Solubility of the Solid Electrolyte Interphase (SEI) in Sodium Ion Batteries

It is often stated that formation of a functional solid electrolyte interphase (SEI) in sodium ion batteries is hampered by the higher solubility of SEI components such as sodium salts in comparison to the lithium analogues. In order to investigate these phenomena, SEI formation and functionality, as well as cell self discharge, are studied for the sodium ion system with comparative experiments on the equivalent lithium ion system. By conducting a set of experiments on carbonaceous anodes, the impact of SEI dissolution is tested. The results show that the SEI layer in sodium ion cells is inferior to that in lithium ion counterparts with regards to self discharge; sodium cells show a loss in capacity at a dramatic rate as compared to the lithium counterparts when they are stored at sodiated and lithiated states, respectively, for a long time with no external applied current or potential. Also, synchrotron-based hard X-ray photoelectron spectroscopy measurements indicate that the major factor leading to increased self-discharge is dissolution of significant parts of the sodium-based SEI. Furthermore, the influence of fluoroethylene carbonate (FEC) electrolyte additive on self-discharge is tested as part of the work.