Sodium Storage and Transport Properties in Layered Na 2 Ti 3 O 7 for Room‐Temperature Sodium‐Ion Batteries

Layered sodium titanium oxide, Na 2 Ti 3 O 7 , is synthesized by a solid‐state reaction method as a potential anode for sodium‐ion batteries. Through optimization of the electrolyte and binder, the microsized Na 2 Ti 3 O 7 electrode delivers a reversible capacity of 188 mA h g −1 in 1 M NaFSI/PC electrolyte at a current rate of 0.1C in a voltage range of 0.0–3.0 V, with sodium alginate as binder. The average Na storage voltage plateau is found at ca. 0.3 V vs. Na + /Na, in good agreement with a first‐principles prediction of 0.35 V. The Na storage properties in Na 2 Ti 3 O 7 are investigated from thermodynamic and kinetic aspects. By reducing particle size, the nanosized Na 2 Ti 3 O 7 exhibits much higher capacity, but still with unsatisfied cyclic properties. The solid‐state interphase layer on Na 2 Ti 3 O 7 electrode is analyzed. A zero‐current overpotential related to thermodynamic factors is observed for both nano‐ and microsized Na 2 Ti 3 O 7 . The electronic structure, Na + ion transport and conductivity are investigated by the combination of first‐principles calculation and electrochemical characterizations. On the basis of the vacancy‐hopping mechanism, a quasi‐3D energy favorable trajectory is proposed for Na 2 Ti 3 O 7 . The Na + ions diffuse between the TiO 6 octahedron layers with pretty low activation energy of 0.186 eV.