Identifying iodide-ion regulation of early-stage zinc nucleation and growth for high-rate anode-free zinc metal batteries†

Anode-free aqueous zinc (Zn) metal batteries (AZMBs) have the advantage of providing higher energy density. However, without excess Zn metal, their cycling life is highly dependent on the reversibility of Zn deposition/dissolution, which is influenced by interfacial issues such as Zn dendrite formation and parasitic side reactions. A simple approach is developed to tackle this challenge by introducing lithium iodide as an additive to the electrolyte, where iodide ions (I⁻) play a crucial role in regulating the early-stage Zn nucleation and growth. Initially, the formation of an I⁻-rich electrochemical double layer reduces the Marcus charge transfer energy barrier of Zn ions (Zn²⁺), hence significantly lowering the heterogeneous nucleation overpotential of Zn. Subsequently, I⁻ ions are preferentially adsorbed onto the Zn (100) and Zn (101) crystal planes compared to the Zn (002) plane, thereby promoting the Zn growth onto these two planes and leading to Zn plating with the dominating Zn (002) orientation. As a result, highly reversible Zn deposition/dissolution is achieved in the Zn‖copper battery, with a superior initial and average coulombic efficiency of 99.9%. Moreover, the anode-free Zn‖iodine battery demonstrates excellent cycling stability and ultra-high-rate performance (0.99 mA h cm⁻² capacity retained corresponding to an 88.2% retention after 10 000 cycles at 50 mA cm⁻²). The I⁻ regulation strategy for early-stage Zn nucleation and growth behavior provides a simple and innovative approach to improving the Zn deposition/dissolution interfacial stability and hence the cycling stability and rate capability of anode-free AZMBs and can also be extended to other anode-free metal batteries.