Microstructure Design And Electrochemical Performances Of Sn-/Si-C Based Composites As Anodes For Li Ion Batteries

Lithium ion batteries are widely used in portable electronic devices such as cell phones and computers.With the rapid development of electric vehicles and smart grids in recent years,the market now has higher requirements for the safety,energy density,cycle life and power density of lithium ion batteries.At present,lithium ion batteries are limited by the theoretical capacity of the commercial electrode materials and their performances cannot fully meet the market demand.Therefore,designing and developing new anode materials with high capacity,long cycle life and high safety are of great importance as promising methods to fully improve the performance of lithium ion batteries.Among the new anode materials,tin-and silicon-based materials are extremely promising for their high theoretical specific capacity,high volume specific capacity,moderate lithiation/delithiation potential,and abundant resources.However,these anode materials also suffer from problems such as poor cycle stability and low initial Coulombic efficiency,which severely limit their applications.Carbon material,on the other hand,has high elastic mechanical properties and can thus be potentially used to improve the cycle stability of tin-and silicon-based anode materials.In this thesis,novel tin-and silicon-carbon based micro-nanosized composite materials were successfully prepared by ball milling and chemical synthesis.The application of the discharge plasma-assisted ball milling（P-milling）was explored to prepare tin-based and oxide composite materials for the first time.During the P-milling,the hard oxide materials served as grinding aids to refine the soft tin-based material.The spherical composites were then obtained with the microstructures of the nanosized tin-and silicon-based particles coated by the thin graphite flakes.Their electrochemical performances were tested as anodes for lithium ion batteries.In addition,the interactions between the various components during the electrochemical processes and the stability of the interface between the anode and the electrolyte were studied.The main achievements are as follows:The Sn-Fe₃O₄@C composite was prepared in a two-step P-milling process.During the first milling step,the hard Fe₃O₄ particles acted as grinding aids to refine the Sn particles.Sn-Fe₃O₄nanocomposite with a uniform microstructure was obtained,which was then dispersed in graphite matrix by a second P-milling step to obtain a spherical Sn-Fe₃O₄@C composite.The influence of microstructure on the electrochemical performance of the Sn-Fe₃O₄@C composite electrode was investigated.In addition,the Sn-Fe₃O₄@C composite exhibited good cycling performance.When tested at a current density of 2 A g^-1 and a voltage window of 0.01-3 V,the Sn-Fe₃O₄@C composite anode could achieve a specific capacity of about 750 m Ah g^-1 even after 500 cycles.The interface stability of the Sn-Fe₃O₄@C electrode-electrolyte was then studied by characterizing the electrochemical curve,observing the interface morphology between the composite anode and the electrolyte,and analyzing the evolution of the chemical composition during the electrochemical reactions.It was found that the transition metal Fe triggered the decomposition of SEI at high potentials.This process provided additional capacity for the battery system,while causing instability of the electrode-electrolyte interface.In order to stabilize the electrode-electrolyte interface,Fe₃O₄ was replaced with SiO_x that has a lower reaction potential.The Sn₂Fe@SiO_x composite was prepared by the planetary ball milling method.During the ball milling,the hard SiO_x particles served as the grinding aids to refine Sn and Fe particles.Thus,the alloy reaction between Sn and Fe particles could be easily proceeded.The final product was the Sn₂Fe@SiO_xcomposite,with the Sn₂Fe nanoparticles well-dispersed in the SiO_x matrix.It was found that the reversibility of the SiO_x conversion reaction was improved by the incorporation of Sn₂Fe based on the electrochemical tests and ex situ microstructural measurments.As a result,the Sn₂Fe@SiO_x composite anodes showed excellent electrochemical performance.When tested at a current density of 1000 m A g^-1 and a voltage window of 0.01-2.5 V,it could provide a discharge specific capacity of about 600 m Ah g^-1 even after 1200 cycles,which corresponded to a capacity retention rate of about 88%.The SiO-Sn₂Fe@C composites were further prepared by the P-milling method,where the Sn₂Fe@SiO_x composite was combined with graphite.A full cell was further assembled with Li Co O₂ as cathode.When tested within a voltage window of 2.7-3.9 V,it could still retain a discharge specific capacity of 520 m Ah g^-1 after 100 cycles.Finally,the Sn_0.95Fe_0.05O_2-δcomposites were prepared by chemical synthesis.The Fe-doped Sn O₂ nanoparticles were firstly capped with polymer chains,which were carbonized during a following heat treatment.The resulting product was carbon-coated ultrafine Sn_0.95Fe_0.05O_2-δparticles.Coulombic efficiency and the electrochemical performance measured in a high-temperature environment showed that carbon coating effectively stabilizes the electrode-electrolyte interface of Sn_0.95Fe_0.05O_2-δand reduced the lithium ion loss in the battery system.As a result,the Sn_0.95Fe_0.05O_2-δhad excellent electrochemical performance as the anode material.It could provide a discharge specific capacity of 1041.7 m Ah g^-1 even at the 100^thcycle,which corresponded to a high capacity retention rate of 91.3%.Overall,the oxide materials assisted the tin-based materials to rapidly form and refine,and novel tin-and silicon-carbon composites were efficiently prepared by the P-milling.These composite anode materials were with highly stable microstrucure and had excellent electrochemical performance.This kind of composite material is highly designable and easy to fabricate and the processing method is straightforward and controllable,which make it desirable when considering mass productions.In addition,the mechanism of the electrochemical reactions of the composite anode material was studied in depth,which is beneficial to both their further optimization and exploration in the anode materials with similar microstructure.