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

V.B.6 Cell Analysis-Interfacial Processes: SEI Formation and Stability on Cycling (HQ)
Zaghib – HQ
mAh/g with the compositions of SiOx:Gr (60:40) and
(50:50), respectively.
Ex Situ /In Situ SEM. For the ex situ analysis, a
number of Li/SiOx-Graphite (1:1) cells with EC-DEC-1M
LiPF 6 were discharged to different states: 0.5V, 0.1V,
0.05V, 0.005V and charged to 2.5V. The anodes from
these cells were analyzed by SEM and compared to the
baseline electrode. The cross section of these electrodes is
shown in Figure V - 29; the baseline electrode shows good
porosity (Figure V - 29a) to absorb the volume expansion of
the anode during cycling. However this porosity is
completely changed after the cell is fully discharged
(5mV); the volume expansion caused densification of the
electrode (Figure V - 29c), and bigger particles start to show
cracks. The elemental chemical analysis showed a
significant increase in the O content (most probably from
Li 2 O formation), particularly in the open porosity/binder
region (Figure V - 29b). When the cell is charged to 2.5V, a
decrease in the O content compared to a fully discharged
cell was observed, but still higher than the baseline
electrode. This indicates the presence of irreversible
transformation. Also, the electrode reverts back to the
porous state after densification when the electrode is
discharged to 5mV.
Conclusions and Future Directions
HQ succeeded to coat the surface of LMNO with C­
LFP by using a dry process. The cycle life and high-rate
performance were impoved compared to the bare material.
Efforts will continue on improving the performance of
stabilazed LMNO cathode material.
The high-capacity fade of the Si-based anode material
is a major challenge, and volume expansion has limited
progress. Different binders and anode compositions were
evaluated, but performance still needs further
improvement. The in situ and ex situ experiments have
provided better understanding of the cycling mechanism of
this anode and the failure mode associated with its capacity
fade. These techniques have revealed that the bigger anode
particles (ca. 13 µm) start to crack at around 0.1V. During
the charging process, all of the cracks remained – however,
some fissures in the smaller particles (< 2µ) did not crack.
The volume expansion caused densification of
the electrode, and bigger particles start to show cracks. The
elemental chemical analysis showed a significant increase
in the O content (most probably from Li 2 O formation),
particularly in the open porosity/binder region. Further
effort is needed to understand the limits of the particle size
and porosity needed to make stable Si-based anode
materials.
FY 2011 Publications/Presentations
1. 2011 DOE Annual Peer Review Meeting Presentation.
2. 219th ECS meeting, 1-6 May 2011, Montreal,
Canada.
3. EV2011 conference, September 26-29, Toronto,
Canada.
Figure V - 29: Reference electrode of SiO:Gr and fully discharged (5mV) in
EC-DEC-1M LiPF6
Energy Storage R &D 494 FY 2011 Annual Progress Report