Structure Design And Dynamic Mechanism Of Silicon-based Anode Materials Based On TiO₂/C Hybrid Coating

With the rapid development of nowadays society,especially the rapid upgrading of portable electronic devices and large energy storage devices,the capacity,safety and stability of lithium ion batteries are increasingly required.Among the newly developed anode materials for lithium ion batteries,the silicon-based materials with higher theoretical specific capacity(4200 m A h g^-1),appropriate lithiation/delithiation potential(?0.4V vs.Li/Li⁺),wide range of sources,low prices and environmentally friendly,are considered as a good substitute for graphite.Neverthless,the practical applications of silicon-based negative electrodes are hampered by problems such as significant volume expansion during repeated lithiation/delithiation processes,low conductivity,and the continuous destruction and generation of solid electrolyte interfaces(SEI).In this paper,the TiO₂/carbon hybrid coating modified silicon-based material is formed by introducing TiO₂,and the electrochemical performance of silicon-based material is improved by adjusting TiO₂ content and crystal state.Meanwhile,we have adoped a unique electrochemical reaction visualizing confocal system(ECCS)and the changes of whole electrode material during charging/discharging process can be directly observed.Thus,the volume expansion and shrinkage of silicon anode are analyzed quantitatively and the mechanism of inhibiting the volume expansion of silicon anode with different TiO₂ content and morphology is expounded.The research contents and results of this paper are mainly divided into following aspects:(1)The improvement of electrochemical performance for silicon/carbon anode materials through the optimization of carbon coating structure in silicon/carbon composites.There are inherent differential conductivity problems in silicon-based materials.Although the carbon coating commonly used improves the conductivity of composite materials,its surface defects and disordered structure hinder the performance of conductivity,which is not conducive to interface stability.Thus,with the micron SiO as active material,coal tar pitch as the carbon source and tetrabutyl titanate as the titanium source,the electrochemical performance of SiO@C composite is enhanced by constructing ordered carbon coating on the surface of micron SiO particles to improve the quality of carbon layer.The research shows that a small amount of TiO₂ can effectively improve the order of carbon coating,reduce the surface defects of carbon layer,and improve the electrical conductivity of silicon/carbon material and the interface stability with electrolyte.The TC-SiO supported with 3%TiO₂ displays significant advantages in cyclic stability,high rate charge/discharge and interface stability compared with C-SiO with disordered carbon coating.And it lays a foundation for further applied research of silicon/carbon anode materials.(2)The combination of high strength mixed coating with internal buffer space improves the long-term cycling stability of micron SiO_x materials.Although it has been proved that a small amount of TiO₂ can improve the order degree of carbon layer and thus improve the conductivity of electrode material,the cyclic stability of SiO anode material still needs to be improved.Therefore,the new type of anode material p-SiO_x@0.3TiO₂@C with internal buffer space and external 200-400 nm thick mixed coating of TiO₂/C is synthesized by utilizing micron SiO_x as the active material,coal tar pitch as the carbon source and tetrabutyl titanate as the titanium source.The internal channels and the rigid TiO₂/C mixed coating buffer the bulk expansion of silicon more effectively,while the TiO₂/C mixed coating has high Li ion/electron transport capability.The electrochemical test and ECCS results show that the p-SiO_x@0.3TiO₂@C anode materials exhibit a volume expansion of only 37%with the capacity of 1006.2 m A h g^-1 in first lithium insertion process.After 100 cycles,compared with capacity of the fifth cycle,the capacity decay of p-SiO_x@0.3TiO₂@C anode materials is 7.83%,reveals a stable cycle life.(3)Inhibition of volume expansion of nano-silicon-based silicon/carbon composites by mixed coating.Based on the above excellent performance of carbon coated silicon oxygen anode materials improved by TiO₂,the TiO₂ modification was further extended to the widely used elemental silicon materials.The silicon-based anode Si@a TiO₂@G is formed by using nano-silicon as the active material and constructing graphene/TiO₂ composite coating bonded by C-O bond on the surface.The amorphous TiO₂ inner shell with high mechanical strength can not only prevent the electrode material structure from breaking,but also is conducive to the rapid transmission of lithium ions.The graphene coating has great electronic conductivity,and promotes the formation of more stable SEI.Meanwhile,the C-O bond between amorphous TiO₂ and graphene can prevent the slippage of graphene layer,thus stabilizing the composite structure.The Si@a TiO₂@G material has a lower irreversible volume expansion compared with pure Si materials.After first charge and discharge cycle,the expansion and contraction of electrode reach a balance,forming a stable electrode structure and extending the cycle life of battery.It further proves the important role of TiO₂/carbon hybrid coating on silicon-based materials.(4)The formation and growth mechanism of SEI on silicon anode materials with different coating structures are studied.With pure nanometer Si materials,graphene-coated nanometer Si materials and amorphous titanium dioxide coated nanometer Si materials as research objects,the influence of different surface layers on SEI formation is investigated by ex situ X-ray photoelectron spectroscopy(XPS)and operando attenuated total reflection-Fourier transform infrared(ATR-FTIR).For pure nanometer Si materials,the SEI formed on its natural SiO₂ surface layer is destroyed by huge volume expansion stress due to its poor inhibitory ability.In addition,due to the difference in intrinsic conductivity,the graphene coating causes FEC to decompose into Li F.While the oxide coating generates CHFOCO₂Li-CH₂OCO₂Li that will further decompose,so the initial SEI generated by graphene coating is more stable.