Experimental and theoretical investigation of Na₄MnAl(PO₄)₃ cathode material for sodium-ion batteries

Finding appropriate structures of cathode materials is crucial to the development of sodium-ion batteries (SIBs). A novel NASICON-related structure (R-32 space group) of Na₄MnAl(PO₄)₃ is synthesized, which exhibits a high specific capacity (116.8 mAh g⁻¹) with two voltage plateaus of ~ 3.56 V and ~ 4.10 V for Mn³⁺/Mn²⁺ and Mn⁴⁺/Mn³⁺ redox couples, respectively. The sodium ion storage mechanism and diffusion mechanism are investigated by ex-situ XRD and ex-situ XPS, with the aid of first principle calculations. Both two-phase and solid solution reactions take place in the charge/discharge process, and the diffusion path along Na(1) ↔ Na(2) ↔ Na(3) is revealed for the first time in NASICON-structured materials. In addition, a voltage plateau of ~ 4.48 V of the Mn⁵⁺/Mn⁴⁺ redox couple is suggested by the DFT calculations in Na₄MnAl(PO₄)₃, which may be more easily accessed than that of other compounds such as Na₃MnTi(PO₄)₃ and Na₃MnZr(PO₄)₃ reported in literature, leaving great potential in elevating the voltage and specific capacity for Na₄MnAl(PO₄)₃ cathode. Because of high theoretical specific capacity, high voltage plateaus and low cost, Na₄MnAl(PO₄)₃ is a strong contender in cathode material for SIBs.