It is worthwhile and promising to explore the direct activation of metal Cu for aqueous solid-state batteries. Herein, a simple strategy of in situ electrochemical interface modification is proposed to produce KCu₇S₄ film on a Cu foam substrate. The as-fabricated KCu₇S₄ products possess a desirable physical hierarchical structure, satisfactory wire array homogeneity as well as high mass loading (∼45 mg cm^-2). The KCu₇S₄ electrode as an anode for solid-state energy storage has an areal capacity of 11.98 mAh cm^-2 (at 10 mA cm^-2), which is far superior to the previous best result (< 5 mAh cm^-2). As revealed by the durability test, the electrode retains a high coulombic efficiency of 86.8% after cycling. The well-assembled KCu₇S₄//NiO single solid-state battery displays an improved energy density of 87.5-66 Wh m^-2 under the power range of 73-730 W m^-2. Besides, the working mechanism based on KCu₇S₄/Cu₇S₄ interconversion, and the reaction kinetics of electrode are deeply analyzed via a series of in situ techniques. This work contributes a promising solution to the fabrication of Cu-based film anodes which may lead to significant progress in large-scale energy storage technology.