Reversible Multi‐Electron Reaction Mechanism of Sodium Vanadium/Manganese Phosphate Cathode for Enhanced Na‐Storage Capability
Xiaochen Lin, Xuefei Ren, Lin Cong, Yue Liu, Xingde Xiang
Abstract
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 −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 −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).