Fig. 2

Fabrication and structural analyses of NiSiNWs@GrμT. a Schematic of NiSiNWs@GrμT. b Simulation of the potential distribution across the NWs@GrμT with an SEI layer on the outer surface of the GrμT. Bottom panel: enlarged images taken from the square in the top panel showing that the formation of the SEI on the outer surface of the GrμT occurred through the development of a rapid potential gradient. A voltage difference of V_o was applied between the left-hand electrolyte and GrμT, and the calculated position-dependent potential values were normalized with respect to V_o. c Schematic of the fabrication process of NWs@GrμT (left) and lithiation-delithiation cycling (right). SEI formation and Li-ion insertion/extraction only occur around the outer surface of the GrμT. d SEM images of as-fabricated (left) and cross-sectioned (right) NiSiNW@GrμT. Scale bars, 50 μm (left) and 2 μm (right). e TEM image of NiSiNW (left) and EDS elemental mapping images of Si (middle) and Ni (right). A Ni to Si atomic ratio of ~1:1 was confirmed by EDS composition analysis. Scale bars, 400 nm (left) and 250 nm (middle and right). f Raman spectra of the GrμT before (black) and after (red) core Ni etching