Reversible Multi‐Electron Reaction Mechanism of Sodium Vanadium/Manganese Phosphate Cathode for Enhanced Na‐Storage Capability

Abstract

Sodium-ion batteries are widely regarded as an important candidate for low cost large-scale storage of intermittent energies. NASICON-type vanadium-based phosphate with formula of Na 3 V 2 (PO 4 ) 3 exhibits promising application as cathode material for sodium-ion batteries due to robust structural framework and high electrochemical activity. However, it is intrinsically limited by low theoretical specific capacity (117 mAh g À 1 ) owing to only two-electron reaction mechanism below 4.0 V. Accordingly, exploring novel vanadium-based phosphates cathode with multi-electron reaction mechanism is essential. Herein, carbon-coated sodium vanadium/manganese phosphate (Na 3.25 V 1.75 Mn 0.25 (PO 4 ) 3 /C) was reported as enhanced cathode for sodium-ion batteries. Its structural properties, charge/discharge performance and electrochemical redox mechanism were investigated by coupling X-ray diffraction, Brunauer-Emmett-Teller measurement, X-ray photoelectron spectroscopy, cyclic voltammetry, and galvanostatic technique. It was unexpectedly found that the material not only achieved a high practical capacity (126.1 mAh g À 1 ) of larger than the theoretical value of traditional Na 3 V 2 (PO 4 ) 3 compound, but also exhibited good cycling stability with 96 % capacity retention after 40 cycles. Experimental evidences revealed that the material underwent a reversible multi-electron reaction mechanism involving V 4 + /V 3 + , Mn 3 + /Mn 2 + and V 5 + /V 4 + redox couples during Na extraction/insertion process, and the improved capacity was attributed to additional contribution of manganese substitution-activated V 5 + /V 4 + redox reaction at the high potential of ~3.8 V (vs Na + /Na).