Transport coefficient of Na-ions in sodium-ion battery (SIB) materials is reported in a wide range of experiments (such as potentiostatic titration, impedance measurements, etc.). However, reported values differ from 2 to 4 orders of magnitude based on the experimental method used. Here in this study, MD simulations using universal force field are employed to provide a direct assessment of the self-diffusion coefficient of Na-ions in the structure of NASICON-type materials such as NaTi2(PO4)3. For this material, inter-atomic potentials, based on partial charge pair-potential model is applied, and the concentration of Na-ion is varied (from 0.82 &lt; x &lt; 0.98) in the supercell (total of 9000 atoms) of NaxTi2(PO4)3 at 323 K. Few runs of 100 ns isothermal-isobaric ensemble (NPT)-MD are performed to stabilize the system. For adequacy of system size, Na-ion transport is also calculated in larger supercell (total of 72000 atoms). Mean square displacement (MSD) of Na+ as a function of time is analyzed and self-diffusion coefficient of Na+ is estimated from the slope of MSD vs time plot, using linear fitting of the curve for the three-dimensional transport, following Einstein’s relation. Calculated diffusivities are in the range of ~10-8-10-9 cm2/s and corresponding ionic conductivity is estimated to be (~10-4 S/cm). The results suggested and supported the fast ion conducting nature of the material. These values are in agreement with experimental values of Na-ions in the similar structure. Furthermore, density functional theory (DFT) calculations are performed to study its electronic properties and Na-ion diffusion barrier via ion hopping mechanism. The activation energies along different paths in the bulk crystal are calculated to be 59.82 kJ/mol, 67.54 kJ/mol, and 64.64 kJ/mol in [001], [100], and [010] direction respectively. This indicated that [001] direction to be preferred for ion transport.

Study of ion transport in NaTi2(PO4)3 electrode material for sodium-ion batteries: A combined MD and DFT simulations

ACS Fall 2021

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