Ti-Doped, Mn-Based Polyanionic Compounds of Na4Fe1.2Mn1.8(PO4)2P2O7 for Sodium-Ion Battery Cathode

Na4Fe3(PO4)2P2O7 (NFPP) is recognized as a prospective electrode for sodium-ion batteries (SIBs) because of its structure stability, economic viability and environmental friendliness. Nevertheless, its commercialization is constrained by low operating voltage and limited theoretical capacity, which result in a power density significantly inferior to that of LiFePO4. To address these limitations, in this work, we first designed and synthesized a series of Mn-doped NFPP to enhance its operating voltage, inspired by the successful design of LiFe1-xMnxPO4 cathodes. This approach was implemented to enhance the operating voltage of the material. Subsequently, the optimized Na4Fe1.2Mn1.8(PO4)2P2O7 (1.8Mn-NFMPP) sample was selected for further Ti-doped modification to enhance its cycle durability and rate performance. The final Mn/Ti co-doped Na4Fe1.2Mn1.7Ti0.1(PO4)2P2O7 (0.1Ti-NFMTPP) material exhibited a high operating voltage of ~3.6 V (vs. Na+/Na) in a half cell, with an outstanding reversible capacity of 122.9 mAh g−1 at 0.1 C and remained at 90.6% capacity retention after 100 cycles at 0.5 C. When assembled into a coin-type full cell employing a commercial hard carbon anode, the optimized cathode material exhibited an initial capacity of 101.7 mAh g−1, retaining 86.9% capacity retention over 50 cycles at 0.1 C. These results illustrated that optimal Mn/Ti co-doping is an effective methodology to boost the electrochemical behavior of NFPP materials, achieving mitigation of the Jahn–Teller effect on the Mn3+ and Mn dissolution problem, thereby significantly improving structural stability and cycling performance.