LETTERS b d Initial Si 百100 1 04 Figure 3 Morphology and electronic changes in Si NWs from reaction with Li. a, b SEM image of pristine Si NWs before (a) and after(b)electrochemical cycling same magnification). The inset in a is a cross-sectional image showing that the Nws are directly contacting the stainless steel current collector. c, d TEM image of a ristine Si Nw with a partial Ni coating before(c)and after(du cycling. e, Size distribution of Nws before and after charging to 10 mv (bin width 10 nm) The average diameter of the NWs increased from 89 to 141 nm after lithiation f, I-v curve for a single nw device(SEM image, inset) constructed from a pristine Si NW. g, I-V curve for a single NW device (SEM image, inset), constructed from a Nw that had been charged and discharged once at the C/20 rate with smooth sidewalls(Fig. 3a)and had an average diameter of helical shape. Although the Nw length increased after lithiation, 89 nm(s d, 45 nm)( Fig. 3e). Cross-sectional scanning electron the NWs remained continuous and without fractures, microscopy(SEM)showed that the Si Nws grew off the substrate maintaining a pathway for electrons all the way from the and had good contact with the stainless steel current collector collector to the Nw tips. With both a diameter and length ( Fig. 3a, inset). After charging with Li, the Si NWs had roughly increase, the Si Nw volume change after Li insertion appears to textured sidewalls(Fig. 3b), and the average diameter increased be about 400%, consistent with the literature to 141 nm(sd, 64 nm). Despite the large volume change, the Efficient electron transport from the current collector to the Si Si NWs remained intact and did not break into smaller particles. NWs is necessary for good battery cycling. To evaluate this, we They also appeared to remain in contact with the current conducted electron transport measurements on single Si NWs collector, suggesting minimal capacity fade due to electrically before and after lithiation (see Methods). The current versu disconnected material during cycling. voltage curve on a pristine Si Nw was linear, with a 25 kn2 The Si NWs may also change their length during the change in resistance(resistivity of 0.02 02-cm)(Fig. 3f). After one cycle, the volume. To evaluate this, 25-nm Ni was evaporated onto as-grown NWs became amorphous, but still exhibited a current that was Si NWs using electron beam evaporation. Because of the shadow linear with voltage with an 8 MO resistance (resistivity of effect of the Si NWs, the Ni only covered part of the Nw surface 3 0-cm)(Fig. 3g). The good conductivity of pristine and cycled (Fig. 3c), as confirmed by energy dispersed X-ray spectroscopy NWs ensures efficient electron transport for charge and discharge (EDS)map (see Supplementary Information, Fig. S4). The The large volume increase in the Si NWs is driven by the Ni is inert to Li and acts as a rigid backbone on the Si Nws. dramatic atomic structure change during lithiation. To After lithiation (Fig. 3d), the Si NWs changed shape and understand the structural evolution of NWs, we characterized the wrapped around the Ni backbone in a three-dimensionally helical NW electrodes at different charging potentials. The X-ray manner. This appeared to be due to an expansion in the length diffraction(XRD) patterns were taken for initial pristine Si Nws, of the Nw, which caused strain because the nw was attached to Si NWs charged to 150 mV, 100 mV, 50 mV and 10 mV, as well as the Ni and could not freely expand but rather buckled into a after 5 cycles( Fig. 4a). XRD patterns of the as-grown Si NWs naturenanotechnologyivol3JaNuaRy2008www.nature.com/r @2008 Nature Publishing Group© 2008 Nature Publishing Group with smooth sidewalls (Fig. 3a) and had an average diameter of 89 nm (s.d., 45 nm) (Fig. 3e). Cross-sectional scanning electron microscopy (SEM) showed that the Si NWs grew off the substrate and had good contact with the stainless steel current collector (Fig. 3a, inset). After charging with Li, the Si NWs had roughly textured sidewalls (Fig. 3b), and the average diameter increased to 141 nm (s.d., 64 nm). Despite the large volume change, the Si NWs remained intact and did not break into smaller particles. They also appeared to remain in contact with the current collector, suggesting minimal capacity fade due to electrically disconnected material during cycling. The Si NWs may also change their length during the change in volume. To evaluate this, 25-nm Ni was evaporated onto as-grown Si NWs using electron beam evaporation. Because of the shadow effect of the Si NWs, the Ni only covered part of the NW surface (Fig. 3c), as confirmed by energy dispersed X-ray spectroscopy (EDS) mapping (see Supplementary Information, Fig. S4). The Ni is inert to Li and acts as a rigid backbone on the Si NWs. After lithiation (Fig. 3d), the Si NWs changed shape and wrapped around the Ni backbone in a three-dimensionally helical manner. This appeared to be due to an expansion in the length of the NW, which caused strain because the NW was attached to the Ni and could not freely expand but rather buckled into a helical shape. Although the NW length increased after lithiation, the NWs remained continuous and without fractures, maintaining a pathway for electrons all the way from the collector to the NW tips. With both a diameter and length increase, the Si NW volume change after Li insertion appears to be about 400%, consistent with the literature5 . Efficient electron transport from the current collector to the Si NWs is necessary for good battery cycling. To evaluate this, we conducted electron transport measurements on single Si NWs before and after lithiation (see Methods). The current versus voltage curve on a pristine Si NW was linear, with a 25 kV resistance (resistivity of 0.02 V-cm) (Fig. 3f ). After one cycle, the NWs became amorphous, but still exhibited a current that was linear with voltage with an 8 MV resistance (resistivity of 3 V-cm) (Fig. 3g). The good conductivity of pristine and cycled NWs ensures efficient electron transport for charge and discharge. The large volume increase in the Si NWs is driven by the dramatic atomic structure change during lithiation. To understand the structural evolution of NWs, we characterized the NW electrodes at different charging potentials. The X-ray diffraction (XRD) patterns were taken for initial pristine Si NWs, Si NWs charged to 150 mV, 100 mV, 50 mV and 10 mV, as well as after 5 cycles (Fig. 4a). XRD patterns of the as-grown Si NWs Number of nanowires Initial Voltage (V) 10 mV Current (µA) Current (nA) –0.4 0.4 0.0 Voltage (V) –0.4 0.4 0.0 0 10 20 –10 –20 0 4 8 –4 Diameter (nm) 200 100 0 0 100 200 300 50 nm 250 nm Si Ni Si Ni Figure 3 Morphology and electronic changes in Si NWs from reaction with Li. a,b, SEM image of pristine Si NWs before (a) and after (b) electrochemical cycling (same magnification). The inset in a is a cross-sectional image showing that the NWs are directly contacting the stainless steel current collector. c,d, TEM image of a pristine Si NW with a partial Ni coating before (c) and after (d) Li cycling. e, Size distribution of NWs before and after charging to 10 mV (bin width 10 nm). The average diameter of the NWs increased from 89 to 141 nm after lithiation. f, I –V curve for a single NW device (SEM image, inset) constructed from a pristine Si NW. g, I –V curve for a single NW device (SEM image, inset), constructed from a NW that had been charged and discharged once at the C/20 rate. LETTERS nature nanotechnology |VOL 3 | JANUARY 2008 |www.nature.com/naturenanotechnology 33