Understanding Sodium Ion Transport in Cathode Materials for Na-Ion Batteries Using MD Simulations

Classical molecular dynamics (MD) simulations are applied to study the transport of Na-ions in cathode materials for Na-ion batteries (SIB). Two different materials (Na x V 4 O 10 [1] and in Na x Ti 2 (PO 4 ) 3 ) are studied to access and compare the level of Na-ion diffusion in electrode structure. Sodium constitute a potential alternative of Lithium in alkali metal batteries, since sources of Li are limited, and Na is readily available. The diffusion coefficient of Na-ions in SIB materials is reported in a wide range of experimental methods (potentiostatic titration, impedance measurements etc.). However, reported values differ from 2 to 4 orders of magnitude based on the experimental method used, since diffusivity estimates specifically from impedance curves are often measured away from the equilibrium and fitting parameters are prone to produce large errors. MD simulations using universal force field have provided a direct assessment of self-diffusion coefficient of Na-ions in the structure of Na x V 4 O 10 with varying concentrations of Na-ions (0.33 < x < 1.33), calculating a diffusion coefficient of 5.75 x 10 -8 cm 2 /s for x=0.66 at 300 K, which is in good agreement with experiments. Mean square displacement (MSD) and Na-ion density plots are further providing a mechanistic insight on the route of Na-ion transport.[2] As a next generation material, NASICON structured type (NaTi 2 (PO 4 ) SIB materials, are investigated for Na-ion transport. Inter-atomic based partial charge potential model is applied, and concentration of Na-ion is varied (from 0.82 <x < 0.98) in the supercell (total of 9000 atoms) of Na x Ti 2 (PO 4 ) 3 at 323 K. Na + transport of five different runs at varying concentration of Na + are calculated and MD trajectory are visualized. MSD of Na + as a function of time is shown as Figure. Self-diffusion coefficient of Na + is calculated from the slope of MSD vs time plot for the three-dimensional transport, following Einstein’s relation. Calculated diffusivities are in the range of ~10 -8 -10 -9 cm 2 /s and corresponding ionic conductivity is estimated to be (~10 -4 S/cm). These values are in agreement with experimentally measured values of Na-ions in the similar structure. References Saroha, R., Khan, T.S., Chandra, M., Shukla, R., Panwar, A.K., Gupta, A., Haider, M.A., Basu, S. and Dhaka, R.S., 2019. Electrochemical properties of Na 66 V 4 O 10 nanostructures as cathode material in rechargeable batteries for energy storage applications. ACS omega , 4 (6), pp.9878-9888. Wani, M.S., Anjum, U., Khan, T.S., Dhaka, R.S. and Haider, M.A., 2020. Understanding Na-Ion Transport in Na x V 4 O 10 Electrode Material for Sodium-Ion Batteries. Journal of Electronic Materials , pp.1-6. Figure 1