Sodium-Ion Batteries Show Higher Crash Safety Threshold Than Lithium Counterparts
In a significant step toward safer electric vehicles (EVs), researchers at Ningbo University have demonstrated that cylindrical sodium-ion batteries (SIBs) exhibit a notably higher mechanical abuse tolerance compared to conventional lithium-ion batteries (LIBs). The findings, published in the December 2024 issue of Chinese Journal of High Pressure Physics, reveal that SIBs only undergo thermal runaway under radial compression when both the state of charge (SOC) exceeds 80% and the compression speed surpasses 14 mm/min—conditions far more extreme than those triggering failure in LIBs.
This research, led by Yuzhe Ma, Jun Yang, Zeyang Cao, Zhijun Qiao, and Professor Dianbo Ruan from the Faculty of Mechanical Engineering & Mechanics and the Institute of Advanced Energy Storage Technology and Equipment at Ningbo University, provides critical empirical data for the automotive industry as it evaluates next-generation battery chemistries. With global automakers racing to reduce reliance on scarce lithium and cobalt, sodium-ion technology has emerged as a compelling alternative due to its lower cost, abundant raw materials, and—now confirmed—enhanced safety under crash-like mechanical stress.
The study focused on commercially produced 18650-type SIBs with a NaNi₁/₃Fe₁/₃Mn₁/₃O₂ cathode and hard carbon anode, a configuration increasingly considered viable for mass-market EV applications. Using a precision-controlled flat-plate compression platform, the team subjected batteries at varying SOCs—60%, 70%, 80%, and 90%—to quasi-static radial crushing at speeds ranging from 9 to 20 mm/min. High-fidelity sensors continuously monitored load, displacement, voltage, and surface temperature to capture the exact moment of internal short circuit and potential thermal runaway.
Crucially, no thermal runaway occurred in batteries charged to 60% or 70% SOC, even under the highest compression speeds tested. Only at 80% and 90% SOC did the cells exhibit catastrophic failure—characterized by flame ejection from the positive-end safety valve, rapid temperature spikes exceeding 160°C, dense smoke, and electrolyte leakage. This establishes a clear safety boundary: SIBs remain mechanically stable under moderate charge levels, a reassuring finding for real-world driving scenarios where batteries rarely operate near full charge for extended periods.
Even more striking was the discovery of a thermal runaway critical speed threshold between 14 and 15 mm/min for 80% SOC cells. Below 14 mm/min, compression induced internal short circuits—evidenced by sudden voltage drops—but did not escalate to thermal runaway. At 15 mm/min and above, however, the same cells ignited violently. This speed-dependent behavior underscores the importance of dynamic loading rates in crash simulations. Unlike slow deformation, rapid impacts generate heat faster than it can dissipate, overwhelming the cell’s thermal management and triggering exothermic chain reactions.
For context, prior studies on lithium-ion 18650 cells report thermal runaway initiating at compression speeds as low as 5–6 mm/min under similar SOC conditions. The Ningbo team’s data thus suggests sodium-ion batteries can withstand mechanical abuse at nearly three times the deformation rate before reaching the point of no return. This resilience stems from inherently milder electrochemical reactions during failure: SIBs in thermal runaway peaked at 160–200°C, significantly cooler than the 250°C+ commonly seen in LIBs. Lower peak temperatures translate to reduced fire intensity, slower propagation, and greater time for occupant evacuation or intervention.
The mechanical response during compression followed a consistent four-stage pattern, offering engineers a diagnostic framework for assessing battery integrity post-impact. Stage I (0–2.10 mm displacement) involved elastic deformation of the steel casing with minimal internal contact. Stage II (2.10–5.35 mm) saw the jelly-roll electrode stack gradually compacted against the shell, entering plastic deformation. Stage III (5.35–9.35 mm) marked rapid load escalation as internal pressure built, culminating in separator rupture and internal short circuit near the peak load of 33–36 kN. Finally, Stage IV (>9.35 mm) featured structural collapse, massive short-circuiting, and—if conditions permitted—thermal runaway with load unloading due to material consumption by fire.
