Electrochemical Properties of Novel Pyrochlore-Type Solid Electrolyte
Authors: Onodera Hitoshi , Shimonishi Yuta , Yoshida Shuhei 
Published: December 2024

For Li7La3Zr2O12 (LLZ) and Li1-3x
La
x
TiO3, which have been investigated as electrolytes for oxide solid-state batteries[1.2], we have proposed a pyrochlore-type oxyfluoride Li2-x
La(1+x)/3M2O6F (M = Nb, Ta etc.) with high ionic conductivity[3]. In this study, electrochemical properties of pyrochlore-type solid electrolyte were investigated.
Li2-x
La(1+x)/3Nb2O6F powder was synthesized by a solid-state reaction. The synthesized powder was identified by XRD analysis and Li2-x
La(1+x)/3Nb2O6F pellets were prepared at 1000°C for 6h in order to evaluate their electrochemical properties. Impedance was measured by sputtering Ag electrodes on both surface of the pellets. The activation energy was calculated from the value of ionic conductivity at -80~120°C. Then, the Li symmetrical cell was assembled, and deposition/dissolution cycle of Li was carried out. Separator (Celgard 3501) containing liquid electrolyte (1M LiPF6 in EC/DEC) were introduced at the interface between the Li metal foil and the solid electrolyte (Li | separator | pellet | separator | Li). The test conditions were 25°C and 0.1 mA/cm2, and the cycle was carried out by repeating charging/discharging for 0.5h each. The garnet-type solid electrolyte LLZ, which is to be compared, was synthesized by a solid-state reaction in the same manner as above, and pellets were prepared at 900°C for 10h.
XRD analysis of the synthesized powder confirmed the diffraction pattern from Li2-x
La(1+x)/3Nb2O6F. The relative density of the pellets prepared using this powder was about 84% and LLZ pellets was about 79%. Figure 1 shows Arrhenius and Cole-Cole plots of Li2-x
La(1+x)/3Nb2O6F and LLZ. The total ionic conductivities of Li2-x
La(1+x)/3Nb2O6F were 3×10-5, 5×10-3, and 2×10-2 S/cm at -80°C, 25°C, and 120°C respectively, and the activation energy was 0.20 eV. Li2-x
La(1+x)/3Nb2O6F showed higher ionic conductivity and lower activation energy than LLZ. Figure 2 shows the results of deposition/dissolution cycle of Li. It was observed that Li2-x
La(1+x)/3Nb2O6F has a lower overpotential than LLZ. In addition, the short circuit behavior of LLZ with the increase of overpotential occurred from around 40 cycles, while Li2-x
La(1+x)/3Nb2O6F was stable after 50 cycles. And it was also found that the value of the overpotential at 1cyc almost coincides with the one estimated from the cell resistance obtained by the impedance measurement. These results indicate that the mobile ion of Li2-x
La(1+x)/3Nb2O6F is Li+ and the ionic conductivity is high. The cycle stability of Li2-x
La(1+x)/3Nb2O6F may be due to the high ionic conductivity of Li2-x
La(1+x)/3Nb2O6F, the suppression of current density heterogeneity, and the uniform progress of Li deposition/dissolution. In this presentation, the results of other physical and electrochemical properties are also reported.

References

[1] Y. Inaguma et al,. J. Phys. Chem. 127, 16041 (2023)
[2] R. Murugan et al. Angew. Chemie Int Ed., 46 7778 (2007)
[3] A. Aimi, H. Onodera, S. Yoshida et al,. Chem. Mater. In press (2024)

Figure 1