Unpacking battery safety: Sodium is safer than Lithium, but beware not all Sodium is fire-safe!
By Bruis Van Vlijmen, Mona Alsubaei and co-authored with Darren Tan, CEO and co-founder of Unigrid
More decarbonisation means more batteries everywhere, which we love. But, not all batteries are fire-safe, and some are less safe than others. Lithium-ion batteries are the workhorse of energy storage, due to good performance and production at scale, yet they can catch fire and this has already happened.
This inherent safety risk slows down mass-adoption of clean energy storage. A new emerging technology, Sodium-ion is being coined a ‘truly safe alternative - poised to make battery fires a thing of the past’.
To validate if all Sodium-ion is truly safe, we dug into actual testing data. The results show that Sodium can be safer than Lithium, but it’s not true for all Sodium technologies.
Why is fire safety a concern?
The widespread adoption of new technologies is intrinsically linked to their safety and public confidence in that safety. The cycle of innovation, safety enhancement, and increased public confidence has been a crucial driver in the development and proliferation of numerous technologies that now shape our modern world.
Batteries are not different. We rely on them for our appliances, vehicles, renewable energy systems, grid integration, and beyond. America alone needs 6 TWh of battery storage to meet its ambitious energy transition goals. Yet, despite their pivotal and growing role, battery safety remains an issue.
Last year, 1 in every 100 fires involved a battery, with residential ESS and e-bikes accounting for half of them—up 68% in just one year. By 2030, global fleets of home batteries and electric two-wheelers are set to grow 6x and >10x, respectively. If batteries keep catching fire at the same rate, we’re on track for a hot and very literal fiery mess.
The cause of battery fires: thermal runaway
This problem will only escalate as decarbonization accelerates. More batteries will be deployed everywhere, for increasingly larger applications. As battery sizes grow, so does their heat retention, amplifying the risk of fires due to thermal runaway—a phenomenon occurring inside battery cells, where increasing temperatures lead to internal material decompositions inside the cell, resulting in further heat release, and creating a dangerous feedback loop.
Bruis van Vlijmen (Transition Venture Partner) wrote a more detailed article on thermal runaway 101.
Fig 1: Schematic depiction of thermal runaway phenomenon, as a function of Temperature and Time.
How to derisk thermal runaway? UL9540a testing.
To ensure safety, we need to eliminate thermal runaway, not simply mitigate the fire consequences.
The risks of thermal runaway in a battery system are currently assessed through various regulatory certification processes, with the UL 9540a test being the most relevant.
The UL9540a testing system follows a hierarchical structure, where passing the test at the cell level (as opposed to just the module or system level) automatically qualifies a battery for UL9540a certification at all higher levels. Hence, cell-level UL9540a is the holy grail of battery safety.
However, in reality, reaching cell-level UL9540a is no simple feat. In fact, there are no Lithium-ion technologies that have passed the test at the cell level based on UL’s database. Amongst the few cell-level technologies that have passed the test are mainly Lead-acid and alkaline batteries. You can check it yourself, as UL publishes their UL9540a test database.
Most BESS (Stationary Battery Energy Storage System), can’t pass the test on cell level. They only pass this test at the module, system or installation level because they have built-in fire containment or suppression systems. For example:
Liquid or air cooling at the module-level.
Insulation at the pack unit-level.
Sprinklers or fire retardants at the installed BESS level.
Fig 2: UL9540a test procedure hierarchy
What does UL9540a test data tell us? Not all Sodium-ion is fire-safe.
Recently, Sodium-ion batteries (Na-ion) have been coined as a safer alternative to Lithium-ion (Li-ion). However, few Sodium-ion technologies have passed the rigorous cell-level UL9540a test, primarily due to their nascent stage of commercialization and limited availability of public safety data. Nonetheless, research institutions are actively exploring their fire safety.
A work that we want to discuss here is Rachel Carter at the U.S. Naval Research Laboratory (US NRL). Dr. Carter conducted in-depth study on Na-ion battery safety, funded by the U.S. Department of Transportation. Dr. Carter's approach to assessing battery fire-safety utilizes the Accelerated Rate Calorimetry (ARC) method, as described in Fig 3. In layman's terms: throw the cell in a battery-BBQ and wait and see till it catches fire. This is also part of standard UL9540a testing.
Fig 3: (left) Heat-Wait-Search ARC test protocol. (middle) Schematic of ARC test setup. (right) expected output data from an ARC test
The study compares Lithium-ion cells (LFP and Ni-rich NCA) and various Na-ion cells. Those Na-ion cells have varying cathode compositions: VPF (Vanadium Fluorophosphate, a.k.a. polyanion), NMF (Nickel Manganese Iron Oxide, a.k.a. layered oxide) and tmCN (transition metal ferricyanide, a.k.a. prussian blue analog).
Fig 4: (left) Temperature vs Time results from ARC test. (right) Incremental Temperature rate, with the green line indicating the thermal runaway threshold at 1°C/min. The red-highlighted NMF cathode Na-ion cells, do go into thermal runaway.
Some takeaways from this study, as derived from Fig 4:
Lithium-ion cells go into thermal runaway, unsurprisingly.
Some Na-ion are indeed fire-safe (Na-VPF and Na-tmCN), but not all.
Na-NMF cathodes show alarming susceptibility to thermal runaway. Surprisingly, these Na-NMF cells exhibit thermal behavior akin to high-energy Li-NCA (Lithium Nickel Cobalt Aluminum Oxide). There is much higher thermal runaway risk for the Na-NMF even than LFP batteries.
This is not entirely unexpected, as NMF cathodes share a very similar Ni-layered structure to NCA and NMC batteries. This similarity gives NMF Na-ion batteries a greater energy density advantage, although it compromises safety. This contradicts many safety claims about this Na-ion chemistry in their marketing.
Can UNIGRID overcome this barrier?
UNIGRID, (Transition portfolio company) developed a high energy density Na-ion technology. Using a high energy density Tin-alloy (Sn) anode and a Chromium-based (NCO) cathode.
Similar to the NMF cells, mentioned in the previous section. The UNIGRID Sn-NCO cells also have a layered-oxide cathode, and are high energy density, but don't come with the higher risk of thermal runaway.
UNIGRID put their cells to the same ARC test, to test for fire safety. However, unlike the NMF cells, the UNIGRID Sn-NCO, do not demonstrate catastrophic thermal runaway. In Fig 6 you can see less fire-risk than LFP. Staying below the >1°C/min threshold, UNIGRID’s high energy cell displays no thermal runaway at all.
Fig 5: (left) Temperature vs Time results from ARC test on the Sn-NCO 18650 cell. (right) Incremental Temperature rate, with the green line indicating the thermal runaway threshold at 1°C/min. The UNIGRID Sn-NCO 18650 does not reach thermal runaway. Check the UNIGRID’s whitepaper* for the full dataset.
So what?
We’ve learned that fire-safety is a real concern already today, and more so in the future as society continues to electrify. UL9540a is the industry standard to test fire-safety. No LiB technology has reached UL9540a on the cell level, the holy grail of safety.
UNIGRID now is on the brink of passing the cell-level UL9540a test. With cells that have an energy density akin to Lithium-ion LFP technology, combined with the safety expected from Sodium-ion. This means that the trade-off between energy density and safety is no longer a concern. Truly a no-compromise solution.
In 2025, UNIGRID will put its first cells to the test to get 3rd party certification. They can be the first ever to meet the holy-grail UL9540a certification with a high-energy commercial cell.
If you care about battery safety and want to help make the energy storage landscape fire-free, please feel free to reach out!