Oxygen vacancies in NaTi2(PO4)3 nanoribbons to enhance low-temperature performance for Na storage

Abstract

Sodium superionic conductor NaTi₂(PO₄)₃ has attracted significant interest as an anode material for sodium-ion batteries (SIBs). However, its practical application is hindered by its low inherent electrical conductivity, particularly at low temperatures. In this study, oxygen vacancies (V_O) were introduced into NaTi₂(PO₄)₃ nanoribbons to enhance sodium storage performance at low temperatures. X-ray diffraction with Rietveld refinement, electron paramagnetic resonance, and X-ray photoelectron spectroscopy confirm that NaTi₂(PO₄)₃-2 nanoribbons (NTP-2) exhibit the richest V_O concentration. These V_O, which bridge TiO₆ octahedra and PO₄ tetrahedra, significantly enhance the antibonding interactions of Ti1–O2 and P1–O1 bonds, while stabilizing the bonding in NaTi₂(PO₄)₃. The energy barrier for Na⁺ migration is reduced to 0.40 eV involving the V_O. The optimized NTP-2 anode demonstrates superior low-temperature performance, maintaining a capacity of 106.1 mAh g⁻¹ (about 96.1 % of its initial capacity) at −20 °C after 300 cycles. Additionally, the NTP-2 anode exhibits a moderate Na⁺ diffusion coefficient of 1.47 × 10^–11 cm² s⁻¹ at −20 °C. Furthermore, the Na₃V₂(PO₄)₃//NTP-2 full cell retains a capacity of 64 mAh g⁻¹ at −20 °C after 250 cycles, highlighting its potential for low-temperature applications. By integrating oxygen vacancies and nanoengineering, both electronic and ionic conductivities are significantly enhanced in NaTi₂(PO₄)₃, positioning promising applications for SIBs in low-temperature environments.