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

Liu – LBNL
V.C.6 Advanced Binder for Electrode Materials (LBNL)
showed poor performance due to the insulating nature of
the polymer matrix. The Si/AB/PVDF and Si/PFFO
electrodes are characterized by high starting capacity but
fast fading, due to the loss of electrical and mechanical
integrity respectively. With the designed electrical and
mechanical properties, PFFOMB achieved both intimate
electric contact for electron conduction and mechanical
integrity, resulting in high specific capacity and stable
cycling performance.
The SiO 2 content as a function of different etching
times was investigated using TGA. As shown in Figure V -
92 SiO 2 content decreases from 26.5 % before etching to
18.9 % after 5 min. and 12.9% after 10 min. of etching. A
simple calculation based on elemental analysis assuming
50-nm Si nanoparticles yields an initial surface SiO 2 layer
of 4 nm, which is at the lower limit of the TEM
observations for the as-received sample. The HF etching
provides a facile process for generating Si nanoparticles
with different SiO 2 content by controlling the etching time.
(b)
Figure V - 91: TEM images of Si nanoparticles [(a) and (b)] as-received from
commercial supplier showing SiO2 layer on the surface and [(c) and (d)] after
30 min of HF etching to remove the SiO2 surface layer.
Figure V - 90: The initial cycling behaviors of Si particles in different
conductive matrixes against lithium metal counter electrodes at C/10 rate.
Commercial Si Nanoparticle Surface and
Modification. An important aspect that has not been well
studied is the surface of the Si nanoparticles and its
impacts on the initial cell performance. Nanomaterials,
including spherical particles, wires or sponge structures,
tend to have enhanced surface area compared to micronsized
particles. Surface impurities play a significant role in
the performance of the nano-material; the initial
performance of Si nanoparticles with similar size
distribution can vary significantly from batch to batch even
if they are from the same supplier. We demonstrate that the
performance variation is due to variations in the native
oxide surface layer on Si nanoparticles, and that the
removal of this layer can significantly improve the Si
nanoparticles’ initial performance.
TEM images of Si nanoparticles before and after
etching for 30 min. to remove surface layers are shown in
Figure V - 91. Besides the large size distribution of the asreceived
sample, a core-shell structure was observed for
most particles in Figure V - 91a,b. Shell thickness is around 4
-10 nm, with thicker shells on bigger particles. This
amorphous shell layer is the native oxide layer from the
manufacturing process. We have investigated different
samples from various sources, and the thickness of this
glassy amorphous layer varies significantly. After HF
etching, this amorphous oxide layer was reduced to 1-2 nm
(Figure V - 91c,d).
SiO 2 wt %
35
30
25
20
15
10
5
0
0 10 20 30
Etching time (min)
Figure V - 92: SiO2 content in the samples, determined using TGA, as a
function of etching time.
The as-received Si nanoparticles and those after 10
min. and 30 min. of etching were used as negative
electrodes in coin cells with Li metal as the counter
electrodes, and their initial electrochemical performance
was evaluated for 10 cycles (Figure V - 93). The specific
capacities in Fig. 3a,b,c are based on the overall weight of
the Si nanoparticles, whereas those in Fig. 6a-1,b-1,c-1 are
after discounting the weight of the SiO 2 . Comparing the
specific capacities based on Si powder weight in Fig.
3a,b,c, there is a clear correlation between the SiO 2 surface
layer thickness and measured reversible capacity. The
reversible capacity of the as-received Si nanoparticles is
less than 970 mAh/g (Figure V - 93a), and even after
discounting SiO 2 , it is 1320 mAh/g (Figure V - 93a-1),
FY 2011 Annual Progress Report 557 Energy Storage R&D