Hi all,
If I were to use any kind of graphite to get fast charging (lithiation), say to get my EV super fast charging, which one would I choose, and which protocol may I use? CCCV or plain CC? Or some pulsed techniques ?
We tend to see the systematic use of CCCV with fairly low current cut-off values for the CV part in the literature, and the lower the current is, the longer that charging gets. The LiC12->LiC6 phase transition (half of the capacity) seems to limit the reaction rate importantly. Can we tackle that in some ways ?
- By breaking down particle size but that increases ICE
- Increase electrode porosity ? Would not that decrease particle electronic interconnectivity. A trade off to be found I guess.
- Add additives to connect particles?
On the side, how bad can an electrolyte additive be for fast charging rate ?
Thanks for reading and sharing your thoughts :)
Dear Loïc! In my understanding beside the intrinsic quality ("speed") of the graphite material (type, grain geometry, grain size distribution, etc.) itself, the anode porosity plays a key role. As the Li-transport via the liquid phase (electrolyte in pores) is normally much faster than via the solid phase (graphite) a high calendar degree of the anode always heavily decreases the max. charge C-rate . Electric conductivity normally should not be a bottleneck at a graphite anode, even if not calendared hard.
Beside that during charging/fast charging also microscopically occurring and not visible Li-plating appears to be the key reason for capacity decrease/ageing/power fade of the cell (see Liu, Q. Q.; Xiong, D. J.; Petibon, R.; Du, C. Y.; Dahn, J. R., Gas Evolution during Unwanted Lithium Plating in Li-Ion Cells with EC-Based or EC-Free Electrolytes. Journal of the Electrochemical Society 2016, 163, (14), A3010-A3015.)
When using graphite, fast charging/"supercharging" always seems to result in fast ageing.
Regards, Michael
Good evening Michael,
Thanks for your answer. I can imagine Li plating at high surface current densities, with large concentration gradients form the surface to the bulk of large particles.
I wonder which porosity is best, I often read 30-40% but some use much larger values for thicker electrodes.
Would these higher values be reasonable for the industry ...?
Moreover, is there any fast method to charge graphite? CCCV, or multiple CV steps, ?
Is 60% capacity achievable in 30mins, say if the battery needs to be charged from 20 to 80%?
What is the optimal temperature? I read battery packs can be kept above 30c to help the kinetics.
So many unknown, or untold !
Best,
Loïc
Dear Loïc!
Yes, in my experience for high-power batteries around 35% porosity is usual. But there are cells for EVs with a design focus on high energy and thick anodes with 25% only. This slows down the max. charge C-rate a lot. Porosities of >40% can have the effect, that the electric bonding between the graphite particles gets to loose and the electric impedance rises (->slow max. discharge rates), which is also not desired.
From the charging programm viewpoint the rules in my experience are: Not to fast at low SOCs (could damage cathode material ->rock salt structures e.g. on NCA) and high SOCs, full speed in the middle. Temperature rise is beneficial for max. charge speed because of speeding up the diffusion processes 2-3 times each 10°C. If not kept to long, 45°C should not be a problem.
60% SOC charging in 30 mins: is possible, but dependant on the battery design. Charging faster than 1.0-1.5C especially on high energy designs with thick anode layers lowers the expected battery cycle lifetime significantly.
Regards, Michael.
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