Importantly, the study also addressed a pragmatic concern: what happens to batteries that suffer minor, non-catastrophic damage? In real-world collisions, not every cell is crushed beyond recognition. Many may experience slight deformation without internal shorting. To evaluate their reusability, the team compressed 0% SOC cells to 1, 3, 5, and 6 mm and then subjected them to standard 0.5C charge-discharge cycles. Cells compressed up to 5 mm retained over 80% of their original capacity—a common industry threshold for end-of-life—and showed only modest increases in charging time. However, the 6 mm compression group suffered a dramatic capacity drop to just 1,079.5 mAh (61.9% retention) from a baseline of 1,742 mAh, effectively rendering them unusable. This establishes a clear mechanical safety margin: radial deformation under 6 mm may permit secondary use, but beyond that, replacement is essential.
These insights carry profound implications for EV battery pack design. Engineers can now leverage the higher mechanical tolerance of SIBs to optimize crash structures, potentially reducing the need for excessive protective armor that adds weight and cost. Moreover, the well-defined failure thresholds enable more accurate simulation models, improving the fidelity of virtual crash testing. For battery management systems (BMS), the data supports the development of impact-detection algorithms that monitor for sudden voltage drops or temperature anomalies within the critical speed and SOC windows.
The automotive industry’s pivot toward sodium-ion technology has accelerated in recent years, driven by supply chain vulnerabilities and geopolitical risks associated with lithium and cobalt. Companies like BYD, CATL, and Northvolt have all announced SIB development programs, with some already deploying them in low-speed EVs and energy storage systems. However, widespread adoption in passenger cars hinges on proving safety parity—or superiority—relative to mature lithium chemistries. This study directly addresses that concern, offering empirical validation that SIBs are not just cheaper, but inherently safer under mechanical stress.
Professor Dianbo Ruan, the corresponding author and a leading figure in China’s advanced energy storage research, emphasized the practical relevance of the work: “As sodium-ion batteries move from lab to road, understanding their failure modes under real-world abuse conditions is non-negotiable. Our data provides a foundation for safer, lighter, and more cost-effective battery systems that can accelerate the transition to sustainable transportation.”
The research also highlights a subtle but critical distinction in failure morphology. Unlike LIBs, which often vent violently from multiple points, the SIBs in this study consistently ejected flames exclusively from the positive-end safety valve. This predictable failure path simplifies thermal runaway mitigation strategies, such as directional venting channels or localized fire suppression systems within the pack.
From a materials perspective, the study indirectly validates the robustness of hard carbon anodes and layered oxide cathodes in SIBs under mechanical duress. While the exact mechanisms behind the higher critical speed remain under investigation, the authors hypothesize that differences in ion size (Na⁺ vs. Li⁺), electrode kinetics, and electrolyte stability contribute to the delayed onset of runaway reactions. Future work will explore these variables in greater detail, including the role of solid-electrolyte interphase (SEI) layer mechanics and separator material properties.
Regulatory bodies may also take note. Current EV safety standards, such as UN GTR No. 20 and FMVSS 305a, are largely calibrated around lithium-ion behavior. As alternative chemistries gain traction, these frameworks will need updating to reflect chemistry-specific failure thresholds. The Ningbo University data offers a robust benchmark for such revisions, ensuring that safety regulations evolve alongside battery technology.
In summary, this comprehensive experimental investigation delivers three key contributions: first, it quantifies the SOC and speed thresholds for thermal runaway in cylindrical SIBs; second, it establishes a four-stage mechanical failure model applicable to crash diagnostics; and third, it defines a practical deformation limit (6 mm) for assessing the reusability of damaged cells. Together, these findings significantly de-risk the integration of sodium-ion batteries into mainstream electric vehicles.
As the global EV market surpasses 14 million annual sales and climbs toward 30 million by 2030, the demand for safer, more sustainable, and economically viable energy storage has never been greater. Sodium-ion technology, long touted for its cost and resource advantages, now has compelling safety credentials to match. With rigorous studies like this one bridging the gap between academic research and industrial application, the road ahead for sodium-powered mobility looks not only greener—but safer.
Study on the Safety Characteristics of Flat Plate Compression of Sodium-Ion Batteries
by Yuzhe Ma, Jun Yang, Zeyang Cao, Zhijun Qiao, and Dianbo Ruan
Faculty of Mechanical Engineering & Mechanics, Ningbo University, Ningbo 315211, China
Institute of Advanced Energy Storage Technology and Equipment, Ningbo University, Ningbo 315211, China
Published in Chinese Journal of High Pressure Physics, Vol. 38, No. 6, December 2024
DOI: 10.11858/gywlxb.20240750