Intergrowth of layered Na2Ti3O7 and tunnel Na2Ti7O15 forming a high-performance anode for sodium-ion batteries

Layered sodium trititanate (Na 2 Ti 3 O 7 ) is a promising anode material for developing safe sodium-ion batteries owning to its appropriate redox potential (about 0.3 V vs. Na + /Na). However, it suffers from structural instability against cycling and sluggish Na + transport kinetics. Here, a new sodium titanate structure (designated as NTTO) formed by ordered intergrowth of Na 2 Ti 3 O 7 and tunnel sodium heptatitanate (Na 2 Ti 7 O 15 ) is synthesized. Temperature-dependent X-ray diffraction results reveal a stepwise formation mechanism of NTTO involving several intermediate phases. In situ characterization data coupled with computational calculations confirm that the Na 2 Ti 7 O 15 phase not only enlarges the interlayer spacing of Na 2 Ti 3 O 7 but also induces the formation of new Ti–O–Ti bonds between Na 2 Ti 3 O 7 layers. These structural features of NTTO synergistically enhance Na + diffusion kinetics and maintain structural integrity during sodiation/desodiation. As a result, NTTO exhibits high reversible Na + storage capacity, excellent rate performance, and ultra-long cycling stability. A full cell fabricated with NTTO as anode and Na 3 V 2 (PO 4 ) 3 @C as cathode delivers an energy density of 282 Wh kg −1 based on the total mass of the electrode materials. This work demonstrates that intergrowth is an enabling approach to constructing high-performance titanate anode materials for developing safe and cost-effective sodium-ion batteries.