This application relates to the field of new energy technology, and in particular, to a method for supplementing an electrolyte solution for a battery cell. The method includes: processing a shell of the battery cell from outside the battery cell to form a first recessed portion, and causing a thickness of a bottom wall of the first recessed portion to be less than a thickness of the shell around the bottom wall; piercing the bottom wall of the first recessed portion from outside the battery cell toward an interior of the battery cell to form an injection structure; and injecting the electrolyte solution into the battery cell through the injection structure.