Highly rough copper current collector: Improving adhesion property between a silicon electrode and current collector for flexible lithium-ion batteries

Two types of Cu foil, conventional flat Cu foil and rough Cu foil, are used to fabricate silicon (Si) electrodes for flexible and high-energy-density lithium-ion batteries (LIBs). Confocal microscopy and cross-sectional SEM images reveal the roughness of the very rough Cu foil to be approximately 3 μm, whereas the conventional flat Cu foil has a smooth surface and a roughness of less than 1 μm. This difference leads to the improvement of the interfacial adhesion strength between the Si electrode and the Cu foil from 89.7 (flat Cu foil) to 135.7 N m^-1 (rough Cu foil), which is measured by a versatile peel tester. As a result, the Si electrode with high Si content (80 wt%) can deliver a significantly higher discharge capacity of 1500 mA h g^-1 after 200 cycles, even at a current rate of 1200 mA g^-1. Furthermore, when the corresponding Si electrode is assembled into a pouch-type cell and cycled in the rolled conformation with a radius of 6.5 mm, the Si electrode with rough Cu foil shows a stable cycle performance due to better interfacial adhesion.