Na2Ti3O7/C rods synthesized by modified sol-gel method as an anode material for sodium ion batteries and the revelation of Na+ intercalation, diffusion mechanism

Na2Ti3O7/C prepared by a simple acrylamide-assisted sol-gel method shows promising electro-chemical performance as an anode material for sodium-ion batteries. It maintains ca. 100 mAh center dot g(-1) capacity after 100 cycles at 0.1C rate. What's more, it displays about 206.4, 181.2, 151.8, 129.0, 100.3, and 63.4 mAh g(-1) at 0.1, 0.2, 0.5, 1, 2, and 5 C current rate, respectively. A capacity of 156.4 mAh & BULL;g(-1) is recovered when the current density is back to 0.2 C. Density functional theory (DFT) together with molecular dynamics (MD) is used to reveal the Na+ intercalation and diffusion mechanism. Theoretical calculation shows highly active Na+ migration on the b-c planes, with a minimal 0.22 eV energy barrier and the highest diffusion coefficient of 3.8 x 10(-7) cm(2) s(-1) along the b-axis, at the fully charged state. The high vitality of Na+ is maintained until the intercalation level x is more than 0.5. After that, the mobility of Na+ is greatly prohibited, leading to an energy barrier of more than 0.7 eV and a diffusion coefficient within the range 10(-14)-10(-15) cm(2) s(-1) at the fully discharged state of Na4Ti3O7. The formation energy convex hull determined by means of cluster expansion enables the calculation of voltage profile and the variation of cell parameters, which reveals the low strain (< 1%) feature during the battery cycles. The calculation of electronic structure also reveals an insulator-metal transition upon the sodiation of the Na2Ti3O7 material.