The direct modification of metal Cu interface for high-performing solid-state anode via KCu7S4/Cu7S4 interconversion mechanism

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.