Cu-doped Fe3O4 supported on porous NC-coated carbon cloth for robust flexible nickel–iron batteries

• Porous NC-coated carbon is employed as coating layer on carbon cloth. • Cu cations are added during Fe 3 O 4 growth to adjust its morphology and improve its performance. • All-solid-state Ni-Fe batteries are assembled with stably high capacities, and high energy/power densities. • Flexible Ni-Fe batteries are fabricated with excellent flexibility and performance stability at different bending angle and in low temperature environments. The pursuit of safe, cost-efficient, environment-friendly and portable energy storage devices has greatly stimulated the exploitation of aqueous flexible batteries that are green and stable. Ni-Fe aqueous batteries have become an ideal choice for flexible energy storage devices due to the advantages such as large theoretical capacity, good safety and low cost. In this work, Fe 3 O 4 is chosen as the active material for the anode of Ni-Fe batteries, which is grown on a modified carbon cloth of PNC/CC with a coating layer of porous nitrogen-carbon. The PNC/CC, as a flexible current collector, has higher hydrophilicity, porosity and conductivity. To further improve the electronic conductivity, ionic adsorption ability, morphology and capacity of the Fe 3 O 4 anode, Cu 2+ is added during Fe 3 O 4 growth, which not only brings more homogeneous and full coverage of spherical Cu/Fe 3 O 4 particles on PNC/CC, but also greatly lowers its electrode–electrolyte charge transfer resistivity, increases its ionic diffusion and accelerates its redox reaction activity. With optimized electrochemical reactions, the Cu/Fe 3 O 4 @PNC/CC anode shows a much higher capacity of 170.7 mAh/g than bare Fe 3 O 4 /CC anode (1 A/g). Solid-state aqueous Ni-Fe batteries are assembled using the Cu/Fe 3 O 4 @PNC/CC anode and a NiCoLDH cathode, exhibiting a capacity of 88.56 mAh/g at 1 A/g, capacity retention of 88.19 % during 2000 cycles, as well as the energy density reaching 96.6 Wh kg −1 and the power density reaching 9.7 kW kg −1 . Moreover, flexible Ni-Fe battery devices are also fabricated, which can maintain relatively stable electrochemical performance at various bending angles and low temperatures. This works brings some insights for the synthesis and modification of Fe 3 O 4 -based anodes for flexible and solid-state Ni-Fe batteries.