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

V.B.7 Role of Surface Chemistry on the Cycling and Rate Capability of Li Positive Electrode Materials (MIT)
Shao-Horn – MIT
Gravimetric power [W/kg]
10 4
10 3
VC
LiCoO 2
10 2
(Chen 08)
Au/C
pristine
electrode weight
discharged
electrode weight
10 1
10 2 10 3 10 4
Gravimetric energy [Wh/kg]
Figure V - 33: Ragone plot for Li-O2 cells with Vulcan carbon (VC) electrode
by pristine electrode weight in black circles; by discharged electrode weight
in black triangles), Au/C electrode by pristine electrode weight in orange
circles; by discharged electrode weight in orange triangles) and conventional
Li-LiCoO2 cell reported previously in blue squares (Tarascon et al.,
Chemsuschem, 2008).
Li 2 O 2 , Li 2 O and LiCoO 2 are shown in Figure V - 35. Figure V -
35a shows that the bulk sensitive fluorescence yield (FY) O
K edge XANES of discharged VC and Au/C electrodes are
in reasonable agreement with both FY and surface
sensitive total electron yield (TEY) spectra of Li 2 O 2
reference powder, suggesting that the O local environment
of the discharge product is Li 2 O 2 -like. The small difference
in the FY O K edge XANES between discharge products
and reference Li 2 O 2 may suggest the presence of structural
defects and/or nonstoichiometry such as lithium vacancies.
Li K edge XANES of the discharge products shown in
Figure V - 35b were found to have a major peak at 62.0 eV
having a small pre-edge (at 58.0 eV) followed by a broad
peak at ~ 70.0 eV. In contrast, the Li K edge XANES
spectra of the discharge products do not match well with
that of reference Li 2 O 2 nor any other references (i.e.,
Li 2 CO 3 , LiOH, not shown). This result suggests that
although XRD data show direct evidence of long-range
ordering of lithium and oxygen in an Li 2 O 2 -like crystal
structure, the Li + local environment in the lithium peroxide
of discharged electrodes is different from that of Li 2 O 2 .
Interestingly, Li K edge XANES spectra of discharge
products resemble that of Li x TiO 2 in both energy and
shape while they resemble that of LiCoO 2 in shape and
have a binding energy about 0.5 eV higher, where Li + ions
are coordinated with 6 oxygen in Li x TiO 2 and LiCoO 2 . It is
hypothesized that the spectrum difference between
discharge products and reference Li 2 O 2 can be attributed to
the presence of structural defects in lithium peroxide such
as oxygen and/or lithium vacancies, which has been
suggested previously in a DFT study (Norskov et al., J.
Chem. Phys. 2010). Further studies to understand the role
of catalyst on affecting the growth mechanism and the
nonstoichiometry and structural defects of the discharge
product(s) are needed to verify the proposed hypothesis.
Figure V - 34: XRD patterns of pristine and discharged electrodes supported
on a Celgard 480 separator (100 and 2000 mA/gcarbon) for VC (a) and Au/C
(b). The reflections appeared in the pristine VC electrode came from Celgard
C480 and those appeared in the pristine Au/C electrode came from Au
nanoparticles and Celgard C480.
We further applied O and Li K edge XANES
technique to probe the local enviornment of oxygen and
lithium in the discharge products. O and Li K edge
XANES of discharged VC and Au/C electrodes at 100
mA/g carbon and the spectra of reference materials including
Energy Storage R &D 498 FY 2011 Annual Progress Report