Constructing electron transport channels in FeSe2 microspheres by a facile strategy for superior long-life and ultrafast sodium-ion storage

•3D fast electron transport channels are constructed inside the FeSe2 microspheres.•Achieved by a simple one-step hydrothermal method.•The conversion from FeSe2 to FeSe is detected for the first time by ex situ Raman.•Ultralong life with 234.5 mAh g−1 after 10,000 cycles at 10 A g−1 is obtained.•High volumetric capacity indicates industrialization potential.