Si nanowires can incorporate large amounts of Li without fracturing and are thus prime candidates for anodes in Li ion batteries. Anodes made from Si nanowires offer a specific capacity per gram more then 10 times larger than the present graphite standard. It is shown how optimized Si nanowire arrays embedded in Cu can be produced in a relatively simple way employing macropore etching in Si followed by chemical etching and Cu galvanic deposition. First tests of these arrays in half‐cells and batteries demonstrated a substantially increased capacity, small irreversible losses and cycle stability. In particular more than 60 charge/discharge cycles could be realized without loss of capacity. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The fabrication process to produce Si wires with superior structural properties for application in Li-ion batteries is described. The process is based on the electrochemical etching of pores in Si, followed by a chemical over-etching step. The wires obtained have a quadratic cross section of around 1.5 × 1.5 μm and are organized in an array stabilized by two in-situ-created supports. An electrodeposited Cu film on the array is used as current collector. The method described is a production-near process with a potential for low costs and high yield.

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Anodically etching macropores in Si substrates followed by chemical over-etching and Cu galvanics allows producing Si nanowire anodes for Li ion batteries with optimized geometry. This paper focuses on the optimizations of the process chain. The times for key processes could be substantially reduced while concomitantly improving the quality and the process window. The process chain now is close to enabling mass production on 200 mm wafers.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 alterna...

Deep macropores can be grown in classical aqueous HF electrolytes only at slow etching speeds, fast macropores can only be grown to modest depths (150 μm). The addition of acid acetic to the electrolyte can roughly double the etching speed of the macropores, enabling quick and easy etching of pore depths as deep as 520 μm, and potentially more, if wafers thicker than 550 μm would be used. The addition of carboxymethylcellulose sodium salt (CMC) to the electrolyte decreases the roughness of the pore walls significantly. It is successfully shown that electrolytes consisting of HF + acetic acid + CMC can be utilized to produce fast, deep, and smooth macropores simultaneously in n‐type silicon, required for a multitude of potential applications. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)