Preparation And Properties Of The Li₂O-TiO₂-P₂O₅ System Ionic Conductor Glass-Ceramics

Solid electrolyte（SSE）can replace the liquid electrolyte in lithium ion battery,and has the advantages of high safety,non-toxic and harmless,and good thermal stability.As NASICON solid electrolytes,the LiTi₂（PO₄）₃ system SSE with high room temperature ionic conductivity,excellent stability,low cost of raw materials and abundant synthesis methods are the most promising solid electrolytes for commercialization.LiTi₂（PO₄）₃ solid electrolyte base powders present poor sintering property after formation,although nano-base powders or hot pressing sintering methods resulted in significant increase in density,high cost and limited production scale.In view of the above problems,the LiTi₂（PO₄）₃ system glass-ceramics solid electrolytes were prepared by bulk crystallization method.LiTi₂（PO₄）₃ system precursor glasses were prepared by adding Si O₂ and B₂O₃ to increase glassforming properties.The effect of Al₂O₃ or Cr₂O₃ replacement of TiO₂ and heat treatment on crystallization behavior and properties of glass-ceramics were studied.Li NO₃is used as a lithium source to control the Ti ion states to increase the glassforming properties the glass.The main research contents and conclusions are as follows:1.Adding 2.87 mol%Si O₂ and 1.44 mol%B₂O₃ on the basis of the chemical formula of the LiTi₂（PO₄）₃ shows good glassforming property.On the basis compose,substituted TiO₂equal to 0-7.18 mol%Al₂O₃ was added.The transition temperature（T_g）and crystallization peak temperature（T_p）of glass gradually decreased with the replacement of TiO₂ by Al₂O₃.The characteristic diffraction peaks of major crystalline phases of glass-ceramics with different heat treatment conditions corresponded with the LiTi₂（PO₄）₃,and Si O₂ were precipitated with the increase of heat treatment temperature and time.With the increase of Al₂O₃ content in glass,crystal doping of different degrees appeared in glass-ceramics.When the Al₂O₃ content is5.38mol%,microcracks occur in the crystallization process of the glass,which leads to the decrease of the volume density.The grain size of the five groups of glass-ceramics fluctuated as Al₂O₃ replaced TiO₂.At 1000℃,spherical crystals grow in the grain gap.The highest ionic conductivity of the glass with 3.59 mol%Al₂O₃ incorporation at Tp+100℃for 1h is 5.43×10^-5S/cm.2.The T_g increases first and then decreases while T_p increases gradually as0-7.18 mol%Cr₂O₃ replaces TiO₂.The characteristic diffraction peaks of the main crystalline phases in five groups of microcrystalline glasses under different heat treatment conditions correspond to the LiTi₂（PO₄）₃,and the crystallinity increases with the increase of heat treatment temperature and duration.With the substitution of Cr₂O₃ for TiO₂,the grain size in the glass-ceramics decreases greatly and then increases gradually,and the grains in the glass-ceramics are connected with each other.With the increase of Cr₂O₃,the contact area between grains gradually increases and the voids gradually decrease.The voids basically disappear in the microcrystalline glass with7.18 mol%Cr₂O₃ content.The highest ionic conductivity of microcrystalline glass with3.59mol%Cr₂O₃ added at T_p+200℃for 1h is 1.31×10^-4S/cm.3.The glass melt with Li NO₃as a lithium source cools into a glass in the air without additional Si O₂ and B₂O₃,due to an increase in Ti⁴⁺in the glass,confirmed by XPS.XRD patterns showed that the diffraction peaks of glass-ceramics were corresponding to standard cards of LiTi₂（PO₄）₃,and Al（PO₄）diffraction peaks were observed in glass-ceramics with crystallization at 1000℃.The grain size increases with the increase of heat treatment temperature,but has no obvious change with the increase of heat treatment duration.The total ionic conductivity of glass-ceramics increases first and then decreases with the increase of heat treatment temperature,and increases with the increase of heat treatment time.The microcrystalline glass treated at 900℃for 4h has the highest ionic conductivity of 1.56×10^-4S/cm.