NASICON-type (Na super ionic conductor) Na 4 VMn(PO 4 ) 3 (NVMP) cathode material has attracted increasing attention due to its higher sodium de-/intercalation voltage, lower cost and greener resources compared with Na 3 V 2 (PO 4 ) 3 . However, the poor electronic conductivity limits the exploitation of its electrochemical performance. In this work, we achieved an ultrahigh rate capable and long cycle life NVMP/C-rGO composite using the simple and scalable spray-drying technique, in which low-fraction rGO nanosheets (2.14 wt%) highly dispersed and bridged the carbon shelled NVMP nanoparticles to form an effective electron conductive network. Benefiting from this favorable nano-architecture, the elaborately designed NVMP/C-rGO composite exhibited excellent performance as cathode in sodium ion batteries, delivering reversible capacities of 105.5 mA h g −1 at 1 C (1 C = 110 mA h g −1 ) and 102.6 mA h g −1 at 50 C with capacity retention of 90.6% (1 C) and 80.4% (50 C) over 500 cycles, respectively. This is the highest rate performance for Na 4 VMn(PO 4 ) 3 materials reported so far. This work provides a practicable tactic for fabricating advanced polyanion-type cathode materials which can be commercially scale up for sodium energy storage.
Jingwen Zhao,Weitao Yan,Shijia Li,Shiyu Li,Wei-Hua Wang,Ying Bai
The Na superionic conductor of (NASICON)-type Na 4 VMn(PO 4 ) 3 (NVMP) with a three-dimensional framework and high operating voltage has been extensively investigated for sodium-ion batteries (SIBs). However, unexpected Jahn-Teller effect and sluggish Na + diffusion kinetics inevitably lead to rapid capacity fade and unfavorable structural distortion. Herein, the double-pinning effect of Al/F co-doping was adopted for the NVMP system to enhance structural stability and facilitate Na + diffusion. The as-synthesized NVMP-Al&F presents a highly reversible capacity of 110 mAh g −1 at 0.1 C, accompanied with a long-term lifespan of 1000 cycles at 5 C (capacity retention of 86.1 %). Moreover, in situ X-ray diffraction affirms the reversible structural evolution and small volume variation (∼4.43 %) during the highly reversible sodiation/desodiation processes. Benefiting from the double-pinning effect, the lowered Na + migration barrier effectively facilitates Na + diffusion and increases to higher reversible capacity by combining theoretical calculation and kinetic analysis . The assembled NVMP-Al&F//hard carbon (HC) full cell manifests a high energy density of 353.9 Wh kg −1 and excellent cycling durability of 91.3 % at 1 C after 100 cycles. This work sheds light on the double-pinning as a novel strategy to boost the cycling stability of high-performance NASICON-type cathodes.
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