| Rechargeable lithium ion batteries are widely used in portable electronic devices and hybrid electric vehicles because of high energy density, long cycle life and environmental benign. Nowadays the development for LIBs has encountered the bottleneck, which demanded people to design and synthesize new materials for solving the problems, especially cathode materials. In cathode materials, the layered Li-rich Li1+z M1-z O2(M is one or more than one type of transition metal elements, z≥0) has been the most potential materials due to its high capacity(>250 m Ah g-1). We have designed a tetrahedral phase diagram of Li Ni O2-Li Co O2- Li Mn O2-Li2 Mn O3, and selected the ternary phase diagram(Li1.2Ni0.4Mn0.4O2-Li1.2Co0.4Mn0.4O2-Li1.2Ni0.2Mn0.6O2) to design Li1.2(Ni0.2Mn0.6)x(Co0.4Mn0.4)y(Ni0.4Mn0.4)1-x-y O2(0≤x+y≤1) materials. This series of materials have been synthesized and proved that they are pure phases with layered structure by acetate combustion method in our previous work. In this paper, we select the carbonate co-precipitation method follow by a heat-treatment reaction which is more suitable for industrial production to prepare this series of materials.The spherical materials with higher tap density have been obtained. The synthesis conditions have been discussed, and the performances of material have been optimized. There is a certain impact on structure、morphology and electrochemical performance of Li-rich materials with the change of Ni、Co、Mn, and it has also been found that some compounds in this series of materials show more excellent comprehensive performance and provide help to the development of the new material for the future lithium ion battery. In this study, the Li-rich cathode materials are obtained via the carbonate co-precipitation method follow by a heat-treatment reaction. Then core-shell structured materials Li1.2Ni0.133Co0.133Mn0.533O2 are designed and prepared by choosing materials with high discharge capacity and good cycle performance of Li Co0.75Al0.25O2, in order to improve the performance. Details are as follows:1. The series of materials are prepared by concurrent carbonate co-precipitation method follow by a heat-treatment reaction. Concurrent carbonate co-precipitation method with 4 L water, and dropwise add salt solution and alkali solution at the same time.The reason is that Mn(OH)2 is easy to be oxided, but CO32- and Mn2+ can combine stable. XRD tests show that the obtained precursors are in accordance with the aimed compositions. The precursors have good sphericity and relatively high tap density. After suffering heat-treatment with Li2CO3,the prepared final products can be identified as a pure phase with a-Na Fe O2-type layered structure, and keep the same spherical morpologies with the precursors. The electrochemical properties of these Li-rich materials have a linear variation law.2. Based on content 1, we choose positive concurrent carbonate co-precipitation method to optimize precursor, in order to obtain more uniform spherical particles. Positive concurrent carbonate co-precipitation method is different from concurrent carbonate co-precipitationmethod, it used salt diluted solution, and dropwise add salt solution and alkali solution at the same time after droping alkaline adjust p H. XRD tests show that the structures of precursors are pure and as same as the design. The sphericity and the tap density are not as good as those of the precursors of concurrent carbonate co-precipitation. After suffering heat-treatment with Li2CO3, although the prepared final products can be identified as a pure phase with a-Na Fe O2-type layered structure, the morphologies are not as same as the precursors. The electrochemical properties of these Li-rich materials are not good.3. Based on the above research, we design and synthesize core-shell structured[Li1.2Ni0.133Co0.133Mn0.533O2]1-x[Li Co0.75Al0.25O2]x(x=0.1,0.2,0.3) materials and carries on the exploratory study. We adopt the method of concurrent via carbonate co-precipitation to prepare core-shell structured precursor. XRD results show that the precursor is single-phase,and SEM results show that the morphology is good spherical, and particle size analysis shows that the core-shell structure has been formed as we expect. After heat-treatment reaction with Li2CO3, all materials are identified as a pure phase with a-Na Fe O2-type layered structure, and keep the same spherical morphologies with precusors. But the core-shell structure need further test and analysis(such as EDS, TEM etc.) to be proved.Electrochemical performance tests show that core-shell structured materials have higher initial charge-discharge capacity and discharge efficiency than homo-materials. |
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