Tunable Oxygen Vacancies of Cobalt Oxides in Lithium-Oxygen Batteries: Morphology Control of Discharge Product.

The discharge product Li2O2 is difficult to decompose in lithium-oxygen batteries, resulting in poor reversibility and cycling stability of the battery, and the morphology of Li2O2 has a great influence on its decomposition during the charging process. Therefore, reasonable design of the catalyst structure to improve the density of catalyst active sites and make Li2O2 form a morphology which is easy to decompose in the charging process will help improve the performance of battery. Here, we demonstrate a series of hollow nanoboxes stacked by Co3O4 nanoparticles with different sizes. The results show that the surface of the nanoboxes composed of smaller size Co3O4 nanoparticles contains abundant pore structure and higher concentration of oxygen vacancies, which changes the adsorption energy of reactants and intermediates, providing more nucleation sites for Li2O2, thereby forming Li2O2 with high dispersion, which is easier to decompose during charging, and eventually improve the performance of the battery. This provides an important idea for the structural design of the cathode catalyst in lithium-oxygen batteries and the regulation of Li2O2 morphology.

Terephthalic Acid Intercalated CoNi-LDH Materials for Improved Li-O2 Battery.

CoNi-LDH (layered CoNi double hydroxides) hollow nanocages with specific morphology are obtained by Ni ion etching of ZIF-67 (Zeolitic imidazolate framework-67). The structure of the layered materials is further modified by molecular intercalation. The original interlayer anions are replaced by the ion exchange effect of terephthalic acid, which helps to increase the interlayer distance of the material. The intercalated cage-like structures not only benefit for the storage of oxygen, and the discharge product reaction, but also have more support between the material layers. The experimental results show that the excessive use of intercalation agent will affect structural stability of the intercalated CoNi-LDH. By adjusting the amount of terephthalic acid, the intercalated CoNi-LDH-2 (with 0.02 mmol terephthalic acid intercalated) is not easy to collapse after 209 cycles and shows the best electrochemical performance in Li-O2 battery.

Crystal Phase Conversion on Cobalt Oxide: Stable Adsorption toward LiO2 for Film-Like Discharge Products Generation in Li-O2 Battery.

Regulating the structure and morphology of discharge product is one of the key points for developing high performance Li-O2 batteries (LOBs). In this study, the reaction mechanism of LOB is successfully controlled by the regulated fine structure of cobalt oxide through tuning the crystallization process. It is demonstrated that the cobalt oxide with lower crystallinity shows stronger affinity toward LiO2 , inducing the growth of film-like LiO2 on the electrode surface and inhibiting the further conversion to Li2 O2 . The batteries catalyzed by the lower crystallinity cobalt oxide hollow spheres which pyrolyzed from ZIF-67 at 260 °C (ZIF-67-260), go through the generation and decomposition of amorphous film-like LiO2 , which significantly reduces the charge overpotential and improves the cycle life. By contrast, the ZIF-67 hollow spheres pyrolyzed at 320 °C (ZIF-67-320) with better crystallinity are more likely to go through the solution-mediated mechanism and induce the aggregation of discharge product, resulting in the sluggish kinetics and limited performance. The combined density functional theory data also directly support the strong relationship between the adsorption toward LiO2 by the electrocatalyst and the battery performance. This work provides an important way for tuning the intermediate and constructing the high-performance battery system.

Atomically Dispersed Ruthenium Catalysts with Open Hollow Structure for Lithium-Oxygen Batteries.

Lithium-oxygen battery with ultra-high theoretical energy density is considered a highly competitive next-generation energy storage device, but its practical application is severely hindered by issues such as difficult decomposition of discharge products at present. Here, we have developed N-doped carbon anchored atomically dispersed Ru sites cathode catalyst with open hollow structure (h-RuNC) for Lithium-oxygen battery. On one hand, the abundance of atomically dispersed Ru sites can effectively catalyze the formation and decomposition of discharge products, thereby greatly enhancing the redox kinetics. On the other hand, the open hollow structure not only enhances the mass activity of atomically dispersed Ru sites but also improves the diffusion efficiency of catalytic molecules. Therefore, the excellent activity from atomically dispersed Ru sites and the enhanced diffusion from open hollow structure respectively improve the redox kinetics and cycling stability, ultimately achieving a high-performance lithium-oxygen battery.