Sodium Solid Electrolytes: NaxAlOy Bilayer-System Based on Macroporous Bulk Material and Dense Thin-Film.

Antonia Hoppe,Simon Carstens,Michael Stelter,Karl Skadell,Cornelius Dirksen,Sharon Koppka,Matthias Schulz,Dirk Enke

doi:10.3390/ma14040854

Antonia Hoppe, Simon Carstens + Show 6 more

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https://doi.org/10.3390/ma14040854

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Abstract

A new preparation concept of a partially porous solid-state bilayer electrolyte (BE) for high-temperature sodium-ion batteries has been developed. The porous layer provides mechanical strength and is infiltrated with liquid and highly conductive NaAlCl4 salt, while the dense layer prevents short circuits. Both layers consist, at least partially, of Na-β-alumina. The BEs are synthesized by a three-step procedure, including a sol-gel synthesis, the preparation of porous, calcined bulk material, and spin coating to deposit a dense layer. A detailed study is carried out to investigate the effect of polyethylene oxide (PEO) concentration on pore size and crystallization of the bulk material. The microstructure and crystallographic composition are verified for all steps via mercury intrusion, X-ray diffraction, and scanning electron microscopy. The porous bulk material exhibits an unprecedented open porosity for a NaxAlOy bilayer-system of ≤57% with a pore size of ≈200–300 nm and pore volume of ≤0.3 cm3∙g−1. It contains high shares of crystalline α-Al2O3 and Na-β-alumina. The BEs are characterized by impedance spectroscopy, which proved an increase of ionic conductivity with increasing porosity and increasing Na-β-alumina phase content in the bulk material. Ion conductivity of up to 0.10 S∙cm−1 at 300 °C is achieved.

Highlights

Electrochemical energy storage is a key feature in transforming the global economy towards sustainability and carbon neutrality
The bilayer electrolyte (BE) are characterized by impedance spectroscopy, which proved an increase of ionic conductivity with increasing porosity and increasing Na-β-alumina phase content in the bulk material
The results clearly show that improvements in the synthesis procedure and in the conditions of the thermal treatment are necessary to increase the content of Na-β-alumina in the macroporous discs

Summary

Introduction

Electrochemical energy storage is a key feature in transforming the global economy towards sustainability and carbon neutrality. The main alternative to Li-ion based batteries (LIBs), which are facing problems in terms of ethical issues and expensive lithium and cobalt production, are Na-ion based batteries (SIB). The only industrially established sodium-based batteries are high-temperature Na/NiCl2and Na/S-batteries. In both technologies, a non-porous ceramic Na-β-alumina tube is used as an electrolyte [3]. The ionic conductivity of both crystal structures is caused by a conduction layer, where the mobile Na-ions are located on the c axis, and can, move along this direction. [4,5] Thereby, an ionic conductivity of >0.2 S·cm−1 at 300 ◦C can be reached [3,6]. Other recent studies undermine the general research interest in sodium-ion batteries [7,8]

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