I'm confused by the following from this webpage:

The cathode is a metal oxide and the anode consists of porous carbon. During discharge, the ions flow from the anode to the cathode through the electrolyte and separator; charge reverses the direction and the ions flow from the cathode to the anode.

On discharge, the anode undergoes oxidation, or loss of electrons, and the cathode sees a reduction, or a gain of electrons. Charge reverses the movement.

This says that the metal oxide electrode is always the cathode and the porous carbon electrode is always the anode. As far as I know, that designation should be correct on discharge but backwards on charge. The anode is always whichever electrode is doing the oxidation, and the cathode is the electrode doing the reduction.

My naive understanding would be this:

  • The cathode is the electrode with the reduction half reaction. The anode is the electrode with the oxidation half reaction. This is true for both charge/discharge.
  • The anode is the one that produces electrons and cathode receives electrons. This is true for both charge/discharge.
  • When switching between charge/discharge, the redox reactions reverse, and the cathode/anode designations also switch to preserve cathode==reduction and anode==oxidation.
  • During discharge, the battery functions as a galvanic cell, where the redox reaction produces electric energy, electrons go down their electrical gradient from the negative electrode to the positive electrode. The anode is the negative electrode, the cathode is the positive electrode.
  • During charge, the battery functions as an electrolytic cell, where electric energy drives a nonspontaneous redox reaction, electrons go up their electrical gradient from the positive electrode to the negative electrode. The anode is the positive electrode, the cathode is the negative electrode.
  • In a lithium ion battery, the positive electrode is the metal oxide and the negative electrode is the porous carbon. The anode/cathode designations switch depending on whether the battery is charging or discharging.

Please help me understand this.

up vote 2 down vote accepted

I have asked experts -> I am right. The common convention is to label the physical electrodes as cathode/anode based on discharge even though that will be backwards when the battery is charging. That is a poor, confusing, and illogical naming convention, but it is what it is.

The more clear and logical naming convention would be to name the physical electrodes either positive or negative since those labels apply in both charge/discharge directions. Some people use this logical naming convention, but the illogical convention seems more prevalent.

  • Have to agree with clay. Naming battery terminals based on the chemical reaction is just plain silly. Negative and positive don't change, and if we did it this way there would be so much less confusion. – Simon Tillson Nov 18 '15 at 2:14

Okay, so your understanding is correct about rechargeable batteries after taking electrochemistry in class.However, electrons migrate from anode to cathode . The reaction at the anode produces electrons.

  • Exactly correct. Electrons migrate from anode to cathode. In a rechargeable battery, the electron flow will reverse when switching between charge/discharge, so the anode/cathode designations of the two electrodes will also switch. – clay May 15 '14 at 23:33

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