An air-stable iron/manganese-based phosphate cathode for high performance sodium-ion batteries

Abstract

• NFMPP could deliver a higher average working potential of 3.27 V with a theoretical capacity of 120 mAh g −1 . • In-situ XRD and ex-situ XPS revealed NFMPP presented good electrochemical reversibility. • Carbon coating NFMPP exhibits excellent electrochemical performance. • The NFMPP-HC full cell shows a maximum power density of 260 W kg −1 with a maximum specifc energy of 130 Wh kg −1 . Iron-based phosphates as a low cost and high structural stability cathode materials for sodium ion batteries (SIBs) have been widely studied. However, the working potential basing on Fe 3+ /Fe 2+ redox is very low (less than 3.05 V vs. Na/Na + ), which has obviously affect on the energy/power density. In this work, we choose the non-precious metal manganese (Mn) to improve the working potential of Na 3 Fe 2 (PO 4 )(P 2 O 7 ) (NFFPP). A series of mixed-phosphate Na 3 Fe 2-x Mn x (PO 4 )(P 2 O 7 ) (x = 0.0, 0.5, 1.0, 1.5 and 2.0) compounds are firstly proposed as cathode materials for SIBs. After optimization, the as-prepared Na 3 FeMn(PO 4 )(P 2 O 7 ) (NFMPP) could deliver a higher average working potential of 3.27 V ( vs. Na/Na + ) with a theoretical capacity of 120 mAh g −1 . Furthermore, the carbon coating NFMPP cathode shows a reversible initial specific capacity of 105 mAh g −1 at 0.1C with a corresponding coulombic efficiency of 99%. The in-situ XRD and ex-situ XPS results indicate that this cathode material goes through the reversible redox Fe 3+ /Fe 2+ and Mn 3+ /Mn 2+ with a reversible two Na-ion insertion/deinsertion process.