NaTi₂(PO₄)₃ hollow nanoparticles encapsulated in carbon nanofibers as novel anodes for flexible aqueous rechargeable sodium-ion batteries

NASICON-structured NaTi₂(PO₄)₃ (NTP) is an attractive anode material for aqueous rechargeable sodium-ion batteries (ARSIBs) thanks to its three-dimensional open framework and appropriate negative voltage window. Nevertheless, the lack of flexible and high-performance binder-free NTP-based anodes remains stumbling blocks to the development of wearable ARSIBs. Herein, hollow-structure NTP evenly encapsulated in cross-linked porous N-doped carbon nanofiber (HNTP@PNC) is prepared through electrospinning technology and subsequent carbonization treatment, directly acting as binder-free anode for flexible ARSIBs. Benefiting from its unique hollow structure, continuous conductive network and favorable synergistic effect, the HNTP@PNC electrode displays as high as of 108.3 mAh g⁻¹ rate capacity at 5.50 A g⁻¹ and an impressive cycling stability of 97.2% capacity retention after 3000 cycles. Further, theoretical calculations reveal that NTP with NC coating significantly enhances electronic conductivity and accelerates Na⁺ diffusion kinetics. Pairing with potassium zinc hexacyanoferrate free-standing cathode, a prototype quasi-solid-state ARSIB with a high-voltage discharge plateau of 1.6 V is successfully constructed, achieving a high volumetric capacity of 24.5 mAh cm⁻³ and an admirable energy density of 39.2 mWh cm⁻³, outperforming most reported flexible aqueous rechargeable energy-storage devices. These exciting results provide valuable intuition into the design of novel binder-free NTP-based anodes for next-generation wearable ARSIBs.