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

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[Mn_1.5Ni_0.5]O₄·（1-x）{Li₂Mn O₃·Li(Mn_0.5Ni_0.5)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 Li_1.2Ni_0.2Mn_0.6O₂ and layered-spinel composite cathode material Li_1.1Ni_0.235Mn_0.735Cr_0.03O_2.3.（2） Li Al O₂-coated Li-rich layered cathode material Li_1.2Ni_0.2Mn_0.6O₂ wasIV synthesized by solution method, Li₄Ti₅O₁₂-coated Li_1.2Ni_0.2Mn_0.6O₂ 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 Li_1.2Ni_0.2Mn_0.6O₂ 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 Li_1.2Ni_0.2Mn_0.6O₂ electrode decrease after coating5 wt% Li Al O₂ and 3 wt% Li₄Ti₅O₁₂. In conclusion, the surface coating of Li_1.2Ni_0.2Mn_0.6O₂ with Li Al O₂ or Li₄Ti₅O₁₂ is a beneficial way to improve the electrochemical performances of Li_1.2Ni_0.2Mn_0.6O₂ 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 Li_1.1Ni_0.235Mn_0.735Cr_0.03O_2.3 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 Li_1.1Ni_0.235Mn_0.735Cr_0.03O_2.3 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.