Improving The Electrochemical Performance Of Ternary Cathode Materials By Metal Oxides Surface Coating For Lithium-ion Batteries

Compared with other batteries, lithium ion batteries have advantages of high energy density, long cycle life, slight self-discharge, no memory effect, and green environmental protection. The key to improve the performances of lithium ion batteries is developing appropriate positive electrode materials. In this thesis, single layer and multiple-layer metal oxides are used to coat ternary material LiNi0.5Co0.2Mn0.3O2 (NCM). We investigate the influence of nature of metal oxides, coating content,and number of coatings on the structure, morphology and the electrochemical properties of NCM.Chapter 1 introduces the development history of chemical power sources and lithium ion batteries, as well as the composition, working principle and applications of lithium ion batteries. It also reviews the structure of the ternary material, characteristics, reaction mechanisms and the surface modification researches in literature.In Chapter 2, the experimental reagents, processes and equipments used in this thesis are presented. Making 2032 coin cells, characterization methods and electrochemical analysis are also introduced.In Chapter 3, different-valence metal oxides are chosen to coat on the surface of NCM material with a single layer. Inert metal oxides are mainly considered, such as MgO, Al2O3, TiO2, ZrO2, Li2TiO3, Nb2O5, Ta2O5, MoO3. The influences of different valence metal oxides from+2 to +6 and coating content on the structure, morphology and the electrochemical properties are studied. We find that 1wt% nanoAl2O3?1wt% ZrO2 and 1mol% Li2TiO3 coated materials show excellent cycling performances and rate capabilities. The three coated materials can deliver initial discharge capacities of 170,171.7 and 171.1 mAh/g at 0.1 C in the voltage range of 2.8-4.3V, with capacity retention of 87.8%,87.6% and 88.2% after 100 cycles at 0.5C, respectively, which are compared with 80.7% for the pristine NCM.In Chapter 4, we choose the above three kinds of materials for the combination of different coatings and sequences. Using double layer and multi-layer coatings, we examine the effects of different combination of metal oxide coatings on the electrochemical performances of NCM. Compared with the single layer coating, double layer and multi-layer coatings can make the coating layers more homogeneous. The EDS results show that the dual layer coating is more uniform and smoother than the single layer coating. Electrochemical tests show that the double-layer 0.5 mol% Li2TiO3 coated NCM can deliver an initial discharge capacity of 180 mAh/g at 0.1 C in the voltage range of 2.8-4.3V, with capacity retention of 94.2% after 80 cycles at 0.5C. The enhanced electrochemical performance is attributed to the stable dual layer Li2TiO3 that serves as a 3D-diffusion pathway for Li ions and a protective layer that prevents side reactions between electrode and electrolyte. Thus, the double-layer coating by Li:TiO3 is a promising method to improve rate capability and cycle performance of NCM.Finally, in Chapter 5, we summarize the innovation and deficiencies of this thesis., and some prospects of the future research are given.