Salt batteries: The battery that defies fire

In 1997, the Mercedes-Benz A-Class tipped out of a bend during the moose test. One of the causes of the infamous incident: the A-Class was originally designed as an electric car. By switching to the combustion engine, the heavy battery was removed and the center of gravity shifted too far upwards.

The battery that should have been installed in the A-Class was a so-called salt battery. In contrast to most other batteries, in which the cathode and anode "float" in a common liquid electrolyte, the electrolyte in a salt battery is a solid, namely a ceramic ion conductor based on sodium aluminum oxide. The solid electrolyte is non-flammable and also enables the anode and cathode to be separated, which increases the service life of the battery. The cathode of a salt battery is based on a granulate of common salt and nickel powder; the sodium metal anode is only formed during charging.

This battery technology has not proven itself for electromobility: Today's electric cars run on lithium-ion batteries, which are lighter and can be charged more quickly. However, salt batteries are superior to their lithium-ion competitors in other areas of application. This is why salt batteries are being actively researched today - at Empa, among others.

Durable and safe

The research collaboration began in 2016 when the Ticino-based salt battery manufacturer HORIEN Salt Battery Solutions, formerly known as FZSoNick, approached Empa. The company wanted to improve the ceramic sodium aluminum oxide electrolyte in its battery cells as part of an Innosuisse project. This led to further projects on the cell geometry and electrochemistry of the salt battery, as it differs greatly from other battery types. "The assembly of salt battery cells for research purposes is very complex and there are hardly any studies on how they work exactly. That's what makes these projects so interesting for us: we can learn a lot and develop our understanding further together with the industrial partner," says Empa researcher Meike Heinz from the "Materials for Energy Conversion" department, which is headed by Corsin Battaglia.

However, their different cell structure also gives salt batteries some advantages over lithium-ion batteries. For example, in terms of safety: although salt batteries need an operating temperature of around 300° Celsius, they can neither burn nor explode. This is why they are also used in places where lithium-ion batteries are not even permitted, such as in mining and tunnel construction and on offshore oil and gas production platforms. Due to their high operating temperature, salt batteries are also much less sensitive to temperature than their lithium-ion counterparts. This makes them ideal emergency power storage systems for critical infrastructure, such as mobile phone antennas. Even in remote and exposed locations, the long-lasting and maintenance-free salt batteries can perform their work reliably for decades.

However, the operating temperature is also a disadvantage of this battery technology: salt batteries need "auxiliary heating" to be ready for use. But is a battery that needs electricity economical at all? "Depending on the application, it is more economical to keep a battery warm than to cool it," explains Meike Heinz. "Heat is generated during charging and discharging due to the natural cell resistance. In an optimal system, a large battery can heat itself as a result," adds Empa researcher Enea Svaluto-Ferro.

Cell chemistry for the future

As a materials researcher, Meike Heinz and her team focus on cell chemistry. The raw materials for molten salt batteries are generally inexpensive and available in large quantities. The architecture of the cell also makes it easy to recycle. However, as the cathode material nickel is increasingly being classified as critical, HORIEN and Empa set about reducing the nickel content of the cells as part of the "HiPerSoNick" project funded by the Swiss Federal Office of Energy (SFOE). This was no easy task, as the composition and microstructure of the cell must be very precisely coordinated to ensure an efficient and long-lasting salt battery.

As part of the EU "SOLSTICE" project, which runs until mid-2025, HORIEN and Empa, together with other project partners, are investigating whether the nickel in molten salt batteries could even be completely replaced by zinc. "However, the low melting point of zinc is a challenge at the current operating temperature," says Meike Heinz. Nevertheless, the researchers have already found promising approaches to stabilize the cathode microstructure.

Further follow-up projects are already being planned in which the Empa researchers will attempt to further improve - and scale up - nickel-free salt batteries. After all, their safety, long service life and lack of critical raw materials would make salt batteries ideal for stationary storage. If salt batteries can be produced cheaply and in large quantities, they could one day supply not only mobile phone antennas but also entire residential areas with electricity.

Source: www.empa.ch