Disodium Pyridine Dicarboxylate vs Disodium Terephthalate as Anode Materials for Organic Na Ion Batteries: Effect of Mol
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Disodium Pyridine Dicarboxylate vs Disodium Terephthalate as Anode Materials for Organic Na Ion Batteries: Effect of Molecular Structure on Voltage from the Molecular Modeling Perspective Yingqian Chen1, Johann Lüder1, and Sergei Manzhos1* 1
Department of Mechanical Engineering, Faculty of Engineering, National University of Singapore, Block EA #07-08, 9 Engineering Drive 1, Singapore 117576 * E-mail: [email protected] ABSTRACT Using Density Functional Theory based modeling, we compare sodium attachment to disodium terephthalate (Na2Tph) and a related molecule disodium pyridine dicarboxylate (Na2PDC). We predict that substitution of the Na2Tph’s aromatic ring with pyridine will lead to an increased voltage by about 0.4 V vs Na2+xTph up to Na2+1PDC and a similar voltage to the terephthalate between Na2+1PDC and Na2+2PDC, i.e. a two-plateau behavior vs. a single plateau for Na2+xTph. INTRODUCTION Organic secondary batteries are promising as they can be scalable and sustainable [1, 2]. They have attracted attention especially for use in sodium ion batteries, as they can easier accommodate the large ionic radius of Na+ than inorganic materials [3]. Organic Na ion batteries would allow for full use of sustainability and scalability of both organic materials and of extremely abundant Na. Contrary to Li ion batteries where good anode solutions such as graphite or silicon exist, new anode materials must be designed for Na ion batteries, as graphite and silicon are not electrochemically active for Na. Lithium and sodium terephthalates have been shown to be effective organic anode materials for lithium and sodium ion batteries, respectively [4-6]. Specifically, disodium terephthalate (Na2Tph) has been studied in several experimental works [5, 6] and showed an ideal voltage for Na ion batteries of 0.2-0.3 V (vs. Na/Na+), a high capacity of about 250 mAh g-1 and a decent cycling rate and cycling life performance in experimental batteries [6]. However, for practical use, cycle rate and life should be improved. Ab initio models of Na attachment to the Na2Tph molecule and Na insertion into the Na2Tph crystal have been reported [7, 8]. These models explained the sodiation mechanism by occupancy of unoccupied orbitals or the conduction band by valence electrons of the inserted Na. It was also understood that Na atoms would segregate into Na2+2Tph resulting in a flat, singleplateau voltage profile, in agreement with experiments [6]. Na2Tph possesses a rather high band gap which results in low electronic conductance. The segregation might also limit the kinetics of sodiation. Disodium pyridine dicarboxylate (Na2PDC) which is obtained by substitution of the Na2Tph’s aromatic ring with pyridine, is expected to improve conductance. This potential anode material remains unstudied. Here, we present a preliminary ab initio investigation of the mechanism of Na attachment to the Na2PDC molecule and, based on available results for the Na2Tph molecule and crystal, make an estimate of the expected voltage curve of Na2+xPDC.
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