Lithium-ion Cell Degradation Mechanisms
Lithium-ion cells and batteries has become an important part of our life. Almost all of our battery powered devices like Mobile, Laptop, Music Players etc. are powered by Li-ion cells. This dependency is only going to increase with mainstream adoption of electric vehicles all of which are powered by Li-ion batteries again.
One of the primary reason for such a high acceptance of Li-ion technology is it’s long life. These batteries can last anything from a year to a decade depending on the application. Although slow, like any other battery these batteries also degrades over time. In this article we will be talking about different mechanisms of cell degradation.
Some of the factors that causes or accelerates degradation are as follows.
- High temperature operation
- Uncontrolled fast charging
- Uncontrolled fast discharging
- High voltage operation (over charge)
- Low voltage operation (over discharge)
When we peek inside these degradation mechanism at chemistry level there are two main factors (although there are many more).
1. SEI Layer Formation
2. Lithium plating
SEI Layer Formation
Anode material such as graphite whose potential is below 1.1V tends to react with the electrolyte which is commercially used in Lithium-ion cells, for example, Ethylene carbonate because of its instability of potential below 1.3V. This incompatibility allows the electrolyte to react with a negative electrode and form an irreversible product like lithium carbonate, lithium bicarbonate, (Li2CO3, LiF, Li2O), and consume lithium in the process.
For a better understanding of SEI layer consider SEI layer as corrosion. Once it’s first layer is formed, it won’t easily allow electrolyte to react with a negative electrode and that’s the reason we operate the cell at very low C-rate during its first cycle so that it will consume less lithium to form a thin layer of SEI and the same formed SEI layer act as a barrier between negative electrode and electrolyte. But it won’t stop the SEI layer formation completely because of mainly two reasons
First: Because of porosity in the layer which allows the SEI layer to form but at a slow rate which leads to cell degradation over time.
Second: During the intercalation or De-intercalation of lithium-ion leads to micro cracking in the SEI layer due to contraction and expansion on the surface. Similar to corrosion when there is a crack in the layer it will fill the void with a new SEI film and in that process, it will consume new lithium-ion.
Along with these two causes, there are other causes too which can highly accelerate SEI formation like operating the cell at a high temperature which leads to the dissolution of SEI or fracture due to mechanical stress
Unlike low potential negative electrode, there are other negative electrodes which operate at high potential like LTO, because of this high potential window we can mostly avoid SEI layer formation. This is one of the main reason why LTO have higher life than other cells. But it has its limitation, due to low overall cell voltage it has less energy density.
In 2000s lot of paper were published on the SEI layer formation and cell degradation
The models were developed to track the SEI growth and capacity loss and resistance increase due to SEI.
The Side reaction expressed as
Solvent(EC) + 2Li+ + 2e- = Product (Lithium carbonate, Li2CO3, LiF, Li2O, etc)
The kinetics of this irreversible side reaction can be described by the Tafel equation.
Now let’s talk about other main reason for cell degradation, Similar to the SEI layer, low potential negative electrode cause rapid lithium plating as compare to the high potential negative electrode.
A high charging rate is one of the main reasons for lithium plating due to lithium limitation on the surface of the negative electrode.
Due to the rapid intercalation of lithium-ion on the surface of the negative electrode, negative electrode gets saturated and the excessive lithium-ion trying to intercalate but no place to go, get plated into lithium metal because of its high kinetic energy. This lithium metal accumulated on the surface and edge of the negative electrode and consume lithium-ion on the process.
Lithium metal accumulated on the negative electrode reacts with electrolyte to form lithium carbonate or other irreversible by-products. This layer act as a barrier between lithium and electrolyte to prevent the formation of new irreversible products. This side reaction can lead to high capacity loss and electrolyte degradation. Operating at low temperatures can also lead to rapid lithium plating.
With the continuous fast charging, more and more lithium metal tends to accumulate on the surface of the plated lithium and form a dendrite structure. This growing dendrite structure of lithium metal penetrates the separator and short the positive electrode and due to its high conductive property, it causes an internal short circuit which either leads to dendrite fuse or thermal runaway due to high heat generation.