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

Abstract

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...

Effect of cation species on surface-induced phase transition observed for platinum complex anions in platinum electrodeposition using nanoporous silicon

“…Solve Eqs. (2) and (3) for the macroparticleaqueous electrolyte solution system using the correlation functions obtained in step (i) as input data. Calculate the correlation functions X LS , X L+ , and X L− (X = h, c).…”

“…[1][2][3][4][5][6][7] In the utilization of porous structure for a reaction field, a larger specific surface area brought by decreasing the pore diameter and increasing the porosity can lead to higher performance of chemical or electrochemical reactions. To prepare porous structure with high performance, the pore size needs to be sufficiently small, i.e., of the scale of a few nanometers.…”

Penetration of Platinum Complex Anions into Porous Silicon: Anomalous Behavior Caused by Surface-Induced Phase Transition

“…We consider positions (1) through (4): (1) is in the bulk, (3) is in the immediate vicinity of the pore-wall surface, (2) is at the center of the space of the larger pore, and (4) is at the center of the smaller pore. The anion concentrations at these positions follow the orders, (1) , and C (4) > C (2) , where C (I) denotes the anion concentration at position (I). Since the anions are only weakly hydrated due to their large sizes, they are excluded from the bulk to the surface: The anion concentration is greatly enriched at the surface.…”

Structural and Electrochemical Investigation during the First Charging Cycles of Silicon Microwire Array Anodes for High Capacity Lithium Ion Batteries

“…Silicon microwire anodes have been produced by an electrochemical-chemical approach thoroughly described in [3,9]. The steps for the preparation of the Si wire anodes could be summarized as follows: (a) Cheap lithography and pre-structuring of Si wafers; (b) electrochemical etching of macropores; (c) chemical over-etching of macroporous Si to obtain a wire array; (d) chemical-electrochemical deposition of a Cu current collector on the wires; (e) detaching the anode from the (reusable) Si substrate.…”

“…Nano- and micro-structured Si anodes have gained much attention in the battery community in the last years because they could allow for very high capacity for Li storage up to the theoretical maximum storage in Si of 4200 mAh/g, as evidenced by some reports [1,2,3,4], which is more than 10 times larger than the capacity of graphite, the most common anode in Li ion batteries. Additionally, nano- and micro-structured Si overcome the problems of pulverization of bulk Si caused by volume expansion during lithiation.…”