V. Focused Fundamental Research - EERE - U.S. Department of ...

V.C.13 Wiring Up Silicon Nanoparticles for High-Performance Lithium-Ion Battery Anodes (Stanford U.) Cui – Stanford U.
contact and eventual capacity fading. 2) Unstable solid
electrolyte interface (SEI). The repetitive volume
expansion and contraction cause the continuous movement
of the interface between Si and organic electrolyte and it is
challenging to form a layer of stable SEI, resulting in low
Coulombic efficiency and capacity loss during cycling. 3)
Challenge to maintain good electrical contact between Si
materials and current collector. Even though mechanical
breaking does not take place in Si nanostructures below
critical sizes, large volume change can still cause the
movement of Si nanostructures and the detachment from
the conducting environment during long-term battery
cycling. 4) Challenge to generate Si materials to address
the above three challenges with low-cost and scalable
processing.
This goal of this project is to study the fundamental
principles related to alloy anodes, to design nanostructured
Si anodes to over the three fundamental challenges and to
develop low-cost and scalable procesing.
Approach
This project develops the ex situ and in situ electron
microscopy to understand the structure and property
correlation of nanostructured Si anodes, which helps
developing the materials design principles. Based on the
principles, this project also designs and synthesizes
nanostructured Si which can overcome the three
fundamentals materials challenges. The project also
develops low-cost and scalable nanomaterials which have
the designed functionality.
Results
Connecting Si particles electrically with “inorganic
glue”. A method was developed to fabricate Si-based
powder electrodes without using polymer binders or
conductive carbon additives. The expansion/contraction of
Si particles in slurry anodes usually results in loss of
electrical contact and capacity decay, even with submicron
particles. To combat this issue, we have developed
“inorganic glue” that serves to bind Si particles together in
an electrode architecture and provides better electrical
contact than conductive additives. In this process, slurry
electrodes containing Si particles or nanowires were
fabricated, and then chemical vapor deposition was used to
deposit ~50 nm of doped amorphous Si onto the
electrodes. This a-Si layer fuses the particles together and
also binds them to the underlying current collector
substrate. In this way, a porous, electrically connected
structure of crystalline Si particles in an a-Si framework is
created. 200 nm Si particles in this framework showed
high capacity (~2500 mAh/g) and good cycling
performance, but micron-sized particles showed fast
capacity decay, as shown in Figure V - 125.
Figure V - 125: Specific capacity with cycling for anodes with different Si
particle sizes bound together with inorganic glue
Ex situ TEM to study Si volume changes. Other
work utilized transmission electron microscopy (TEM) to
study in detail the volume changes in individual Si
nanostructures. In this technique, Si nanostructures are
dispersed directly on a metal-coated TEM grid, and single
nanostructures can be imaged before and after
electrochemical charge/discharge. TEM is useful because
the volume and structural changes in single NWs during
lithiation can be studied simultaneously. Figure V - 126 shows
TEM images and selected-area diffraction patterns of the
same two nanowires before and after lithiation. The
crystalline nanowires become amorphous and expand in
diameter after lithiation.
Figure V - 126: TEM images and diffraction patterns of the same two
nanowires before (left) and after (right) lithiation
Using this technique, we have studied differences in
volume changes between nanowires with and without a
surface oxide layer. We observed that smaller NWs with
oxide shells tend to expand to a lesser extent than those
without oxide. We expect that this could be useful in
engineered hollow nanostructures, where a constraining
layer could be used to control volume expansion away
from the surface in contact with the electrolyte.
Energy Storage R &D 586 FY 2011 Annual Progress Report