The preparation, electrochemical properties, Na+ migration mechanism and kinetics of a new anode material Na4TiO(PO4)2 with open framework for Na ion batteries

Abstract

A new anode material for NIB Na4TiO(PO4)2 and the carbon coating counterpart Na4TiO(PO4)2/C, which were never reported elsewhere, were synthesized by a traditional high-temperature solid-phase method and explored both experimentally and theoretically. The morphology and the composition of Na4TiO(PO4)2 and Na4TiO(PO4)2/C were examined by XRD, SEM, TEM, ICP, EDS and XPS. Na4TiO(PO4)2/C material shows better rate capability and cycle stability than Na4TiO(PO4)2. It provided an initial charge and discharge capacity of 117/234.5 mAh g−1 at a current rate of 1C, and retained a capacity of 103/107.4 mAh g−1 after 240 cycles. Except the initial several cycles, the Coulomb efficiencies of Na4TiO(PO4)2/C were higher than 95%. When the current density experienced 0.1 C, 0.2 C, 0.5 C, 1 C and 2 C and returned to 0.1 C, the discharge capacity of 149.29 mAh g−1 can be obtained by the Na4TiO(PO4)2/C electrode, closed to the initial 171.12 mAh g−1. The chemical diffusion coefficient in discharge process varies from 1.3 × 10−9 to 2.6 × 10−9 cm2 s−1 and from 9.2 × 10−10 to 4.9 × 10−9 cm2 s−1 for charge process, showing features of fast ion conducting materials. Both nudged elastic bands (NEB) method and molecular MD show a relatively low diffusion barrier of ca. 0.4 eV on a-c plane and a 0.1 eV higher energy barrier along b-axis. The theoretical calculations show that the high mobility of Na+ originates from the ample cavities and the complicated three dimensional conducting framework in the lattice. Ex-situ experiments reveal an ultra-stable framework of Na4TiO(PO4)2 material in the intercalation/deintercalation process and also a less than 0.92% contraction in this process.