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Myth-buster: Lithium-ion Battery Chemistries and Safety — Part 1

Li-ion, is considered to be one of the most sophisticated pieces of technology in energy storage right now. It is also the most commercialized technology for its cost to performance ratio.

Expectedly, for such a versatile and widely used technology, there is a lot of misleading information. These are usually a result of ill researched facts, often undertaken by the marketing department of companies, who are largely or wholly restricted to working with a certain kind of battery chemistry. Subsequently, this results in the sharing of half — truths or sometimes, even outright wrong claims, which instead of providing clarity, creates further fogginess in the minds of the end user!

In this series, we will try to understand the full picture of Lithium-ion battery safety and their co-relation to its chemistry.

Different Chemistries in Lithium-ion Family

What we call Li-ion is actually a family of Chemistries. They differ by Cathode material (mostly) or by Anode material. There are major variations which should be included in Li-ion family or not is debatable, and then there are minor or very custom variations made for very specific purpose and majority of people are not even aware of those.

For the sake of simplicity we will limit our discussion to most common Chemistries which are well known and commercialized by multiple companies. These are

  • NMC (Nickel Manganese Cobalt)
  • LFP (Lithium Ferro Phosphate or Lithium Iron Phosphate)
  • NCA (Nickel Cobalt Aluminium Oxide)
  • LMO (Lithium Manganese Oxide)
  • LCO (Lithium Cobalt Oxide)
  • LTO (Lithium Titanate Oxide)

We will also occasionally talk about Chemistries or technologies outside of this list, mostly for making a point.

How Lithium-ion Cell Works

To understand the Li-ion battery safety concerns, we need to understand how these batteries works at cell level.

Without going in too much of detail, what we need to understand from above image is that when we are discharging the cell, the Lithium ions move from Anode to Cathode through electrolyte while the electrons move in same direction but through the external circuit (the load). While charging, the reverse occurs with Lithium ions and electrons moving back to Anode.

Lithium-ion cell components

Li-ion cell is made of (although not limited to)

  • Cathode: NMC, LFP, LMO, NCA, or LCO
  • Anode: Carbon (In Graphite form) or LTO
  • Separator: Ceramic or similar material
  • Electrolyte: Lithium salt (e.g. LiPF6), in organic solvent (e.g. Ethylene Carbonate)
  • Current Collectors: Aluminium and Copper

Lithium-ion Safety Concerns

Li-ion cell safety is compromised when any of the above mentioned components gets damaged or becomes unstable.

Any safety breach leads to a sudden release of stored energy raising the cell temperature to extremely high levels, a phenomenon, which is called, “thermal runaway”. This heat energy then reaches nearby cells pushing them into thermal runaway. It creates a chain reaction, jeopardizing the safety of the complete battery pack and ultimately can result in a fire hazard.

We will look at some of the major safety breaches and how they are related to chemistry of the cell.

Cathode Material Disintegration

Common Materials

Common cathode materials are NMC, LFP, LMO, NCA, and LCO. What is notably missing from this list is LTO because that is an Anode material. When we have an LTO battery, the Cathode will still be any of the common ones, although NMC and LFP are most widely used.

Failure mechanism

It is a well-known fact that cathode materials start to become unstable at high temperatures. This is called thermal runaway temperature and the temperature threshold is different for each chemistry, as seen in the table below:

Thermal runaway temperature of different Li-ion Chemistries

As can be seen, the LCO and NCA are having lowest thermal runaway temperature of 150 degrees centigrade. When a cell starts heating, (either because of thermal runaway of nearby cells or any external heat source or high current draw), these chemistries will be the first to start disintegrating and going into a state of thermal runaway. On the other hand, LFP has the highest thermal runaway temperature of 270 degrees centigrade. This makes it the safest Cathode material. LMO and NMC are midway at 250 degrees and 210 degrees centigrade, respectively.

This is the reason, why LFP is advertised to be the most safe Lithium-ion chemistry but it overlooks a lot of important factors. Let us understand what those are.

Factors overlooked

Safety of LTO Battery: LTO batteries are considered to be the safest of all Li-ion batteries. LTO batteries have NMC for their Cathode material and LTO is the Anode material. Even with NMC in place these batteries are extremely safe. This tells us that most accidents with Li-ion batteries are not related to Cathode material. Instead the stability of the Anode material, plays a much bigger role, and here LTO makes it much safer than Graphite.

Energy density: Energy density plays a big role in the safety of the battery. It is not hard to understand that more energy is released, from these high energy density cells, in the event of a thermal runaway. This high amount of energy increases the probability of a nearby cell reaching their thermal runaway temperature threshold. This can be understood from the fact that any cell chemistry, which fails the “nail penetration test” at full charge condition, passes the same test in a partially charged state. Partially charging a cell, in a way, is making it a low energy density cell. This is the reason why LTO and LFP are so safe, as they have much lower energy density of 70Wh/kg and 120Wh/kg respectively, when compared to 250+Wh/kg for NMC and others.

Conclusion

To conclude, we can say, that for a cell to go into thermal runaway, the deciding factors are:

i) The temperature at which the Cathode material will start disintegrating

ii) The amount of energy the cell is holding, as this will tell us the probability of propagation of thermal runaway in the adjacent cells.

If all other factors are same, LFP is the most stable Cathode material and LCO/NCA, the least stable. But this is only, half of the story. The much bigger factors are the stability of the Anode material and the energy density of the cells. These factors ultimately overshadow the advantage of LFP and dictates the overall safety of the battery.

In part 2 we will talk in detail about the safety risk associated with the Anode materials.

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