Nanostructured Cathode Architecture By Atomic Layer Deposition for Improving Performance of Li-O2 Battery

Abstract

Due to its extremely high theoretical energy density among all the existing energy storage systems, Li-O2 battery has received significant interest since it was firstly reported in 1996. However, its sluggish kinetics, especially during the oxygen evolution reactions, usually results large voltage polarization and low efficiency, and therefore hinders its practical applications in commercialized devices. There are various factors influencing the electrochemical reactions in Li-O2 battery, such as the nature, morphology, and surface area of the catalysts. Therefore, designing efficient catalysts which could facilitate the electrochemical reactions have become an emerging topic. Some of the early work was based on carbonate electrolytes, which have been found unstable against the active anion such as O2 2- and O2 -, leading to the decomposition of the electrolytes. The electrolyte decomposition could also result a high charge potential and lead to the possible formation of Li2CO3. To reduce the electrolyte decomposition and benefit the catalytic performance, ether-based electrolyte (e.g. TEGDME) is currently widely employed in the study of Li-O2battery due to the relatively stability under high potential. However, the decomposition is still observed, even at the charge potential as low as 3.5 V, which may result from reactions of the electrolyte with defect sites on the porous carbon surface. It is reasonable to expect that the electrolyte decomposition would be significantly reduced by a decrease in the surface defect sites. Our previous study[1] reported a reduction in charge overpotential to 0.2 V by an alumina atomic layer deposition (ALD) on porous carbon surface which prevents electrolyte decomposition on carbon surface. By taking advantage of the difference in surface reaction rate, ALD technique can provide selectively deposition of the coating materials onto the targeted surface. Accordingly, the cathode based on the passivation layer architecture shows promising results for solving the charge overpotential problem by passivating the surface defect sites. Here we describe an approach based on a cathode architecture that has a protective ZnO coating passivation layer on a porous carbon substrate. Bulk ZnO has a direct band gap of 3.3 eV, much smaller than that of bulk Al2O3 (8.8 eV). ZnO ALD using alternating exposures to diethylzinc and water is well understood and provides conformal coatings. In addition, the growth rate of Pd is faster on the ZnO surface than the Al2O3 surface. All the above advantages make ZnO a promising material as the passivating layer on carbon. In this study, we demonstrated that a cathode architecture with uniformly dispersed palladium nanoparticles onto a ZnO-passivated porous carbon substrate prepared by ALD shows high electrochemical catalytic activity in Li-O2 battery. Transmission electron microscopy (TEM) showed discrete crystalline nanoparticles decorating the surface of the ZnO-passivated porous carbon support in which the size could be controlled in the range of 3–6 nm, depending on the number of Pd ALD cycles performed. X-ray absorption spectroscopy (XAS) at the Pd K-edge revealed that the carbon-supported Pd existed in a mixed phase of metallic palladium and palladium oxide. The ZnO-passivated layer effectively blocks the defect sites on the carbon surface, minimizing the electrolyte decomposition. As a result, this cathode architecture reduced the charge overpotential to almost 0 V, which is the lowest ever reported. The discharge products are characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of the Pd loading on the electrochemical performance of the Li-O2 cell is also investigated. Our results not only show promising results for solving the charge overpotential problem, and provide the basis for future development of lithium-oxygen cathode materials that can be used to improve the efficiency and to extend cycle life. Figure 1