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

Shao-Horn – MIT
V.B.7 Role of Surface Chemistry on the Cycling and Rate Capability of Li Positive Electrode Materials (MIT)
the F 1s regions (not shown) have binding energy near 689
eV and can be attributed to fluorine chemistry in Nafion.
Figure V - 36: O 1s photoemission lines of pristine and discharged carbon,
Pt/C and Au/C electrodes along with Li2O and Li2O2 standards.
Figure V - 35: (a) O K edge and (b) Li K edge XANES FY spectra of
reference compounds Li2O2, Li2O, LiCoO2 and discharged VC and Au/C
electrodes at 100 mA/gcarbon.
Li 1s Li 2 CO 3
Li 2 O 2 /LiOH
Li 2
O 2
- ref.
XPS studies of discharge products in Li-O 2 cells.
We have used conventional XPS to identify the role of the
Pt and Au catalysts on the discharge products from O 2
reduction with lithium ions in Li-O 2 cells. The cell
consisted of a Li foil, two pieces of Celgard separator
(C480) and a Nafion-bonded cathode coated on a Celgard
C480 using pure VC, Au/C (Au/C) and 40 wt% Pt/C (Pt/C)
from Premetek. The cells were discharged at
100 mA/g carbon to 2.0 V Li in 0.1 M LiClO 4 in DME. The
XPS measurements were conducted using conventional Al
X-rays. Li 2 O and Li 2 O 2 powders were used as standards.
All samples were transferred from an argon-filled glove
box to the test chamber of the spectrometer without
exposure to ambient conditions. The XPS multiplexes of
the Li 1s, C 1s, O 1s and F 1s were recorded and those of
O and Li are presented in Figure V - 36 and Figure V - 37,
respectively. The C 1s regions (not shown) for the pristine
electrodes display a high binding energy band (291-292
eV) due to fluorinated carbon from Nafion in addition to
the hydrocarbon component at 284.6 eV. Those for the
discharged electrodes display some carbon bonded to
oxygen with binding energies at 286.2 and 289.4 eV in
addition to the hydrocarbon component. The majority of
Li 2
O - ref.
Dis. Pt/C
Dis. Au/C
Dis. C
60 58 56 54 52 50
Binding Energy (eV)
Figure V - 37: Li 1s photoemission lines of pristine and discharged carbon,
Pt/C and Au/C electrodes along with Li2O and Li2O2 standards.
The O 1s binding energy of Li 2 O 2 reference powder
(Figure V - 36, right panel) was found to be 530.7 eV, which
is comparable to that reported for LiOH (531.2 eV). This
observation can be explained by 1) difficulties in
differentiating Li 2 O 2 and LiOH based on XPS data and 2)
the surface of reference Li 2 O 2 is covered by LiOH.
FY 2011 Annual Progress Report 499 Energy Storage R&D