NASICON-NaV_0.25Al_0.25Nb_1.5(PO₄)₃/C: A High-Rate and Robust Anode for Fast Charging and Long-Life Sodium-Ion Batteries

Nb-based NAtrium Super Ionic CONductor (NASICON) frameworks (e.g., Nb₂(PO₄)₃ and Na_1.5V_0.5Nb_1.5(PO₄)₃) are emerging as the attractive Na-ion anodes due to their lower intercalation voltage (≈1.4-1.2 V vs Na⁺/Na⁰) and higher storage capacities (≈140-160 mAh g^-1). However, their practical realization is limited by moderate cycle life and rate performances. In this work, a carbon-coated NASICON-NaV_0.25Al_0.25Nb_1.5(PO₄)₃ (NaVAlNb/C) anode is unveiled for fast charging Na-ion batteries. The incorporation of Al³⁺ increases covalency of NASICON and creates disordered Na-ion sublattice as observed by X-ray diffraction and nuclear magnetic resonance spectroscopy measurements. Besides, the carbon-coating and particle downsizing produces facile electron and ion percolation network. Accordingly, the NaVAlNb/C anode renders extraordinary rate performances (80 mAh g^-1 at 20C) with longer cycling stability (95.2% retention after 5000 cycles at 10C). Climbing image nudged elastic band calculations reveal reduced Na-ion migration barrier (202 meV) for NaVAlNb/C. Most importantly, a full Na-ion cell based on Na₄V₂(PO₄)₃ cathode and NaVAlNb/C anode is demonstrated with a high-power density (6493 W kg^-1) and long-cycle life (3000 cycles at 20C), which are far excellent compared to the state-of-the-art NASICON-based cells. This work demonstrates the significance of carbon coating and chemical tuning to tailor high-rate NASICON anodes, which can produce fast-charging Na-ion batteries.