(i) is the active material electrode area (half cell) affecting the capacity as well as OCV ?
(ii) is the counter electrode area (lithium foil, half cell) affecting the capacity as well as OCV ?
(iii) is the Separator thickness (thicker one, half cell) affecting the capacity as well as OCV ?
(iv) is it good to do aging of the half cell for 10-12 hrs in glove box before charge/discharge measurement ?
Please respond with your valuable comments.
Thanks
Regarding the OCV--this is a thermodynamic value based on the reduction potential difference of the two electrodes with no current flow and therefore is not affected by the area of the electrodes. However, cell capacity is dependent on the individual electrode surface areas and even half-cell configurations can be fabricated such that either the active material or the counter electrode can be the capacity-limiting component. For active material characterization, typically a large excess of the counter electrode (i.e. lithium) is used but in an actual cell, careful mass/capacity/area calculations can maximize material utilization.
With the proper electrolyte, the separator should have a negligible effect on the OCV or the capacity, however, it can have a considerable effect on the cell operation once current is flowing. Things like salt concentration, separator porosity, and electrolyte wetting can also be important parameters.
For cell aging, if the separator is visibly wetted by the electrolyte during cell fabrication then any aging step is most likely unnecessary.
Good luck!
I think it is good to keep the cell for an equilibrium time of 2-3 h. For large format cells it can be extent to 12- 24 hrs.
Prakash
Hi Dr Alok
I think and i agree almost with Marshall. OCV doen't affect with area , of course oxidation and reduction peaks height may change but difference remains constant. In case case of capacity, lot of factor contributing, area, active material weight, thickness of all components, electrolyte quality.
Pura Ram
Parameters which can affect OCV:
- Electrode Surface Area (not form factor of electrode large or small)
- Electrode making recipe (AM:Carbon:Binder)
- Corrosion of the current collector
- Reaction of the electrolyte with active material
- State of Charge of the electrode
- Stack pressure on the cell etc.
Parameters which can affect Capacity:
- Separator thickness if it's limiting the transport (may affect at high rate)
- Electrode thickness (Kinetic limitation, find out the optimum rate)
- Long term aging may affect the capacity, as it increases the impedance (kinetic limitation, find out the optimum rate) etc.
So if you are making coin cell make sure :
- Make good electrode
- Use always large Li as counter and make sure you remove the surface impurity
- Make sure no hardware corrosion from coin cell parts
- Overlap between the electrode and stack pressure
Hope it helps.
Thanks
Thanks a lot Nupur Nikkan Sinha.
Cu-Foil Current collector thickness has also any affect on capacity ?
Hi Dr. Alok Kumar Rai,
In typical fabrication of a half cell, e.g. Coin cell (CR2025), the most important thing, i think, is to make a fine electrode. Of course, the proficiency in fabrication can also affect the performance of the half cell. Detailed favricating procedure can be seen everywhere in papers about Li-ion batteries. The thickness of the electrode should be controlled to 25-30 μm, cannot be too thick. Active material loading 3.0-4.0 mg cm-2 is good. The capacity of a half cell depends on the Specific Surface Area of active material because larger surface area is favourable for lithium ions diffusion. You have to make sure the area of the counter electrode (Li foil) is larger than that of the working electrode. Mershall has provided some valuable answers to your separator and OCV questions and I agree with that. As for the aging part, 3-5 h is just fine. We did some comparable experiments about aging times for those half cells and shown no significant influence.
Hope it helps.
Good luck !
Try to contact Dr. David Anseán González (University of Oviedo, Gijon, Spain) that has just very successfully defended his PhD thesis High Power Li-Ion Battery Performance: A Mechanistic Analysis of Aging, with extensive utilization on half cell fabrication and testing.
Regards
All the questions have been clearly answered by Marshall. Only I should add on aging time , this can be optimized by recording OCV after a given interval of time (say 1 hour),when equilibrium will reach OCV will not change with time.
Hi Dr. Alok
The information from these questions is very fundamental and will be very beneficial when we optimize the cell conditions.
First of all, you have to make a fabrication and design form the specification (such as thickness of electrode with current collector, separator, spacer and spring), meaning that you better calculate the expected thickness of all components. I think, you can optimize the condition of cell fabrication with CR2032, or CR2025 and so on, by trying electrochemical test with those, many times.
1. Area of electrode materials doesn't make difference of the intrinsic properties like thermodynamic characterization (OCV; Open Circuit Voltage). However, the observed capacity(mAh) can be changed in terms of the electrode area if you mean the area as electrode area(2 cm2 with 16 mm of diameter), not the specific surface area of materials.
2. The counter electrode is should be larger than cathode electrodes. For example, you have to cut into bigger than 16 diameter if you cut the cathode into 16 diameter . It can be simply understood, provided that the direction of reaction of lithium ions from cathode is parallel to anode surface, meaning that lithium should be fully extracted from cathode and reacted with lithium metal.
3. As same as question #1, OCV should be identical regardless of the difference of separator thickness. The capacity might be altered due to the porosity of separator, which can be affected to the movement of lithium ions. The rate performance will be changed as the thickness of separator.
4. The cell assembled in glove box should be aged for wetting the electrode enough. But, if you use wet separator in electrolyte and assemble a cell in good manner, there is no significant difference between those.
Best Regards.
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