Molten-salt Combustion Synthesis And Electrochemical Performance Of Mg And Cu Doped Spinel LiMn₂O₄

A series of Mg or Cu doped Li Mx Mn2-x O4（M=Mg or Cu） samples with high purity, well crystalline and excellent electrochemical performance were rapidly synthesized at a relatively low temperature by the “Molten-salt combustion synthesis”. Lithium acetate, manganese acetate, magnesium acetate and copper acetate were used as the raw materials. The effects of different Li/Mn molar ratio and synthesis process including reaction temperature, time, two-stage calcinations process and M dopant amount on the phase structure, micromorphology and the electrochemical properties of Li Mn2O4 were studied by X-ray diffraction（XRD）, scanning electron microscope（SEM）, galvanostatic charge/discharge tests, cyclic voltammetry（CV） and electrochemical impedance spectroscopy（EIS）.The results show that all the cathode materials prepared via the “Molten-salt combustion synthesis” are spinel Li Mn2O4 with Fd3 m space groups. Some impurity phases of Mn2O3, Mn O or Mn3O4 can be detected in the samples synthesized under different synthetic conditions, such as M dopant amount and reaction temperature. Firstly, the molten starting materials were pretreated at 200 oC for 5 min. At 600 oC, when the molar ratio of Li n（Li） increasing from 1.0 to 1.1, the impurity phase of sample converts from Mn3O4 into Mn2O3, but the content of impurity almost does not change. The content of impurity phase increases when the calcination temperature increasing from 700 oC to 800 oC. The starting materials were stoichiometrically weighed according to the molecular formula of Li Mn2O4, then they were pretreated at 200 oC for 5 min, the main phase of sample calcined at 300 oC is not spinel Li Mn2O4. When the calcination temperature ≥ 400 oC, spinel Li Mn2O4 can be obtained. The content of Mn3O4 impurity decreases with the increasing calcination temperature. At 600 oC, the Li Mn₂O₄ with high purity and well crystallinity can be obtained. The content of Mn3O4 impurity in samples increases again, when the calcination temperature above 700 oC. Samples calcined at 700 oC for 3 h and 6 h are spherical-like particles with homogeneous granularity. The initial discharge capacity of sample calcined at 600 oC obtained the highest discharge capacity of 116.5 m Ah/g. CV and EIS tests indicate that this sample has well electrochemical performance with well symmetry, good reversibility, high electrochemical activity and weak electrochemical polarization.In order to explore the effects of different temperatures on the two-stage calcination products, 500, 600, 700 and 800 oC where chosen as the research objects, and the calcination time is 3 h. The dopant Li Mx Mn2-x O4 calcined at 600 oC and 700 oC are pure Li Mn2O4. But the dopant Li Mx Mn2-x O4 calcined at 800 oC contains much Mn3O4 impurity, and the diffraction peak of Mn3O4 becomes weaker with increasing dopant element. At 600 oC, samples with aspheric particles with single phase, narrower size distribution, smaller particle size and well crystalline can be obtained. The spherical particles of Mg dopant Li Mgx Mn2-x O4 samples become smaller and more uniform with increasing Mg content, but the particles size is about 190-280 nm with different Mg content. The crystallinity of sample is improved with the increasing calcination temperature for the same dopant Mg content, and particles develop into spinel octahedral crystals at 800 oC. The spherical particles of Cu dopant Li Cux Mn2-x O4 samples are smaller and more uniform granules, but the particles become larger and the interface between granules is more distinct with increasing Cu content. The crystallinity of sample is improved with the increasing calcination temperature for the same dopant Cu content, and particles develop into standard spinel octahedral crystals at 800 oC. At the same calcination temperature, both the crystallinity and the spinel octahedral crystals at 800 oC of Cu dopant Li Cux Mn2-x O4 samples are better than that of Mg dopant Li Mgx Mn2-x O4 samples.Electrochemical performance of the two-stage calcination products demonstrates that the initial discharge capacity gradually decreases with the increasing Mg dopant content for the Mg dopant Li Mgx Mn2-x O4 samples. The initial discharge capacity of Li Mn2O4, Li Mg0.02Mn1.98O4, Li Mg0.05Mn1.95O4, Li Mg0.06Mn1.94O4 and Li Mg0.10Mn1.90O4 is 124.0, 123.4, 122.0, 117.7 and 109.2 m Ah/g, respectively. The discharge capacity after 100 cycles of them is 91.1, 101.8, 105.4, 92.1 and 97.6 m Ah/g. Both the discharge capacity and cycling efficiency of Mg dopant Li Mgx Mn2-x O4 samples are higher than that of the pristine Li Mn2O4. Comprehensive considering the discharge capacity and cycling efficiency, the electrochemical performance of Li Mg0.05Mn1.95O4 is the best. For example, the initial discharge capacity of Li Mg0.06Mn1.94O4 first increases to a maximum and then gradually decreases when the calcination temperature increasing from 500 oC to 800 oC. The initial discharge capacity of it is 101.9, 117.7, 112.8 and 79.8 m Ah/g, and the discharge capacity after 100 cycles is 82.6, 92.1, 89.5 and 54.9 m Ah/g, respectively. Thus the electrochemical performance of 800 oC two-stage calcination sample is rather inferior. CV results show that both the peak current and area of Mg dopant samples are larger than that of the pristine Li Mn2O4. The peak current and area of 600 and 700 oC two-stage calcination samples are much larger than that of 500 and 800 oC.The initial discharge capacity of 600 oC two-stage calcination Li Cux Mn2-x O4（x=0, 0.02, 0.05 and 0.10） samples is 124.0, 120.8, 119.0 and 104.0m Ah/g, respectively. Their capacity retention after 100 cycles is 73.5%, 83.4%, 95.0% and 88.4%, respectively. After doping Cu, the cycling performance of spinel Li Mn2O4 is effectively improved. For the Li Cu0.04Mn1.96O4 samples obtained at different two-stage calcination temperatures（500, 600, 700 and 800 oC）, the initial discharge capacity of 500 and 600 oC are 116.0 m Ah/g, but both the discharge capacity（107.9 m Ah/g） and the capacity retention（93.0%） of 600 oC after 100 cycles are better than that of 500 and 700 oC. The electrochemical performance of 800 oC is the worst, its initial discharge capacity is 83.1m Ah/g, and after 100 cycles its discharge capacity is just only 47.7 m Ah/g with the capacity retention of just 57.4%.