Giant two-dimensional titania sheets for constructing a flexible fiber sodium-ion battery with long-term cycling stability

Following the rapid development of wearable technology , fiber-shaped energy storage systems integrating the merits of being lightweight and flexible are receiving unprecedented attention. Meanwhile, considering the increasing concerns for the scarcity of lithium (Li) resources, research on rechargeable batteries that use sodium (Na) as the shuttling ion is rapidly growing. As yet, there have been very few successful demonstrations of fiber-shaped Na-ion batteries, probably due to the lack of mechanically stable electrodes that can sustain the insertion of large-sized Na ions . Herein, by stacking giant two-dimensional sheet forms of an electrochemically active material, titania, along with reduced graphene oxide (rGO), into a belt-shape and placing one-dimensional carbon nanotubes (CNTs) sporadically between the neighboring sheets, a fiber electrode with decent Na-ion storage performance was successfully obtained. Benefiting from the conformal stacking of the electrochemically active and charge-collecting components, the fiber electrode did not require any conducting backbones, and thus exhibited outstandingly robust integrity. The incorporation of CNTs with a differing dimension disordered the stacking structure, which opened up paths for Na-ion intercalation, thus enabling exceptionally long-term reversible Na-ion intercalation/deintercalation, even under bending conditions. This work developed a new paradigm toward designing novel fiber electrodes that can host large alkaline ions for use in future flexible and portable electronic devices.