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.