Accessing a high-voltage nonaqueous hybrid flow battery with a sodium-methylphenothiazine chemistry and a sodium-ion solid electrolyte

The development of redox flow batteries (RFBs) with nonaqueous electrolytes offers the possibility of accessing a high cell-operation voltage (no restrain of hydrogen evolution and oxygen evolution potentials) and a low operation temperature (can be operated below the freezing point of water). Therefore, nonaqueous RFBs have recently garnered increasing attention. However, the cross-mixing of liquid electrode/electrolyte materials has been plaguing the progress of the nonaqueous RFBs. Herein, we present a crossover-free, high voltage nonaqueous hybrid flow battery (HFB) with a novel sodium-methylphenothiazine (MPT) chemistry and a single-ion solid-electrolyte separator. The Na-MPT redox couple delivers a high voltage of similar to 2.6 V when the cell was operated at a medium current density. A NASICON-type solid electrolyte membrane (Na3Zr2Si2PO12) could circumvent the crossover of the liquids between the positive and negative electrodes, and meanwhile could maintain a single-ion (Na+-ion) conduction between the two electrodes to sustain the electrochemical reactions. Under such an electrochemical mechanism, the nonaqueous Na-MPT HFB shows remarkable cycling performance.