Preparetion And Performance Improvements Of Layered LiMnO₂ Cathode Materials For Lithium-ion Batteries

Layered LiMnO₂ cathode material for lithium-ion batteries was synthesized by solid-state reaction and sol-gel method. The effects of calcination temperatures and time on the material performance were studied in solid-state reaction. Calcination temperatures were determined by thermal gravimetric analysis. The solvent and pH value were researched in the sol-gel method. Then the layered LiMnO₂ was doped and coated. The crystal structure features and particle morphologies of samples were investigated using X-ray diffraction（XRD）, scanning electron microscopy（SEM）,transmission electron microscopy（TEM） and energy dispersive spectrometer（EDS）.The electrochemical performances of all samples as cathode materials were studied by electrochemical impedance spectra（EIS）, cyclic voltammetry（CV） and charge-discharge test.Layered LiMnO₂ cathode material was prepared by solid-state reaction using lithium hydroxide（Li OH?H2O） and home-made Mn2O3（calcing MnO2 at 750 ℃ for5 h） as starting material. The mixture of LiOH?H2O and Mn2O3 was analyzed by thermal gravimetric analysis, then the influences of different temperatures（550 ℃,650 ℃, 750 ℃, 800 ℃） and time（8 h, 12 h, 16 h） for layered LiMnO₂ were studied.The experiment results showed that the best synthetic process was as follows: The mixture of LiOH?H2O and home-made Mn2O3 were first calcined for 4 h at 300 ℃ in air, then fired at 750 ℃ for 12 h under N2 atmosphere. The sample synthesized at the optimal conditions has layered orthorhombic structure and the smooth surface of particle morphology. The discharge capacity of the sample reached 181.1mAh/g at0.1 C rate for 20 cycles in the cut-off voltage of 2.0-4.3 V, and the capacity retention rate was 97.3%, after more 10 cycles. With Na, Al and K, Cr as doping elements, the electrochemical performances of doped samples were researched. The results showed that the Na:Al optimal atomic ratio was 0.02:0.05, the discharge capacity of the Li0.98Na0.02Mn0.95Al0.05O2 sample reached 201 m Ah/g at 0.1C rate for 10 cycles, and the capacity retention was 98.4%, after more 20 cycles. The Cr: Al optimal atomic ratio was 0.01:0.04, the discharge capacity of the LiMn0.95Cr0.01Al0.04O2 sample reached 158.8 mAh/g at 0.1 C rate for 10 cycles, less then Li MnO2 material, but the capacity stability of the LiMn0.95Cr0.01Al0.04O2 sample was greatly improved. Last, thelayered LiMnO₂ material was coated by CoO and MgO. CoO-coated samples were synthesized by liquid phase method, while MgO-coated samples were synthesized via precipitation. When coating solvent was anhydrous ethanol and the amount of CoO coating was 4wt.%, the highest discharge capacity of the sample reached 156.2mAh/g at 0.1 C rate for 24 cycles, and the capacity retention rate were 96.4%, 95.0%and 87.4% at 0.2 C, 0.5 C and 1 C for 20 cycles, respectively. Compared with uncoated LiMnO₂ material, the capacity stability of the CoO-coated LiMnO₂ material was greatly improved. MgO-coated LiMnO₂ materials were synthesized by precipitation. The highest discharge capacity of 6wt.% MgO-coated LiMnO₂ was153.2 mAh/g lower than uncoated Li MnO2 material, but the the capacity retention rate of this sample was greatly improved.The effects of solvents（anhydrous ethanol, deionized water） and pH value（6, 7,8） were studied in the synthesis process of layered LiMnO₂ by sol-gel method. The experiment results showed that the sample synthesized with deionized water as solvent showed highest discharge capacity and the best cyclic performance. The discharge capacity of this sample reached 139.0mAh/g at 0.1 C rate for 20 cycles in the cut-off voltage of 2.0-4.3 V, and the final capacity retention rate was 98.0%. The first discharge capacity of this sample reached 87.1mAh/g at 1 C rate, and the capacity retention rate was 95.4% after 20 cycles. K- and Cr- doped LiMnO₂ materials were synthesized via sol-gel method. The results were as follows: when the K: Cr atomic ratios were 0.02:0.01 and 0.02:0.25, the discharge capacities of samples were 151.8 mAh/g and 143.3 mAh/g at 0.1C rate, higher than LiMnO₂ materials,while the discharge capacity of the Li0.98K0.02Mn0.99Cr0.01O2 sample and LiMnO₂ materials were almost equal at another rates. Discharge capacities of another doped sample were lower than undoped LiMnO₂ material.