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

V.C.2 Interfacial Processes – Diagnostics (LBNL)
Kostecki – LBNL
provide a real-time probe of the formation of a surface
electrolyte interphase (SEI) on the particle surface during
cycling of the electrode.
During the first potential sweep to 5.0 V, a sharp rise
in fluorescence intensity occurred (Figure V - 80) at the
beginning of the Ni 2+ oxidation reaction and continued
until the reverse scan caused Ni 4+ reduction. The
fluorescence signal declined after this point until Ni 2+
oxidation during the potential sweep on the second cycle
occurred again. This pattern, repeated for all three cycles
strongly suggests that electrolyte decomposition is
catalyzed by the changes that occur upon Ni oxidation
during the delithiation process. In addition, the loss of
fluorescence intensity indicates that most of these
fluorescent decomposition products either dissolve into the
electrolyte, or decompose to other products right after
formation. However, the rising fluorescence background
intensity demonstrates that some fluorescent species
remained at the particle surface. A similar increase in
fluorescence has also been observed from 1 M LiClO 4 in
EC:DEC electrolyte solutions. Therefore, the fluorescent
species likely originate primarily from decomposition of
the carbonate-based electrolytes, as opposed to LiPF 6 .
250 Fluorescence Intensity (a.u.)
3e-6
I (A)
2e-6
1e-6
0
-1e-6
-2e-6
-3e-6
200
150
100
50
-4e-6 0
0 10 20 30 40 50
Time (hr)
Figure V - 80: Current (left axis) and fluorescence intensity (right axis) vs. time during three CVs between 3.5 and 5.0 V at 0.05 mV/s.
Transmission FTIR and Raman spectroscopy were
used to probe the structural changes of the LMNO spinel
powder (synthesized and provided by Jordi Cabana) after
aging in 1 M LiPF 6 in EC:DEC (1:2 w/w) electrolyte.
Noticeable changes in FTIR spectra after 6 weeks at 55°C
indicate changes in the local structure of the crystal. The
Raman spectra of the aged LMNO powder suggest the
surface Mn enrichment and possible Ni dissolution
The FTIR spectrum of delithiated MNO after one
week at 60°C (Figure V - 79d) appears almost identical to the
fully lithiated sample. Spontaneous relithiation of MNO
(and corresponding reduction of the active metal) must be
accompanied by electrolyte oxidation at the particle
surface. The increased reactivity of LNMO in the charged
state toward the electrolyte and its inability to retain charge
during storage at elevated temperatures, are both critical
issues that must be addressed for this material to be used in
commercial Li-ion batteries.
Our second goal was to investigate the SEI layer
composition on model Sn electrodes to gain insights into
the different electrolyte degradation mechanisms at the
highly reactive Sn (100) surface and the more stable Sn
(001) surface. Preliminary soft X-ray XAS measurements
were performed in the ALS (beam line 8.0.1) on Sn single
crystals with the (100) and (001) orientations after cycling
in EC/DEC 1M LiPF 6 electrolyte. Carbon, oxygen, and
fluorine K-edges were probed with both florescent X-ray
and Auger electron signals, which are proportional to the
X-ray absorption. The surface sensitive Auger electron
signal (escape depth of 50Å) indicates different
compositions of the surface layers. Observation of C π*
and O π* near-edge features at 290 eV and 534 eV
respectively, on Sn (100), are identical to NEXAFS spectra
of Li 2 CO 3 . The F K-edge pattern on Sn (001) suggests that
F is in the form of LiF within the SEI layer.
Our third objective was to explore the feasibility and
benefits of near-field microscopy and spectroscopy
techniques to study interfaces and interphases in Li-ion
systems at subwavelength resolution. Preliminary nearfield
measurements on HOPG model electrodes, Sn anodes
and Li x FePO 4 single particles were carried out using a
near-field spectroscopy instrumentation at JASCO (Japan)
and Neaspec (Germany). The initial results proved to be
extremely encouraging revealing new surface features at
subwavelength resolution, which is unavailable by any
other existing techniques. A similar instrumental setup
from Neaspec will be purchased in October 2011 with
capital equipment funding recently received and will
enable future in-house spectroscopic measurements at the
nanoscale (nanoRaman, i.e. TERS and nano-FTIR) to
characterize cathode and anode structure interfacial
behavior.
Energy Storage R &D 542 FY 2011 Annual Progress Report