LETTERS High-performance lithium battery anodes using silicon nanowires CANDACE K CHAN. HAILIN PENG2 GAO LIU3 KEVIN MCILWRATH4. XIAO FENG ZHANG4 ROBERT A HUGGINS2 AND YI CUI2* Department of Chemistry, Stanford University, Stanford, California 94305, USA Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA eNvironmental Energy Technologies Division, Lawrence Berkeley National Lab, 1 Cyclotron Road, Mail Stop 70R108B, Berkeley, Califomia 94720, USA "Electron Microscope Division, Hitachi High Technologies America, Inc, 5100 Franklin Drive, Pleasanton, California 94588, USA *e-mail: yicui@stanford. edu Published online: 16 December 2007; doi: 10. 1038/nano. 2007.411 There is great interest in developing rechargeable lithium Initial substrate batteries with higher energy capacity and longer cycle life for pplications in portable electronic devices, electric vehicle wAr d implantable medical devices. Silicon is an attractive node material for lithium batteries because it has a low F discharge potential and the highest known theoretical charge capacity(4, 200 mAh g-; ref. 2). Although this is more than ten times higher than existing graphite anodes and much larger than various nitride and oxide materials, silicor anodes have limited applications because silicon,'s volum changes by 400% upon insertion and extraction of lithium, which results in pulverization and capacity fading 2. Here, we show that silicon nanowire battery electrodes circumvent thes issues as they can accommodate large strain without Facile strain pulverization, provide good electronic contact and conduction, and display short lithium insertion distances. We achieved the theoretical charge capacity for silicon anodes and maintained a discharge capacity close to 75% of this maximum, with little fading during cycling Good contact with current collector Previous studies in which si bulk films and micrometre-sized particles were used as electrodes in lithium batteries have shown that o r in si duri capacity fading and short battery lifetime due to pulverization electrochemical cycling. a, The volume of silicon anodes changes by about and loss of electrical contact between the active material and the 400% during cycling. As a result, Si films and particles tend to pulverize during current collector (Fig. la). The use of sub-micrometre pillars and micro- and nanocomposite anodes3-9 led to only limited cycling. Much of the material loses contact with the current collector,resulting stable capacity over many cycles, but have insufficient mates in poor transport of electrons, as indicated by the arrow. b, NWs grown directly mprovement. Electrodes made of amorphous Si thin films hav on the current collector do not pulverize or break into smaller particles after for a viable battery. The concept of using one-dimensional(ID) cycling. Rather, facile strain relaxation in the NWs allows them to increase in nanomaterials has been demonstrated with carbon, Co,O diameter and length without breaking. This NW anode design has each NW (refs 11, 12),SnO2(ref. 13)and TiO, ( ref. 14)anodes, and has connecting with the current collector, allowing for efficient 1D electron transport shown improvements compared to bulk materials. A schematic of own the length of every Nw. our Si nanowire(NW)anode configuration is shown in Fig. Ib Nanowires are grown directly on the metallic current collector 二 substrate. This geometry has several advantages and has led to is electrically connected to the metallic current co mprovements in rate capabilities in metal oxide cathode all the nanowires contribute to the capacity. Thir materials. First, the small Nw diameter allows for better have direct ID electronic pathways allowing for accommodation of the large volume changes without the transport. In electrode microstructures based on particles initiation of fracture that can occur in bulk or micron-sized electronic charge carriers must move through small interparticle materials(Fig. la). This is consistent with previous studies that contact areas. In addition, as every NW is connected to the have suggested a materials-dependent terminal particle size below current-carrying electrode, the need for binders or conducting which particles do not fracture further 6. 