Source: Electrive, Journal of Power Sources Advances

Image credit: Fraunhofer EMI

Researchers in Germany have conducted tests comparing the behaviour of sodium-ion batteries to lithium-ion batteries under certain conditions, discovering that safety mechanism design is not a one-size-fits-all scenario.

The research was jointly conducted by the Federal Institute for Materials Research and Testing (Bundesanstalt für Materialforschung und -prüfung, BAM), the European Synchrotron Radiation Facility (ESRF) and the Fraunhofer Institute for High-Speed Dynamics (EMI). The research was undertaken to investigate the viability of sodium-ion batteries as an alternative to more tried-and-tested lithium-ion systems, as there is potential for savings in resources and costs, and sodium-ion cell chemistry is considered to be relatively safe.

The researchers noted that, to compete with lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) need to be built with increased energy density. This in turn entails new requirements regarding battery safety, which need to be evaluated through rigorous battery abuse testing, aiming to deliberately initiate a thermal runaway event.

The testing process

Three types of batteries were tested, all cylindrical cells in the 18650 format.:

• NFM cells (sodium-ion cells with nickel, iron and manganese)
• LFP (lithium iron phosphate)
• NMC532 (lithium nickel manganese cobalt oxide)

BAM wanted to explore the behaviour of the SIBs in comparison to the more familiar LIBs types, and to examine if the built-in safety mechanisms are equally effective. Mechanical damage to the battery cells was simulated using a nail penetration test, where the cell is pierced to “trigger a critical damage event.” This normally leads to an internal short circuit, which, combined with the mechanical damage, can lead to thermal runaway.

High-speed X-ray imaging technology developed by Fraunhofer EMI was used by ESRF researchers to visualise the internal results of this critical damage event.

The results

The tests on the familiar LIBs went as expected, with BAM stating: “The lithium iron phosphate (LFP) battery proved to be particularly stable. The lithium-ion battery with a nickel-manganese-cobalt (NMC532) cathode reacted in a controlled manner – its safety mechanisms worked as intended.”

The researchers were surprised by the behaviour of the SIB, however, with the test leading to “an almost explosive reaction.”

The use of the high-speed X-ray images enabled the researchers to determine that the cause of this behaviour was not due to the SIB’s cell chemistry, but instead the structure of the cell itself – specifically, a “failure of the cell’s venting system.”

The cells’ venting systems are designed to ensure that excess pressure, in the event of an internal thermal reaction, is reduced by targeted venting. BAM noted that, “however, due to the rapid increase in pressure, the venting system was blocked by other components of the safety equipment, which led to the abrupt and violent reaction.”

Nils Böttcher, head of the BAM battery testing centre, expanded: “Our investigations show that safety mechanisms cannot simply be transferred from one battery technology to another. 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.”

As a result, BAM is actively involved in the development of standards and norms in the field of sodium-ion battery safety.

The research was published in issue 36 of the Journal of Power Sources Advances, where video recordings of the X-ray imaging of the tests can be viewed, revealing the internal behaviour of all three tested battery types.