Na4Fe1.5Mn1.5(PO4)2(P2O7): A low-cost and earth-abundant cathode for robust sodium storage

The mixed compounds of phosphates and pyrophosphates are attractive cathodes for sodium-ion batteries (SIBs) owing to their robust open framework structure and superior diffusion dynamics. However, most reported mixed phosphate cathodes generally suffer from low operating potential. Herein, we develop a bimetallic Na4Fe1.5Mn1.5(PO4)2(P2O7)/C-rGO (NFMPP/C-rGO) cathode, which possesses two working plateaus at 2.92 and 3.95 V. The obtained NFMPP/C-rGO demonstrates a stable high capacity of over 120 mAh g−1 at 0.1 C. In-situ XRD characterization discloses a solid solution reaction for the Fe2+/3+ redox couple and a two-phase reaction for the Mn2+/3+ redox couple. First-principles calculations reveal the migration of Na+ in NFMPP has low barriers. This work provides a new, low-cost, earth-abundant, and stable cathode choice for practical SIBs.