Nb-Doped titanium phosphate for sodium storage: electrochemical performance and structural insights

Abstract

The effect of Nb⁵⁺ doping on the electrochemical and structural characteristics of NaTi₂(PO₄)₃ was investigated. The Nb⁵⁺ substitution lowered the band gap energy from 2.8 to 1.4 eV according to density functional theory calculations. Subsequent carbonization of pitch carbon on the surface of NaNb_xTi_2−x(PO₄)₃ (x = 0.05) significantly increased the electrical conductivity, thereby improving the capacity and high-rate capability. The excellent cyclability and superior electrode performance result from facile Na⁺ insertion and extraction that occur via a biphasic redox mechanism, namely, the Ti^4+/3+ redox couple for the cathode and a sequence of two biphasic redox reactions associated with the Ti^3+/2+ redox couple for the anode, as revealed by operando X-ray diffraction and ex situ X-ray absorption spectroscopic studies. Moreover, the possibility of application of Na_2.9Nb_0.05Ti_1.95(PO₄)₃ as both a cathode and anode material was demonstrated in a symmetric cell, which delivered a capacity of 105 mA h g⁻¹ after 100 cycles at 0.2C with a capacity retention of 83%.