High-Areal-Capacity Manganese-Based Redox Flow Batteries via Sodium Diphosphate-Modified Electrolyte

Manganese (Mn)-based redox flow batteries (RFBs) have emerged as promising candidates for large-scale energy storage owing to their high redox potential (Mn 2+ /Mn 3+ : 1.58 V vs SHE), cost-effectiveness, and sustainability. Nevertheless, Mn-based RFBs face a critical challenge: the undesired formation of crystalline MnO 2 through Mn 3+ disproportionation and irreversibility of MnO 2 deposits on electrode surfaces severely diminish accessible reaction sites and restrict achievable areal capacity. Herein, it is demonstrated that ligand chelation-mediated structural transformation of MnO 2 by sodium diphosphate (Na 4 P 2 O 7 , PPi) modulates MnO 2 deposition behavior. The PPi coordinate with Mn 2+ to form a stable [Mn(HPO 4 ) 2 (H 2 O) 2 ] 2− chelate, which facilitates the incorporation of defective O and P into the evolving MnO 2 structure, effectively disrupting its crystallographic ordering and converting MnO 2 from crystalline to amorphous configuration. The amorphous MnO 2 tends to flow into the electrolyte instead of depositing on the electrode due to the electrostatic interplay, thereby resulting in a breakthrough areal capacity of 91.5 mAh cm −2 (300 cycles @ 99.7% CE) and 141.8 mAh cm −2 (150 cycles @ 98.2% CE, the highest reported values), representing a 10-fold enhancement compared to additive-free counterparts. The ligand chelation modification of MnO 2 provides a new pathway for developing high-areal-capacity Mn-based RFBs.