| After years of development and improvement,LiNixCoyMn1-x-yO2(NCM)materials have become the important cathode materials for power lithium-ion batteries due to its excellent comprehensive electrochemical performance and relatively low cost.However,with the rapid development of the demand for electric vehicles,people have higher requirements for the power/energy density of lithium-ion batteries,and NCM materials are constantly developing in the direction of high voltage and high nickel,but the problem of performance degradation also increases such as high voltage resistance,air storage stability,thermal stability and cycle stability,these are the key issues that NCM materials must face in practical use.When working at high voltage,in addition to the performance degradation of the NCM materials,the reaction at the electrode/electrolyte interface will also directly affect the overall electrochemical performance of the battery.For storage performance,residual alkali and adsorbed impurities will grow on the surface of the NCM materials due to the direct contact with atmospheric environment.The thermal stability of NCM materials has always been a topic of concern,which is mainly caused by the weakening of the oxygen binding ability of the materials in the delithiation state and the occurrence of oxygen release and other behaviors.The cycle performance is an important indicator of battery life and economy.The decline of the cycle performance of NCM materials is mainly related to properties such as crystal reconstruction during cycling.It can be seen that many problems of NCM materials are closely related to the properties of surface and bulk.Therefore,this thesis will combine the surface and bulk regulation to optimize the surface contact and enhance the structural stability of the material to improve comprehensive properties of high-nickel NCM materials.The main research contents of this thesis are as follows:(1)High voltage electrolyte for high specific energy NCM811/Si@C system.As people’s requirements for energy and power density of lithium-ion electric vehicles continue to increase,the working voltage of lithium-ion power batteries with NCM materials is also increasing.Although increasing the cut-off voltage of the battery can greatly improve its specific capacity,it will often bring some negative effects.Among them,the oxidative decomposition of the electrolyte under high voltage is a very serious problem,which will bring about serious degradation of battery performance.The Chapter 3 of this thesis introduces the combination of the advantages of each solvent through theoretical calculations to prepare an electrolyte that can effectively improve the high-voltage performance of the NCM811-Si@C system battery.Fluoroethylpropyl ether(HFE)and fluoroethylene carbonate(FEC)are used as components to improve the oxidation resistance of the electrolyte,and at the same time,the good film-forming properties of FEC can improve the compatibility of the electrolyte with the Si@C anode.In addition,sulfolane(SL)was chosen as the cathode film-forming additive to form a stable passivation layer on the cathode surface,and dimethyl carbonate(DMC)was added to reduce the viscosity of the entire electrolyte.The electrolyte based on the above design can effectively improve the battery capacity by protecting the positive and negative electrodes,respectively,suppressing the structural damage of the electrode material under high voltage,thereby greatly improving the electrochemical performance of the NCM811-Si@C system under high voltage.(2)Pre-blending of conductive agent to improve the storage performance of NCM materials.During the period from the preparation of NCM materials to the time they are put into use,exposure to the atmospheric environment is inevitable.However,we know from previous studies that high nickel NCM materials are very susceptible to moisture and carbon dioxide in the air when stored in the atmosphere.Therefore,it is particularly necessary to improve the storage performance of NCM materials.Although predecessors have also modified the storage performance by means of water washing,calcination or coating,most of these methods will destroy the material structure,or introduce other components to reduce the capacity advantage of NCM materials,and bring more tedious process steps.In the Chapter 4 of this thesis,by pre-blending the conductive agent with the NCM materials,the fine particles of the conductive agent are used to fill the gaps of the NCM materials,and supplemented by calcination at a certain temperature,the binding force between the conductive agent and the materials is enhanced,thereby reducing the formation of surface impurities during storage of the NCM materials.This method is simple,effective,easy to implement,and does not introduce other components,which can well suppress the deterioration of NCM materials during storage.(3)Research on storage performance of single-crystal and polycrystalline NCM materials.By studying the storage performance of NCM materials,we found that in the process of storage of polycrystalline NCM,impurities always grow first in the gaps of primary particles.As single-crystal NCM has gradually entered people’s field of vision due to its excellent cycle performance,its structure with only primary particles also means that its storage performance may also be different from that of polycrystalline materials.In the Chapter 5 of this thesis,two kinds of high-nickel NCM materials LiNi0.8Co0.1Mn0.1O2 and LiNi0.6Co0.2Mn0.2O2 are used as the research objects to study the effects of single-crystal and polycrystalline structures on the storage properties of the NCM materials.This chapter found that the single-crystal high nickel NCM materials have better storage performance than the polycrystalline NCM materials.Under the same storage conditions,the number of impurities generated on the singlecrystal materials are significantly fewer,and the electrochemical performance can be better maintained.And after the polycrystalline materials are stored,the impurities will enter the materials along the gap of the primary particles,while the internal of singlecrystal materials will not be invaded by the impurities after storage.(4)Single-crystal structure helps improve the thermal stability of NCM materials.Although single-crystal materials show better performance than polycrystalline materials in many aspects such as cycle and storage performance,the poor thermal stability of NCM materials has always been an obvious disadvantage.Therefore,it is necessary to study the safety difference between single-crystal and polycrystalline NCM materials.In the Chapter 6 of this thesis,high-nickel LiNi0.8Co0.1Mn0.1O2 and LiNi0.6Co0.2Mn0.2O2 NCM materials are also used as research objects to study the heat generation behavior during battery cycling and thermal stability of delithiated materials at high temperature.The study of thermal properties mainly includes the heat generated during the cycle and the thermal stability of the two charged state materials at high temperatures,and the thermal properties of single-crystal NCM materials are significantly better than those of poly crystalline materials.During the cycling process,due to the existence of internal crystal gaps,polycrystalline materials have higher lithium-ion transport impedance and lower ion diffusion coefficients,resulting in significantly higher heat production during cycling.At the same time,due to the existence of crystal gaps,the inhomogeneity during the charging and discharging process of polycrystalline materials is significantly higher than that of single-crystal materials,and the degree of delithiation in some areas of the materials is higher than the average degree of delithiation,the structural damage in these regions is more serious,and the binding of oxygen is weakened,which becomes the triggering factor for the polycrystalline material to release oxygen in advance at high temperature,thereby reducing the safety performance of the polycrystalline material.(5)Synthesis and mechanism exploration of modified single-crystal NCM materials.It can be seen from the research in the previous chapters that the comprehensive properties of single-crystal NCM materials are obviously better than that of polycrystalline ternary materials.However,the current marketization of single-crystal ternary is low,mainly because the current preparation process of single-crystal NCM is not perfect,resulting in its irregular morphology and uneven particle size,which affects the performance of single-crystal NCM to a certain extent.In the Chapter 7 of this thesis,single-crystal NCM materials with uniform morphology and size were prepared by the molten salt method.Their electrochemical performance and safety performance were further improved by element doping.Doping modification can enhance the structural stability,increase Li+transport channels,reduce lattice slippage of single-crystal materials at high voltages,and inhibit side reactions at the interface between materials and electrolytes.At the same time,a high-nickel NCM lithium-ion battery system with significantly improved comprehensive performance is obtained by combining the modified single-crystal NCM material with the high-voltage-resistant electrolyte. |
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