Effects of Polymer Electrolytes on the Interfacial Contact with Positive Electrode Materials for β”-Al2O3 Based All-Solid-State Sodium Batteries
Authors: Inoue Yoshihiko , Tsujimoto Shota , Sakamoto Ryo , Lee Changhee , Miyahara Yuto , Miyazaki Kohei , Abe Takeshi 
Published: December 2024

Introduction

All-solid-state sodium batteries (ASSSBs) have attracted large attentions as one of the alternatives to lithium-ion batteries from the viewpoint of the relative abundance and low cost of sodium and safety. Oxide-based inorganic solid electrolytes have been proposed as a promising candidate among various solid electrolytes because of their high ionic conductivity and electrochemical stability, but their hardness causes increased charge transfer resistance at the interface between electrode and solid electrolyte. A major key to the practical application of ASSSBs is to enhance the interfacial contact between active materials and electrolytes. In contrast, polymer electrolytes are relatively soft and easy to contact at the interface, although their ionic conductivity is inferior. Therefore, composite electrolytes that appropriately mix inorganic solid electrolytes and polymer electrolytes have been proposed, and many studies have been conducted, but the ionic conduction paths are so complex that there is no clarity regarding the effect of the polymer electrolyte on the resistance at the interface.
In this study, we introduced a softer polymer electrolyte than inorganic solid materials between Na3V2(PO4)3 positive electrode / Na-β"-Al2O3 solid electrolyte to reduce the interfacial resistance by the enhancement of the solid | solid contact. This allows a detailed analysis of the resistive components of composite electrolyte systems with complex ionic conduction paths.

Experimental

Na3V2(PO4)3 (NVP), which has been reported to exhibit sodium-ion insertion/de-insertion at relatively high 3.4 V (vs. Na+/Na) when used with liquid electrolyte and good cycle characteristics, was used as the positive electrode material. [1] Acetylene black was added as conductive material and polyvinylidene fluoride (PVdF) as binder to form composite electrode. The polymer electrolyte was prepared as the dry polymer film by casting an acetonitrile solution containing sodium bis(fluorosulfonyl)amide (NaFSA) and polyethylene oxide (PEO), followed by completely volatilizing the solvent. [2] The polymer electrolyte membrane was introduced between the oxide solid electrolyte Na-β"-Al2O3 and positive electrode, and Na metal was used as a negative electrode to construct a two-electrode cell. Electrochemical impedance spectroscopy (EIS) was performed with an AC amplitude of 10 mV in a frequency range of 1.0 MHz–10 mHz, after constant current charge-discharge measurements (measurement temperature: 50°C, current density: 5.9 mA g−1, voltage range: 2.7~3.7 V) to investigate the effect of the polymer electrolyte membrane on the junction interface between positive electrode and solid electrolyte.

Results and discussions

The charge-discharge curves without and with PEO dry polymer electrolyte membrane are shown in Fig. 1(a)(b), respectively. The reversible reaction did not occur in the cell without the polymer electrolyte, whereas a reversible capacity of more than 70 mAh g−1 with a coulombic efficiency of more than 99% was obtained in the cell with the polymer electrolyte.
The Nyquist plots of the potential dependence are shown in Fig. 1(c)(d). In the cell without polymer electrolyte, the very large semicircle with potential dependence is observed. Since the bulk and grain boundary resistance of Na-β"-Al2O3 and the charge transfer resistance at Na-β"-Al2O3 / Na interface was very small, this semicircular component was attributed to the charge transfer resistance at Na-β"-Al2O3 / NVP interface. This result suggests that the frequency factor is very small because Na-β"-Al2O3 / NVP interface is the solid | solid contact. On the other hand, in the cell with polymer electrolyte, the overall resistance is greatly reduced. The main components of the semicircles observed in the high frequency up to 5 kHz, the medium frequency from 5 kHz to 1 Hz, and the low frequency region from 1 Hz were attributed to the bulk resistance of the polymer electrolyte, the ion transfer resistance at the polymer electrolyte / Na-β"-Al2O3 interface, and the charge transfer resistance at the polymer electrolyte / NVP interface, respectively. The results suggest that the introduction of the polymer electrolyte increases the frequency factor by the enhancement of the solid | solid contact at Na-β"-Al2O3 / NVP composite electrode junction interface.

Conclusion

The effect of polymer electrolytes on Na3V2(PO4)3 positive electrode / Na-β"-Al2O3 junction interface of ASSSBs was investigated. In a composite electrolyte system with complex ionic conduction paths, the introduction of a polymer electrolyte as a membrane allows us to analyze the effect of the polymer electrolyte on the junction interface.

References

[1] Zelang Jian, Liang Zhao, Huilin Pan, Yong-Sheng Hu, Hong Li, Wen Chen, Liquan Chen, Electrochem. Commun.
14 (2012) 86-89.
[2] Xingguo Qi, Qiang Ma, Lilu Liu, Yong-Sheng Hu, Hong Li, Zhibin Zhou, Xuejie Huang, and Liquan Chen, ChemElectroChem
3 (2016) 1741-1745.

Figure 1