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Li-ion battery positive electrode attraction

  1. Aug 10, 2014 #1
    I don't understant how the positive electrode can attract both electrons and the positive Li ions as they both flow to it. Also, the way to start the whole process is to let the electrons flow, right? (Close the circuit.) Without it, the ions also don't want to start flowing, although there is nothing else stopping them.
     
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  3. Aug 10, 2014 #2

    mfb

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    If you do not close the circuit, some ions go to the electrode until it accumulates a small positive charge - this charge then repels the other ions so the process stops.
    If you close the circuit, that does not happen and the concentration difference and chemical potential between the two sides leads to a flow of lithium ions towards the positive electrode.
     
  4. Aug 11, 2014 #3
    So some ions diffuse to the other side and will form a weak positive side there. But won't there be much more Li ions in the graphite still, so won't that side be stronger? How come the electrons want to migrate to the other side where there is just a "thin layer" of Li ions that diffused there?
     
  5. Aug 11, 2014 #4

    mfb

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    What do you mean with "stronger"? The graphite side has ions and electrons, so it is uncharged. As soon as some ions go to the other side, it gets a negative charge and the other side gets a positive charge, driving electrons through the circuit.
     
  6. Aug 11, 2014 #5
    Sorry, I was under the impression the graphite was full of Li ions, presenting an overall positive charge. (Neutral graphite+Li ions=positive.) Or do the additional electrons come from the electrolyte to the graphite? That's my problem, all those nice drawings and animation are misleading in explaining this.
    Also, if I remember correctly, the flowing electrons are supposed to be ripped off the Li ions, making them even more positive. So the real question is...which ingredient is where originally...and what goes where. But I can feel it's getting clearer in my mind :-)
     
  7. Aug 11, 2014 #6

    mfb

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    A general rule: macroscopic objects are always extremely close to neutral. You'll never see an imbalance of 0.01%, for example (like 9999 electrons per 10000 protons). Such a material would instantly explode from the mutual repulsion of the positive charges.

    The graphite is full of Lithium atoms, so it is a collection of "Lithium ion plus electron".
    They have a single positive charge when they go to the other side.
     
  8. Aug 12, 2014 #7
    Oh, I'm getting it, then. So it is actually Li metal incarcerated in the graphite. By closing the circuit, we allow an oxidation process to start, so the electrons can flow thru the circuit (ripped off from the Li metal) and the resulting Li ions migrate to the other side, going after the electrons?
     
  9. Aug 12, 2014 #8

    mfb

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    There is no "after", those processes happen at the same time, but yes.
     
  10. Aug 13, 2014 #9
    "After" as in the spacial sense, not temporal. I could have said "following" but it has a temporal meaning as well :-) So the Li ions are sort of dragged along.
    Okay, thank you, then!
     
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