Reducing Interfacial Resistance between Li7La3Zr2O12 and Liquid Electrolyte through Wet Polishing for Quasi-Solid Batteries
Authors: Kondo Jumpei , Otani Kazushi , Noba Ryota , Miyamoto Suguru , Hikosaka Hideaki , Kondo Yasuyuki , Katayama Yu , Yamada Yuki 
Published: December 2024

Introduction

Composite electrolytes, fabricated by combining solid and liquid electrolytes, are promising for next-generation batteries with high safety and energy density. However, they face a critical challenge of notably large Li+-transport resistance at the unstable solid/liquid interface. Recent studies have indicated that both Li+ de-solvation energy and the interfacial/interphasial layer dominate the interfacial resistance.1,2 Particularly, Li7La3Zr2O12 (LLZO), which is one of oxide-based solid electrolytes with high ionic conductivity, is sensitive to moisture and carbon dioxide, leading to the formation of a contamination layer mainly composed of lithium hydroxide (LiOH), lithium carbonate (Li2CO3), and adventitious carbon species. This contamination layer might increase the interfacial resistance.3 Although dry polishing under an inert atmosphere can partially remove the contamination layer, the bare LLZO surface immediately reacts with contaminations to form a resistive layer again.4

Herein, we report a novel wet polishing process of LLZO with a liquid electrolyte. This process removes the contamination layer while forming a stable interfacial layer derived from the electrolyte components on the LLZO surface, which can remarkably reduce the interfacial resistance.

Experimental

The Li6.95Mg0.15La2.75Sr0.25Zr2O12 (LLZO-MgSr) sintered pellets with a diameter of 20 mm were synthesized using a solid-state method. The LLZO-MgSr pellets were treated with dry and wet polishing under an Ar atmosphere. Wet polishing was performed using a 2.7 M LiN(SO2F)2 (LiFSI)/sulfolane (SL) electrolyte solution. After dry or wet polishing, the surface of LLZO-MgSr was analyzed using X-ray photoelectron spectroscopy (XPS). The interfacial resistances between the solid and liquid electrolytes (2.7 M LiFSI/SL) were measured by electrochemical impedance spectroscopy with a four-probe cell employing LLZO-MgSr as solid electrolyte membranes, 2.7 M LiFSI/ SL as liquid electrolytes and Li metal for all electrodes.

Results and Discussion

XPS analysis revealed that the dry-polished LLZO-MgSr surface was composed of low amounts of lanthanum (La) and zirconium (Zr) elements from LLZO-MgSr, suggesting that it was covered with a thick contamination layer such as LiOH (Fig. 1a). In contrast, the wet-polished LLZO-MgSr surface showed more abundant La and Zr elements together with nitrogen (N), fluorine (F), and sulfur (S) from the liquid electrolyte. These results indicate that the wet polishing process forms a thin interfacial layer derived from the liquid electrolyte components, which may suppress the re-formation of the contaminant layer. Fig. 1b showed the Nyquist plots of four-probe cells containing the LLZO-MgSr pellet after dry or wet polishing. Compared to the interfacial resistance of 79 Ω cm2 after the dry polishing, the wet-polished LLZO-MgSr showed a remarkably lowered value of 18 Ω cm2. The reduced interfacial resistance may be due to removing the contamination layer.
In conclusion, this wet-polishing method effectively controls the interfacial resistance on LLZO composite electrolytes by forming a stable and less resistive interfacial layer derived from liquid electrolytes and suppressing the re-formation of the resistive contamination layer. This method is expected to contribute to developing next-generation batteries with high safety and energy density.
(1) T. Abe et al.,
J. Electrochem. Soc. 11, 152 (2005). (2)M. R. Busche et al., Adv. Mater. Interfaces. 19, 7 (2020). (3) A. Sharafi et al., J. Mater. Chem. A, 5, 13475–13487 (2017). (4) A. Sharafi et al., Chem. Mater. 29, 7961–7968 (2017).

Figure 1