High-entropy stabilized NASICON cathode with accelerated Na+ transport for flexible aqueous sodium-ion batteries

Flexible aqueous rechargeable sodium-ion batteries (ARSIBs) have become attractive options for wearable energy-storage systems. To achieve high-performance flexible ARSIBs, addressing the structural instability and sluggish Na + transport kinetics of Na 3 V 2 (PO 4 ) 3 (NVP) cathodes is highly important. Herein, high-entropy Na 3 V 1.3 (MnFeNiCuTi) 0.14 (PO 4 ) 3 (HE-NVP) materials are constructed by partially substituting V sites with multiple transition metal elements. The capacity retention of HE-NVP maintains 74%, with a Coulombic efficiency of 96% after 2000 cycles at 1 C. The high-entropy-induced lattice distortion optimizes the local crystal environment and facilitates Na + transport, endowing HE-NVP with excellent cycling stability and enhanced Na + diffusion kinetics. Electrochemical kinetic analysis and density functional theory (DFT) calculations show that the weakened Na binding interaction contributes to favorable Na + extraction/insertion behavior. Ex-situ characterizations demonstrate that HE-NVP undergoes near solid-solution phase transition behavior during Na + storage, leading to outstanding electrochemical reversibility and structural stability. Furthermore, fully printed flexible ARSIBs based on the HE-NVP cathode and NaTi 1.6 Y 0.4 (PO 4 ) 3 (NTYP) anode are fabricated via screen-printing technology. The assembled batteries exhibit 1.6 V wide voltage window, excellent mechanical flexibility, and 60% capacity retention after 1000 cycles. This work provides a high-entropy engineering strategy for optimizing NVP cathodes and promotes the advancement of high-performance flexible ARSIBs.