Why is the aluminum foil in the lithium ion battery eroded ?
During reverse charging, capacitance works as primary cell with two other power supply shorted. This means aluminum as the negative pole will react with the electrolyte, thus being eroded. Aluminum foil can not work as the negative pole, for Li and Al will alloy with each other at low voltage. The hard oxidation film covering the surface of the aluminum product will prevent itself from further oxidizing. On the contrary, oxygen and water can still react with Iron through the rust. Therefore, aluminum is more active than Iron, but it has better anti-corrosion abilities.
High voltage (above 5V) might cause erosion of aluminum foil, but it is impossible for lithium ion battery whose voltage ceiling is 5V. In fact, the erosion is caused by HF produced by the electrolyte decomposition, especially when the water in the battery is not enough. While charging, capacitance works as electrolytic tank and aluminum as the negative pole. As the electrons flows through external circuit, aluminum will not react with the electrolyte (aluminum loses electrons)
