A Stable and Energy-Dense Polysulfide/Permanganate Flow Battery

Redox flow batteries (RFBs) as promising technologies for energy storage have attracted burgeoning efforts and have achieved many advances in the past decades. However, for practical applications, the exploration of high-performance RFB systems is still of significance. In this work, inspired by the high solubility and low cost of both polysulfides and permanganates, the S/Mn RFBs with S₄^2-/S₂^2- and MnO₄^-/MnO₄^2- as negative and positive redox pairs are demonstrated. Moreover, to solve the poor cycling performance caused by the sluggish kinetics of polysulfide-involved redox reactions and instability of the carbon felt (CF) electrode in the strong oxidative and corrosive catholyte, both the anode and cathode are designed to obtain high performance. Herein, the NiS_x/Ni foam exhibiting electrocatalysis activity toward polysulfide ions is prepared and works as the anode while the graphene-modified carbon felt (G/CF) with high stability is fabricated and utilized as the cathode. Additionally, NaMnO₄ with a high solubility limit (3.92 M) in the alkaline supporting electrolyte is preferred to KMnO₄ as the redox-active molecule in the catholyte. The resulting S/Mn RFB cells show outstanding cell performance, such as high energy density (67.8 Wh L^-1), long cycling lifetime with a temporal capacity fade of 0.025% h^-1, and low chemical cost of electrolytes (17.31 $ kWh^-1). Moreover, a three-cell stack shows good cycling stability over 100 cycles (226.8 h) with high performance, verifying the good scalability of the proposed S/Mn RFB system. Therefore, the present strategy provides a reliable candidate for stable, energy-dense, and cost-effective devices for future energy storage applications.