Hexagonal Close-Packed RuCo Alloys Modulate Li2O2 Formation and Decomposition Pathways in Lithium-Oxygen Batteries.

Aprotic lithium-oxygen (Li-O2) batteries suffer from large voltage hysteresis and inferior reversibility, originating from the sluggish redox kinetics of insulating Li2O2. Here, we incorporate Ru atoms into Co nanoparticles encapsulated within carbon nanotubes and induce a phase transition from a face-centered cubic (fcc) to a hexagonal close-packed (hcp) structure to facilitate cathode reaction kinetics. The hcp-RuCo alloy exhibits significant charge redistribution and a downshifted d-band center, promoting LiO2 desorption into the electrolyte and inducing the formation of solution-mediated, sheet-like Li2O2 nanoarrays with rapid charge/mass transport. Notably, it reduces the delithiation energy barrier to drive a kinetically dominant, stepwise, single-electron oxidation pathway (Li2O2 → LiO2 → O2), bypassing the highly polarizing two-electron route upon charging. Consequently, Li-O2 batteries with the hcp-RuCo catalyst achieve remarkably reduced polarization of 0.72 V and ultralong cycle durability without performance decay. This work demonstrates that crystal-phase engineering is a powerful strategy for tuning oxygen electrocatalysis.