How to Make Optimized Arrays of Si Nanowires Suit-

Abstract #522, 219th ECS Meeting, © 2011 The Electrochemical Society
How to Make Optimized Arrays of Si Nanowires Suitable as Superior Anode for Li-Ion Batteries
Enrique Quiroga-González, Emmanuel Ossei-Wusu,
Jürgen Carstensen, and Helmut Föll
Institute for Materials Science, Christian-AlbrechtsUniversity of Kiel
Kaiserstr. 2, D-24143 Kiel, Germany
E-mail: enqg@tf.uni-kiel.de; Phone: (+49) 431 880 6181
Silicon has a theoretical nominal anode capacity of 4200
mAh/g [1], more than ten-fold that of standard graphite
anodes with a capacity of 370 mAh/g [2]. However, the
volume expansion during lithiation of bulk Si invariably
leads to rupture and pulverization of the Si. Groundbreaking work of Cui et al [2, 3] has shown that Si in the form
of nanowires does not suffer dramatic rupture problems.
The capacity of the anodes presented in those reports was
indeed around 3100 mAh/g and remained nearly constant
for the subsequent cycles.
Most of the Si nanowire anodes have been grown by the
“Vapor-Liquid-Solid” technique or were produced by a
metal-assisted catalytic etching process of singlecrystalline silicon. Both methods have some problems
with respect to mass production, e.g. the aspect ratio obtained in a reasonable time might be too small. For large
aspect ratios the wires tend to collapse and stick to each
other. Encountering this “stiction” effect makes it difficult
to deposit metal layers needed for electric contacts on the
top. Moreover, both methods produce a dispersion of sizes and shapes, which reduces the packing factor.
We have recently reported about the production of Si nanowires prepared by chemical over-etching of macropores
in Si, a method that allows achieving optimal geometries
at reasonable cost [4]. Nevertheless, the process in [4],
while yielding very good battery test results, was not yet
optimized with respect to cost and process window.
In the present work we report an improved way for producing Si nanowire anodes contacted with Cu that meet
all requirements for mass production.
An aspect ratio of 150 was chosen; it can be easily adjusted to larger or smaller values (Fig. 1). The key is to
run an optimized pore diameter – depth profile during pre
etching. This allows introducing thin stabilizing planes
between the nanowires (obtained after -anisotropic- overetching of the pores) and thus avoids stiction completely.
These stabilizing planes result from locally decreasing the
macropore diameter (Fig. 2a) since areas with thicker
pore walls last longer during chemical etching and thus
some Si still connects the nanowires, cf. Fig. 1 and 2b.
The monodispersity of sizes and shapes is accomplished
by (very simple and cheap) photolithography. Additionally, using an anisotropic chemical KOH-based overetchant the etched wires are square-shaped in the cross
section, with flat walls (Fig. 3), allowing removing the
minimum amount of Si needed to make room for volume
expansion. An additional advantage of the etchant developed for this work is that it etches with almost the same
rate in the full depth of the macropores; i.e. supplying a
large process window (in contrast to standard acidic
etches). The time needed to over-etching is further reduced by introducing a “global” taper to the pore diameter
that reduces the amount of Si to be etched-off in the depth
of the pore.
An “end taper” increases the pore diameter close to the
pore tip with the result that the nanowires are very thin
close to the substrate. This allows easy detachment of the
“nanfur” resulting after Cu is (electroless) deposited on
the still well defined wire surface (Fig. 4).
Peeling of the nanofur leaves a still pre-structured Si wafer that could be used for producing a few more nanofurs.
This optimizes the usage of expensive Si wafers and eliminates the need of lithography for most batches.
Preliminary electrical studies showed a capacity of the
wires of up to 3600 mAh/g, no fading for up to 60 cycles,
and irreversible losses <15 %.
Fig. 1. Si nanowires with optimal structural properties.
a)
b)
Fig. 2. a) Macropores in Si with localized diameter narrowings. b) Stabilizing layer in a detached nanofur.
Fig. 3. Top view of the Si nanowires.
Fig. 4. Surface of a sample after detaching the Si nanowires. Its structure is suitable for further processing.
[1] B.A. Boukamp et al., J. Electrochem. Soc. 128 (1981)
725.
[2] C.K. Chan et al., Nat. Nanotechnol. 3 (2008) 31.
[3] Y. Yang et al., Nano Lett. 10 (2010) 1486.
[4] H. Föll et al., phys. stat. sol. RRL 4 (2010) 4.
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