How Safe Are Sodium-Ion Batteries?

Sodium-ion batteries are considered a sustainable alternative to lithium-ion batteries. But how safe is this type of battery? A recent BAM study provides information on battery safety. 

Research into novel battery technologies has accelerated enormously. The development and use of sodium-ion batteries (SIB) is particularly promising in terms of sustainability and potential cost advantages. However, SIBs must be built with a higher energy density in order to compete with established lithium-ion batteries (LIB). This increase in energy density, in turn, brings with it new requirements for battery safety.

The Federal Institute for Materials Research and Testing (BAM), together with the European Synchrotron Radiation Facility (ESRF) and the Fraunhofer Institute for High-Speed Dynamics, Ernst-Mach-Institut (EMI), recently investigated how safe sodium-ion batteries are in a study. The result: A holistic approach is necessary. According to the researchers, proven safety mechanisms are not automatically equally effective for all battery technologies, but should be specifically adapted to the chemical composition and cell design of new batteries.

Analysis Using a Nail Penetration Test

The investigations simulated the mechanical damage to a sodium-ion battery using a so-called nail penetration test. In this test, a nail or metal pin is driven through the battery to simulate the penetration of sharp objects and trigger a critical damage event, such as might occur in accidents or collisions, as explained in the book chapter State-of-The-Art Reviews to Develop Lithium-ion Batteries for Electric Vehicles.

The aim of the analysis was to find out whether the battery – similar to lithium-ion batteries – enters into a dangerous thermal reaction in which the cell heats up significantly and possibly ignites or explodes, and whether the built-in safety mechanisms are effective.

Internal Processes in LIBs Made Visible

Using high-speed X-ray imaging in a test chamber specially developed by Fraunhofer EMI, researchers at the ESRF in Grenoble were able to visualize the internal processes in sodium-ion batteries during a critical event in real time for the first time.

Two other battery types with different safety mechanisms and chemical properties were also examined in a direct comparison: a classic lithium-ion battery with a nickel-manganese-cobalt cathode, which is widely used in electric vehicles and portable devices, and a lithium iron phosphate battery. According to the researchers, this type of battery is considered particularly safe and is often used in stationary storage systems.

Considering Cell Chemistry and Safety Design Together

According to the scientists, the results show clear differences in behavior: The lithium iron phosphate battery proved to be particularly stable. The lithium-ion battery with a nickel-manganese-cobalt cathode reacted in a controlled manner – its safety mechanisms functioned as intended. The researchers were surprised by the behavior of the sodium-ion battery, which reacted in an almost explosive manner. However, the cause was not the cell chemistry itself, but a failure of the cell's venting system, which is actually designed to relieve excess pressure. "Due to the rapid increase in pressure, however, the venting system became clogged by other components of the safety devices, leading to the abrupt and violent reaction", it is stated.

"Our investigations show that safety mechanisms cannot simply be transferred from one battery technology to another", explains Nils Böttcher, head of the BAM battery testing center. "Especially with new battery types such as sodium-ion cells, mechanical components such as venting systems must be specifically adapted and tested. Our findings do not call into question the fundamental safety of sodium-ion technology, but they do underscore the need to consider chemical composition and safety design together". BAM is therefore actively involved in the development of standards and norms in the field of sodium-ion battery safety.

Market Applications and Opportunities

Sodium-ion batteries are likely to be used initially in the automotive sector in low-speed electric vehicles such as e-bikes, scooters, and small city vehicles, "where the lower energy density can be offset by the cost savings and safety benefits of SIBs", as the Springer authors explain in the book chapter An outlook on sodium-ion battery technology toward practical application. As the technology matures and energy density improves, SIBs could compete with LIBs in more demanding applications such as passenger cars and commercial vehicles. Further market opportunities would arise in the areas of stationary energy storage, emergency power supply systems, and portable electronic devices.

This is a partly automated translation of this German article.