| Abstract:The spinel LiMn2O4has excellent properties such as high voltage, good safety and low cost and it has been widely applied in the cathode materials for power lithium ion batteries. However, LiMn3O4has obvious capacity fading and short cycling life especially at elevated temperature during the repeated charge and discharge process, and this drawback seriously affects the marketization and industrialization. So it desperately needs process optimization and modification to improve the electrochemical properties of spinel LiMn2O4.In this paper, LiMn2O4was synthesized via high temperature solid state reaction method. The thermal characteristics of the precursor have been studies with TG/DSC to optimize the condition of presintering. XRD, SEM, TEM, charge and discharge curves, CV and EIS analysis were used to characterize the influences of Al-doping on the properties of LiMn1.9Al0.1O4and the A12O3pre-coating on the properties of LiMn2O4. The main research contents and results are as follows:(1) LiMn2O4was obtained via separate sect sintering solid state method from Li2CO3and EMD. It can be seen from the thermal analysis that the raw materials decomposed and reacted with each other from300℃to750℃. When presintered at400℃, the presintering product contains a large number of MnO2and Mn2O3. And the content of intermediate products grandually decreased with the presintering time increased from6h to24h. While at500℃, the presintering product with burn time for6h contains a small amount of MnO2and Mn2O3. With the increase of presintering time, the product with burn time of12h only contains MnO2and the products with burn time for18and24h have the single spinel phase. After second calcination at750℃for6h the final products presintered at the temperature of400℃and500℃all have typical spinel structure, and the particle size of500℃is larger than that of400℃. For the final products presintered at400℃and500℃respectively in the burn time for18h and12h have good electrochemical properties, and at55℃the capacity retention after50cycles is91.0% and92.4%, respectively.(2) LiMn1.9Al0.1O4was synthesized via separate sect sintering solid state method from Li2CO3, EMD and Al(NO3)3·9H2O used as doped material. It can be seen from the thermal analysis that the formation of LiMn1.9Al0.1O4occurs between400℃and600℃. When the presintering temperature is500℃, the lattice constants decrease first and then increase with the presintering time increased from6h to24h and the intervals between the two peaks are0.33/0.28V,0.21/0.19V、0.51/0.40V、0.25/0.12V, respectively. The capacity retentions are78.0%,96.4%,85.7%and94.3%respectively at a high temperature (55℃). The product with burn time for12h has a good symmetry in the CV curve and a good cycling performance.When the presintering time is12h, the product presintered at500℃has the largest lattice constant and the intervals between the two peaks are0.38/0.33V,0.21/0.19V\0.59/0.39V for products presintered at400℃,500℃and600℃, respectively. At a high temperature (55℃), the capacity retentions of the products presintered at400℃and500℃are90.6%and96.4%, but the discharge capacity of the latter is higher than the former. The specific discharge capacity of the products presintered at600℃is fluctuant and it has a poor cyclic stability.(3) Al2O3pre-coated LiMn2O4(P-LMO) and A12O3cal-coated LiMn2O4(C-LMO) were synthesized by a chemical deposition and used Li2CO3and EMD as raw materials. The lattice constants for both C-LMO and P-LMO increase with the coating amount of A12O3increased from lmol.%to lOmol.%. C-LMO powers exhibit well-develop polyhedral morphology, and the particle morphology of P-LMO reveals spherical shape. The particles size of P-LMO is smaller than that of the C-LMO. The two potential plateaus for P-LMO coated with different amount of A12O3were maintained well during the cycling process. At55℃, the capacity retentions of P-LMO (77.2%,87.3%,87.2%and84.0%) are higher than those of C-LMO (75.0%,76.4%,72.5%and61.3%) and P-LMO-2have the best electrochemical performances. The Al2O3coating layer of the P-LMO could effectively reduce the increase of charge transfer resistance (Rct) and Mn dissolution into the electrolyte. |
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