Assessment on the Stable and High-Capacity Na-Se Batteries with Carbonate Electrolytes

Herein, the factors affecting proper functioning of Na-Se system are investigated mainly focusing on the polyselenide formation in ether- and carbonate-based electrolytes. In this study, selenium cathode is prepared by a simple and cost-effective ball milling method with commercially available carbon and selenium powders. It is relieved that the soluble polyselenide species form in ether-based electrolytes while there is no signature in carbonates proven by the in-situ 4-electrode cyclic voltammetry (CV), 3-electrode electrochemical impedance spectroscopy (EIS) and ex-situ UV-vis spectroscopy measurements as well as monitoring self-discharge behaviors. Staircase potentio electrochemical impedance spectroscopy (SPEIS) is also applied to indicate discharge mechanism in those different electrolyte combinations. Followed by the systematic studies of the electrolyte solvent impact, the formation of volume expansion is targeted using different type of binders. Among those, the composite with carboxyl methyl cellulose (CMC) - styrene butadiene rubber (SBR) delivers the highest reversible capacity and the best rate performance that assists in defeating drastic volume change of Se. To further improve the cell performances along with the defined electrolyte composition, fluoroethylene carbonate (FEC) is used as a film forming additive that preserves the Na metal integrity proven by the Na-Na symmetric cells and voltage relaxation monitoring upon cycling. As a result, it is demonstrated that the selection of the proper binder and electrolyte compositions are the two crucial factors to obtain a stable and high-capacity Na-Se cell. Contrary to most of the reports in the field of Na-Se batteries, this study underlines that the confinement of Se into the state-of-the-art porous cathode architectures are not required since there is no soluble polyselenide formations in carbonate electrolytes.