Oxygen defect regulation, catalytic mechanism, and modification of HfO2 as a novel catalyst for lithium–oxygen batteries

Abstract

Developing efficient cathode catalysts in lithium–oxygen batteries is urgent to solve their challenging problems, such as sluggish Li–O reaction kinetics, low reversible capacities, and short lifespan. This work reports HfO₂ as a cathode catalyst for Li–O₂ batteries for the first time and studies its working mechanism, activation behavior during cycling, and reversible oxygen defect evolution. First-principles calculations show that fluorite-structured HfO₂ has suitable adsorption energy for LiO₂, which helps Li₂O₂ grow in the shape of nanosheets, thereby improving the ultimate discharge capacity and cycling performance. Ex situ techniques (scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy) verified the changes in discharge products during cycling. LiO₂ decomposes preferentially over Li₂O₂ during charging, which differs from previous reports. HfO₂ undergoes reversible redox reactions upon cycling, accompanied by rocking-chair oxygen defects. Appropriate oxygen defect contents (6.81–9.32%) can promote the Li–O reaction. On this basis, sub-10 nm oxygen-defect-riched HfO₂ particles are homogeneously dispersed on nitrogen-doped carbon nanosheets through the sol–gel method, boosting the battery performance.