Weakly coordinating anions offer an opportunity to explore ion-solvent interactions to establish electrolyte design rules and apply these rules to develop new electrolytes for high energy density chemistries. Chemistries based on multivalent working cations, such as Mg, are one class where significant effort is required to develop reductively and oxidatively stable electrolytes to support high efficiency metal deposition and cation insertion. Electrolyte stability is dictated by how strongly Mg(II), anion and solvent interact with published examples of reductive stability being impaired by Mg(II)- anion contact ion pairing and oxidative stability being limited by anion-solvent interactions. These effects can be decoupled through selective partnering of anions and solvents using experiment and computation to define the extent of interaction and correlating this knowledge with electrochemical activity and stability measurements. In this presentation, we employ three classes of weakly coordinating anions including carboranes, fluoroalkoxyborates, and fluoroalkoxyaluminates with high voltage solvents to establish stability limits and determine the chemistry that limits stability. As an example, we demonstrate that the stability of the carba-closo-decaborate anion is limited by solvent independent neutral radical generation and subsequent reaction to form electronically passivating films on cathode surfaces. Derivatization of the carborane cage is shown to increase anion stability, but radical formation continues to impose the electrolyte anodic limit. In contrast, the ligands of borate/aluminate anions provide opportunities to tailor anion-solvent interactions extending the electrolyte stability window. These three anion systems form the basis of an emergent electrolyte design capability for divalent metal cations.

Ion-solvent interactions and their impact on electrolytes for Mg-based energy storage

256th National Meeting (2018)

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