Preparation And Characterization Of Li_3-xM_xN Anode Materials For Lithium Ion Batteries

Lithium ion batteries are regarded as one of the most promising candidates for power sources due to their high reversible capacity, long service life and environmental advantages. The lithium nitridometalates, Li_3-xM_xN (M=Co, Ni, Cu), are considered to be an alternative anode material to graphite in commercial lithium-ion batteries, owing to their high specific capacity. Li_3-xCo_xN has shown a higher capacity among these nitridometalates, but the capacity retention is poor. In this work, we studied the preparation and modification of Li_3-xCo_xN compounds. The valuable results of this thesis will be helpful for the practical applications of Li_3-xCo_xN materials.Li3N and Co powders were used as source materials to prepare the materials. Li_2.6Co_0.4N and Li_2.6Co0.25Cu0.15N compounds withα-Li₃N structure were prepared by a solid-state reaction. A slight of Li₂O impurity phase was observed in the XRD spectrum. The Li₂O phase was probably formed on the surface during the experimental process because the material is very sensitive to O2 and H₂O. SEM micrograph was taken for the Li_2.6Co_0.4N and Li_2.6Co_0.25Cu_0.15N specimens. The molten-like surface of the compounds implies the deterioration of the material during exposure in air. We investigated the lithium storage mechanism and electrochemical properties of the nitride compounds by charge-discharge experiments. The discharge capacity of Li_2.6Co_0.25Cu_0.15N was slight lower than Li_2.6Co_0.4N. However, the cycling stability of Li_2.6Co_0.4N was improved by Cu substitution. Electrochemical impedance spectroscopy was used to study the interfacial behavior of the samples. The total resistance of Li_2.6Co_0.25Cu_0.15N was smaller than that of Li_2.6Co_0.4N. This implies that the interfacial compatibility of Li_2.6Co_0.4N was improved by Cu substitution. Therefore, Cu substitution would prevent the electrolyte decomposition, leading to a better electrochemical performance. We fabricated Li_2.6Co_0.4-xCu_xN/Cu_0.04V₂O₅ full-cells and investigated the electrochemical properties of the batteries. The Li_2.6Co_0.25Cu_0.15N/Cu_0.04V₂O₅ cell exhibited better electrochemical performance than the Li_2.6Co_0.4N one. The former cell delivered a high specific capacity of 260 mAh/g. The corresponding specific energy was 505.7 mWh/g,which is much higher than that of commercial C/LiCoO₂ batteries.Natural graphite has very abundant sources all over the world. It has a lower price and a comparable high specific capacity than artificial graphite materials. However, the practical use of natural graphite is still hindered by several shortcomings such as its low initial columbic efficiency and poor cycle life. Based on the above analysis, it is seen that Li_2.6Co_0.4N had a higher columbic efficiency in the first charge-discharge cycling. This promoted us to used Li_2.6Co_0.4N to compensate the high initial irreversible capacity of natural graphite material. The Li_2.6Co_0.4N/graphite composites were prepared in different Li_2.6Co_0.4N/graphite ratios. We investigated the lithium storage mechanism and electrochemical properties of the composite materials by charge-discharge experiments. The charge-discharge curves of the composite materials exhibited the mixing characters of the Li_2.6Co_0.4N and natural graphite materials. The composite materials exhibited better cycling performance than Li_2.6Co_0.4N and a higher specific capacity than natural graphite. Especially, the material containing 20 wt.% of Li_2.6Co_0.4N seems to have the best electrochemical performance among all of the composite materials.Further more, we studied the magnetic properties of Li_2.5Co_0.5N material. The magnetic hysteresis and Arrott plots indicate a character of soft room-temperature ferromagnetism behavior. The magnetic hysteresis displays a lack of magnetization saturation with the decreasing of temperature. This indicates the existence of certain magnetic transition, together with the reserved ferromagnetism. The studies of dc magnetization and ac susceptibility reveal spin glass transition with the decreasing of temperature. The spin glass ground state observed in Li_2.5Co_0.5N was likely to be caused by the random substitution of magnetic Co ions in the two-dimensional triangular lattices. Due to the random substitution, there are two possible factors for the formation of the spin glass state. First, the spin glass state could be caused by the competition between magnetic clusters. And the other explanation lies in the geometrical frustration induced by the random substitution.To summarize, the innovation and scientific worth of this thesis can be concluded as following:(1) We synthesized Li_2.6Co_0.4-xCuxN anode materials by solid state reaction. These materials delivered a specific capacity of about 650 mAh/g and showed good capacity retention. According to this study, the electrochemical stability of Li_2.6Co_0.4N could be improved by Cu substitution.(2) We used Li_2.6Co_0.4N to compensate the high initial irreversible capacity of natural graphite material. The composite materials exhibited good electrochemical performance. This will be helpful for the further applications of natural graphite in lithium ion batteries.(3) We assembled a new kind of lithium ion battery using Cu0.04V2O5 and nitride as the cathode and anode materials. The Li_2.6Co0.25Cu0.15N/Cu0.04V2O5 full-cell delivered a specific energy of 505.7 mWh/g, which was much higher than commercial lithium ion batteries that using carbon as anode materials.(4) We investigated the physical properties of the Li2.5Co0.5N materials. Spin glass ground state in the Li_3-xCo_xN system was observed for the first time.