Stabilizing NASICON-type Na₄MnCr(PO₄)₃ by Ti-substitution toward a high-voltage cathode material for sodium ion...

The sodium superionic conductor Na₄MnCr(PO₄)₃ gains increasing attention owing to its three-dimensional structure and the three-electron reaction. However, rapid structure degradation during cycling is the major challenge for its practical application. Herein, Ti⁴⁺ is utilized to replace a portion of Mn²⁺ in Na₄MnCr(PO₄)_3. The low redox voltage and d⁰ electronic configuration of the Ti⁴⁺ ions are helpful to suppress the structure alteration and improve electronic conduction. Consequently, the as-prepared Na_3.4Mn_0.7Ti_0.3Cr(PO₄)₃/C cathode exhibits a remarkable good 91.0% capacity retention after 500 cycles at 10C rate, with exceptional rate capacities of 99.5 mAh g^-1 and 81.0 mAh g^-1 at 5C and 10C rate, respectively. Furthermore, based on ≈2.86-electron reactions involving Mn²⁺/Mn³⁺ (3.5 V), Mn³⁺/Mn⁴⁺ (4.1 V), Cr³⁺/Cr⁴⁺ (4.3 V), and Ti³⁺/Ti⁴⁺ (2.1 V), the material can provide an energy density of approximately 541.6 Wh kg^-1, slightly surpassing that of Na₄MnCr(PO₄)₃. Ex-situ XRD investigation further elucidates that throughout the entire charge-discharge process, the Ti-substituted material experiences highly reversible solid-solution and two-phase reactions. Additionally, Ti substitution can greatly promote the interfacial charge transfer of the material and suppress the decomposition of the electrolyte during cycling. This work might open a new insight for designing sodium-ion battery cathode materials with good cycling stability and high energy density.