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

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-O-2 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.