Composition and Evolution of the Solid-Electrolyte Interphase in Na<sub>2</sub>Ti<sub>3</sub>O<sub>7</sub> Electrodes for Na-Ion Batteries: XPS and Auger Parameter Analysis Miguel A. Muñoz-Márquez Maider Zarrabeitia Elizabeth Castillo-Martínez Aitor Eguía-Barrio Teófilo Rojo Montse Casas-Cabanas 10.1021/acsami.5b01375.s001 https://acs.figshare.com/articles/journal_contribution/Composition_and_Evolution_of_the_Solid_Electrolyte_Interphase_in_Na_sub_2_sub_Ti_sub_3_sub_O_sub_7_sub_Electrodes_for_Na_Ion_Batteries_XPS_and_Auger_Parameter_Analysis/2176378 Na<sub>2</sub>Ti<sub>3</sub>O<sub>7</sub> is considered a promising negative electrode for Na-ion batteries; however, poor capacity retention has been reported and the stability of the solid-electrolyte interphase (SEI) could be one of the main actors of this underperformance. The composition and evolution of the SEI in Na<sub>2</sub>Ti<sub>3</sub>O<sub>7</sub> electrodes is hereby studied by means of X-ray photoelectron spectroscopy (XPS). To overcome typical XPS limitations in the photoelectron energy assignments, the analysis of the Auger parameter is here proposed for the first time in battery materials characterization. We have found that the electrode/electrolyte interface formed upon discharge, mostly composed by carbonates and semicarbonates (Na<sub>2</sub>CO<sub>3</sub>, NaCO<sub>3</sub>R), fluorides (NaF), chlorides (NaCl) and poly­(ethylene oxide)­s, is unstable upon electrochemical cycling. Additionally, solid state nuclear magnetic resonance (NMR) studies prove the reaction of the polyvinylidene difluoride (PVdF) binder with sodium. The powerful approach used in this work, namely Auger parameter study, enables us to correctly determine the composition of the electrode surface layer without any interference from surface charging or absolute binding energy calibration effects. As a result, the suitability for Na-ion batteries of binders and electrolytes widely used for Li-ion batteries is questioned here. 2015-04-15 00:00:00 Na 2Ti Electrodes Auger parameter electrochemical cycling Auger Parameter AnalysisNa 2Ti Na 2Ti electrodes Na 2CO NaCO 3R fluorides batterie binder photoelectron energy assignments Auger parameter study SEI electrode surface layer NMR capacity retention battery materials characterization binding energy calibration effects XPS limitations polyvinylidene difluoride