# Influence of Alkali Cations on Redox Matching and Capacity Access in Redox-Mediated Flow Batteries

## TL;DR

This paper explores how different alkali cations affect the performance of redox-mediated flow batteries, aiming to improve their energy density.

## Contribution

The study introduces a method to optimize redox mediation by comparing the impact of alkali cations on battery performance.

## Key findings

- K+ shows superior intercalation dynamics compared to Li+ and Na+ in the booster material.
- Maximum booster utilization of 35% is achieved with 200 mM potassium ferri/ferrocyanide.
- Optimizing mediator concentration and cation species is crucial for higher energy density in RMFBs.

## Abstract

Cost-effective redox
flow batteries (RFBs) offer reliable energy
storage for intermittent solar and wind sources; however, their energy
density is inherently lower than that of lithium-ion batteries because
of solubility limitations. The redox-mediated flow battery (RMFB)
concept addresses this limitation by combining the operational flexibility
of RFBs with the high energy density of solid-state batteries. In
this system, a solid material which is immobilized inside the electrolyte
(the booster) undergoes charge/discharge indirectly through electron
transfer mediated by a dissolved active species (mediator). As a result,
the energy density of the RMFB is ideally determined by the amount
of solid material incorporated. Beyond booster engineering and material
screening, the intercalating cation to the booster upon discharge
is able to aid in this potential alignment. In this work, electrochemical
techniques including cyclic voltammetry (CV), electrochemical impedance
spectroscopy (EIS), galvanostatic charge–discharge (GCD), and
operando ultramicroelectrode cyclic voltammetry (UME-CV) are employed
to identify the optimal utilization window of the booster and quantify
trends in redox mediation kinetics by comparing the impact of three
alkali cations on redox mediation in Prussian blue (booster)/ferri/ferrocyanide
(mediator) systems. The results indicate that Li+ and Na+ diffuse more slowly within the booster compared to K+, leading to superior intercalation dynamics with K+. Under the tested conditions with varying mediator concentrations
and cation species, the maximum booster utilization was found to be
35% using 200 mM potassium ferri/ferrocyanide. These findings highlight
that achieving higher energy densities in RMFBs requires careful optimization
of mediator concentration, electrolyte composition, and redox mediation
kinetics.