Electrochemical Characteristics of the Composite Electrodes of Li4Ti5O12 Crystals/Li3BO3 Glass and Li4Ti5O12 Crystals/Li3BO3-Li6.75La3Zr1.75Nb0.25O12 Hybrid Solid Electrolyte System for All-Solid-State Lithium-Ion Secondary Battery

Abstract

Introduction All-solid-state lithium ion rechargeable batteries (LIBs) have been desired for higher energy density, long term durability and safety. A great challenge of all-solid-state LIBs for practical use is reduction of the interfacial resistance. Effective interface modification techniques have been proposed, however, the interfacial resistance is still large because the charge transfer resistance at the active material/ solid electrolyte interface would be strongly affected by interfacial contact condition. On the basis of these backgrounds, we have proposed a new route to prepare smart interfaces for all-solid-state lithium ion batteries through the growth of highly-faceted active material crystals in molten Li ion conductive glass as a flux. In facts, we reported the growth of LiCoO2 crystals with well-defined facets from molten Li3BO3 glass. High-resolution TEM and SEM-EDS revealed that the no voids and defects were formed at the interface between LiCoO2 and Li3BO3, suggesting that the homogeneous nucleation and following growth and liquid-phase sintering of the LiCoO2 crystal promoted the formation of highly sintered segregated particle in molten Li3BO3 flux resulted in the formation of tightly connected interface at atomic level with wide dimension. We have further extended this concept to hybrid electrolyte system composed of Li3BO3 and Li6.75La3Zr1.75Nb0.25O12, exhibiting total Li+ conductivity of 4.7×10-5 S·cm-1 at room temperature. And composite electrodes of LiCoO2 crystals/ Li3BO3 glass-Li6.75La3Zr1.75Nb0.25O12 crystals with volumetric ratio of 50 % exhibited 1.1×10-4 S·cm-1 at room temperature. In this work, in order to extend our concept to making anode electrodes, we demonstrate the fabrication of Li4Ti5O12 crystals/ Li3BO3 glass and Li4Ti5O12 crystals/ Li3BO3 glass-Li6.75La3Zr1.75Nb0.25O12 crystals composite electrodes as well as the characterization of their electrochemical properties by using electrochemical impedance spectroscopy. Experimental Anatase-phase TiO2, Li2CO3, and Li3BO3 were used as a starting materials for Li4Ti5O12 crystal growth in Li3BO3 glass matrix. The volume fraction of Li4Ti5O12 crystal was controlled from 50 to 90% to Li3BO3. The mixed powders was heated at 900oC for 10 h. Subsequently the powders were cooled to 100°C under temperature controlled conditions. The products were pulverized and heated again at 900 oC for 10 h after pelleting. The as obtained composites were structurally characterized by XRD, SEM-EDS, XRD, and TEM. Electrical conductivity measurements of the sintered pellets were performed using Li+ blocking Au-electrodes with area of 0.5 cm2 in the frequency range from 7 MHz to 0.1 Hz using an impedance analyzer at room temperature. Results and Discussions Primary single crystals with well-developed facets with 5 μm diameter were formed from a molten Li3BO3. Powdered XRD patter was good agreement with reference data of ICDD PDF 049-0207, indicating that the crystals formed spinel structure crystallized into Fd-3m cubic system. There are no any other diffraction lines were observed, suggesting that Li3BO3 formed amorphous phase. SEM-EDS revealed the idiomorphic Li4Ti5O12 crystals were homogeneously dispersed in the Li3BO3 glass matrix. Furthermore, Ti element was homogeneously dispersed in the all crystals, indicating that no Li+-rich phase, such as Li2TiO3 phase was formed at the interface. TEM observation revealed that the interface was atomically connected without any sub-phases formation, suggesting that the interfaces potentially provide efficient Li+ transport pathways. We will further discuss the effect of mixing ratio on the densification and the lithium ion conductivity of both Li4Ti5O12 crystals/ Li3BO3 glass and Li4Ti5O12 crystals/ Li3BO3 glass-Li6.75La3Zr1.75Nb0.25O12 crystals composite electrodes at PRiME 2016. Acknowledgments This research is partially supported by JST-ALCA and JST-CREST.