Optimization of Disodium Naphthalene Dicarboxylates Negative Electrode for Organic-Inorganic Hybrid Sodium Batteries

Abstract

The impressive growth in demand for energy storage systems and the constraints it might bring on the availability of raw materials has promoted research into new battery technologies alongside the dominant lithium ion technology[1,2]. In the last years, organic-based materials have gained attention as promising candidates to replace inorganics: these compounds are characterized by low production costs and high recyclability, as well as the possibility of being obtained from natural sources, for an overall lower environmental impact[3,4]. Despite the extensive literature on the study of organic molecules able of storing energy, nowadays there are still few examples of optimizations of the formulation of organic-based electrodes, to obtain electrochemical performances comparable to commercially available materials. Herein we would like to report on the optimization of an organic electrode for hybrid organic-inorganic sodium-ions battery. Among the different organic compounds, disodium naphtalene dicarboxylate was chosen as the active material for a negative electrode[5,6], as this molecule showed outstanding electrochemical properties, to be coupled to a positive electrode based on the NVPF[7]. Two formulations were investigated, one with an organic solvent (NMP) and the other one in water, with the PVDF and the CMC binders, respectively. Differences in crystallinity, microstructure and particle size distribution were analyzed for both formulations and correlated with several electrochemical performance, such as reversible capacity and capacity retention. The aqueous formulation showed a better specific capacity (208 mAh g-1) in the first cycle and a superior capacity retention over cycling. Furthermore, based on these results, a hybrid full cell was built with NVPF cathode and Na2NDC anode. The cell showed remarkable electrochemical performance: at low charge rates (C/20), a specific capacity of 94 mAh g-1 (based on the positive electrode mass) were observed, with only 20% irreversibility, in the first cycle. The cell also showed good capacity retention: 81 mAh g-1 (CE 99,5%) of reversible capacity were measured after 50 cycles of the full-cell. Moreover, the hybrid cell showed outstanding result in terms of power density, making it a possible replacement for hard carbon: a reversible capacity of 73 mAh g-1 and 55 mAh g-1 was observed after 50 cycles 1C and 5C, respectively. Keywords: Naphtalene carboxylate, sodium batteries, organic electrode, hybrid batteries.