Optimized Cu Contacted Si Nanowire Anodes for Li-Ion Batteries Made in a Production Near Process

The promise of affordable electrical cars in the near future can only be met if major progress will be made with respect to the Li ion battery performance. The energy per kg of present state-of-theart Li ion batteries is at best around 2% of that of liquid fuel, and batteries with a substantially higher specific capacity are needed. The capacity of a Li ion battery is directly proportional to the amount of Li that can be intercalated into a weight unit of the anode (and cathode); it is typically expressed in mAh/g. Just as important for mass applications are specific costs expressed, e.g., in €/Wh. In what follows only the anode of a Li ion battery will be considered. The maximum capacity of the present state-of-the-art graphite anodes is about of 370 mAh/g. In practice this number is somewhat lower and found in the 330 mAh/g range. 1 It has been known for some time that Si would make a much better anode with a nominal anode capacity of 4200 mAh/g, more than ten-fold that of standard graphite anodes. 2 Just as important, most (>80%) of the Li can easily be taken out again, and a Si/Li anode just reduces the possible battery voltage by about 0.5 V upon discharging, thus not much more than the standard graphite anode. Despite these obvious advantages, bulk Si is useless as an anode, because the intercalation of Li leads to a volume expansion of up to a factor of 4, and the resulting stress will invariably fracture bulk Si into dust. In a groundbreaking paper Chan et al. showed in 2008 that this problem could be overcome by using nano-structured Si in the form of nanowires. 1 Si nanowires, while doubling their diameters during the intercalation of Li, do not fracture if they are thin enough. Some random arrangements of nanowires, with a diameter distribution centered around 90 nm were tested in Ref. 1; it was found that they could withstand more than ten charging/discharging cycles without significant loss of capacity. Meanwhile, substantial progress has been made concerning nano-structured Si as anode material 1,3,6–8 and the viability of the approach is now beyond reasonable doubt. While large-scale tests in batteries are not yet available, all results obtained so far indicate that nano-structured Si might meet all battery requirements and thus might be found in commercial batteries of the near future. The nanowires in most papers addressing this topic were grown with the standard vapor-liquid-solid (VLS) technique, using mostly “Au droplets” as catalytic growth sites, 1,7,9,10 or by metal-assisted catalytic etching of single-crystalline silicon. 5,11 This paper addresses an alternative way for producing suitable Si nanowire anodes. First the basic techniques for producing Si nanowire arrays with optimized geometry via macropore etching in Si and Cu galvanics will be briefly described. Next, relevant test results with these anodes tried in standard batteries will be presented. The main part of the paper then deals with the substantial improvement of the structure of the anodes concerning to the drawbacks observed in the previous results; the concomitant reduction of process costs by optimizing the process chain is also discussed.