Ultra-thin ceramic electrolyte substrates for sodium solid-state batteries

In solid-state batteries, an ion-conducting solid (solid electrolyte) replaces the liquid separator of conventional batteries. By eliminating flammable and environmentally harmful liquid components, leakage and fire accidents are avoided. In addition, the energy density of the cell can be significantly increased if the solid electrolyte is combined with metallic alkaline metal anodes. Although the current focus of research is on Li-ion batteries, sodium(Na)-based cell concepts are gaining increasing interest due to the growing global scarcity of lithium.

At Fraunhofer IKTS, ceramic solid electrolyte concepts have been researched and demonstrated in solid-state batteries for 15 years. The aim of these developments is to produce suitable (ultra-)thin ionic conductors to reduce the amount of material used and to enable higher power densities and lower resistances. A key technology is the tape casting of various ceramic solid electrolytes, such as Na₅RSi₄O₁₂ (NaRSiO) or Na_3.4Zr₂Si_2.4P_0.6O₁₂ (NASICON). These materials are calcined at Fraunhofer IKTS and milled to fine powders, which are then processed into thin green tapes by tape casting.

In order to achieve the ionic conductivity required for battery applications, the tapes are conventionally sintered at temperatures above 1300 °C. As a way to significantly reduce the sintering temperature of NASICON separators, Fraunhofer IKTS is investigating the cold sintering process. Using aqueous sintering additives and the simultaneous application of temperature and pressure enable the sintering of thin electrolyte substrates at temperatures below 400 °C. Within the BMBF-funded “HeNa” project, it was possible to achieve the production of translucent electrolyte substrates with a thickness of 260 µm and an ionic conductivity of 0.27 mS/cm (Fig. 1).

The significant reduction of the sintering temperature by cold sintering opens up new possibilities for the production of composite electrodes consisting of electrolyte material, active material (e.g. NVP with the molecular formula Na₃V₂ (PO₄ ) ₃ ) and electron conductor (carbon). A functional solid-state cell consisting of electrolyte substrate and composite cathode can be produced in a single sintering step, with no reaction between the materials (Fig. 2).

The full cell is completed by applying a Na metal electrode on the opposite side. These Na solid-state batteries can already be charged and discharged at 80 °C with high capacity retention (Fig. 3). The Na ion transport in the cold-sintered composite cathode is currently still limiting the rate capability of the battery and is the focus of current research at Fraunhofer IKTS.