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

QY Wang and KS Dai and YT Xie and M Shui and J Shu, ELECTROCHIMICA ACTA, 381, 138272 (2021).

DOI: 10.1016/j.electacta.2021.138272

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 Na-4 TiO(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 x10(-9) to 2.6 x10(-9) cm(2) s(-1) and from 9.2 x10(-10) to 4.9 x10(-9) cm(2) 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. (c) 2021ElsevierLtd. Allrightsreserved.

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