| Summary: | 博士 === 輔仁大學 === 化學系 === 102 === This study has three topics and is described as follow:
1. We report a novel synthetic approach for creating spherical and multi-grain lithium cobalt oxide (LiCoO2) powders. The synthetic steps include co-precipitations of spherical CoCO3, sintered to spherical Co3O4 precursor, then solid-state calcination to LiCoO2. The effects of calcination temperature on the structural and electrochemical properties of the LiCoO2 were systemically studied. Electrochemical testing results at room temperature showed that the best discharge capacities of LiCoO2 calcined at 800 ºC are 146.7 and 125.8 mAh g-1 at discharging rates of 0.1 and 6 C, respectively. Low temperature (-10 ºC) discharging results at a rate of 0.2 C exhibited that LiCoO2 calcined at 800 ºC has a highest capacity (129.7 mAh g-1). Electrochemical impedance spectrometry (EIS) analysis showed that the charge transfer resistance(Rct) of LiCoO2 calcined above 800 ºC is much lower than that of LiCoO2 calcined at 750 ºC due to their higher crystallinity. The results of potentiostatic intermittent titration technique (PITT) showed that the apparent chemical diffusion coefficients (Dapp) of LiCoO2 decreases with increasing the calcination temperature from 750 to 850 ºC, which is attributed to increase the primary-particle size of LiCoO2. Our results showed that LiCoO2 calcined at 800 ºC is optimized trade-off between electronic conductivity and lithium ion diffusion, therefore the electrochemical performances are better than those spherical LiCoO2 calcinized at other temperature.
2. We have successfully synthesized two concentration-gradient cathode materials (LiNi0.72Co0.28O2 (CG-LNCO) and LiNi0.72Co0.18Mn0.1O2 (CG-LNCMO)), via a co-precipitation route. According to the careful examinations by physical, electrochemical, and thermal analyses, we found that (1) The CG-LNCO has a structure with different chemical compositions of primary particles from the surface toward core of each of the secondary particles. The primary particle with rich Co content near the surface and the primary particle with rich Ni content in the core of secondary particle of the CG-LNCO have provided the advantages of high safety and high capacity. (2)the CG-LNCMO has a structure with different chemical compositions of primary particles from the surface toward core of each of the secondary particles. The primary particle with rich Mn content near the surface and the primary particle with rich Ni content in the core of the secondary particle of the CG-LNCMO have provided the advantages of high safety and high capacity.
3. Spherical LiNi0.5Mn1.5O4 (LNMO) cathode material gradient-doped with Mg is successfully synthesized via a co-precipitation method. The average Mg doping concentration in the LNMO is 2 mol% with 10 mol% on the particle surface and a gradual reduction to 0% in the core. The Mg gradient-doped (GD) LNMO shows an improved electrochemical performance with discharge capacities of 121.5 and 91.4 mAh g-1 at discharging rates of 0.1 and 4.0 C, respectively. The corresponding values for a pristine LNMO sample are 125.7 and 80.8 mAh g-1. After 80 charge/discharge cycles at room temperature, the Mg(GD)-LNMO retains 92 % of the initial capacity, compared with a retention rate of 80% for the pristine LNMO. A high-temperature (55 ºC) cycling test shows that the Mg(GD)-LNMO has a lower loss of capacity (8%) compared with the pristine LNMO (24 %) after 20 cycles. Electrochemical impedance spectroscopy (EIS) reveals that the Mg-rich surface of Mg(GD)-LNMO suppresses the reaction of the electrolyte with the LNMO and decreases the total resistance. In addition, gradient-doping LNMO with Mg can also improve Li ion diffusion based on results of the potentiostatic intermittent titration technique (PITT). Finally, differential scanning calorimetry (DSC) analysis shows that the exothermic peak of the Mg(GD)-LNMO is shifted at higher temperatures and that the amount of heat is decreased by comparison with the values for pristine LNMO.
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