Constructing Flexible and Binder-Free NaTi₂(PO₄)₃ Film Electrode with a Sandwich Structure by a Two-Step Graphene Hybridizing Strategy as an Ultrastable Anode for Long-Life Sodium-Ion Batteries

Flexible energy storage devices show promising advantages in next-generation bendable, wearable, and implantable electronic systems, and therefore, they attract considerable interest for researchers to fulfill requirements of future energy storage devices. Here, we report a facile two-step strategy to construct flexible and binder-free NaTi₂(PO₄)₃/graphene film with a sandwich structure (GN/NaTi₂(PO₄)₃/GN). Such a design makes the resulting film with an excellent flexibility, which can be used for a binder-free anode for flexible sodium-ion batteries (SIBs). GN nanosheet supported NaTi₂(PO₄)₃ nanocrystals (NaTi₂(PO₄)₃/GN) are first prepared by a facile hydrothermal route. Then, the resultant NaTi₂(PO₄)₃/GN hybrid is embedded homogeneously in interconnected GN nanosheet framework to form three-dimensional flexible GN/NaTi₂(PO₄)₃/GN film. The flexible film exhibits excellent cycling stability (91% capacity retention over 1000 cycles at 500 mA g^-1) for sodium half cells. Furthermore, it can be assembled into flexible full SIBs (Na_0.44MnO₂ as the cathode) for practical application. The flexible full battery shows good cycling stability both under flat and bent states, maintaining 92.9% retention after the first 30 cycles under a flat state compared to the original capacity and 86% retention after another 40 cycles under a bent state. This synthesis route offers a general method for constructing other flexible films for SIBs.