Intercalated Redox-Active Anions in Layered Double Hydroxides: Toward New Electrode Designs

Abstract

Layered Double Hydroxides (LDHs) are a versatile class of 2D materials, which consist of layered di- and trivalent metal cation hydroxides with intercalated anions (1). They are used in many applications including energy storage electrodes, water purification or drug delivery (2, 3).In this new study, Anthraquinone-2-sulfonate (AQS) intercalated in the LDH structure of Mg2Al(OH)6, has been investigated and a charge capacity of 100 mAh g-1 has been measured. However, the dissolution of reduced AQS into the aqueous electrolyte occurs from the first reduction leading to a drastic capacity fade upon cycling.To get around the dissolution issue, we synthesize a new intercalated LDH using ferrocene as the redox active moiety. Electrodes with this new LDH material Mg2Al(OH)6-Fc (Figure 1A) showed more stable cycling when used in combination with aqueous or organic electrolytes. However, the most significant increase in cycle life was achieved by operating this electrode in highly concentrated, highly viscous ionic liquid based electrolytes such as Pyr13TFSI. Over 97 % capacity retention was achieved after 200 cycles with a coulombic efficiency close to 100 % (Figure 1B). The higher stability of redox active anions working at positive redox potentials like ferrocene in the LDH structure is a key parameter to improve cycling ability. Moreover, the limited solubility of the intercalated redox anions into the high concentrated ionic liquid electrolyte is also a key factor for improving cycle life of this new electrode.Figure 1: A) Schematic structure of Mg2Al(OH)6-Fc, C) Improved cycling stability of Mg2Al(OH)6-Fc based electrodes in ionic liquid electrolyte Pyr13TFSI. Figure 1