Triphasic Electrolytes for Membrane-Free High-Voltage Redox Flow Battery

Liquid/liquid biphasic batteries without a membrane could be promising models of redox flow batteries. However, such batteries inherently undergo self-discharge at the liquid/liquid interface, resulting in a low Coulombic efficiency and poor capacity retention. Herein, we report a triphasic system comprising all-nonaqueous electrolytes to overcome the challenges in biphasic systems. By incorporation of a third electrolyte layer to separate the anolyte and catholyte, the system effectively impeded the self-discharge event without compromising the Coulombic efficiency and capacity retention. Li metal anode and phenothiazine derivative C18-PTZ (0.5 M) cathode have been used to construct the electrochemical cell. Moreover, we have designed two all-nonaqueous triphasic models either stacking the phases vertically or physically separating two phases sidewise along with a common interfacial layer at the top. Interestingly, both designs showed high capacity retention under static (∼98%, 31 days, 100 cycles) and flow (∼99%, 39 days, 100 cycles) conditions. Moreover, all batteries exhibited an average Coulombic efficiency of nearly 100%.