Exceeding three-electron reactions in Na3+2xMn1+xTi1−x(PO4)3 NASICON 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)3 have greatly enhanced the capacity of NASICON-structured materials, the low potential from Ti3+/4+ redox reaction and undesirable initial coulombic efficiency (ICE) have inhibited its practical application. Herein, NASICON structured Na3+2xMn1+xTi1−x(PO4)3 was 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. Na3+2xMn1+xTi1−x(PO4)3 can deliver an extra-high capacity of 181.4 mA h g−1 at 0.1C (1C = 150 mA h g−1), and 100.4 mA h g−1 at 10C during the rate tests. When x = 0.15 and 0.2, the energy density is up to 560.2 and 539.5 W h kg−1 at 0.1C, which is significantly higher than 442.4 W h kg−1 with x = 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 by ex situ XRD, 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.