Li-Ion battery SEI layer, the critical process step in fabrication of Li-Ion batteries.

RCinFLA

Photon Sorcerer
A very critical process step in the fabrication of lithum Ion chemistry batteries, including LiFePO4 battery, is the initial formation of the Solid Electrolyte Interface (SEI) layer. It is crucial in protecting electrolyte from chemical decomposition facilitating long cycle life of battery. It was not originally recognized as a important part when LiIon batteries were first being developed. It is still not fully understood.

The formation process is done on a completed cell assembly. It is the first charging sequence that must be carefully done to ensure the optimum growth of the protective SEI layer with minimal damage to electrolyte layer, and minimal consumption of available Lithium. It begins with a low voltage charge starting at about 0.25v and cycles through several incremental steps of increasing voltage and current to reach its final state. Heating of cell is also used to accelerate the process. This process can take up to a couple weeks to complete. Cell impedance is measured to determine the progression of the SEI layer growth. Charging for SEI growth process is done by manufacturer and is not significantly modifiable by end user when battery is received by customer. The layer is a chemical transformation that is created between the graphite anode (negative side of battery) and the electrolyte interface. It consumes about 5% of the cell's available Lithium and thus reduces the cell's capacity.

Some continued growth of the SEI layer will occur during normal end user cycling of battery. Its continued growth consumes more of the Lithum reducing cell capacity and its increased growth thickness is an additional obsticle to the transfer of Lithium Ions increasing the impedance of cell over time.

This initial growth process is done in multi-cell batches and is one of the criteria in selecting matched cells for multi-cell battery construction. Because it is so process dependent, different batches cannot assure identical results. Because it consumes Lithium, and the amount of Lithium in a cell determines its capacity, it is important to matching capacity between cells. It also impacts cycle life degradation progression which should be closely matched for optimum longevity of multiple cell battery packs.

It is a very time consuming process in the fabration of LiIon batteries and therefore an expensive process step. A lot of work has been put into reducing the time and preciseness required of this process. Additives (manufacturers' special sauce recipes) have been put into electrolyte that are intended to help facilitate the formation of SEI layer, reduce the time required, and reduce the amount of Lithum consumed to get more useable capacity from cell. As in the case of medications, electrolyte additives side-effects may not always be fully understood.

The solid state battery, or 'silicon' battery under development has the promise of storing up to five times more ions then present graphite layer and hopefully reduce, or eliminate, the complexity of SEI layer formation.

LPF battery layers.png
 

RCinFLA

Photon Sorcerer
Additional info:

The graphite anode phyically expands by about 13% when fully charged. Because the SEI layer is a thin crust layer, the graphite expansion when fully charged puts extra stress on SEI layer creating small cracks, allowing more electrons to transition back into electrolyte which causes chemical decomposition of electrolyte. Subsequent charging will heal the cracks in the SEI but consumes some more of the available lithium, reducing capacity of cell, and thickening the SEI layer increasing battery impedance. This is part of cell aging degradation.
 

Sharky722

Solar Enthusiast
Additional info:

The graphite anode phyically expands by about 13% when fully charged. Because the SEI layer is a thin crust layer, the graphite expansion when fully charged puts extra stress on SEI layer creating small cracks, allowing more electrons to transition back into electrolyte which causes chemical decomposition of electrolyte. Subsequent charging will heal the cracks in the SEI but consumes some more of the available lithium, reducing capacity of cell, and thickening the SEI layer increasing battery impedance. This is part of cell aging degradation.
Thank you for sharing this.
I've been trying to find info on the mechanism of degradation and I think I just found it.

Do you think the process of the sei growth is accelerated via this mechanism when the cell is kept or stored at high SOC or even float charged constantly?
 

PeterBC

Solar Enthusiast
Is the initial formation of SEI layer normally done at the factory ? How would one know if new batteries have been thru this process ?
 

Picasso

Solar Addict
SEI layer is also known as cell forming, it tends to be the first charge from what I understand.
 

Attachments

  • cellgform.jpg
    cellgform.jpg
    27.8 KB · Views: 8

RCinFLA

Photon Sorcerer
Is the initial formation of SEI layer normally done at the factory ? How would one know if new batteries have been thru this process ?
The cell may bloat more at full charge and will not last the expected number of cycles with capacity and impedance degrading faster.

A lot would depend on how long it is maintained at full charge during useage. Holding at full charge without a proper SEI layer is most damaging because it allows more electron leakage back into electrolyte causing decomposition of electrolyte (bloating of cell). 'Normal' cell impedance is a good indicator. Abnormally low initial impedance can mean the process was short cycled. They usually target the process to an optimum cell impedance for a given AH/cell model indicating the optimum SEI growth thickness. Too much growth of SEI just increases cell impedance and consumes more lithium reducing cell capacity a few percent more.

Don't necessarily brag because your new cells have a lot lower impedance and more capacity then spec'd. It may just mean the SEI protective layer is thinner then it should be, so you live fast and die young.

It would be a major manufacturing screw up not to have it done.
 
Last edited:

Sharky722

Solar Enthusiast
The cell may bloat more at full charge and will not last the expected number of cycles with capacity and impedance degrading faster.

A lot would depend on how long it is maintained at full charge during useage. Holding at full charge without a proper SEI layer is most damaging because it allows more electron leakage back into electrolyte causing decomposition of electrolyte (bloating of cell). 'Normal' cell impedance is a good indicator. Abnormally low initial impedance can mean the process was short cycled. They usually target the process to an optimum cell impedance for a given AH/cell model indicating the optimum SEI growth thickness. Too much growth of SEI just increases cell impedance and consumes more lithium reducing cell capacity a few percent more.

Don't necessarily brag because your new cells have a lot lower impedance and more capacity then spec'd. It may just mean the SEI protective layer is thinner then it should be, so you live fast and die young.

It would be a major manufacturing screw up not to have it done.
Very interesting, seems we put a lot of trust on the manufacturer to do the right thing when cell forming. I exercised my cells a number of times before putting into service so I'm hoping this has assisted in forming a healthy SEI.
In relation to bloating/swelling when the cell is at full charge i understand the anode is full and a certain amount of swelling is natural.
What happens at the opposite end of the scale when a cell is severely discharged and becomes swollen?
 
Top