Size- and Crystallinity-dependent Oxygen Vacancy Engineering to Modulate Fe Active Sites for Enhanced Reversible Nitrogen Fixation in Lithium-nitrogen Batteries

Abstract

Lithium-nitrogen (Li-N₂) battery is not only an electrochemical energy storage platform, but also an environmentally friendly nitrogen fixation technology. However, a great challenge remains in regulating catalyst activity to accelerate cathode reaction kinetics. Herein, we proposed an oxygen vacancy-mediated modulation of Fe active sites in FeO_x nano-particle catalysts and Fe single-atom catalysts to enhance nitrogen reduction reaction in Li-N₂ batteries. High-concentration oxygen vacancy is generated through a size- and crystallinity-dependent oxygen vacancy engineering based on the precise atomic layer deposition of reducible oxides. The oxygen vacancy on FeO_x drives the electron redistribution of Fe³⁺ d-orbitals to provide electron-donating Fe active sites for N₂ fixation. Meanwhile, oxygen vacancy-rich MoO_y is used as a support to anchor Fe single atoms. Adjacent oxygen vacancy drives the stable coordination between Fe single atoms and O atoms to facilitate the directional electron transfer from MoO_y to Fe to N₂. Therefore, the Li-N₂ batteries exhibit large discharge capacity, excellent rate performance, and reliable cycle stability. In addition, the formation and decomposition of the discharge product Li₃N indicate a reversible N₂ fixation. This work provides a precise regulation mechanism of catalytic active sites based on oxygen vacancy engineering, which is expected to promote the development of high-performance Li-N₂ batteries.