Improvement of ionic conductivity and relative density of Li6.4La3Zr1.4Ta0.6O12 electrolytes by optimization of composition and sintering aid
Authors: Kouno Yuya , Masuo Yuta , Onodera Hitoshi , Yoshida Shuhei , Doi Takayuki , Haruta Masakazu , Inaba Minoru 
Published: December 2024

Introduction

All-solid-state lithium metal batteries (ASSLMBs) are expected to have high energy density. Among various solid electrolytes, oxide-based solid electrolytes have attracted attention in recent years due to their higher air stability compared to sulfide-based solid electrolytes. However, oxide-based solid electrolytes exhibit lower ionic conductivity compared to sulfide-based or liquid electrolytes, leading to inferior high-rate cycle performance. Additionally, when the relative density of solid electrolytes is low, the formation of lithium dendrites through structural defects in the solid electrolyte causes short circuits. Therefore, improving the ionic conductivity and relative density of solid electrolytes is essential for enhancing the performance of oxide-based all-solid-state batteries.
In our previous research, we increased the relative density of lithium lanthanum zirconium tantalate (Li6.4La3Zr1.4Ta0.6O12, LLZT) solid electrolytes to 99.3% by co-doping LLZT with alumina (Al2O3)1) and lithium tungstate (Li2WO4)2). The increase in relative density resulted in improved short-circuit resistance performance. However, the drawback of adding these sintering aids is the reduction in ionic conductivity of LLZT due to the substitution effect on the solid electrolyte. According to the paper by N. Hayashi et al., reducing the amount of lanthanum in LLZ-CaBi solid electrolytes can improve ionic conductivity and relative density3). In this study, we attempted to enhance the ionic conductivity and relative density of LLZT by reducing the amount of lanthanum in the raw materials.

Experimental

The Li6.4La3-xZr1.4Ta0.6O12-1.5x (x=0, 0.03, 0.06, 0.09, 0.12,0.15) powders were synthesized via solid-state synthesis method. Lithium carbonate, lanthanum oxide, zirconium oxide, and tantalum oxide were used as raw materials. The raw materials were mixed and then heated at 900°C for 12 hours to synthesize LLZT powder. During the weighing process, lithium carbonate was added in excess by 20% of the target composition to account for lithium carbonate volatilization. LLZT powder and sintering aids were mixed using a planetary ball mill, and then the powder was pressed into pellets at 98 MPa. The pellets were sintered at 1150°C for 5 hours. Next, dry polishing was conducted to adjust the thickness of LLZT to 0.8 mm, and Au blocking electrodes were sputtered on both sides of LLZT to perform AC impedance. AC impedance measurements were performed at 30°C with an applied voltage of 10 mV over a frequency range from 1 MHz to 1 Hz. Li metal electrodes (diameter 6 mm) were attached to both sides of the Au thin film on LLZT to fabricate Li-Au|LLZT|Au-Li symmetric cells to conduct short-circuit test. In the short-circuit tests, charge-discharge cycles were repeated every 30 minutes, with the current density increasing by 0.1 mA cm-
2 every 20 cycles at 60°C.

Results and Discussion

Table 1 shows the relative density, resistance, and ionic conductivity of as-prepared LLZT without sintering aid. Decreasing the amount of lanthanum led to improvements in both relative density and ionic conductivity. The highest relative density achieved at x=0.06 and 0.09. Additionally, at x=0.06, bulk ionic conductivity was measured at 1.12 mS, and total ionic conductivity at 0.976 mS. The low grain boundary resistance at x=0.06 is believed to be due to the reduction in voids resulting from the improved relative density. For samples with x=0.09 and above, a gradual decrease in both relative density and ionic conductivity was observed.
Figure 1 depicts the XRD patterns of LLZT from 20° to 22°. Pronounced peaks of Li2ZrO3 were observed in samples with x=0.12 and x=0.15. This is believed to result from the reaction of excess lithium, zirconium, and oxygen due to the reduction in lanthanum. Previous our studies have shown that Li2ZrO3 suppresses abnormal grain growth in LLZT particles and improves relative density. Therefore, the enhancement in relative density is likely attributed to Li2ZrO3.
Figure 2 shows cross-sectional SEM images of LLZT. Abnormal grain growth and voids were observed in samples with x=0 and 0.03, while samples with x=0.06 and above exhibited no abnormal grain growth, indicating that dense solid electrolytes were successfully fabricated. This is considered evidence of abnormal grain growth suppression due to the formation of Li2ZrO3.
In addition to the above, the reasons for the enhancement in bulk ionic conductivity, the characteristics of LLZT (x=0.06) added sintering aids, and the results of short-circuit tests will be discussed in more detail on the poster.

References

[1] K. Zhang, et al., Energy Research, 44, 9177–9184. (2020)
[2] C. Zheng, et al., Materials Today Energy, 27, 101034 (2022)
[3] N. Hayashi, et al., J. Mater. Chem. A, 11, 15681-15690 (2023)

Figure 1