Stabilization of Naphthalene Diimide Anions by Ion Pair Formation in Nonaqueous Organic Redox Flow Batteries

In
redox flow batteries, a compelling strategy for enhancing the
charge capacity of redox-active organic molecules involves storing
multiple electrons within a single molecule. However, this approach
poses unique challenges such as chemical instability by forming radicals,
elevated energy requirements, and unsustainable charge concentration.
Ion pairing is a possible solution to achieve charge neutrality and
engineer redox potential shifts but has received limited attention.
In this study, we demonstrate that Li+ can stabilize naphthalene
diimide (NDI) anions dissolved in acetonitrile and significantly shift
the second cathodic potential close to the first. Our findings, supported
by density functional theory calculations and Fourier transform infrared
spectroscopy, indicate that dimeric NDI species form stable ion pairs
with Li+. Conversely, K+ ions exhibit weak interactions,
and cyclic voltammograms confirm significant potential shifts when
stronger Lewis acids and solvents with lower donor numbers are employed.
Galvanostatic examinations reveal a single voltage plateau with Li+, which indicates a rapid redox process involving doubly charged
NDI2– with Li+. These aggregated ion
pairs offer the additional benefits of hindering crossover events,
contributing to excellent cyclability, and suppressing undesirable
side reactions even after 1000 redox cycles.