42technology

Power storage: On the safe side with the right battery technology

Lithium iron phosphate (LiFePO) batteries

Not all electricity storage systems are the same. The installed battery technology varies and so does the safety of battery storage. We untangle the threads and describe here the different battery technologies and their associated advantages and disadvantages.

Safety always comes first. Because if this is not the case, the consequences can be devastating: Worst-case battery storage can also burn, creating a blast wave - a deflagration. A deflagration is a rapid combustion process, also called "deflagration", and is caused by slightly slower combustion than detonation.

The lever at which battery technology should start is therefore fire prevention. And the way to achieve this is by choosing the right form of technology.

 

The most important facts at a glance:

  • In principle, it is very unlikely that battery storage units will start to burn.
  • Especially in the stationary area, there are hardly any external influences that can damage the storage unit and thus endanger it.
  • However, even if the probability of damage to the storage unit due to external influences is very low, there are some safety-relevant aspects that should be taken into account when choosing the storage technology. After all, there are a number of good reasons why we at neoom Batterien deliberately choose lithium iron phosphate technology. We explain what these are in the following technical explanations.

 

Rising Star: Lithium-ion batteries

Within lithium-ion batteries, two main types are distinguished on the basis of cell technology: Lithium nickel manganese cobalt oxide (NMC) and lithium iron phosphate (LFP) based cells.

Lithium-nickel-manganese-cobalt oxides (NMC)

NMC technology also has advantages: The batteries can be charged quickly and have a high energy density, but they do not get old. They are the right choice for electric cars because of their high energy density and small footprint. The disadvantage: In the case of microcracks, a lot of energy escapes in a small space, which leads to high heat and thus a fire. But it is not only microcracks that can cause fires in cells with chemically and thermally unstable cathode material such as lithium cobalt oxide (LCO) or lithium nickel manganese cobalt oxide (NMC). Severe heat generation from overcharging, an internal or external short circuit, mechanical damage, production-related contamination, or severe external heat exposure can also cause an in-cell exothermic chemical reaction. What then follows is a devastating chain reaction: the released thermal energy increases the reaction rate of the cell chemistry and causes the cell-internal temperature to rise further. If a specific temperature limit is exceeded, this self-accelerating process can no longer be stopped. This temperature limit depends on the cell chemistry used. In the case of lithium cobalt oxide (LCO), for example, it is 150°C. When the temperature limit is reached, thermal runaway occurs, which can eventually lead to fire or explosion of the cell. Because the oxygen bound in the cathode material (it is, after all, lithium cobalt oxide technology) is released, such a cell fire is very difficult to extinguish and burns even under water.

Tests with a simulated short circuit show the occurrence of very high temperatures of over 700°C, which can melt the separator between the individual cells and thus spread to the other cells and result in the aforementioned extinguishable fire.

This is less critical in a car because a fire is often noticed quickly and the electric car can usually be abandoned quickly. Tesla therefore installs LFP batteries in its electric cars, among other things. Finally, one more advantage: NMC batteries are now cheaper due to the mass production of the automotive industry.

Lithium iron phosphate (LiFePO) batteries

Lithium iron phosphate batteries, on the other hand, have a lower energy density than many other cathode materials such as NMC or NCA due to their lower nominal voltage of 3.2 V, and thus require more space.

Therefore, they are better suited for home storage because space is not an issue in the basement - unlike a car. If micro-cracks do occur, LiFePO cause less heat and thus do not pose a fire hazard. In tests, even fully charged lithium iron phosphate batteries have not shown a comparable reaction to NMC or NCA cells. The Li-Fe-PO batteries did not burn or develop critical temperatures that could melt the separator or spread to other cells. This was also found by an independent study sponsored by the German Federal Ministry for Economic Affairs and Energy ("Compendium: Li-ion batteries") by the Association for Electrical, Electronic & Information Technologies (VDE) and the German Commission for Electrical, Electronic & Information Technologies (DKE) found this. The study compares the various lithium battery technologies and concludes, "In contrast to oxides, lithium iron phosphate (LFP) shows no thermal effects up to 300 °C. Among other things, this puts LFP out of contention in terms of safety." And emphasizes once again: "In the event of an accident, the oxides can lead to the development of oxygen with fire consequences. NCA (lithium nickel cobalt aluminum oxide) is particularly critical in this context."

Tortoise vs hare

The differences between NMC/NCA and lithium iron phosphate technology can be illustrated pictorially:

Do you know the fable of the race between the tortoise and the hare? If not, here is the summary in a nutshell: In the fable by the ancient Greek fabulist Aesop, a hare taunts the slow tortoise. The tortoise then challenges the hare to a race. Quickly, the hare is ahead by the hare's length and far ahead of the tortoise. Confident that he will win the race anyway, he takes a nap during the race. But when the hare wakes up, he finds that the tortoise, who was walking slowly but steadily, had won the race. The moral of the story: perseverance wins.

And that's why we prefer to rely on tortoises rather than hares. In our example, the tortoises are the lithium-iron-phosphate batteries: they charge more slowly, have a low energy density, but live a very long time.

The rabbits, on the other hand, are the batteries with NMC / NCA technology: fast, a high energy density - but they do not get old. The fact that they charge faster also does not bring any advantage, because home storage is not charged in 5-10 minutes, but in 3-4 hours. As a result, they lose the race.