How safe are polyanion sodium cells?
At Kurt.energy safety has alwyas been one of our main concerns. This is the reason why we exclusively focus on hybrid supercapacitors. Recently we added the novel soium-ion cells and batteries. We analysed manufacturer’s report and claims and also did our own test like overcharging and short-circuiting them.
So what does the literature tell us about poly-anion sodium cells? Thse cells offer upto 6000 cycles and have acceptable parameters for many applications, without using lithium and problematic materials.
Polyanionic sodium-ion (Na-ion) cathode materials are considered among the safest options for sodium-ion batteries due to their rigid structure and strong covalent bonding between transition metals and poly-anionic groups, which provides excellent thermal stability and minimal volume change during Na⁺ insertion/extraction, thus reducing thermal runaway risk .
Key Safety Features of Polyanionic Sodium Cells
– Thermal Stability: Poly-anionic compounds exhibit strong covalent bonding (stronger than Metal-Oxygen bonds in layered oxides), resulting in the least thermal runaway behavior among cathode families . They also have a low internal resistance, reducing the heat build-up due to the current.
– Mechanical & Abuse Testing: Cells using poly-anionic materials (e.g., Na₃V₂(PO₄)₂F₃) have passed rigorous tests, including overheating, overcharging, nail penetration, and short circuits, with no thermal runaway observed .
– Cycling Stability: Excellent cycling stability. 6000 cycles and more and high thermal stability make them suitable for long-term use .
– Low Risk of Sodium Plating: unlike hard carbon anodes (which risk sodium plating near 0–0.1 V vs. Na/Na⁺), polyanionic systems often use anodes with higher redox potentials (e.g., Ti-based anodes at 0.5–1.0 V), avoiding dendrite formation .
Comparison with other Sodium based cathode types
| Cathode Type | Energy Density | Thermal Stability | Key Safety Notes |
| Polyanionic | Lower | Best | Strong covalent bonding, minimal oxygen release |
| Layered Oxides | Highest | Poor | Prone to oxygen loss and structural instability at high temps |
| Prussian Blue Analogs (PBAs) | Moderate | Moderate | Risk of toxic HCN/cyanogen gas under extreme conditions |
Limitations & Considerations
– Lower Energy Density Poly-anionic materials trade energy density for safety, as their higher redox potential reduces specific capacity .
– Environmental Concerns: Some poly-anionic chemistries (e.g., vanadium-based) raise sustainability issues; research is ongoing to replace V with earth-abundant elements like Al.
– BMS and system set-up must still ensure that maximum current limits are observed.
Conclusion
Polyanionic sodium cells are **exceptionally safe**, with robust performance under abuse conditions and minimal thermal runaway risk. Their primary drawback is lower energy density compared to layered oxides, but they are ideal for applications prioritizing safety and longevity .
Poly-anion cells and batteries available from Kurt.energy
– 50 Ah, prismatic. Typical battery: 600V nom., 30 kWh, 5C capable dicharging, charging at 0.5C
– 160 Ah, prismatic. Typical battey: 48V nom., 7.68 kWh, 1C capable discharging, charging at 0.5C.
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