A new study, headed by the Chinese Academy of Sciences’ Qingdao Institute of Bioenergy and Bioprocess Technology and published in Energy & Environmental Science, found considerable safety issues in sodium-ion batteries (SIBs), particularly when compared to lithium-ion batteries.
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The study demonstrates that SIBs are more susceptible to thermal runaway than previously thought, calling into question their apparent safety despite cost and resource advantages.
SIBs have attained energy densities of up to 160 Wh/kg and cycle lifetimes of more than 4,000 charge-discharge cycles in the last decade alone. Despite these developments, safety remains a top priority. This work focuses on the role of sodium clusters in hard carbon (HC) anodes, which can cause early thermal runaway. These clusters have electronic activity that outperforms even metallic sodium, resulting in localized "electronic storms" within the battery.
When the battery reaches critical charge levels, sodium clusters lower the self-heating onset temperature to as low as 92 °C, triggering thermal runaway much earlier than in LIBs.
Guanglei Cui, Professor, Chinese Academy of Sciences
The researchers used solid-state nuclear magnetic resonance (ssNMR) spectroscopy to explore sodium clusters on a quantum scale. They discovered that these clusters have significant metallicity, with more conduction electrons at the Fermi energy level than bulk metallic sodium. This makes them highly reactive and speeds up thermal runaway.
Unlike LIBs, where exothermic reactions are unaffected by the state of charge (SOC), SIBs show a high correlation between reaction initiation and SOC. Sodium clusters form at high SOCs, lowering the onset temperature for self-heating much before the SEI disintegration seen in LIBs. According to the study, even during normal operation, SIBs may provide safety issues similar to those associated with sodium plating in overcharged batteries.
These findings fill a key knowledge gap regarding the relationship between cell safety and salt storage microenvironments. The researchers propose substituting liquid electrolytes with solid-state materials could greatly minimize thermal runaway concerns, making it a safer option for future energy storage.
Journal Reference:
Niu, J. et. al. (2025) Sodium cluster-driven safety concerns of sodium-ion batteries. Energy & Environmental Science. doi.org/10.1039/D4EE05509H