| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 95 === For developing lithium-ion secondary batteries with higher capacity and longer cycle life, the employment of surface modification and composite materials were introduced to explore positive and negative electrode materials, respectively, in this study. It was expected that electrochemical performance of lithium-ion batteries could be improved by optimizing the intrinsic characteristics in electrode materials.
Raising the upper cut-off voltage can increase the capacity of commercial LiCoO2 cathodes, yet degrade the cycleability. Surface modification by metal-oxide coating was verified to be an effective way to retard the abrupt capacity fading of LiCoO2 cycled at cut-off voltage higher than 4.2 V. In this study, ZnO was coated on the surface of LiCoO2 particles via a wet-chemical process. The influences of the amount of coated ZnO and calcination temperatures on electrochemical performance of LiCoO2 were discussed. Furthermore, how ZnO modification improved the cycling behavior was also probed from the view points of surface and structural properties of LiCoO2.
The bright-field transmission electron microscopy (TEM) images and selected area diffraction (SAD) patterns of as-coated LiCoO2 particles calcined at 450oC revealed the existence of continuous ZnO films with a 10-nm thickness deposited on LiCoO2 surfaces. This showed that a Li-Zn-O phase would be formed on the surface region besides ZnO. This also indicated that Zn2+ ion diffused into the surface region of LiCoO2. In addition to the phase identification results, uniform distribution of Zn atoms in the ZnO-modified LiCoO2 after calcination at 650oC revealed by X-ray color mapping with field-emission electron probe microanalyzer (FE-EPMA) also confirmed the diffusion of Zn2+ ions into LiCoO2 particles during the surface modification process. Crystallography data derived from XRD analysis demonstrated that appropriate amounts of diffused Zn2+ ions was beneficial to the layer property of LiCoO2, while unsuitable Zn doping derived from excess amount of deposited Zn2+ ions (>0.2wt.%) and calcinations at excessively high temperatures (>650oC) would aggravate the cation mixing of LiCoO2.
The cycle life of LiCoO2 cathodes cycled at a high cutoff voltage was greatly improved by surface modification with ZnO coating. In addition to high-voltage cycleability, cycle-life degradation caused by inappropriate conductive carbon could also be moderated by ZnO coating. Furthermore, the rate capability at high current density was also notably improved. In this material system, the LiCoO2 cathode coated with 0.2 wt.% Zn and calcined at 650°C exhibited the most superior electrochemical performance. As tested between 3.0 and 4.5 V for 30 cycles, the capacity retention of the optimal ZnO-modified LiCoO2 was as high as 93%, which was much higher than that of the pristine LiCoO2 (55%). The mechanism of the cycleability improvement was proposed with respect to degree of cation mixing, the surface conditions, and structural evolution during cycling.
As for the negative electrodes, raising the specific capacity of anodes by introducing tin-based compounds was the task in this work. To overcome the rapid capacity fading of tin-based anodes, carbonaceous materials were used as the matrix to accomodate the stress induced by the large volume changes of Li-Sn alloys during cycling. A modified electroless plating technique was adopted to prepare the composite electrodes of Sn compounds/carbonaceous material. Multiphased Sn compounds were deposited on mesophase graphite powders (MGP) by this process. The multiphase compounds containing metallic Sn, SnP3 and SnP2O7 were expected to provide a higher spectator to Sn ratio for improved cycleability. During cycling between 0.001 and 1.5V, the charge capacity was significantly enhanced without appreciable fading. From the voltage profiles and cyclic voltammetry (CV) curves, it was revealed that the capacity fading was caused by either the formation of insulated LiP in the early stage or by aggregation of metallic Sn after prolonged cycling. As the cut-off voltage was lowered from 1.5 to 0.9V, the capacity retention was improved to be as high as 96% of the highest capacity after 45 cycles.
In addition, amorphous tin compounds were deposited on graphite by the modified electroless plating process. The charge capacity retention of the amorphous Sn-P-O/graphite composite anode tested between 0.001 and 1.5V was 89% after 20 cycles. This was nearly identical to that of the pristine graphite and indicated very good cycleability. High-resolution transmission electron microscopy (HR-TEM) images demonstrated that Sn clusters in the composite anode were nano-sized and well separated even after prolonged cycling. This clearly revealed that deposited P and O provided effective buffer effect during cycling, which was the major reason for the improved cycling stability of the Sn-P-O/graphite composite anode.
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