A Novel NASICON‐Typed Na 4 VMn 0.5 Fe 0.5 (PO 4 ) 3 Cathode for High‐Performance Na‐Ion Batteries

Abstract The Na + superionic conductor (NASICON)‐type Na 3 V 2 (PO 4 ) 3 cathodes have attracted extensive interest due to their high structural stability and fast Na + mobility. However, the substitution of vanadium with low‐cost active elements remains imperative due to high cost of vanadium, to further boost its application feasibility. Herein, a novel ternary NASICON‐type Na 4 VMn 0.5 Fe 0.5 (PO 4 ) 3 /C cathode is designed, which integrates the advantages of large reversible capacity, high voltage, and good stability. The as‐obtained Na 4 VMn 0.5 Fe 0.5 (PO 4 ) 3 /C composite can deliver an excellent rate capacity of 96 m Ah g ‐1 at 20 C and decent cycling durability of 94% after 3000 cycles at 20 C, which is superior to that of Na 4 VFe(PO 4 ) 3 /C and Na 4 VMn(PO 4 ) 3 /C. The synergetic contributions of multimetal ions and facilitated Na + migration of the Na 4 VMn 0.5 Fe 0.5 (PO 4 ) 3 /C cathode are confirmed by the first‐principles calculations. The processive reduction/oxidation involved in Fe 2+ /Fe 3+ , Mn 2+ /Mn 3+ , V 3+ /V 4+ /V 5+ redox couples are also revealed upon the charging/discharging process by ex situ soft X‐ray absorption spectroscopy. The reversible structure evolution and small volume change during the electrochemical reaction is demonstrated by in situ X‐ray diffraction characterization. The rational design of NASICON‐type cathodes by regulating composition with substitution of multimetal ions can provide new perspectives for high‐performance Na‐ion batteries.