Tailoring monoclinic Na3V2(PO4)3-based cathode via bimetallic substitution for high-energy and long-lifespan Na-ion batteries

NaV(PO) is a promising cathode for Na-ion batteries (NIBs) owing to the high electrochemical reversibility. The NaV(PO) has two typical polymorphs including rhombohedral and monoclinic phases; the former has been extensively studied, whereas the latter is rarely reported. Here, we successfully designed monoclinic NaV(PO)-based cathode via Ga and Fe substitutions owing to the lowered lattice energy. In addition, we revealed that Ga substitution improves average voltage owing to the activation of the V/V redox couple and the Fe substitution enhances rate capability due to the decreased band gap and Na-ion diffusion activation energy. As a result, the designed monoclinic NaVGaFe(PO) cathode exhibits high voltage plateaus (3.4 V and 4 V) and high-rate capability (from 116.8 mA h/g at 0.2 C to 103 mA h/g at 20 C) as well as superior cycling stability (99.9% capacity retention over 4,500 cycles at 5 C). Moreover, the assembled NaVGaFe(PO)//hard carbon full cell delivers a high-energy density of 313.8 Wh/kg with 86.4% capacity retention after 100 cycles at 1 C. This work demonstrates the design of monoclinic NaV(PO)-based cathode via bimetallic substitution, providing a new route for development of high-energy and long-lifespan NIBs.