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

Abstract

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. 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 theoretical specific energy densities (11,140 Wh/kg and 2,600 Wh/kg respectively). However, several fundamental issues hinder their commercialization; for example, slow kinetics of the oxygen evolution reaction (OER) during charging results in low round-trip efficiency of the Li-O2 battery. High voltage required during the OER process leads to degradation of non-aqueous electrolytes and carbon-based air electrodes resulting in rapid capacity fade and further increasing the discharge-charge voltage. 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, most research focus has been on improving the cycle life of the batteries by using thin electrodes (<2 mg/cm2 of active sulfur loading). However, for practical applications, it is imperative to validate the different approaches on fabricating electrodes at a relevant scale. 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. We have recently showed that PAMAM dendrimers can be used as hosts to encapsulate monodispersed ruthenium oxide (RuO2) nanoparticles, and employed as catalysts in the air electrode for Li-O2 batteries. The dendrimers stabilized the RuO2 nanoparticles while ensuring the availability of the entire nanoparticle surface for catalysis, thus reducing the amount of noble metal catalysts used in these systems by almost ten times than the state-of-the-art, without sacrificing their catalytic efficiency. We will further discuss the formation of highly catalytically efficient sub-nanometer clusters of Ru atoms using dendrimers as templates, i.e. dendrimer-based superatoms, in organic solvents. Additionally, we also explored the application of chemically modified PAMAM dendrimers with high electronic conductivity in Li-S batteries. These modified dendrimers showed a better cycling stability compared with hyperbranched polymers used as cathode materials. While challenges exist in the properties of the cathode materials themselves, the binder in the electrode is also critical in bonding the active materials together as well as maintaining a good electrical contact with the current collector. We have been able to obtain high loading (> 4 mg/cm2) of active sulfur using dendrimer-based functional binders with various surface functionalities. Here, we will discuss the structure-function relationships of dendrimer-based binders and cathode materials in enhancing the performance of high areal capacity Li-S batteries. 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. By using novel dendrimer chemistry and assembly, our research provides new clues to embody the nanoscale properties of soft and porous dendritic nanostructures for improving the state‐of‐the‐art of sustainable, high energy density battery technology applications.