Syntheses,Characterization And Modification Of Nickel Rich Cathode Materials For Lithium-ion Batteries

Considering the serious environmental issues and energy shortage,many governments around the world have already started to promote the electric vehicles(EVs),thus bringing about the rapid development of EVs.However,the EVs cannot yet compete with fuel cars in the aspects of continuous driving mileage and price due to the limited performance and high costs of power batteries.On the one hand,the battery manufacturing processes should be optimized to increase the energy density and decrease the costs.On the other hand,employing the cathode materials with higher energy densities,such as the Ni-rich cathode materials,is the most important method to increase the energy density of batteries.Therefore,this thesis has systematically investigated the practical issues for the Ni-rich cathode materials,including inferior cycling stability,rate capability and thermal stability,expecting to provide some solutions to their applications in EVs.Chapter 1 briefly introduces the working mechanism and the typical structure of a lithium ion battery.Then a short review about several promising cathode materials,especially for the Ni-rich materials,as well as the anode materials is given.Finally,the research background is elaborated.In Chapter 2,the reagents used in this thesis and the devices and apparatuses for the material synthesis and characterizations are briefly introduced.Then the procedure of making a coin cell and the electrochemical test methods are presented in detail.In Chapter 3,two typical Ni-rich cathode materials,LiNi0.8Co0.15Al0.05O2(NCA)and LiNi0.8Co0.1Mn0.1O2(NCM811),are synthesized by using a thermal polymerization method.The comparative study indicates that it is easier for NCA than for NCM811 to form a good layered-structure under high temperature sintering because the lower average valance of Ni resulting from the existence of Mn4+ causes serious cation mixing in NCM811.And in the charging process,NCM811 suffers a higher volume change of 3.6%than NCA does(1.1%).However,the DSC analysis indicates that both NCA and NCM811 have inferior thermal stability.In Chapter 4,taking NCA as an example of the Ni-rich cathode materials,we investigate the relationships between the rate capability and the Li+/Ni2+ antisite defects by combing experimental results and theoretical calculations.The experimental results indicate that the NCA sample with 2.3%Li+/Ni2+ exchange has the highest Li+ diffusion coefficient and exhibits the best rate capability,and the calculation results reveal that the activation barrier for the diffusion of Li+ ions significantly decreases when there is a minor degree of Li+/Ni2+ antisite defect in the Ni-rich layered oxides.In Chapter 5,we propose a facile pre-oxidization method with Na2S2O8 aqueous solution on the NCA hydroxide precursors.It can effectively eliminate the crystal defects on the surface of NCA particles and form a complete and ordered surface layer.The cycling stability both under 4.3 V and 4.5 V and rate capabilities of treated NCA sample have been significantly improved compared to the pristine one.In Chapter 6,we firstly coat the weakly acidic NH4H2PO4 on the surface of hydroxide precursor Ni0.815Co0.15Al0.035(OH)2 before mixing with Li2CO3.In the following high temperature sintering process,NH4H2PO4 can react with the lithium residues and simultaneously form a uniform Li3PO4 coating layer.The electrochemical properties and thermal stability of this Li3PO4-coated NCA material have been largely improved.In Chapter 7,by taking advantage of the low specific surface energy of acetylene black,a carbon coated cathode material NCM523 has been successfully prepared via a simple milling process.The coated NCM523 exhibits greatly improved rate capability owing to the good electronic conductive network consisting of carbon coating layer and conductive agent in electrode,and enhanced cycling stability because the side reactions with electrode are suppressed by the coating layer.In Chapter 8,we investigate the differences between the fresh NCM523 and deteriorated NCM523 in aspects of morphology,crystal structure and electrochemical properties.Then we study the effects of CO2 and H2O on NCM cathode materials and propose a possible mechanism of the deteriorating process for NCM or NCA when stored in air.What's more,we have proved that a secondly heating treatment for deteriorated NCM by using a high-power microwave oven can effectively and quickly recover its electrochemical properties.In Chapter 9,we give a summary of the originalities and inadequacies in this thesis.Some perspectives and suggestions for the future research on the Ni-rich cathode materials are also given.