By definition, the cathode is the electrode at which reduction occurs, and the anode is the electrode at which oxidation occurs. When the battery is providing current to the phone, the cathode and anode are as shown in the diagram in the video. Essentially, lithium atoms at the anode give up their electrons and become oxidized; the anode, through which the electrons flow out of the cell, is thus negative. At the cathode, electrons flow into the cell to reduce lithium ions back to neutral lithium atoms; the cathode is positive.During charging, current flow is reversed, and reduction and oxidation occur at the opposite electrodes to those in the previous situation; the cathode and anode switch places. Since in this case, the external potential exceeds the cell potential, you could claim that the cathode is negative and the anode is positive.

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