±Rs Potential versus Li/i*) Capacity (mAng CC/2C/5 ::: 3,000 画10 2,000 01,0002000300400 Cycle number 2 Electrochemical data for Si NW electrodes. a, Cyclic voltammogram for Si NWs from 2.0V to 0.01 V versus Li/Li* at 1 mV s-I scan rate. The first seven re shown. b, Voltage profiles for the first and second galvanostatic cycles of the Si NWs at the C/20 rate. The first charge achieved the theoretical capacity of 4, 200 mAh g-i for Li4 &Si C, The voltage profiles for the Si NWs cycled at different power rates. The C/20 profile is from the second cycle. d, Capacity versus cycle number for the Si NWs at the C/20 rate showing the charge(squares) and discharge capacity (circles). The charge data for Si nanocrystals(triangles) from ref. 8 and the theoretical capacity for lithiated graphite(dashed line) are shown as a comparison to show the improvement when using Si NWs our Si Nw battery electrode can be easily realized using the capacities remained nearly constant for subsequent cycles, with vapour-liquid-solid(VLS) or vapour-solid(VS)template-free little fading up to 10 cycles(Fig. 2d), which is considerably better growth methods 8-2 to produce NWs directly onto stainless steel than previously reported results.. As a compa current collectors(see Methods) A cyclic voltammogram of the Si Nw electrode is shown in (372 mAhg )for the graphite currently used in lithium battery Fig. 2a. The charge current associated with the formation of the anodes, and the charge data reported for thin films containing Li-Si alloy began at a potential of 330 mV and became quite 12-nm Si nanocrystals(NCs)in Fig. 2d. This improved capacity large below 100 mV. Upon discharge, current peaks appeared at and cycle life in the Si Nws demonstrates the advantages of our about 370 and 620 mV. The current-potential characteristics were Si NW anode desig consistent with previous experiments on microstructured Si The Si NWs also displayed high capacities at higher currents anodes,. The magnitude of the current peaks increased with Figure 2c shows the charge and discharge curves observed at the ycling due to activation of more material to react with Li in each C/20, C/10, C/5, C/2 and IC rates. Even at the IC rate, the scan. The small peak at 150-180 mV may have been due to capacities remained >2, 100 mAh g, which is still five times reaction of the Li with the gold catalyst, which makes a negligible larger than that of graphite. The cyclability of the Si NWs at the contribution to the charge capacity (see Supplementary faster rates was also excellent. Using the C/5 rate, the capacity Information, Figs SI and S2). was stable at 3, 500 mAh g for 20 cycles in another device Si NWs were found to exhibit a higher capacity than other (see Supplementary Information, Fig S3). Despite the improved forms of Si (ref 5). Figure 2b shows the first and second cycles at performance, the Si Nw anode showed an irreversible capacity he C/20 rate(20 h per half cycle). The voltage profile observed loss in the first cycle, which has been observed in other work was consistent with previous Si studies, with a long flat plateau Although solid electrolyte interphase(SEI) formation has been during the first charge, during which crystalline Si reacted with observed in Si (ref. 28), we do not believe this is the cause of our Li to form amorphous Li Si. Subsequent discharge and charge initial irreversible capacity loss, because there is no appreciable cycles had different voltage profiles characteristic of amorphous capacity in the voltage range of the SEI layer formation Si (refs 24-27). Significantly, the observed capacity during this(0.5-0.7 V) during the first charge(Fig. 2b). Although SEl first charging operation was 4, 277 mAh g, which is essentially formation may be occurring, the capacity involved in SEI uivalent to the theoretical capacity within experimental error. formation would be very small compared to the high charg The first discharge capacity was 3, 124 mAh g, indicating a capacity we observed. The mechanism of the initial irreversible coulombic efficiency of 73%. The second charge capacity capacity is not yet understood and requires further investigation. decreased by 17% to 3, 541 mAh g,although the second The structural morphology changes during Li insertion were discharge capacity increased slightly to 3, 193 mAh g, giving a studied to understand the high capacity and good cyclability of coulombic efficiency of 90%. Both charge and discharge our Si NW electrodes. Pristine, unreacted Si Nws were crystalline 9 2008 Nature Publishing Group© 2008 Nature Publishing Group our Si NW battery electrode can be easily realized using the vapour–liquid – solid (VLS) or vapour– solid (VS) template-free growth methods18–23 to produce NWs directly onto stainless steel current collectors (see Methods). A cyclic voltammogram of the Si NW electrode is