Spatially Controlled Li Electrodeposition on 3-D Patterned Cu Current Collectors Through Nanometer-Scaled Surface Roughness in Li-Metal Batteries

Ever-growing demands for electric vehicles and unmanned aerial vehicle systems require higher energy density of energy storage. The metallic lithium (Li) has been considered a promising negative electrode in lithium-ion batteries, owing to its high specific capacity (3860 mAh/g) and a negative electrochemical reduction potential (-3.04 V vs. SHE). Recently, the Li-free copper (Cu) current collectors have also been developed, called anode-free Li batteries, to improve gravimetric energy density further. However, the dendritic growth of Li during the plating process has caused an electric short-circuit and risk of catching fire for both batteries. Various strategies have been suggested to mitigate this detrimental Li deposition. One of the ideas is to utilize three-dimensional (3-D) current collectors, comprised of alternate receding and protruding parts, to induce selective Li deposition into the receding space. However, many studies showed the preferential Li deposition on the protruding area or edges, which rather promoted vertical Li growth and the electric short-circuit.<br/>Here we show that the nanometer-scale surface roughness of the Cu current collector aided the Li plating at the receding space, which improved cell performance. We designed a 3-D Cu pattern through photolithography, followed by chemical etching using 3.5 mol/kg ammonium persulfate solution. The resulting Cu foil had square patterns of the receding and protruding parts, where the former had ~10 μm of offset from the latter surface. They also had a similar Cu oxidation state and surface crystallinity to the pristine Cu foil. However, due to the chemical etching process, the receding area had a rougher surface than the protruding one, measured to be &lt;10 nm of surface roughness. This nanometer-scale roughness in the 3-D Cu foil contributed to two-fold higher capacitance than the flat and pristine Cu foil. In addition, the surface roughness determined the exchange current-density. COMSOL simulation for the 3-D patterned Cu predicted superior local current density at the receding area compared to the protruding part where the enhanced electric field was typically expected. Experimentally, we also observed the selective Li deposition at the receding space thanks to the high surface roughness, better wettability, and high surface energy. COMSOL simulations also indicated that the surface roughness should be dependent on the depth of the receding area if we intended to more intense local current density in this space. Namely, deeper receding areas need more significant roughness. We demonstrated improved Li|Cu cell performances using the resulting 3-D Cu current collectors. This concept guides the design of selective Li deposition in the Li metal and anode-free Li batteries.