Interaction between Electrolytes and Sb₂O₃-Based Electrodes in Sodium Batteries: Uncovering the Detrimental Effects of Diglyme

Conversion materials are promising to improve the energy density of sodium-ion-batteries (NIB). Nevertheless, they suffer from the drawback of phase transitions and pronounced volume changes during cycling, which causes cell instability. When using these types of electrodes, all cell-components have to be adjusted. In this study, a tremendous influence of the electrolyte solution on Sb₂O₃ conversion electrodes for NIBs is discussed. Solutions based on three solvents and solvent combinations established for NIBs, ethylene carbonate/dimethyl carbonate (EC/DMC), EC/DMC+5 % fluoroethylene carbonate (FEC), and diglyme, lead to a massively divergent electrochemical behavior of the same Sb₂O₃ electrode. Sb₂O₃ demonstrates the highest stability in solutions containing FEC, because this component forms a flexible, protecting surface film that prevent disintegration. One key finding of this work is that electrolyte solutions based on ether solvents like diglyme can remove Sb-ions from Sb₂O₃ during cycling. Diglyme has the ability to coordinate and extract Sb³⁺ during the oxidation of Sb₂O₃. This leads to contaminations of all cell components and a strong capacity loss together with an irregular electrochemical signature. Due to its poor reactivity at low potentials, diglyme forms a thin or even no surface layer. Thereby, there are no protecting films on the Sb₂O₃ electrodes that can avoid Sb³⁺ ion dissolution. A critical examination of the electrolyte solutions components’ impact is essential to match them with conversion reaction anodes.