Exploring the sodium storage mechanism in disodium terephthalate as anode for organic battery using density-functional theory calculations

We present an ab initio study of sodium storage mechanism in disodium terephthalate (Na2TP) which is a very promising anode material for organic sodium (Na)-ion batteries with reported experimental capacities of similar to 255 mAh g(-1), previously attributed to Na attachment to the two carboxylate groups (coordinating to oxygen atoms). We show here that the inserted Na atoms prefer to bind at carboxylate sites at low Na concentrations and are dominant for insertion of up to one Na atom per molecule; for higher Na concentrations, the hexagonal sites (on the aromatic ring) become dominant. We confirm that the Na2TP crystal can store a maximum of two Na atoms per molecule, as observed in experiments. Our current results are intriguing as we reveal that the Na binding at carboxylate sites contributes to the initial part of Na2TP sodiation curve and the Na binding at hexagonal sites contributes to the second part of the curve. The inserted Na atoms donate electrons to empty states in the conduction band. Moreover, we show that the Na diffusion barriers in clean Na2TP can be as low as 0.23 eV. We also show that there is significant difference in the mechanism of Na interaction between individual molecules and the crystal. (C) 2016 Elsevier B.V. All rights reserved.