Effectiveness of salification against shuttle effect in p-type organic batteries: Case studies of triflimide and iodide salts of N, N '-dimethylphenazine

Abstract

Salification is one solubility reduction strategy for limiting the deleterious shuttle effect in organic batteries, although its applicability for oxidizable (p-type) cationic compounds is less established. Using as case studies the salts N,N'-dimethylphenazinium iodide, [DMPZ] [I], and triflimide, [DMPZ] [TFSI], we demonstrate that solubility reduction by the anion does not necessarily translate into improved battery performance. As exemplified by the formation of the well-known I-/I-2 shuttle in [DMPZ] [I] cathode, intermolecular interactions that reduce solubility can be lost as state-of-charge changes during redox reaction (i.e. charge/discharge). Another point of consideration is the compatibility in terms of (electro)chemical stability of the electrode components and the charge/discharge parameters when placed together within a cell, even if they are individually stable. Here, the iodide salt underwent decomposition within the literature-optimized electrolyte to form a cathode-electrolyte interface, encapsulating the redox-active compound and changing the charge storage mechanism to one of pseudo-capacitance, thus deteriorating capacity retention. Considering the multitude of requirements as listed here, salification appears challenging to implement for improving battery performance for p-type molecular compounds.