Direct observation of electronic conductivity transitions and solid electrolyte interphase stability of Na2Ti3O7 electrodes for Na-ion batteries

This communication reports the first experimental evidence of an interesting change of transport properties, and particularly of electron conductivity, during the Naþ insertion/extraction process in Na2Ti3O7 negative electrodes. Probed by electrochemical impedance spectroscopy, for 0.0 x < 1.4 in Na2þxTi3O7 the material exhibits insulator behaviour, the bulk electronic conductivity being the limiting factor in the insertion process. After further Naþ insertion, the material becomes electronic conductor and at around 0.13 V vs. Naþ/Na the rate of interfacial charge-transfer becomes the limiting factor. The observed conductivity transition is reversible upon cycling. Additionally, this impedance study sheds new light on the solid electrolyte interphase layer performance which is found to be unstable upon electrochemical cycling and negatively contributes on the capacity fading observed for this electrode material.