Polarization impedance at the Na-Na₅YSi₄O₁₂ interface

Na-ion solid-state batteries operating at room temperature are typically supposed to have a metallic Na anode for sufficiently high energy density. The less known but due to easy processing very promising Na-conducting Na₅YSi₄O₁₂ glass ceramic is investigated regarding its interface resistance in contact with metallic Na electrodes. Two types of this material, with and without the addition of P₂O₅ as a glass-forming sintering additive, are investigated. The surface is treated by polishing to induce a defined roughness. Highly polished surfaces lead to incomplete covering with Na metal. In contrast, surfaces with higher roughness can be covered more homogeneously with Na and thus exhibit significantly reduced interface resistance. These measurements reveal the intrinsically small interface resistance between Na₅YSi₄O₁₂ glass ceramics and metallic Na, which makes Na₅YSi₄O₁₂ a promising electrolyte for Na-ion solid-state batteries. Prolonged contact of the electrolyte with humid air, like several days or weeks, leads to a strongly increased interface resistance. Thus, storage conditions and surface treatment of the samples after air contact need to be considered carefully.