Study Of Si/oxide Composite Negative Electrode Interface Effects And Strategies For Realizing Si-based Thick Films

Silicon（Si）is considered as a promising negative electrode material for lithium-ion batteries（LIBs）with the advantages of ultra-high theoretical specific capacity,low lithiation potential and abundant natural reserves.However,the structural instability caused by its huge volumetric change during the electrochemical process severely restricts the practical applications of Si-based negative electrodes.In order to mitigate the volumetric expansion problem,researchers have proposed various solutions.One of the common solutions is to compound a layer of other materials on its surface to buffer the volume change,and the commonly used buffer materials are carbon and various oxides such as TiO₂,Si O₂ and Al₂O₃.Another common solution is to reserve space for the volume expansion of Si,such as in core-shell and porous structures.Thin film electrodes are widely used in consumer electronic devices and aerospace,due to the feasible and controllable preparation,environmental friendliness,high compactness and good reproducibility.The use of Si as the main active material in thin film electrodes still suffers from the problem of large volumetric expansion,and the use of buffer materials and porous structure design can effectively mitigate the negative effect of the huge volume change.However,on the one hand,it is still unclear which oxide combined with Si has a more stable electrode structure and more excellent electrochemical performance,and how the interfacial effect affects the electrochemical performance;on the other hand,the current thickness of Si-based thin film electrodes is too thin to meet the applied requirements for high-energy-density energy storage devices,and contradictorily an incease in film thickness might cause structural instability,and therefore it is necessary to explore an effective method to design thicker Si-based film electrodes.In addition,the lower electronic conductivity of Si as a semiconductor material affects its electrochemical properties.To address the above issues,the following three researches were conducted in this thesis:（1）In order to alleviate the volume expansion problem of Si,Kirkendall effect was utilized to design and prepare three different porous multilayer film composite electrodes of Si/TiO₂,Si/Si O₂ and Si/Al₂O₃ using TiO₂,Al₂O₃ and Si O₂ as the buffer layer of Si.A contrast study of the structural and electrochemical properties of the three types of electrodes demonstrated that the composite system of Si/TiO₂ produced a most satisfactory electrode performance.It showed that a specific heterogeneous interface was formed at the interface of the two phases and under the heat treatment condition of350°C it achieved good cycling stability,the lowest charge transfer resistance and the highest lithium ion diffusion rate.（2）Retaining structural stability in thick film electrodes during electrochemical cycling is considered as a big challenge.In order to overcome this challenge,composite electrode of Si/TiO₂/Cu was designed and the electrode thickness was extended to be 1μm,1.5μm and 2μm based on the synergistic effect of Kirkendal effect and Cu metal coating.It was found that,under appropariate heat treatments,due to the diffusion of Si from the inner to the surface of the film cause by the Kirkendal effect,larger pores could be created in thicker films,which on the one hand provided space for the volumetric expansion of Si and on the other hand facilitated the lithium ions transport.The electrochemical measurements showed that such electrode architecture possessed increased areal capacity,improved cyclic stability,boosted lithium ions diffusion coefficient and smaller charge transfer resistance.Furthermore,the Cu coating showed beneficial effects on improving electron transport and the structure stability.It enhanced electronic conductivity obviously,helped form more stable solid electrolyte interface（SEI）,reduced side reactions at the electrode/electrolyte interfaces,and promoted the initial coulombic efficiency and the electrode structural stability.（3）In order to investigate the effects of different metal coatings,a nano layer of Cu,Ti,Ag and W was sputtered on the surface of the electrode.It was found that Cu coating is more favorable in stabilizing the electrode structure and achieving better electrochemical performance due to its good ductility.