Charge is Lithium move from cathode to anode and discharge is when lithium moves from anode to cathode.Then how discharge capacity is higher than charge capacity in lithium ion half cell.
Hello Ashish,
What are the charge-discharge rates ? What technology of Li-ion are you studying ?
The measured capacity may depends on the way the half-cell has been charged prior to the measurements. That is to say:
- if the half-cell has been charged by using a CCCV protocol in the first place, it has reached a high SoC (state of charge) value, let say 100%.
- then, by discharging at constant current you may reach 5% SoC, which gives you 95% of the total capacity for this discharge.
- finally, by charging at constant current you may reach 95% SoC (because of the cut-off voltage), which gives you only 90% of the total capacity for this charge.
The temperature variations during discharging and charging may also affect the measured capacity.
I hope this answer will help you.
Regards.
Nicolas Damay
Dear Ashish,
You are right: in a lithium ion battery during charge lithium ions moves from cathode to anode and invertedly during discharge. In lithium ion half cell (as a rule with lithium foil as counter and reference electrode) charge capacity (lithiation of anode) is higher than discharge capacity (delithiation one). Irreversible losses in charge capacity (Qir = Qch – Qd) are due to the binding of lithium ions into some products taking no part in the further electrode discharge process: solid electrolyte interface (SEI) formation, irreversible capture in the electrode material surface and bulk is also possible, moreover some lithium irreversible can be captured on the surface of Cu foil (as a result of CuO/Cu2O electroreduction with formation of lithium oxides and accommodation of some lithium ions on the grain boundaries structure).
Best regards,
Dr. Kuksenko
More information is necessary to answer this question. At first, you have to be sure that it is happening repeatedly. For that, you need to do multiple experiments. Actually, Coulombic efficiency cannot be greater than unity. A maximum value is one.
Actually, Coulombic efficiency cannot be greater than unity as explained by Kuksenko. A maximum value is one.
Several possible reasons:
1. As suggested by Nicolas, your testing protocol may have great effect on the charge-discharge capacity. Conduct the test under lower rate may help you to know better of your system. You may also want to do a CV scan at extreme low rate
If you charge and discharge your anode half-cell at high C-rate, an oscillation may occur in voltage profile of discharge process, which can prolong the discharge time. This happens because the voltage reaches to its upper limit (for example 2 or 3V) in a time longer than normal time. But this will not happen at low C-rate of discharging process. The reason of this behavior is disability of Anode for delithiation at high C-rate.
We did some rate-performance experiments on anode at different c-rate and we observed that the charge capacity of the anode (delithiation) is higher than the charge capacity (lithiation) at low c-rate. The charge and discharge capacities of the anode are the same at high c-rate. Any comment? Do you have any reference to cite for this observation?
Best regards
In a half-cell set up the Li chip is always negative and your material is always positive (regardless of it being graphite, NMC, silicon).
From a thermodynamics point of view, in a half cell, lithium ions spontaneously want to lithiate your material. It means that once they do the potential decreases.
So decrease in potential = lithiation
and
increase in potential = delithiation
So discharge means lithiation of your working electrode material and charge delithiation of the working electrode material.
