Unlocking the Ultrastable Sodium Storage via Cation Engineering in Vanadium‐Iron‐Based NASICON Framework

Abstract Vanadium‐iron‐based Na _x VFe(PO ₄ ) ₃ NASICON has been renowned as a low cost and exceptional ionic conductivity cathode material for sodium‐ion batteries (SIBs). However, the irretrievable phase transition of V ⁴⁺ /V ⁵⁺ and large cationic strain result in structural instability, fettering their practical utilization. This study reports here a cost‐effective sodium‐deficient Na _3.4□0.6 VFe _0.5 Al _0.5 (PO ₄ ) ₃ (NVFAP) NASICON as an ultrastable cathode for SIBs by suppressing the cationic strain generated at the octahedral site of V ³⁺ (0.64 Å) by replacing the large ionic radii of Fe ²⁺ (0.78 Å) with small ionic radii Al ³⁺ (0.53 Å). The structural balance and strong MO ₆ bonding provided by the cationic engineering (V ³⁺ /Fe ²⁺ /Al ³⁺ ) broaden the Na diffusion channels and allow more Na to participate in electrochemical reaction, hence achieving high specific capacity of 158.31 mAh g ⁻¹ at 0.1 C, with an energy density of 434.5 Wh kg ⁻¹ The inserted Al ³⁺ ion can provide a strong coordination environment at the octahedral site, decreases the forbidden bandgap, enabling the reversible redox activation, which prevents the structural collapse beyond 3.8 V. Consequently, an outstanding long‐term stability is achieved, retaining 91.7% of its capacity after 5000 cycles at 20 C. This study has prodigious implications for the development of low‐cost, cutting‐edge advanced NASICON cathode materials for SIBs.