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

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.

Automated summary

Salification is a solubility reduction strategy for limiting the shuttle effect in organic batteries, but its applicability for oxidizable compounds is not clear. In this study, using two salts as case studies, it is shown that solubility reduction by the anion does not necessarily improve battery performance. The compatibility of electrode components and charge/discharge parameters also needs to be considered.