β-Cyclodextrin inducing confinement effect enabling spherical Na3V2(PO4)3 with multielectron reaction and superior performance at extreme conditions for sodium-ion batteries

Abstract

Currently, simultaneous regulation of external morphology and internal electronic structure for Na₃V₂(PO₄)₃ (NVP) is rarely realized. Herein, complexes of β-cyclodextrin (βCD) and ethylenediaminetetraacetic acid ferric sodium salt (EDTAFeNa) are utilized for the one-step preparation of NVP with spherical morphology as well as Fe substitution. βCD is initially hydrolyzed into glucose, and then carbon microspheres with numerous pores are formed through continuous dehydration and carbonization. The intermediate hydroxymethylfurfural is rich in active functional groups, which are attractive for the V/P-contained raw materials. Accordingly, the nucleation sites for NVP are successfully limited in the spherical framework, possessing a superior surface area of 97.15 g m⁻². Furthermore, the beneficial Fe in EDTAFeNa enters into the NVP bulk to construct a novel Fe-doped Na₃V_1.95Fe_0.05(PO₄)₃ (NVP/β-ISC) material. Fe-substitution induces significant optimizations of electronic structure for NVP, which has been verified by the newly generated abundant oxygen vacancies and extended V–O bond length. Moreover, a multielectron reaction is activated, resulting from the V⁴⁺/V⁵⁺ redox couple. The charge compensation mechanism of NVP/β-ISC is also deeply investigated. Density functional theory (DFT) calculations theoretically elaborate the mechanism of Fe-doping. Consequently, NVP/β-ISC reveals superior sodium storage performance in both half and full cells and even at different extreme conditions (needling, soaking, bending, and freezing).