Applications of Dendrimer-Based Nano-Architectures in Li-Air and Li-Sulfur Batteries

Dendritic branched polymers, particularly the well studied, most commonly synthesized and commercially available polyamidoamine (PAMAM) dendrimers have a high degree of surface functionalities, high surface-area, large molecular weights, and ample well-defined interior space. Additionally, due to their high porosities, high room temperature ionic conductivities of their lithium (Li) salts, and good chemical and thermal stabilities, they may provide improved properties as potential electrode materials and polymer electrolytes. PAMAM dendrimers are particularly attractive candidates as templates for the synthesis of catalyst metal nanoparticles due to a high density of interior tertiary amines, which are able to complex with metal ions and variable surface groups to tailor their solubility. These remarkable physico-chemical properties of dendrimers enable them to be applied in next-generation energy storage technologies.

Amongst all sustainable rechargeable Li-batteries, Li-air (Li-O₂) and Li-sulfur (Li-S) batteries are the most promising with very high specific energy densities (11,140 Wh/kg and 2,600 Wh/kg respectively). However, several fundamental issues hinder their commercialization. Oxidation of the insoluble, poorly conducting Li₂O₂ discharge products during charging, slow kinetics of the oxygen reduction reaction during discharge, and the oxygen evolution reaction (OER) during charging resulting in low round-trip efficiency of the battery, high voltage required during the OER process leading to degradation of non-aqueous electrolytes and carbon-based air electrodes resulting in rapid capacity fade and further increasing the discharge-charge voltage hysteresis in the battery are some of the critical challenges with the Li-O₂ battery. In Li-S batteries, the charge/discharge reaction creates highly soluble polysulfide intermediates in electrolytes, causing irreversible loss of active sulfur and low cycling efficiency. Furthermore, the battery’s cycle life and safety can be greatly improved by replacing liquid electrolytes with gel or solid polymer electrolytes.

Here, we will discuss a fundamental understanding of branched polymers as advanced energy storage materials expanding their applications to sustainable high-energy density storage and conversion systems such as those mentioned above. Specifically, dendrimer-encapsulated ruthenium nanoparticles (DEN-Ru) were used as catalysts in Li-O₂ batteries for the first time. The DEN-Ru significantly improves the cycling stability of Li-O₂ batteries with carbon black electrodes and decreases the charging potential even at low catalyst loading. Furthermore, the potential of using DEN-Ru as a standalone cathode material for Li-O₂ batteries also will be discussed.

Additionally, we also explored the application of chemically modified PAMAM dendrimers with electronic conductivity in Li-S batteries. These modified dendrimers showed a better cycling stability compared with hyperbranched polymers used as cathode materials.

We will also discuss the potential of ionically conducting dendrimers as electrolyte systems. Detailed characterizations of these novel nano-architectures will be discussed to understand how their structure and chemistry influence the electrochemical performance of the aforementioned energy systems. It is anticipated that by using novel dendrimer chemistry and assembly, our research will lead to the development of energy materials that embody the nanoscale properties of soft and porous dendritic nanostructures and provide innovative solutions for improving the state‐of‐the‐art of sustainable, high energy density applications.