We’re seeing sodium-ion batteries in cars, data centres and grid- scale applications. There are several benefits related to sodium-ion batteries, including rapid charge/discharge, large temperature ranges, zero-volt storage, and improved safety.
Within these batteries, there are several components that impact safety, the anode, the cathode, the electrolyte, and the separator. The anode materials currently used are primarily hard carbons. However, the cathode and electrolyte are going to play the biggest role in safety. Early commercial offerings typically use carbonate electrolytes. The flammability and the reaction of the electrolyte plays a big role in battery safety, and new non- flammable electrolyte formulations are going to play a role. There are a wide variety of cathodes available, and that’s going to be what that makes the biggest difference in these introductory product lines.
There are three classes of cathode materials that we’re seeing in initial offerings: layered oxides – similar to the traditional NMC and LCO, Prussian blue or white analogues, and polyanionic compounds – both the NASICON and NFVP types, and also iron- based polyphosphates, NFPPs.
Thermal runaway is one of the biggest concerns when it comes to lithium-ion batteries, and the industry can work towards preventing this as new chemistries are developed. Evaluating battery safety can be conducted through abuse testing including: calorimetry, mechanical, electrical, and thermal testing. Additional tests help assess specific hazards for different applications, focusing on thermal property management, and modelling performance before and during thermal runaway (Fig 1).
Initially, looking two different commercially available 18650s (Fig 2), both are using a layered oxide cathode, with the manganese, iron and nickel structure of relatively equal proportions. One of these cells had a copper current collector on the negative electrode, enabling the use of existing lithium manufacturing lines. However, one of the main advantages of sodium-ion batteries is the ability to use aluminium as both the negative and positive current collector. Both cells used a polyethylene-based separator material, with one featuring an aluminium oxide coating for additional structural and thermal stability. Both used carbonate- based electrolytes with a mix of solvents. Nail penetration testing induced thermal runaway upon penetration, similar to NMC and LFP cells.
During testing, both sodium- ion and NMC cells showed similar temperatures leading to thermal runaway– when corrected for stored energy– but lithium-ion cells reached higher maximum temperatures. Although sodium-ion cells had comparable runaway behaviour, the maximum temperature difference can impact their ability to ignite other materials nearby.
The response of sodium-ion cells during thermal runaway showed an improvement over lithium-ion, as sodium cells did not produce flame jets that could lead to propagation. Accelerating rate calorimetry measured self- heating during temperature changes, revealing that sodium-ion cells have a higher thermal stability than NMC cells, though LFP cells showed better safety profiles overall.
Further investigation showed that sodium-ion cells could still undergo thermal runaway even at reduced states of charge, emphasising the need for proper thermal management. While the layered oxide sodium-ion cells showed promise, they were less stable than LFPs. Examining other chemistries revealed that Prussian blue analogues produced lower energy during self-heating and did not accelerate into thermal runaway, indicating better stability under thermal stress compared to layered oxide cathodes.
At larger formats, with more density this isn’t necessarily going to hold true, but it does show better stability of the oxygen within the structure. So we are seeing clear indications that the cathode structure plays a big role in the thermal stability and in the propagation resistance of these cells, and the Prussian blue analogue cells had no reaction to nail penetration. NASICON cells peaked at ~100°C while layered oxide cells peaked at >400°C.
Understanding the vent gases produced during thermal events is crucial for safety, as sodium-ion cells can generate gases similar to lithium-ion cells, including toxic and flammable compounds such as CO2, CO, and hydrocarbons. The potential for explosive risks and toxicity from gases, such as hydrogen fluoride, generated during thermal runaway, must also be assessed to understand environmental impacts and safety protocols.
Any sort of vent gas that’s being produced is going to have a lower and an upper explosivity limit and often the explosivity of these gasses is higher than a traditional hydrocarbon. This can result in a secondary combustion reaction following from thermal runaway.
Any cell that has sodium hexafluorophosphate as a salt runs the risk of hydrogen fluoride formation– a highly toxic and corrosive gas. And so, the layered oxides, will produce HF at similar levels to a traditional lithium-ion battery. However, some elevated levels of formation with the NASICON-type cell compared to the other cells have been observed. This needs to be assessed and quantified on a chemistry level.
Prussian blue and white analogues, are based on a ferrocyanide-based chemistry in the cathode, which can undergo thermal decomposition, resulting in the formation of hydrogen cyanide gas. The formation of gasses is well in excess of both the immediate danger and the permissible exposure limits for these electrodes. So, even though these electrodes show better thermal stability behaviour, if they are subject to thermal hazards that can elevate the temperature above 100°C the formation of these toxic gasses will be seen, which can have significant environmental impacts.
