Tailoring the electronic structure of the NaTi2(PO4)3 anode for high-performing sodium-ion batteries via defect engineering†
Abstract
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 Ti2 t2g orbitals. Simultaneously, the electronic structure of O atoms bridging these cations (Ti1, Ti2, 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.