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

Cui – Stanford U.
V.C.13 Wiring Up Silicon Nanoparticles for High-Performance Lithium-Ion Battery Anodes (Stanford U.)
Determining size effects on fracture upon
electrochemical lithiation/delithiation of silicon
nanopillars. The circumstances causing fracture of Si
nanostructures during lithiation/delithiation are not
completely understood. To determine the effect of
nanostructure size and structure on fracture characteristics,
nanopillars with controlled size and crystallographic
orientation were fabricated and observed with SEM after
lithiation/delithiation. Surprisingly, we found that many
nanopillars fractured upon initial lithiation, which has not
been predicted by many computational simulations. Pillars
with , , and axial orientations all show
cracks that run along their length after lithiation.
Interestingly, the cracks are usually located between
directions of preferred radial anisotropic expansion.
Because of this, we propose that the cracks develop due to
regions of concentrated tensile hoop stress between the
preferred radial expansion directions. In addition, we
found that pillars with diameters > 360nm consistently
fractured upon lithiation, while pillars with diameters <
240nm usually did not fracture. The fraction of fractured
pillars was dependent on lithiation rate only for pillars of
intermediate diameter (240nm). Overall, these data
indicate a critical size for fracture upon lithiation between
~240 and 360nm.
Interconnected hollow Si nanoparticles as anodes.
We developed a novel interconnected Si hollow
nanosphere electrode that is capable of accommodating
large volume changes without pulverization during cycling
(Figure V - 127). We developed a finite element model to
simulate the diffusion-induced stress evolution and
investigated the volume expansion of the same single
hollow spheres before and after lithiation using TEM.
More interestingly, we achieved high initial discharge
capacity of 2725 mAh/g and 700 cycles in electrochemical
tests (Figure V - 128). Less than 8% capacity degrades for
every 100 cycles. Even after 700 cycles, this Si hollow
sphere electrode shows 1420 mAh/g capacity. Superior rate
capability is demonstrated as well and attributed to fast
lithium diffusion in the interconnected Si hollow structure.
Conductive, mechanically robust,
electrochemically inactive nanoscale scaffolding for
supporting Si active material. We fabricated titanium
carbide/carbon core-shell nanofibers directly on a steel
substrate, and then used CVD to coat amorphous Si onto
this scaffolding. The nanoscale electronically conductive
scaffolding allows for relatively high mass loading and
good electronic connectivity, and the large mechanical
stiffness and strength of the TiC/C nanofibers permit the
silicon to mechanically deform during alloying/dealloying
without damaging the underlying conductive backbone.
This is in contrast to other one-dimensional nanostructures,
such as Si nanowires, in which the entire structure reacts
with Li during charge/discharge and is altered in the
process. The specific capacity data with cycling shown in
Fig. 5 shows that a capacity close to 2800 mAh/g can be
maintained after 100 cycles for the TiC/C/Si composite
(red dots), which exceeds that of Si nanowires (black dots).
The Coulombic efficiency is also improved compared to
silicon nanowires. This study shows the importance of
interfacing large volume-expansion alloying anode
materials with mechanically robust inactive materials for
good battery performance.
Figure V - 127: Hollow Si nanoparticle synthesis and images.
FY 2011 Annual Progress Report 587 Energy Storage R&D