Joining of copper foils via Al/Ni reactive multilayer nanofoils

• Al/Ni reactive multilayer nanofoils are used as a local heat source in the joining of copper foils. • Grain coarsening and grain refining are both occurred in the reactants of Al/Ni reactive multilayer nanofoils. • The joints strength of copper foils has been improved with the assistance of Al/Ni reactive multilayer nanofoils. Interconnected individual battery cells of lithium batteries are unavoidably involved in joining in parallel to improve the battery capacity. Due to the advantage of the small joint area and short process time, micro resistance welding is one of common applied joining techniques of copper foils. However, strength improvement of micro resistance welded copper foils was restricted by the insufficient Joule heat accumulation at the faying interface. Here we used the exothermic reaction of Al/Ni reactive multilayer nanofoils as a local heat source at the faying interface of micro resistance welding of copper foils, designing to increase the heat accumulation and improve the joints strength. Two faying interfaces were formed at the joints of copper foils, one was at the thickness direction of AlNi reactants caused by the diffusion of Cu element based on the high temperature and fracture of coating layer, and another was formed at the surface of AlNi reactants and copper foils. Due to grain boundary relaxation, grain refining of AlNi alloy compound of final product with the average grain size of 27.9 nm at the faying interface attributed to the hardness increase and improvement of joints strength. The joints strength of two-layer copper foils increased to 100 N and the value was 500 N for the joints of multilayer copper foils with the assistance of Al/Ni reactive multilayer nanofoils. These results pave the way for the application of micro resistance welding technique in the battery cells joining of lithium batteries manufacture.