Carbon Nanotube-Encapsulated Noble Metal Nanoparticle Hybrid as a Cathode Material for Li-Oxygen Batteries

Although Li-oxygen batteries offer extremely high theoretical specific energy, their practical application still faces critical challenges. One of the main obstacles is the high charge overpotential caused by sluggish kinetics of charge transfer that is closely related to the morphology of discharge products and their distribution on the cathode. Here, a series of noble metal nanoparticles (Pd, Pt, Ru and Au) are encapsulated inside end-opened carbon nanotubes (CNTs) by wet impregnation followed by thermal annealing. The resultant cathode materials exhibit a dramatic reduction of charge overpotentials compared to their counterparts with nanoparticles supported on CNT surface. Notably, the charge overpotential can be as low as 0.3 V when CNT-encapsulated Pd nanoparticles are used on the cathode. The cathode also shows good stability during discharge-charge cycling. Density functional theory (DFT) calculations reveal that encapsulation of "guest" noble metal nanoparticles in "host" CNTs is able to strengthen the electron density on CNT surfaces, and to avoid the regional enrichment of electron density caused by the direct exposure of nanoparticles on CNT surface. These unique properties ensure the uniform coverage of Li2O2 nanocrystals on CNT surfaces instead of localized distribution of Li2O2 aggregation, thus providing efficient charge transfer for the decomposition of Li2O2.