Batteries: Is the current a diffusion current?

  • #1

Main Question or Discussion Point

Greetings,

I realized that I don't understand a fundamental fact of common Li-ion batteries.

During the charging process, electrons are forcefully extracted from the cathode and pushed into the anode. Charge balance then yields a flow of positive Li ions from the cathode to the anode (through the electrolyte), where they are bound by the additional electrons.
That implies that both electrodes are still electrically neutral. However, the anode has higher chemical potential as more particles accumulated there (electrons and Li ions).
But this would mean that the current during the discharge process is actually a diffusion current. Since both electrodes are electrically neutral, there should be no electrostatic force between them.

Is this correct?
 

Answers and Replies

  • #2
Baluncore
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Since both electrodes are electrically neutral, there should be no electrostatic force between them.

Is this correct?
If there is a voltage difference between two electrodes then there is an electric field and also an attractive force between the electrodes.
 
  • #3
If there is a voltage difference between two electrodes then there is an electric field and also an attractive force between the electrodes.
So where does it come from then? There should be no net charge accumulation at the electrodes shouldn't it?
 
  • #4
nsaspook
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Remember the electron flow path is external so we have charge separation (an electric field) inside the cell.

 
  • #5
Baluncore
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Is the current a diffusion current?
It depends on interpretation. The language becomes difficult and the terminology changes where electronics, current and voltage, meets with the chemistry. You should stand on one side or the other.

An electrochemical cell is composed of two separate half-cell reactions. The reactions proceed together only while electrons are removed from the negative electrode and returned through the external circuit to the positive electrode.

There must also be a diffusion of ions through the electrolyte to supply or balance the external electron flow. The maximum current available from a cell is determined by the rate of the chemical reaction, limited by the rate of ion diffusion, which increases with temperature.
 
  • #6
Remember the electron flow path is external so we have charge separation (an electric field) inside the cell.

Thanks for the video. The part I don't understand is 4:30.
Why would Li ions want to move back? Fine, they might prefer the state in the metal oxide, but at the moment, they are spatially separated from there. They will only move there if there is a force that pulls them there. But where does this force come from? The same positive and negative charge travelled from left to right, there should be no electric field.

It depends on interpretation. The language becomes difficult and the terminology changes where electronics, current and voltage, meets with the chemistry. You should stand on one side or the other.
That's pretty frustrating. One would have thought that a MSc. in physics teaches you the correct terms to describe an every-day-thing such as a battery. I always feel like the explanations found online are way too simplified. Or my understanding is just too poor.

Btw. why can the Li ions cross the separation layer, but the electrons can't? The latter are thousands of times smaller. It can't be a potential barrier, otherwise the Li ions would not be able to return on their own.
Man, this is confusing o_O
 

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