Beta-alumina solid electrolyte (BASE) is a fast-ion conductor material used as a membrane in several types of molten salt electrochemical cell.
It is a hard polycrystalline ceramic, which, when prepared as an electrolyte, is complexed with a mobile ion, such as Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+ or Ba2+ depending on the application.
The crystal structure of the β-alumina provides an essential rigid framework with channels along which the ionic species of the solid can migrate.
Its special characteristics on ion and electrical conductivity make this material extremely interesting in the field of energy storage.
The main advantage of solid electrolytes over liquid ones are increased safety and higher power density.
At the Ford Motor Company, researchers (Yung-Fang YuYao, J. T. Kummer and Neill Weber) rediscovered the high ionic conductivity of β-alumina, which meant it could be used as solid electrolyte.
In the early 1970s, instigated by the oil crisis, most research focused on industrial application of β-alumina in energy storage solutions.
[11][12][13] These are three possible positions for the sodium ion, named Bever-Ross (BR), anti-Bever-Ross (aBR) and mid-oxygen (mO).
It has rhombohedral symmetry and its unit cell consists of three spinel blocks, including adjacent conduction planes.
) non-stoichiometric β-alumina the mobile ions can migrate easily to different sites, because of low energy barriers, even at room temperature.
[9] The mobile ions move through the conduction plane by hopping between the different possible sites (BR, aBR, mO).
[10] In β-alumina, in contrast to β”-alumina, the gap between oxygen atoms is generally too small for larger alkali ions, such as
[9][14] For the large-scale and cost-efficient energy storage needs, sodium batteries operating at high temperatures are showing signs of success.
The ion-conductive β-alumina plays a key part in the battery cells performance, requiring development of optimal microstructure and purity to ensure beneficial electrical and mechanical properties.
Current high-end manufacturing methods for producing the β-alumina electrolytes includes: isostatic pressing and electrophoretic deposition (EDP).
Eletrophoretic deposition is the process where migrating colloidal particles suspended in a medium using an electrical field to get the desired material.
Both processes, although resulting in good products, require numerous steps to create a batch, contributing significantly to the battery cost.
Extrusion, pressing stock material through a die to get the desired cross-section in the final product, offers this possibility.
Currently it shows promising results with acceptable ceramic quality having potential to significantly lower manufacturing costs.
The relevant properties of β-alumina solid electrolytes are high ionic conductivity, but low electronic transference number and chemical passivity.
The electricity is generated in such a way that, during discharge, metal atoms are released form the sodium moving to the positive electrode through the electrolyte.
[16] The development of a new high energy density class of primary cells using β-alumina membranes has been an advancing process.
These cells intended to function at room temperature and exhibit long shelf and operating lifetime.
[17] In the heart of a sodium heat engine, a beta alumina ceramic tubular membrane is placed at the centre.
The system can be viewed as a sodium vapor cell where a differential in pressure is controlled by two heat reservoirs.
Stationary energy storage, particularly the segments with 2-12 h half cycle time, appear to be well-suited for sodium-beta alumina batteries.
IMore specifically, the degradation of beta-alumina, such as the formation of sodium metal dendrites between the grains in the solid electrolyte, seems to be the main reason for a poor adoption of this technology in all market niches.
[20] Currently the research on the topic of doping the crystal structure of the solid electrolyte could lead to more favourable characteristics of the material.
When adding iron over the composition range, it could reach higher ionic conductivity with respect to the undoped version.
Using high amounts of doping has as counterproductive negative effect that the electrical conductivity of the electrolyte rises.