While doing repeated cycling (charge & discharge) of LFP cells from different manufacturers with Arbin tests shows coulombic efficiency(CE) more than 100%. The CE says between 99% to 102%.
The cells are in ambient temperature throughout while charging and discharging at 0.5C. The 0.5C rate is specified by the manufacturer as standard charge and discharge rate. Why is this happening?
I know LI-ion cells can have side reactions or there can be extra lithium metal plating on anode that can cause this.
But without going inside the cell, how can I know for sure what is causing this? Are LFP more prone to this?
I believe this can happen during early cycling in cells that contains extra lithium-metal foil on the anode to temporarily mask early capacity fading. BTW This is not my area of expertise, I just have an electric car and got interested in the common lithium cell chemistries used in vehicles.
Hi Khalid,
1st. I won't relate this topic to cathode chimestry i.e. LFP or NMC etc.
2nd. 0.5 Crate is a high current to measure CE so in this case the cell impedance effect can't be neglected.
3rd. You mentioned the CE distributed between 99% and 102%. How many samples were above 102%?
4th. How is the CE calculated? You did multiple cycles then integrated the charge/ discharge currents?
Generally speaking, CE is measured using low currents
Firas Alhaider,
1. That is fair. These behaviors are agnostic of the cathode chemistry.
2. 0.5C was specified by the manufacturer as "standard discharge current". The rated capacity is obtained at this standard discharge current and 25degree C. Wouldn't changing the C-rate from the standard discharge rate change the discharge/ charge capacity?
3. All the samples are below 101%.
4. CE is calculated by reported charge and discharge capacity in Ah given by the Arbin test equipment.
Any thoughts?
Khalid Khan
2. True when you measure the cell capacity but for CE applications low current e.g. C/20 is used to assure the impedance won't bias the results.
3. & 4. Then I might have an explanation:
Resistances during charge and discharge are NOT identical. For example, if the charging resistance was slightly higher than the discharging resistance, this means you can extract more Ah during discharging than charging before reaching the cut-off voltages. In this case, CE can show values bigger than 100%. This is not true but simply because the resistance biased the CE calculations.
That is why I believe if you used low current you could get more realistic CE calculations.
Hope that helps :-)
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