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Complete discharge of spent lithium-ion batteries (LIBs) is a crucial step in LIB recycling, with the physical discharge method being particularly noted for its high discharge efficiency and environmental friendliness. However, previous studies and standards have focused on the performances of the discharge methods, neglecting the battery materials changes caused by discharge. Here we demonstrate that although prolonged discharge of spent batteries keeps the voltage around 0 V, an obvious current flow can be still observed, resulting from the dissolution and subsequent deposition of the copper foil. The deposited copper, primarily in the forms of Cu, Cu2O, and CuO, shows a gradient distribution on the surface of the anode and cathode active materials. This copper deposition significantly compromises the electrochemical performance of the discharged battery, with evident deterioration observed in the first charge-discharge capacity, cycling performance, and coulombic efficiency when compared to the original battery. This study provides guidance for the discharge methods and offers new insights into the materials failure mechanisms during discharge of spent batteries.