A novel cell concept for sodium-based medium-temperature batteries

The Energy Transition requires batteries

Stationary electrochemical energy storage systems are essential for excess energy storage and to ensure secure energy supply. High-tem­perature batteries are already being used com­mercially for these purposes (“ZEBRA cell”). In contrast to Li-ion batteries, ZEBRA cells are operated without cobalt and lithium, which are rare elements, and therefore expensive. In the new cell system sodium acts as the nega­tive electrode. The amount of sodium in the earth´s crust is a thousand times higher than the amount of lithium. Hence, it is cheap and available in almost unlimited supply. Another crucial component is the ceramic solid electro­lyte: sodium-beta alumina. ZEBRA cells have been successfully improved by Fraunhofer IKTS and are currently being commercialized in cooperation with a business partner.

Sodium-beta alumina – the heart of the battery

Fraunhofer IKTS has engineered materials and processes for the production of sodium-beta alumina, achieving high phase purity and den­sity. Therefore, the solid electrolyte exhibits outstanding chemical, electrochemical and mechanical stability and is ideal for use as an ion-conducting solid electrolyte in batteries. In contrast with conventional organic electro­lytes, it is also non-toxic and non-flammable.

The success of transition metal oxides

Today´s Li-ion cells rely on transition metal oxides (“NMC“), which function as positive electrodes. Their high energy density and cost efficiency make them extremely successful – even gaining their discoverers a Nobel Prize in Chemistry in 2019. However, powerful sodium-based lithium analogs are also avail­able, so these high-performance transition metals can be used in sodium-based cell systems as well.

A novel cell concept – combining powerful cell components

At Fraunhofer IKTS, sodium, sodium-beta alu­mina and transition metal oxides are combined in a novel cell concept, which is suitable for stationary energy storage.

Utilizing a special polymeric composite cathode creates a safe solid-state battery with high specific energy. By integrating the composite positive electrode, IKTS was able to reduce the operating temperature to only 80 °C. The disk-shaped sodium-beta alumina solid electrolyte is advantageous for production and battery pack construction.

Due to the active material´s phase transitions, the battery´s lifetime is still limited. However, improvements are being made, for example by introducing stabilizing agents into the crys­tal lattice (“doping”). Simultaneously, the cell resistance is optimized by producing even thinner sodium-beta alumina solid electrolytes. Fraunhofer IKTS’s development goal is a com­mercial solid-state battery with high specific energy, which is sustainable and safe and can be utilized for stationary energy storage.