Continuous-Flow Synthesis of High-Entropy Sodium Vanadium Fluorophosphate for High Rate Capacity in Sodium-Ion Batteries

Abstract

High-entropy sodium vanadium fluorophosphate (HE-NVPF) with good structural stability, high electrical conductivity, and rapid redox dynamics has been considered to be a promising cathode material for sodium-ion batteries. However, the HE-NVPF prepared by the conventional sol–gel method is usually challenged by time-consuming reaction processes and uncontrollable particle sizes due to slow mass/heat transfer and uneven reaction space, which restrict its electrochemical performance. In this study, we developed a continuous-flow synthesis strategy via a microreactor to construct high-entropy Na₃V_2–xM_x(PO₄)₂F₃ (0 < x < 2, M = Ca, Mg, Al, Cr, Mn) (M-HE-NVPF). Notably, the rapid mass/heat transfer, confined reaction space, and high-entropy doping effect can realize the efficient synthesis (2 h) of M-HE-NVPF with a small particle size (∼131 nm), high electrical conductivity because of the introduction of local disturbances leading to overlapping energy distributions of sites, and distinct redox dynamics by suppressing the detrimental phase transitions in the low plateau region. As a result, M-HE-NVPF exhibits high rate capacities of 125.3 mAh g^–1 at 0.5 C and 110.8 mAh g^–1 at 50 C, marvelous cycle stability of 90.5% capacity retention at 0.5 C, and 95.5% capacity retention at 20 C after 400 cycles.