shown in Fig. 2a. The charge current associated with the formation of the Li–Si alloy began at a potential of 330 mV and became quite large below 100 mV. Upon discharge, current peaks appeared at about 370 and 620 mV. The current–potential characteristics were consistent with previous experiments on microstructured Si anodes6 . The magnitude of the current peaks increased with cycling due to activation of more material to react with Li in each scan6 . The small peak at 150–180 mV may have been due to reaction of the Li with the gold catalyst, which makes a negligible contribution to the charge capacity (see Supplementary Information, Figs S1 and S2). Si NWs were found to exhibit a higher capacity than other forms of Si (ref. 5). Figure 2b shows the first and second cycles at the C/20 rate (20 h per half cycle). The voltage profile observed was consistent with previous Si studies, with a long flat plateau during the first charge, during which crystalline Si reacted with Li to form amorphous LixSi. Subsequent discharge and charge cycles had different voltage profiles characteristic of amorphous Si (refs 24–27). Significantly, the observed capacity during this first charging operation was 4,277 mAh g21 , which is essentially equivalent to the theoretical capacity within experimental error. The first discharge capacity was 3,124 mAh g21 , indicating a coulombic efficiency of 73%. The second charge capacity decreased by 17% to 3,541 mAh g21 , although the second discharge capacity increased slightly to 3,193 mAh g21 , giving a coulombic efficiency of 90%. Both charge and discharge capacities remained nearly constant for subsequent cycles, with little fading up to 10 cycles (Fig. 2d), which is considerably better than previously reported results8,9. As a comparison, our charge and discharge data are shown along with the theoretical capacity (372 mAh g21 ) for the graphite currently used in lithium battery anodes, and the charge data reported for thin films containing 12-nm Si nanocrystals8 (NCs) in Fig. 2d. This improved capacity and cycle life in the Si NWs demonstrates the advantages of our Si NW anode design. The Si NWs also displayed high capacities at higher currents. Figure 2c shows the charge and discharge curves observed at the C/20, C/10, C/5, C/2 and 1C rates. Even at the 1C rate, the capacities remained .2,100 mAh g21 , which is still five times larger than that of graphite. The cyclability of the Si NWs at the faster rates was also excellent. Using the C/5 rate, the capacity was stable at 3,500 mAh g21 for 20 cycles in another device (see Supplementary Information, Fig. S3). Despite the improved performance, the Si NW anode showed an irreversible capacity loss in the first cycle, which has been observed in other work5 . Although solid electrolyte interphase (SEI) formation has been observed in Si (ref. 28), we do not believe this is the cause of our initial irreversible capacity loss, because there is no appreciable capacity in the voltage range of the SEI layer formation (0.5 –0.7 V) during the first charge (Fig. 2b)8 . Although SEI formation may be occurring, the capacity involved in SEI formation would be very small compared to the high charge capacity we observed. The mechanism of the initial irreversible capacity is not yet understood and requires further investigation. The structural morphology changes during Li insertion were studied to understand the high capacity and good cyclability of our Si NW electrodes. Pristine, unreacted Si NWs were crystalline Potential versus Li/Li+ (V) 0.0 0.5 1.0 1.5 2.0 Current (mA) 2 0 –2 –4 –6 7 1 1 7 Potential versus Li/Li+ (V)2.0 1.5 1.0 0.5 0.0 0 1,000 2,000 3,000 4,000 Capacity (mAh g–1) C/2 C/5 C/10 C/20 C C/2 C/5 C/10 C/20 C Capacity (mAh g–1) 0.0 0.5 1.0 1.5 2.0 0 1,000 2,000 3,000 4,000 5,000 Capacity (mAh g–1) Cycle number 0 2 4 6 8 10 Potential versus Li/Li+ (V) Si NW discharge Si NW charge Si NC charge Discharge Charge 0 1,000 2,000 3,000 4,000 Graphite charge Figure 2 Electrochemical data for Si NW electrodes. a, Cyclic voltammogram for Si NWs from 2.0 V to 0.01 V versus Li/Liþ at 1 mV s21 scan rate. The first seven cycles are shown. b, Voltage profiles for the first and second galvanostatic cycles of the Si NWs at the C/20 rate. The first charge achieved the theoretical capacity of 4,200 mAh g21 for Li4.4Si. c, The voltage profiles for the Si NWs cycled at different power rates. The C/20 profile is from the second cycle. d, Capacity versus cycle number for the Si NWs at the C/20 rate showing the charge (squares) and discharge capacity (circles). The charge data for Si nanocrystals (triangles) from ref. 8 and the theoretical capacity for lithiated graphite (dashed line) are shown as a comparison to show the improvement when using Si NWs. LETTERS 32 nature nanotechnology | VOL 3 | JANUARY 2008 | www.nature.com/naturenanotechnology