Growth of Li6.75La3Zr1.75Nb0.25O12 Crystals in Molten Li3BO3 Glass and Their Electrochemical Characteristics As a Hybrid Solid Electrolyte By Using Electrochemical Impedance Spectroscopy
Authors: Kaneko Sakina , Yamashita Miho , Zettsu Nobuyuki , Teshima Katsuya 
Published: February 2020

INTRODUCTION

Solid state fast lithium ion conductors are believed to be potential candidates to replace the organic liquid electrolytes, leading the improvement of the energy density and safety of next generation lithium ion secondary batteries. Among various approaches on the preparation of all-solid-state battery architectures were demonstrated, total lithium ion conductivity was found to be more than three orders of magnitude higher, comparing to commercialized lithium ion secondary battery using organic liquid electrolytes. Main difficult issue for the enhancement of the lithium ion conductivity is the formation of atomically-connected tight solid-solid interfaces. On the basis of these backgrounds, we propose hybrid electrolyte systems composed of Li6.75La3Zr1.75Nb0.25O12 (LLZN) having lithium ion conductivity of 8.0×10-4 S·cm-1 and Li3BO3 (LBO) with ~10-6 S·cm-1 for achieving good interfacial connection and high lithium ion transportation. To enhance sintering process for tight interfacial contact and efficient Li+ transportation, we expect that LBO would be a promising material as a liquid-state additive at high temperature and Li+ conductive glass matrix at low temperature. In this work, we systematically investigate the effects of mixing ratio on the densification and the lithium ion conductivity of the LLZN-LBO hybrid electrolytes and the reactivity with LiCoO2crystals at 1000°C to fabricate composite electrodes for all-solid-state lithium ion battery.

EXPERIMENTAL

Grinded powders of La2O3，ZrO2，Li2O and Nb2O5 were mixed to be stoichiometric composition of Li6.75La3Zr1.75Nb0.25O12, to which the LBO powder as a flux was added. The solute concentration was adjusted to 25, 50 and 75 mol%. The samples were heated at 900°C for 20 h. The crucible was subsequently cooled to 400°C at a rate of 200°C·h–1 using a cooling program. The products were pulverized and heated again at 1000°C for 30 min after pelleting. Electrical conductivity measurements of the sintered pellets were performed using Li+ blocking Au-electrodes with area of 0.5 cm2in the frequency range from 7 MHz to 0.1 Hz using an impedance analyzer at room temperature.

RESULTS and DISCUSSION

The all sintered pellets formed garnet structure crystallized into cubic lattice. Any other diffraction patterns were not observed, indicating that LBO formed amorphous phase. Fig.1 summarized the effects of LLZN composition ratio on the relative density and total ion conductivity in the LLZN-LBO hybrid electrolyte. Relative density became larger as the reduction of LLZN concentration. Cross-sectional SEM observation showed that the LLZN crystals grew and condensed into secondary particles after heating procedure. Furthermore, the density of the sintered pellet relative to the theoretical density, determined from the weight and physical dimensions, maximally reached at 85.4%. A typical Nyquist plot is composed of two semicircles and a tail. The separate contributions from the grain and grain boundary could be distinguished. Total bulk Li+ conductivity of the LLZN-LBO hybrid electrolytes was calculated from the inverse of the resistivity derived from the intercepts of the high frequency semicircles with the real axis at approximately 500 kHz and those of grain-boundary at approximately 1 kHz. The tail in the low frequency range arises from the ionic blocking electrode. Bulk conductivity tended to be higher as increase the LLZN concentration, however, that of grain-boundary showed opposite trends. Total conductivity reached showed 4.69×10-5 S·cm-1 at the maximum in the highly sintered LLZN (25 mol%) - LBO (75 mol%) hybrids. As comparing to the sample sintered at 900°C with same composition ratio, the total conductivity was enhanced two order magnitudes while the relative density is comparable. These all results strongly implies that the LBO helped to enhance the relative density through liquid phase sintering, leading to enhance Li+ion conductivity at grain boundary.
Finally, we performed the fabrication of composite electrodes composed of 50 vol% of LiCoO2 (LCO) and 50 vol% of LLZN (25 mol%) - LBO (75 mol%) hybrids. The XRD profiles of the mixture sintered at 1000oC for 10min is consist with those of LCO and LLZN. No other any diffraction lines were observed, LBO formed amorphous and no subphase through solid state reaction with LCO and LLZN were formed. In previous reports, LLZN reacted with LCO at the interface at above 700°C, LBO also play a key role for protect layer for the subphase formation. The relative density was 85%, and total conductivity was 1.0 ×10-4 S·cm-1 at room temperature.

Figure 1