharging analysis test shows that:with the increasement of Ni amount,the reversible capacity of the sample increased,but the cycle performance deteriorated seriously.When y=0.1,Li_1.2Fe_0.2Ni_0.1Mn_0.5O₂ has the best rate performance（the first discharge specific capacity is as high as 252.4 m Ah/g at a rate of 0.1C,and the capacity is still 94.6 m Ah/g at 2C）.The capacity attenuation is severe during the cycle（under 1C rate,the capacity retention rate is only 58.36%）.Secondly,by adjusting the iron and nickel content z（take z as 0,0.05,0.1）,using self-propagating combustion to synthesize different Fe/Ni ratio precursors,and then calcining at 700℃to synthesize nano-Li_1.2Fe_0.2-zNi_0.1+zMn_0.5O₂material.XRD analysis confirmed that when z=0.05（the Fe/Ni ratio is 1:1）,the Li_1.2Fe_0.15Ni_0.15Mn_0.5O₂（LFNMO-1F1N）has a typicalα-Na Fe O₂ layered structure and a more complete layered degree,with a grain size of 10.96 nm.SEM test analysis shows that LFNMO-1F1N is a uniformly distributed nanoparticle,and its particle size is mainly in the range of 38～62 nm.Constant current test shows that LFNMO-1F1N has a higher reversible specific capacity and the best cycle performance.Under 0.1C rate,the first reversible capacity of LFNMO-1F1N is as high as 258.9 m Ah/g,which is much higher than the reversible capacity of LFNMO-2F1N（191.6 m Ah/g）and the reversible capacity of LFNMO-1F2N（155 m Ah/g）.After 100 cycles of charge and discharge at a rate of 1C,the reversible capacity is still 122.6 m Ah/g,and the retention rate is 78.6%.