Research On Crystallization And Electrochemical Performance Of TiO₂-SiO₂ System Glasses

Oxide glasses are considered as promising electrode material for lithium-ion batteries owing to their open network structures and no grain boundaries.This paper mainly focuses on the crystallization behavior and electrochemical properties of Ti O₂-Si O₂ glass system.Firstly,the influence of the structure of Sb₂O₃-based glasses with different components on their electrochemical properties was studied.Then,the stable Ti O₂-Si O₂ system glass was selected as the anode for Li-ion batterys.The change of electrochemical properties was studied by changing the composition and heat treatment.The main contents of the thesis are as follows:（1）A series of Sb₂O₃-based glasses were prepared by the melt-cooling method.The influence of components on the structure were studied by DSC,XRD and FTIR.The effects of structure change and crystallization on the electrochemical properties of Sb₂O₃ based glass anode materials were compared by charge discharge cycle and EIS.The results show that the glass network of 70Sb₂O₃-30[Sb（PO₃）₃]_n is poor and its crystallization trend is strong.Despite it has high discharge/charge capacity,the cycle stability is extremely poor.However,the cycling stability of the sample which crystallized by heat treatment has a few improved.When Na₂O and B₂O₃ are introduced,the fragility index（m）decreases,the viscosity（η）increases,the content of bridge oxygen in the glass network increases,and the cycle stability of the anode is greatly enhanced,but it has the lowest capacity.Further introducing Ti O₂ into the sample,the glass structure continued to be strengthened,and the cycling stability and capacity of the anode was further improved.The glass transition temperature（T_g）of the sample introduced with Silicate glass（Float glass）is significantly higher than the above three samples,it means that the glass network structure is more stable,and it has the best electrochemical performance.By studying the structure and electrochemical properties of Sb₂O₃ based glass,we decided to take the more stable Ti O₂-Si O₂ system glass as the further research target.（2）40BaO-20TiO₂-40SiO₂glass was prepared by melt-cooling method.The influence of composition on glass structure and crystallization was studied by doping Cu O and Sn O₂,and their electrochemical properties were further studied.The results show that with the introduction of Cu O the glass transition temperature（T_g）decreases,the density decreases,the oxygen packing density decreases,and the glass network was less packed.The introduction of Cu O also promoted the crystallization of this glass,and the preferential growth of Ba₂Ti Si O₈ on the surface appeared.With the introduction of Cu O the glass transition temperature（T_g）increases,the density increases,the oxygen packing density increases,and the glass network structure becomes dense.The introduction of Sn O₂ also promoted the crystallization of this glass,but no preferential growth of Ba₂Ti Si O₈ on the surface occurred.It is found that the residual glass phase in the glass-ceramics has some influence on the electrochemical properties.Among them,the electrochemical performance of Sn O₂ containing anode is improved slightly,while the glass phase in Cu O containing anode improves its conductivity and lithium ion transfer rate,and its first coulomb efficiency and capacity are enhanced.（3）46.33SiO₂-9.30B₂O₃-13.00Li₂O-10.30K₂O-21.07Ti O₂ glass was prepared by melt-cooling method.The crystallization mechanism was studied by XRD and DSC.The morphology and structure were characterized by SEM,TEM and HRTEM.The results show that with the increase of heat treatment temperature,the crystals precipitated in the glass matrix gradually change from anatase to rutile to Li₂Ti Si O₅.Li₂Ti Si O₅ glass-ceramic shows layered structure,and the Li₂Ti Si O₅ nanocrystals are uniformly dispersed in the glass matrix.The electrochemical performance test shows that the Li₂Ti Si O₅ glass-ceramic anode exhibits higher capacity and cycle stability than the pure Li₂Ti Si O₅ crystal anode prepared by solvothermal-sintering method.At the current density of 1 A g^-1,the capacity is maintained at about 260 m Ah g^-1 after 1500cycles.Even at the current density of 5 A g^-1,the capacity of 180 m Ah g^-1 can be maintained after 5000 cycles and the crystal structure was not destroyed in the HRTEM photos.The excellent electrochemical performance of Li₂Ti Si O₅glass-ceramic anode is attributed to its unique layered structure,which provides enough space for Li⁺storage.At the same time,the open network of glass matrix can not only provide Li⁺diffusion path,but also greatly relieve the stress caused by volume change during discharging/charging.