| Lithium ion batteries have drawn great attention due to its high voltage, high energy density, long cycle life, low self-discharge, non-menory effect and environmental friendliness. In this paper, layered cathode material LiNi0.75Mn0.25O2 and LiNi0.5Mn0.5O2 compounds were successfully synthesized with layered Ni-Mn hydrotalcite as precursor, and the performances of LiNi0.75Mn0.25O2 and LiNi0.5Mn0.5O2 were studied.Ni-Mn hydrotalcite was prepared by co-precipitation method. Effects on the structure and morphology of Ni-Mn hydrotalcite are studied from feeding mode, reaction time, reaction temperature and Ni/Mn molar ratio, and the best synthesis technics are optimized. The results show that the samples of Ni-Mn hydrotalcite was acted at 20℃for 16h, used the mode that mixed solution of Mn(NO3)2 and Ni(NO3)2 and solution of NaOH cocurrent added to solution of Na2CO3, and Ni/Mn=3:1 are of higher crystallinity and purity.In this work, layered LiNi0.75Mn0.25O2 and LiNi0.5Mn0.5O2 powders were successfully synthesized by solid state reaction using LiOH·H2O and Ni-Mn hydrotalcite as procursors. The properties of LiNi0.75Mn0.25O2 and LiNi0.5Mn0.5O2 were extensively and systematically explored by means of XRD,SEM,TGA/DT,CV,EIS and charge-discharge tests.Effects on the structure and properties of LiNi0.75Mn0.25O2 samples were studied by sintering temperature, time and the initial Li/(Ni+Mn) molar ratio, and the best synthesis technics are optimized. The results show that the capacity of LiNi0.75Mn0.25O2 sintered at 850℃for 16h in air and Li/(Ni+Mn)=1.1 is 180.7 mAh·g-1 at the first discharge between 2.5 and 4.5V, and the capacity is 142.44 mAh·g-1 after 30 cycles.Effects on the structure and properties of LiNi0.5Mn0.5O2 samples were studied by sintering temperature, sintering time, Li/(Ni+Mn) molar ratio, lithium source and sintering process. The results show that 800℃is the best sintering temperature. The cycling performance of LiNi0.5Mn0.5O2 is improved by increasing the sintering time. The initial Li/(Ni+Mn)=1.1 is the best molar ratio. The samples of LiNi0.5Mn0.5O2 which were used LiOH·H2O and Li2CO3 as lithium source, Li/(Ni+Mn)=1.1, and were sintered at 800℃for 24h , deliver up 149.8 mAh·g-1 and 153.1 mAh·g-1 respectively at the first discharge between 2.5 V and 4.5 V, and both of the samples without capacity fading after 30 cycles. The sample of LiNi0.5Mn0.5O2 which were used LiNO3 as lithium source is worse electrochemical performances. Twice calcination is beneficial to electrochemical performance of LiNi0.5Mn0.5O2. The sample which was sintered at 650℃for 4h before sintered at 800℃for 12h possessed integrated layered structure, smooth surface and good electrochemical properties, the capacity is 171.4 mAh·g-1 at the first discharge without capacity fading after 30 cycles.Cyclic voltammetry (CV) profiles show that the manganous in LiNi0.75Mn0.25O2 and LiNi0.5Mn0.5O2 is in the tetravalent state. Results of electrochemical impedance spectroscopy(EIS) test indicated that the impedance of the batteries were full changed is smaller than that of the batteries were full dischanged. |
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