A biphasic copper-zinc alloy passivated anode for zinc-ion batteries: Kinetically controllable alloying of monolayered copper nanoparticles

Recently, zinc-ion batteries have gained significant attention as a safer alternative to commercial lithium-ion batteries due to their use of non-flammable aqueous electrolytes. Zinc foil has been adopted widely as an anode, but it adversely suffers from critical issues regarding dendrite growth and relevant side reactions. This study suggests a facile strategy of forming the biphasic Cu-Zn alloy passivation layer comprised of primary Zn-rich ε-CuZn₅ and secondary Cu-rich α-Cu_xZn phases by a simple one-step green laser annealing process. We designed a chemical scheme where the Cu nanoparticles with localized surface plasmon resonance are adsorbed as a monolayer on the citric acid-treated Zn foil. We revealed that based on the transient photothermal simulation, the green laser irradiation enables instantaneous heating in the vicinity of the Cu nanoparticles, up to temperatures ranging from 392 – 848 °C in a timeframe of 0.5 msec with quenching rates from 3.7 to 8.3 °C/msec. This instantaneous photothermal annealing enables the regulation of the spatial migration of Cu nanoparticles toward the underlying Zn foil depending on the level of laser power. It allows the formation of the biphasic passivation layer where Cu-rich and Zn-rich alloy phases co-exist simultaneously within the ∼ 1 μm-thick shallow layer. We demonstrated that the biphasic passivation layer significantly enhances the electrolyte interfacial wettability and addresses the key limitations of the Zn anode, unlike the conventional single-phase Cu-Zn alloy passivation layer. A full-cell test paired with a zinc vanadium oxide cathode further demonstrated the superiority of the biphasic passivation layer compared to the single-phase counterpart, achieving a capacity retention of up to 60 % at a current density of 5 A g⁻¹ over 3,000 cycles.