High-performance lithium battery anodes using silicon nanowires [chapter]

2010 Materials for Sustainable Energy  
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 devices 1 . 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 g 21 ; ref. 2). Although this is more than ten times higher than existing graphite anodes and much larger than
more » ... much larger than various nitride and oxide materials 3,4 , silicon anodes have limited applications 5 because silicon's volume 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 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 pillars 6 and micro-and nanocomposite anodes 5,7-9 led to only limited improvement. Electrodes made of amorphous Si thin films have a stable capacity over many cycles 5,8 , but have insufficient material for a viable battery. The concept of using one-dimensional (1D) nanomaterials has been demonstrated with carbon 10 , Co 3 O 4 (refs 11, 12), SnO 2 (ref. 13) and TiO 2 (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 materials 15 . 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 further 16, 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 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
doi:10.1142/9789814317665_0026 fatcat:2zpv2u2fxrgpxpnzhlr3wbumf4