Double-pinning effect assisting Na₄VMn(PO₄)₃ with superior structural and electrochemical stabilization for sodium-ion batteries

The Na superionic conductor of (NASICON)-type Na4VMn(PO4)(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 (similar to 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.