Synthesis And Electrochemical Properties Of LiAl_xMn_2-xO₄ Prepared By Flameless Combustion

Spinel Li Mn₂O₄ has become one of the most promising lithium ion batteries cathode materials because of its high safety,high energy density,high power density,abundant manganese resources,low toxicity and environmental friendliness.In this paper,based on the overview of the working principle of lithium-ion battery,the development of cathode materials and the progress of research,the spinel Li Mn₂O₄ were doped and modified by selecting Al metal element which is non-toxic,low cost,lighter than transition metal elements to overcome the disadvantages of the poor cycle performance and rapid capacity decay of spinel LiMn₂O₄,especially at elevated temperature.Looking forward to improve the cycling and rate performances of spinel Li Mn₂O₄ and revealing the effects of Liquid phase flameless combustion synthesis,Al doping and Li,Al equal molar co-doping on the phase composition,particle size,crystallinity,microstructure and the reaction mechanism of the kinetic process during the charging and discharging process,a series of Al-doped spinel LiAl_xMn_2-x O₄（x≦0.20）and Li,Al co-doped spinel Li_1.05Al_0.05Mn_1.90O₄ lithium ion batteries cathode materials were prepared by Liquid flameless combustion method to improve the specific capacity,cycle performance,rate performance.The effects of calcination temperature and calcination time on the structure,morphology and electrochemical properties of the LiAl_xMn_2-x O₄ lithium ion batteries cathode materials were investigated.X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FT-IR）,field emission scanning electron microscopy（FE-SEM）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）,energy dispersive spectroscopy（EDS）,galvanostatical charging and discharging performance test,cyclic voltammetry（CV）and electrochemical impedance（EIS）were used to detailedly study the structure,phase composition,microstructure,particle size distribution,particle growth direction,Mn average valence and electrochemical properties of as-prepared samples.（1）The as-prepared samples belong to the Fd3 m space group of cubic spinelLi Mn₂O₄,Al3⁺ successfully entered the lattice of LiMn₂O₄,partially replacing the Mn³⁺ position.With the increase of Al doping amount,the lattice constants and unit cell volumes of the products tend to decrease.The as-prepared samples have a high crystallinity,regular octahedral morphology.With the increase of Al doping,the grain boundary became more clear,the grain size distribution became more uniform,the agglomeration of the particles was weakened,and the particle size was about200-300 nm.（2）Galvanostatical charge and discharge performance test showed that the as-prepared sample with Al doping content of 0.10 had the best cycle performance and capacity retention rate.The initial discharge specific capacity and the capacity retention rate were 112.2 m Ah·g-1 and 57.3% at 5 C and 25 ℃after 2000 cycles,respectively.At 10 C and 25 ℃,the initial discharge specific capacity was 99.5mAh·g^-1,the discharge specific capacity after 2000 cycles was 56.7 mAh·g^-1,and the capacity retention rate was 57.0%.At 1 C and 55 ℃,the initial discharge capacity was 112 mAh·g^-1,the discharge specific capacity after 500 cycles was 91.3mAh·g^-1and the capacity retention rate was 81.5%.CV and EIS tests showed that the as-prepared sample with Al doping content of 0.10 had the best electrochemical reversibility and faster charge transfer kinetics.Li Al0.10Mn1.90O4 had a higher lithium ion diffusion coefficient than that of undoped LiMn₂O₄,and the apparent activation energy Ea of LiAl_0.10Mn_1.90O₄ was 23.1 kJ·mol^-1,which was significantly lower than that of the value of Ea of undoped LiMn₂O₄(32.5 kJ·mol^-1).（3）Increasing the calcination temperature can improve the electrochemical performance of the product.While the calcination temperature was 650 ℃,the as-prepared sample had the best electrochemical performance.The initial discharge capacity was 114.6 mAh·g^-1,the discharge specific capacity after 300 cycles was 94.5mAh·g^-1 and the capacity retention rate was 82.5% at 1 C and 25 ℃（4）Increasing the calcination time can improve the electrochemical performance of the product.While the calcination time was 9h,the as-prepared sample had the best electrochemical performance.The initial discharge capacity was 121.9 mAh·g^-1,the discharge specific capacity after 500 cycles was 79.8 mAh·g^-1at 1 C and 25 ℃.（5）On the basis of Al doping,the Li-rich and Al co-doped spinelLi_1.05Al_0.05Mn_1.90O₄ lithium ion batteries cathode materials were prepared.The as-prepared samples were single phase,excess Li+ and the doping Al3⁺ successfully entered the lattice of spinel Li Mn₂O₄,the lattice constant and cell volume of Li+,Al3⁺co-doped spinel LiMn₂O₄ decreased slightly.The Li-rich and Al co-doped sample had higher crystallinity,regular octahedral morphology,uniform particle size distribution.XPS analysis results showed that Li-rich and Al co-doping increased the average oxidation state of manganese,from +3.51 for undoped sample to +3.55 for doped sample.（6）Galvanostatical charge-discharge performance test showed that Li-rich and Al co-doping significantly improved the cycling stability of LiMn₂O₄.The initial discharge specific capacity was 126.2 mAh·g^-1,the capacity retention rate after 1000 cycles was 82.9% at 1 C and 25 ℃.At 5 C and 25 ℃,the capacity retention rate was82.4% of Li_1.05Al_0.05Mn_1.90O₄ sample after 1000 cycles,while the capacity retention rate of the undoped sample under the same conditions was only 60.4%.In addition,at1 C and 55 ℃,the discharge capacity of undoped sample decreased from 128.1mAh·g^-1 to 18.7 mAh·g^-1,the capacity retention rate was only 14.6%,while the capacity retention rate of Li_1.05Al_0.05Mn_1.90O₄ sample was 73.9% under the same conditions.The rate performance of Li_1.05Al_0.05Mn_1.90O₄ sample was more excellent than that of the undoped sample.CV and EIS tests showed that Li_1.05Al_0.05Mn_1.90O₄ had a better electrode kinetics process,specifically: Li_1.05Al_0.05Mn_1.90O₄ had better electrochemical reversibility,lower charge transfer impedance and higher Lithium ion diffusion coefficient,the apparent activation energy Ea(19.10 kJ·mol^-1)of Li_1.05Al_0.05Mn_1.90O₄ sample was lower than that of undoped LiMn₂O₄(32.5 kJ·mol^-1).