Single Ion Conducting Polymers of Boron Rich Nanoclusters: Synthesis and Fundamental Electrochemical Properties

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Non-Technical Summary: Innovative electrochemical energy storage technologies beyond lithium-ion batteries (LIBs) are essential for a new era of information and communication technologies. Emerging applications such as smart grid, telecommunications, and the Internet of Things demand new rechargeable batteries characterized with low costs, high safety, and excellent cycle stability. Rechargeable magnesium (Mg) ion batteries can potentially fulfill these requirements and are scientifically intriguing due to their distinctly different electrochemical mechanisms from those known in LIBs. The key bottleneck for Mg-ion batteries is the lack of Mg2+ ion electrolytes that offer excellent Mg2+ conductivity, chemical and electrochemical stability. In this project, supported by the Solid State and Materials Chemistry Program in the Division of Materials Research, researchers develop a completely new class of solid polymer electrolytes (SPEs) that have high Mg2+ ion conductivity and excellent stability. The researchers achieve this by covalently tethering anionic species to the polymer backbones, which attract and transport Mg2+ cations within the polymer network. The specific anions in this project are boron rich nanoclusters (BRNs), which are cage-like bulky molecular structures composed of carbon and boron atoms and carrying a negative charge. The uniqueness and advantage of these BRN anions is their relatively weak bonding with Mg2+ cations so that Mg2+ can be dissociated and moved within the electric field in solid state. The BRN SPEs also exhibit excellent stability due to the strong carbon-boron and boron-boron bonds. Additionally, this project directly involves the participation of graduate, undergraduate, and high school students from underrepresented groups. Funds have been allocated for high school student stipends to continue an outreach program the PI and Co-PI developed. These activities will provide society with more greatly needed STEM educated people to enter the workforce. Technical Summary: Boron Rich Nanoclusters (BRNs) are weakly coordinating polyatomic anions composed of boron and carbon. Their salts exhibit extraordinary solid-state ionic conductivity at low temperature and unmatched (electro)chemical stability. With this project, supported by the Solid State and Materials Chemistry Program in the Division of Materials Research, the research team translates the favorable properties of magnesium (Mg) BRN salts into Mg2+ single ion conducting ionomers as solid polymer electrolytes (SPEs) for rechargeable Mg-ion batteries. The central hypothesis is that a new class of single ion Mg2+ conducting polymers can be created by covalently linking BRNs anions to polymer backbones. The obtained Mg2+ ionomers maintain the high ionic conductivity and (electro)chemical stability observed for their crystalline powders. It is also hypothesized that the Mg2+ ion transport in the proposed BRN SPEs follows a new mechanism superior to the conventional ion hopping due to the weakly coordinating nature of the BRN anions. Surface functionalization of BRNs is also investigated to fine tune their electrochemical properties, which is not possible or very limited with traditional polyatomic anions. Two methods including (1) base initiated epoxide ring opening polymerization and (2) ring opening olefin metathesis polymerization are proposed to synthesize the Mg BRN SPEs. The physical, chemical, and electrochemical properties of the synthesized SPEs are systematically investigated. The investigations on the electrochemical properties focus on ion conductivity, interfacial stability with Mg metal anodes, and electrochemical reactions with the cathodes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.