Surface Modification Study On Li₂MO_3-xF_2x?M=Si,Sn;0?x<1.5? Compound Modified Lithium-rich Layered Materials

The work mainly studies the electrochemical improvement of Lithium-rich?Li-rich?material via surface modification,and we also do deep exploration on the modification mechanism.Due to the advantages of light quality,no pollution and so on,Lithium ion batteries?LIBs?as the current commercial effective energy storage equipment have been widely applied to the portable electronic devices,and acquire some achievements in hybrid electric vehicles,aerospace,military and other fields.For the traditional LIBs cathode material,the performances such as capacity and efficiency have been difficult to meet the increasing needs of people,so the high energy density and high capacity Li-rich layered materials come into people's horizons.However,there are also some shortcomings of the material:Firstly,the loss of oxygen in the initial charge and discharge process will influence the coulomb efficiency,further lead to irreversible capacity loss;Secondly,the phase transformation during cycling will cause capacity and voltage decay,and then is detrimental to the structure stability;In addition,when the cell charges to high voltage,the surface side reaction and oxidative decomposition of the electrolyte will give rise to several bad impacts on the electrochemical properties.So in order to overcome the above shortcomings,people usually adopt the doping and/or surface coating methods because of the advantage of simple operation,low cost.Many researchers have confirmed the surface modification can improve the structure and thermal stability,and which are mainly due to the coating layer can effectively restrain the side effects between the electrode materials and electrolyte,thus improve the ion diffusion of electrodes on the surface.Therefore,based on materials genome in the first principles,under the guidance of theory of calculation,considering these factors such as structure matching,diffusion channels and conductivity,in order to achieve the rational choice of surface modified materials in LIBs,the materials Li₂SiO₃ and Li₂SnO₃ have similar structure to Li₂MnO₃ are selected to modify the Li-rich material in the paper.The experiment results confirm that the electrochemical performances are indeed improved after modification,and then the surface modification mechanism are further discussed,respectively.After analysis,we attributed the performance improvement to the effect of the interface matching.Subsequently,we study the influence of doping on the interface based on the surface coating.Finally,in order to solve the interface problem more effectively,the integration modification scheme of surface coating and doping is proposed.The main works of the paper are summarized as follows:1.Li₂SiO₃ surface modified Li-rich material is prepared by sol-gel method.Physical characterization shows that the surface coating has no change the crystal structure and morphology compared with the pristine,but the charge and discharge test data indicate that surface coating indeed improve the cycling,structural and thermal stabilities.To explore the function mechanism of the modified layer,we make the immersion aging experiment.Research results show that the modified materials can react with the electrolyte,and generate Lewis acid,then,the acid neutralizes the alkaline insulating impurity on the active material surface;In addition,the dissolution of transition metal ions and ion exchange could expedite the formation of solid solution on the surface,thus improving the electrochemical properties of the materials.2.Li₂SnO₃ as ion-induced compound is surface modified on the Li-rich material.The structure of the Li₂SnO₃?C2/c?has a space group similar to that of the main phase material Li₂MnO₃?C2/m?,so it has inherent advantages in interface lattice matching and interface structure stability.By electrochemical test of different Li₂SnO₃ coating amount,the optimum electrochemical performance is 0.5 wt.%.Physical characterization suggests the surface coating layer didn't change the bulk structure,and electrochemical data indicates the Li₂SnO₃ surface modified material has better cycling capacity.Through the aging experiment,we thought that the Li₂SnO₃ coating layer can protect the material from the corrosion of the electrolyte,and show a good structural stability by inhibiting the dissolution of transition metal ions.In addition,the coating layer could serve as an inorganic additive to enhance the transmission characteristics of lithium ions on the interface.3.On the basis of Li₂SnO₃ surface coating Li-rich material,compared with F-LMNCO materials, in order to further improve the electrochemical performances,we use F doped Li₂SnO₃ as the modification layer to modify the Li-rich material.The electrochemical data show that the F doping Li₂SnO₃ material has a better initial coulomb efficiency,kinetic behaviors,cycling,structure and thermal stability in contrast to the only surface coating and doping.Through the aging experiment,we found that F doping Li₂SnO₃ as surface coating layer is not only protect the material,but also the moderate F doping can ameliorate the transport interface of lithium ions between the surface coating layer and active materials.