Exceeding three-electron reactions in Na_3+2xMn_1+xTi_1−x(PO₄)₃NASICON cathodes with high energy density for sodium-ion batteries

Sodium super ionic conductor (NASICON) materials are considered as an attractive cathode in sodium-ion batteries. Although the three-electron reactions in Na₃MnTi(PO₄)₃have greatly enhanced the capacity of NASICON-structured materials, the low potential from Ti^3+/4+redox reaction and undesirable initial coulombic efficiency (ICE) have inhibited its practical application. Herein, NASICON structured Na_3+2xMn_1+xTi_1−x(PO₄)₃was designed and synthesized by the atomic-ratio-controlled method. Impressively, the increase in the Mn content not only significantly enhances the average voltage, but also increases the theoretical capacity with more than three-electron reactions. Na_3+2xMn_1+xTi_1−x(PO₄)₃can deliver an extra-high capacity of 181.4 mA h g⁻¹at 0.1C (1C = 150 mA h g⁻¹), and 100.4 mA h g⁻¹at 10C during the rate tests. Whenx= 0.15 and 0.2, the energy density is up to 560.2 and 539.5 W h kg⁻¹at 0.1C, which is significantly higher than 442.4 W h kg⁻¹withx= 0,i.e.Na₃MnTi(PO₄)₃. The capacity retention is 87.4% at 1C after 500 cycles and 83% at 5C after 1000 cycles, respectively. In addition, the ICE is as high as 89.2% after the introduction of more Na ions in the pristine structure. The structural evolution and electrochemical reaction mechanism were further confirmed byex situXRD, XPS and TEM. This work provides a new insight into controllable design of low cost, high capacity and energy density NASICON-structured materials for SIBs.