7 Second, each Si Nw additives, which add extra weight, is eliminated. Furthermore, naturenanotechnologyivol3JaNuaRy2008www.nature.com/r @2008 Nature Publishing Group© 2008 Nature Publishing Group High-performance lithium battery anodes using silicon nanowires CANDACE K. CHAN1 , HAILIN PENG2 , GAO LIU3 , KEVIN McILWRATH4 , XIAO FENG ZHANG4 , ROBERT A. HUGGINS2 AND YI CUI2 * 1Department of Chemistry, Stanford University, Stanford, California 94305, USA 2Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA 3Environmental Energy Technologies Division, Lawrence Berkeley National Lab, 1 Cyclotron Road, Mail Stop 70R108B, Berkeley, California 94720, USA 4Electron Microscope Division, Hitachi High Technologies America, Inc., 5100 Franklin Drive, Pleasanton, California 94588, USA *e-mail: yicui@stanford.edu Published online: 16 December 2007; doi:10.1038/nnano.2007.411 There is great interest in developing rechargeable lithium batteries with higher energy capacity and longer cycle life for applications in portable electronic devices, electric vehicles and implantable medical devices1 . Silicon is an attractive anode material for lithium batteries because it has a low discharge potential and the highest known theoretical charge capacity (4,200 mAh g21 ; ref. 2). Although this is more than ten times higher than existing graphite anodes and much larger than various nitride and oxide materials3,4, silicon anodes have limited applications5 because silicon’s volume changes by 400% upon insertion and extraction of lithium, which results in pulverization and capacity fading2 . Here, we show that silicon nanowire battery electrodes circumvent these issues as they can accommodate large strain without pulverization, provide good electronic contact and conduction, and display short lithium insertion distances. We achieved the theoretical charge capacity for silicon anodes and maintained a discharge capacity close to 75% of this maximum, with little fading during cycling. Previous studies in which Si bulk films and micrometre-sized particles were used as electrodes in lithium batteries have shown capacity fading and short battery lifetime due to pulverization and loss of electrical contact between the active material and the current collector (Fig. 1a). The use of sub-micrometre pillars6 and micro- and nanocomposite anodes5,7–9 led to only limited improvement. Electrodes made of amorphous Si thin films have a stable capacity over many cycles5,8, but have insufficient material for a viable battery. The concept of using one-dimensional (1D) nanomaterials has been demonstrated with carbon10, Co3O4 (refs 11, 12), SnO2 (ref. 13) and TiO2 (ref. 14) anodes, and has shown improvements compared to bulk materials. A schematic of our Si nanowire (NW) anode configuration is shown in Fig. 1b. Nanowires are grown directly on the metallic current collector substrate. This geometry has several advantages and has led to improvements in rate capabilities in metal oxide cathode materials15. First, the small NW diameter allows for better accommodation of the large volume changes without the initiation of fracture that can occur in bulk or micron-sized materials (Fig. 1a). This is consistent with previous studies that have suggested a materials-dependent terminal particle size below which particles do not fracture further16,17. Second, each Si NW is electrically connected to the metallic current collector so that all the nanowires contribute to the capacity. Third, the Si NWs have direct 1D electronic pathways allowing for efficient charge transport. In electrode microstructures based on particles, electronic charge carriers must move through small interparticle contact areas. In addition, as every NW is connected to the current-carrying electrode, the need for binders or conducting additives, which add extra weight, is eliminated. Furthermore, Initial substrate After cycling X X X Film Particles Facile strain relaxation Good contact with current collector Efficient 1D electron transport Nanowires Figure 1 Schematic of morphological changes that occur in Si during electrochemical cycling. a, The volume of silicon anodes changes by about 400% during cycling. As a result, Si films and particles tend to pulverize during cycling. Much of the material loses contact with the current collector, resulting in poor transport of electrons, as indicated by the arrow. b, NWs grown directly on the current collector do not pulverize or break into smaller particles after cycling. Rather, facile strain relaxation in the NWs allows them to increase in diameter and length without breaking. This NW anode design has each NW connecting with the current collector, allowing for efficient 1D electron transport down the length of every NW. LETTERS nature nanotechnology |VOL 3 | JANUARY 2008 |www.nature.com/naturenanotechnology 31