Na₂Ti₃O₇ is a promising negative electrode for Na-ion batteries (NIBs) with a very low insertion voltage (0.3 V vs. Na⁺/Na) and high specific capacity (178 mAh/g) [1, 2]. However, Na₂Ti₃O₇ shows poor capacity retention when synthesized from Na₂CO₃ as sodium precursor, reaching only 50-70% of capacity retention after 10 cycles [3, 4, 5]. The capacity fading is correlated, among other factors, with the presence of Na₂CO₃ on the particles [3], which is originated by the interaction of Na₂Ti₃O₇ particles with atmospheric H₂O and CO₂. Another important factor to take into account is the formation of a stable solid-electrolyte interphase (SEI) layer. In fact, the reversible Na⁺ insertion/extraction reaction occurs at low potential and, therefore, electrolyte reduction occurs. The stability and composition of this SEI layer has been previously studied by X-ray photoelectron spectroscopy (XPS) [6], concluding that the SEI layer formed upon Na⁺insertion is partially dissolved during extraction.

 In order to better understand the reasons behind the poor capacity retention, an electrochemical impedance spectroscopy (EIS) study was carried out to determine the electronic and ionic transport properties of Na₂Ti₃O₇ electrodes. An interesting change of transport properties, and particularly of electron conductivity, during Na⁺ insertion/extraction process is revealed for Na₂Ti₃O₇negative electrodes by EIS.

 Na₂Ti₃O₇ was synthesized via a ceramic route from precursors: TiO₂ anatase and Na₂CO₃·H₂O in excess. Three electrode Swagelok type cells were tested using metallic sodium disks as counter and reference electrodes; electrochemical measurements were performed at room temperature in the voltage window 0.05-1.6 V vs. Na⁺/Na. EIS measurements were performed by controlling the electrode potential through PITT (potentiostatic intermittent titration technique).

 The EIS study here presented is the first experimental demonstration of a transition from electronic insulator to conductor in Na₂Ti₃O₇ electrodes for NIBs [Fig. 1]. This reversible transition is originated by Na⁺ insertion/extraction and was recently predicted by DFT calculations. Moreover, the instability of the SEI layer has been also observed, in agreement with previous XPS studies, contributing to the capacity fading widely reported for this material. This confirms that prior to Na⁺ insertion the Na₂Ti₃O₇ is an insulator and the ionic transport kinetics are limited by the electronic conductivity, but when the intercalated Na⁺ increases the Na₂Ti₃O₇behaves as a metallic conductor and the kinetics are limited by the interfacial charge-transfer step.

Acknowledgments

M. Zarrabeitia thanks the Government of the Basque Country for funding through a PhD Fellowship. Financial support from the Basque Government (Etortek 10 CIC Energigune) and from the Ministerio de Economía y Competitividad of the Spanish Government (ENE2013-44330-R) is also acknowledged.

References