ZEBRA batteries (Zero Emission Battery Research Activities) for their high energy and power density are one of the possible solutions to electrical storage for stationary applications. These systems are based on nickel chloride-sodium cells operating at high temperatures (about 270°–350 °C), and rely on a ceramic β”-alumina tube or planar membrane as solid electrolyte. In this work, recent results generated from 3 cm² button cells, including base line cell performance, cell design and chemistry will be presented. The activity was directed to the realization of two new planar configuration of a beta sodium battery, that means the study of a planar sealing between the metal and the ceramic components. This battery, as many other electrochemical storage technologies, may play an important role for the transmission and distribution of electricity, because it can satisfy, in a very flexible way, many different needs (voltage and frequency control, self consumption of stochastic renewable energy, peak shaving etc.). For this purpose the storage system has to be characterized not only by a suitable energy capacity, but by quick response and high power performance too. A planar configuration could optimize the battery in terms not only of energy density (already satisfactory for this kind of batteries), but also of power density (still to be improved). The planar configuration could also provide a better stack design and thermal regulation. This battery utilizes a ceramic material as the ionic membrane between the anodic and cathodic semi cells and so it has to operate at 300°C. The current form of the β”-alumina membrane is “glass shaped” in order to contain the reagents, so the possible planar shape has to include a sealing system able to contain the reagents, which are in liquid status at the high temperature of the battery and they are also highly chemical reactive, especially the liquid sodium. In particular the research activity was previously directed to the study of the components of the seal between the metal bodies of the semi cells and the ceramic beta alumina and afterwards to a new configuration of the sealing system. The studied materials were insulating paints, ceramic pastes and glass seals and they were tested in order to assess their chemical stability in presence of sodium and their mechanical stability during thermal cycling. The research activity was addressed to define the right geometry of the ceramic support of the β”-alumina. Another purpose was to define a mechanical seals between the ceramics support and metallic body of the two, anodic and cathodic, electrodic compartments. A new activity was started to realize a new configuration in which also the body of battery was realized in ceramic material (α-Alumina), which allows a greater wettability in the anodic compartment.

The mono cell was realized with a new sealing system, composed by a new geometry of a ceramic ring to contain the beta alumina membrane and to guarantee the mechanical stability. In both planar configurations of the cell made, it has been noticed that this type of design

reduces the volume of the cathodic and anodic semi cell; in this way, not only the energy density is increased but also the power density is improved. Furthermore, the planar design would improve the stack geometry and its thermal distribution. The problems faced in this work were the material resistance to the chemical attack of liquid sodium at high temperature, the mechanical resistance and the compensation of the different thermal expansion coefficient between the solid electrolyte (β-alumina) and the metallic body.

The first button cell consisted of two battery cases (stainless steel) for cathode and anode sides, a Nichel network cathode current collector, a stainless steel anode current collector, an α-alumina (99.5% purity) fixture and a planar composite yttria-stabilized zirconia (YSZ)/ BASE (3 cm² active area) disc.

The second buttom cell consisted of battery cases (α-Alumina) for cathode and anode sides,
a steel plate held by a spring as a current collector both at the anode and at the cathode (the use of springs makes it possible to better exploit cathodic and anodic active materials), an α-alumina (99.5% purity) fixture and a planar composite yttria-stabilized zirconia (YSZ)/ BASE (3 cm² active area) disc.

It is using an inexpensive stacked design to improve performance at lower temperatures, leading to a less expensive overall storage technology. The new design greatly simplifies the manufacturing process, providing a subsequent pathway to the production of scalable, modular batteries at half the cost of the existing tubular designs.