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Study On The Preparation And Properties Of Fast Ion Conductor Coated Lithium Rich Layered And Layered-spinel Composite Cathode Material

Posted on:2017-01-09Degree:MasterType:Thesis
Country:ChinaCandidate:Q L WangFull Text:PDF
GTID:2271330503464060Subject:Metallurgical engineering
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Li-ion batteries are extensively used in electric tools and electronic devices due to their merits like high energy density, low weight, low pollution, long life, etc. The lithium-ion battery with specific energy is demanded by the development of electronic products, especially the development and application of electric vehicles, but the traditional cathode material is difficult to meet the needs of high energy density for the Li-ion battery cathode materials, so the exploration and development of cathode materials with higher specific discharge capacity are great important. The Li-rich layered solid-solution cathode material x Li2 Mn O3·(1-x)Li MO2(0<x<1,M=Co、Ni、Cr 、 Ni1/2Mn1/2, etc.) and layered-spinel composite cathode material x Li[Mn1.5Ni0.5]O4·(1-x){Li2Mn O3·Li(Mn0.5Ni0.5)O2} are the new cathode materials developed in recent years. Compared with the conventional cathode material Li Co O2,it has the ability to deliver capacities of 250 m Ahg-1 or more, lower cost and better safety. Despite of all these advantages, there are several issues which restrict the further development of this material, such as large initial irreversible capacity,capacity fading during cycling, poor rate capability, etc. So it needs to be improved to meet the ever-increasing demand of high-power and high capacity for electric vehicles.In this thesis, fast ionic conductor Li Al O2 and Li4Ti5O12 were used to improve the electrochemical performance of cathode material. The morphology, structure and electrochemical properties of the prepared material were studied by X-ray diffraction,scanning electron microscopy, transmission electron microscopy, impedance test, and charge/discharge test of the synthetic material. The main parts of work are shown as follows:(1) Based on the study of research group, the research object of this paper are Li-rich layered solid-solution cathode material Li1.2Ni0.2Mn0.6O2 and layered-spinel composite cathode material Li1.1Ni0.235Mn0.735Cr0.03O2.3.(2) Li Al O2-coated Li-rich layered cathode material Li1.2Ni0.2Mn0.6O2 wasIV synthesized by solution method, Li4Ti5O12-coated Li1.2Ni0.2Mn0.6O2 was synthesized by sol-gel method. The XRD、SEM and TEM results show that the Li Al O2 and Li4Ti5O12 particles have been coated on the surface of Li1.2Ni0.2Mn0.6O2 material without any crystal structural change. The electrochemical performance results indicate that the 5 wt% Li Al O2-coated and 3 wt% Li4Ti5O12-coated sample have the best electrochemical performance. For 5 wt% Li Al O2-coated sample, the initial discharge capacity is 254.6 m Ah/g, the coulomb efficiency is 75.2%, the capacity retention is 98.7% after 50 cycles, and its discharge capacity is still 110.8 m Ah/g at10 C. For 3 wt% Li4Ti5O12-coated sample, the first discharge specific capacity is258.4 m Ah/g, the coulomb efficiency is 82.1%, the capacity retaining ratio is 98.7%after 50 cycles, the discharge capacity is 110.8 m Ah/g at 10 C. EIS results show that the charge transfer impedance of Li1.2Ni0.2Mn0.6O2 electrode decrease after coating5 wt% Li Al O2 and 3 wt% Li4Ti5O12. In conclusion, the surface coating of Li1.2Ni0.2Mn0.6O2 with Li Al O2 or Li4Ti5O12 is a beneficial way to improve the electrochemical performances of Li1.2Ni0.2Mn0.6O2 In addition, by the contrast analysis,the coating effect of Li4Ti5O12 is better than Li Al O2.(3) Based on the study of research group, the layered-spinel composite cathode material Li1.1Ni0.235Mn0.735Cr0.03O2.3 was synthesized by co-precipitation-calcination and coated with Li4Ti5O12(1, 3, 5 wt%) by sol-gel method. The results show that Li4Ti5O12 has only modified the surface of the active material without changing the crystal structure. The 3 wt% Li4Ti5O12-coated sample has the best electrochemical performance. The first discharge specific capacity can reach 260.4 m Ah/g, the coulomb efficiency can reach 82.1%, the capacity retaining ratio is 99.9% after 50 cycles, the discharge capacity still can reach 127.7 m Ah/g at 10 C, so the rate performance is improved greatly. EIS results show that the coated sample has the lowest charge transfer impedance.Those results suggest that Li4Ti5O12 surface coating can probably be an effective way in improving the electrochemical performances of Li1.1Ni0.235Mn0.735Cr0.03O2.3 cathode material. The improved electrochemical performance can be attributed to that the surface modification of Li4Ti5O12 can obstruct the direct contact and suppress the side reactions between the electrode andelectrolyte, which also can facilitate Li+ diffusion through the interface.
Keywords/Search Tags:lithium-ion battery, cathode material, fast ionic conductor, LiAlO2, Li4Ti5O12, Li1.2Ni0.2Mn0.6O2, Li1.1Ni0.235Mn0.735Cr0.03O2.3, surface modification
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