Engineering Na3V2(PO4)3Cathodes via High-Entropy Substitution for Enhanced Pseudocapacitive Sodium Storage and Extended Cycle Life

Abstract

Na3V2(PO4)3(NVP) is regarded as a promising cathode for sodium-ion batteries (SIBs) owing to its high operating voltage and robust NASICON-type framework. However, its practical application is hindered by poor rate capability and limited long-term cycling stability. In this work, a high-entropy substituted cathode, Na3V1.75(Ca, Zr, Al, Mn, Mg)0.25(PO4)3(denoted as HE-NVP), is successfully synthesized via a conventional solid-state method. The introduction of multiple aliovalent cations leads to local structural regulation, where elongation of Na(1)–O bonds and contraction of Na(2)–O bonds enhance structural stability and facilitate fast Na+migration. Simultaneously, the V4+/V5+redox couple is activated at ∼4.0 V, significantly boosting energy density and cycling stability. High-entropy substitution also promotes pseudocapacitive behavior through the modulation of the local chemical environment and vanadium redox chemistry. As a result, HE-NVP delivers excellent rate performance with a high specific capacity of 93.2 mAh g–1at 50 C and outstanding long-term durability, retaining 81.8% of its initial capacity after 2,000 cycles at 20 C. Even under harsh conditions of 50 C, it maintains 78% capacity retention after 4,000 cycles, with an ultralow capacity decay rate of 0.006% per cycle after 6,000 cycles. Ex-situ XRD analysis confirms a single-phase solid-solution reaction mechanism with a minimal volume change of only 1.5% during Na+insertion/extraction. This study highlights a practical and effective strategy for designing high-performance SIB cathodes through rational high-entropy engineering of the crystallographic structure and redox chemistry.