Tailoring the electronic structure of the NaTi₂(PO₄)₃ anode for high-performing sodium-ion batteries via defect engineering

NASICON-type electrode materials suffer from poor intrinsic electronic conductivity, which significantly limits their capacity and rate capability for further application in sodium-ion batteries. Herein, we aim to address this issue by introducing oxygen vacancies (VO) into core-shell C@NaTi2(PO4)(3-x) composites to tailor the electronic structures and enhance the Na storage performance. Various characterization techniques, including Rietveld refinement of X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS), confirm the successful generation of VO in the core-shell C@NaTi2(PO4)(3-x) composites. Density functional theory calculations demonstrate that VO induce partial hole states in O 2p orbitals, localizing the electronic structures of P1 3p, Ti1 t2g, and Ti-2 t2g orbitals. Simultaneously, the electronic structure of O atoms bridging these cations (Ti1, Ti-2, and P1) becomes delocalized. This unique electronic modulation facilitates sodiation/desodiation and enhances fast Na+ diffusion kinetics. Among the C@NaTi2(PO4)(3-x) composites, the C@NaTi2(PO4) (3-1).0 anode, which possesses the highest content of VO, exhibits the most stable cycling performance (retaining 108.9 mA h g(-1) after 10 000 cycles at 20C) and the best rate capability. The CV and GITT tests confirm about an order of magnitude of DNa+ increase in C@NaTi2(PO4)3-1.0 composites. Furthermore, the EPR, scanning electron microscopy and transmission electron microscopy results confirm the robustness of intrinsic VO and the stable core-shell structure even after 10 000 cycles at 20C, firmly confirming their ability to enable high electrochemical activity. Overall, the engineering of oxygen vacancies provides a promising approach to address the poor electronic and ionic conductivities of NASICON-type materials for future applications of SIBs.