Flexible Quasi-Solid-State Sodium-Ion Batteries Built by Stacking Two-Dimensional Titania Sheets with Carbon Nanotube Spacers

The rapid advances in portable and wearable electronics have triggered an ever-increasing demand for flexible energy storage devices. Because of concerns on the limited lithium resources and constantly increasing lithium prices, it is predicable that flexible rechargeable sodium (Na)-ion batteries could pave the way for broader acceptance of wearable energy storage devices. However, developing flexible electrodes for rechargeable Na-ion batteries has been seriously limited on account of the reduced availability of electrodes that can host bulky Na ions while maintaining structural integrity. Here, a stacking assembly of Na-ion intercalation materials, titania sheets, was designed and fabricated by a simple vacuum-assisted filtration method. Chemically reduced graphene oxide was molecularly hybridized and served as the current collector, and trace carbon nanotubes were introduced between the sheets for improved electrolyte infiltration and ion intercalations. As a result, the designed electrode manifested good mechanical flexibility integrated with excellent electrochemical Na-ion storage performance, delivering an initial discharge capacity of 130 mA h g–1 at 15 mA g–1 and still preserving 74% after 500 cycles. A quasi-solid-state Na-ion full battery employing the designed electrode coupled with Prussian blue maintained excellent Na-ion storage performance. Importantly, the electrochemical behavior was not affected by the battery shape or external mechanical deformation, demonstrating practical applicability to powering various wearable electronic devices. The work is expected to accelerate the utilization of two-dimensional sheet materials in flexible energy storage systems.