Electrochemical Analyses of the Li-Ion Transfer Resistances at the Chloride Solid Electrolyte | Sulfide Solid Electrolyte Interfaces
Authors: Yoshida Kotaro , Ikezawa Atsunori , Okajima Takeyoshi , Yanagihara Shoma , Ohno Saneyuki , Arai Hajime 
Published: December 2024

Introduction

The all-solid-state Li-ion battery is expected to be a next-generation battery owing to its enhanced safety and high rate-capability ascribed to sulfide solid electrolytes (SEs) having incombustibility and high ionic conductivity. However, low cyclability hinders their practical application. Since the oxidative decomposition of the sulfide SE in the positive electrode is known as one of the main degradation modes, bilayer SE cells are promising to suppress the degradation where the chloride SE having relatively high stability against oxidation is placed on the positive electrode side and the reduction-resistive sulfide SE is placed on the negative electrode side. [1]. While it is reported that the interface between the sulfide and chloride SEs is not chemically stable and interphases are formed between the SEs [1], the effect of the interphase on the Li-ion transport between the sulfide and chloride SEs has not been investigated. As a way to analyze the resistance between two SEs, we have recently established an all-solid-state electrochemical four-electrode cell using a partially reduced lithium titanate (R-LTO) reference electrode (RE) [2,3].

Experimental

In this study, we apply the electrochemical four-electrode cell to the sulfide SE | chloride SE interfaces to analyze the Li-ion transport resistance. The glass-ceramic Li2S-P2S5-LiI (LPSI) was synthesized by the method reported by X. Feng et al. [4]. The chloride SEs, Li3InCl6 (LIC), Li3SrCl6 (LSC), and Li3YCl6 (LYC) were synthesized following the previous report [1]. The ionic conductivities and activation energies of the LPSI, LIC, LSC, and LYC were measured to be 2.1, 0.36, 0.5 and 0.28 mS cm–1 at 25 °C, and 29.8, 33.5, 38.5 and 38.6 kJ mol–1, respectively. The R-LTO REs were fabricated by the previously reported method [2]. All-solid-state four-electrode cells (Fig. 1(a)) were fabricated in the following way. First, the LPSI pellet incorporating the R-LTO was fabricated by hand pressing in a mold. Then, the chloride SE powder was sandwiched with the LPSI layers followed by cold pressing at ca. 290 MPa. Finally, Li-In foil synthesized with a chemical lithiation of the In foil was put on the outer sides of the stacked pellet as the counter electrodes (CEs) [2]. Electrochemical impedance spectroscopy (EIS) was conducted between 50 mHz and 7 MHz with an AC amplitude of 10 mV at OCV using an electrochemical measurement system (VSP-300, BioLogic).

Results and Discussion

Fig. 1(b) shows the impedance spectrum obtained by EIS using a pristine LIC | LPSI cell at 25 °C. Two semicircles
P

1 and
P

2 are found in higher and lower frequency regions and assigned as bulk and interfacial Li-ion transfer from the frequency response [2]. The interfacial resistance of the LPSI | LIC | LPSI cell shows a continuous increase at 25 °C, though it does not change significantly at –5 °C. This result indicates that the resistive interphase is formed by a chemical reaction. The growth ratios of the interfacial resistances of the LPSI | LIC | LPSI, LPSI | LYC | LPSI, and LPSI | LSC | LPSI cells for 50 h are 2.2,1.6 and 1.4 respectively.

References

1) M. Chandrappa et al., J. Am. Chem. Soc., 144, 18009 (2022).
2) K. Yoshida et al., submitted to Electrochim. Acta (2024).
3) A. Ikezawa et al., Electrochem. Commun., 116, 1067433 (2020).
4) X. Feng et al., Energy Storage Mater., 22, 397 (2019).

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number JP22H04608.

Figure 1