Facilitating Dense Zn Deposition via Spontaneous Cu-Zn Alloying for High-Energy Aqueous Batteries

Multivalent redox chemistry-based batteries (including Zn/Zn²⁺, Al/Al³⁺, Mg/Mg²⁺, etc.) have long been recognized as promising next-generation energy storage solutions due to the abundance of raw materials and their ultrahigh theoretical capacities. Among them, aqueous Zn-ion batteries (AZIBs) have gained significant attention from researchers as promising alternatives to Li-ion batteries (LIBs) due to the use of inexpensive and non-flammable aqueous electrolytes coupled with Zn metal anodes, which exhibit remarkable volumetric and gravimetric capacities (5854 mAh/l and 820 mAh/g, respectively). However, their practical implementation is impeded due to their insufficient energy density and lifespan, mainly originating from the heterogeneous and dendritic Zn metal growth. Adopting an alloying reaction during metal plating is one of the most effective ways to prevent dendritic metal growth. To date, the deposition of noble metals (e.g., Ag, Au) on the current collector as seeds for alloying with Zn has been attempted to regulate Zn electrodeposition. Nevertheless, the spontaneous alloying between Zn deposits and Cu substrates, the commercialized anode current collector, is ideal for Zn metal anodes, but its realization has not been experimentally achieved yet.
Herein, we provide the first experimental demonstration of extensive Cu-Zn alloying occurring in tandem with Zn plating on Cu foil. We show that using a low-cost, non-flammable, and environmentally benign deep eutectic solvent (DES) as the electrolyte solvent enables Cu-Zn alloying during Zn electroplating. Through comprehensive characterization, we identified the formation of a Zn-rich hexagonal close-packed (HCP) alloy on the Cu foil, facilitated by the removal of native oxides from the Cu surface by DES. This Zn-rich alloy phase, which is uniformly distributed between the Cu and Zn layers, exhibits zincophilic properties and an isotypic structure to Zn metal, enabling the formation of densely packed Zn anodes with near-ideal thickness and appropriate Zn loading for AZIBs.
Remarkably, when paired with conventional aqueous electrolytes, the compact Zn anode retains its morphology over repeated plating/stripping cycles, preventing corrosion and cell swelling. This results in excellent reversibility, with the Zn anode delivering an accumulated capacity of over 600 mAh cm^-2 in a symmetric cell, even at a high depth of discharge (DOD) of 40%. Additionally, full-cell tests with various cathode materials in conventional aqueous electrolytes demonstrated the longevity of this system, with stable cycling performance exceeding 7,000 cycles under appropriate negative-to-positive areal capacity (n/p) ratios.
This strategy introduces a new paradigm for achieving highly reversible metal anodes by directly activating the spontaneous alloying reaction of the commercial current collector, without the need for complex procedures or expensive additives. The newly identified mechanism paves the way for the development of AZIBs with improved lifespan and energy density, advancing their potential for commercialization.