Beta-alumina solid electrolyte

Fast ion conductor material
Beta alumina solid electrolyte (BASE) is a fast ion conductor material used as a membrane in several types of molten salt electrochemical cell. Currently there is no known substitute available. The sodium–sulfur battery was a topic of intense worldwide interest during the 1970s and 1980s, but interest in the technology for vehicle use diminished for a variety of technical and economic reasons. Its "successor", the sodium nickel chloride battery, is of commercial interest. The sodium nickel chloride battery (or ZEBRA battery) has been under development for almost 20 years. The crystal consists of closely packed 'spinel' blocks, separated by loosely packed conduction planes. The spinel blocks are linked by Al O Al bonds. These conduction planes contain mobile sodium ions, which makes the β alumina an ionic conductor. β alumina is generally non stoichiometric. The general formula is given by Na {1+x}Al {11}O {17+x/2}, with the excess of sodium atoms, balanced by an excess of oxygen atoms. would be the stoichiometric compound, but is normally not stable. Generally is around 0.3. The ions K+, Ag+, Tl+, NH 4+, H 3O+ can replace the sodium in the conduction layer. Three important sites for the conduction mechanism in the conduction planes have been identified. These are three possible positions for the sodium ion, named Bever Ross (BR), anti Bever Ross (aBR) and mid oxygen (mO). The first two are named after the scientist who first identified these positions. The last is named mid oxygen, as it is the position exactly between two oxygen ions in the conduction plane. These three position are at , and respectively, when taking one of the symmetry points as the origin. The sodium ion is most likely at the BR position,

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). Isostatic pressing is the process where casting are pressed into compact solids using a mould and pressure. 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. A large volume production desires a simplified low cost and continuous process. This is offered by extrusion. 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.

In the past decades several devices based on β alumina have been researched for energy conversion and storage. The relevant properties of β alumina solid electrolytes are high ionic conductivity, but low electronic transference number and chemical passivity. It is also able to be formed into useful shapes.

Research and development on the sodium–sulphur cell has reached a point where this technology is now commercialised. Average units have power output in the range from 50 400 kWh. Its lifetime is estimated to be around 15 years, around 4500 cycles at an efficiency of 85%. The quick response times, claimed speeds in the order of 1 ms, add to the overall utility of the battery. A high temperature of 300 400 degrees Celsius is needed during operation

Sodium–sulphur batteries have a basis of molten salt technology where molten sodium and sulphur are used as the electrodes of the battery. A high temperature of 300 400 degrees Celsius is needed during operation for the components to remain molten. 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. The electrolyte consists of a beta alumina tube. Due to the fast and efficient ion transportation, β alumina allows the battery to function at these high speeds and efficiency. The application of these batteries are commonly in the field of renewable energy, the main function being peak shaving and energy stabilization. For this purpose the high ion transport beta alumina provides is crucial.

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. Intended applications are for example pacemakers and electronic watches.

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. The temperature difference between the two regions gives rise to a certain sodium activity differential, the sodium expands almost isothermally. Since the beta alumina electrolyte does not conduct electrons favourably the expansion causes sodium ions across the membrane and the electrons through an external circuit. At a porous electrode the ions are neutralized on the low pressure side, the neutral atoms evaporate through a vapor chamber ending up in a condenser. The cooled liquid sodium is then pumped back to  the high temperature region. For this application beta alumina is especially applicable, since the most efficient features of the heat engine are a result form the properties of the work fluid. The heat engine application calls for an electrolyte with long term durability. This is one of the features that hot sodium gives, electrolyte resistivity is particularly low at high operating temperature. Since the conversion efficiency is almost independent of size, this heat engine has a modular form and could form a candidate for local generation of power in energy systems. To date it has seen most application in combination with solar thermal electric systems. was considered in the past for both stationary energy storage and electric vehicle applications. The main drawback of these batteries is that they operate at 300 degrees Celsius, when the vehicle is not in use it needs an external heat source to keep the battery operational. It has been researched if this external heating will use more energy than ambient temperature batteries. The conclusion was that the ZEBRA battery does not use more electricity than a traditional battery due to the variation in daily driving habits. The most efficient use case for this battery would therefore be in fields where the battery sees the most usage, such as public transport. Stationary energy storage, particularly the segments with 2 12 h half cycle time, appear to be well suited for sodium beta alumina batteries. General Electric attempted to commercialized ZEBRA batteries for stationary enery storage in 2011 2015, but failed to do so. It appears, that the reasons for the GE's failure were technical rather than economical. 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.

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. The concentration and type of dopant are the variables that can change the properties of the material. Using high amounts of doping has as counterproductive negative effect that the electrical conductivity of the electrolyte rises. Research is focussed on finding the trade off between ionic and electrical conductivity.