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V.B.7 Role of Surface Chemistry on the Cycling and Rate Capability of Li Positive Electrode Materials (MIT)
Shao-Horn – MIT
Considering XPS analysis of powder LiOH (not shown)
indicated that the surface region of LiOH consisted mostly
of lithium carbonate, the surface region of reference Li 2 O 2
powder observed here more likely consists of Li 2 O 2 . The O
1s spectrum of Li 2 O reference powder (Figure V - 36, right
panel) consists of two components: 1) a major component
at 530.9 eV, which is close to those reported for LiOH; 2)
a minor component (~2%) at 528.4 eV, which is attributed
to O 2- ion in Li 2 O. Based on the major component at 530.9
eV, it is postulated that the surface of reference Li 2 O is
largely LiOH in nature as Li 2 O is known to be extremely
moisture-sensitive.
The O 1s regions for the pristine electrodes (Figure V -
36, left panel) display a double peak structure that can be
associated with oxygen bond to CF x groups (i.e., CF-O­
CF 2 ) at 535 eV and SO 3 groups at 532 eV in Nafion. After
discharge (Figure V - 36, right panel), the CF-O-CF 2
component at 535 eV from Nafion disappeared, which
indicates the buildup of discharge products on the
electrode surface. The majority of the oxygen signal for
the discharged electrodes is attributed to the formation of
Li 2 O 2 at 531.0 eV for all discharge electrodes. However,
the formation of LiOH cannot be ruled out. The formation
of Li 2 O can be ruled out by the lack of an oxygen
component with binding energy in the range of 528-529
eV. In addition, a second component was identified at
~533.0 eV for the discharged electrodes, which can be
associated with Li 2 CO 3 and/or O-C=O (532.1 eV) and
oxygen singly bonded with carbon atom in O-C=O (533.5
eV). The formation of Li 2 O 2 (majority) and minority
Li 2 CO 3 for all discharge electrodes is also supported by the
Li 1s spectra of Figure V - 37.
Electrochemical activity of high-energy Li-rich
(Li 2 O) x (MO 2 )y systems. Positive electrodes of lithium
peroxide and lithium oxide (Li 2 O x ) were shown to exhibit
higher energy densities compared to LiCoO 2 cells (Bruce
et al., JACS 2006). However, the overpotential required to
charge the Li-Li 2 O x cells is ~1.5 V owing to the slow
reaction rate associated with decomposing Li 2 O x . In order
to efficiently operate the high energy Li-Li 2 O x cells, we
have applied nanoparticles of platinum (Pt) and gold (Au)
to facilitate the charging reaction rates where Pt was
shown to have the highest charging activity among Pt, Au
and carbon (Shao-Horn et al. ESSL, 2010). However, the
utilization of precious Pt metal is not ideal. Therefore, it is
crucial to identify highly active catalyst with abundant
elements such as Mn, Fe. Co, and Ni for high energy Li­
Li 2 O 2 and Li-Li 2 O cells. Here we report the
electrochemical activity for electro-oxidation of Li 2 O 2 on
pure carbon, Pt/C, and 40 wt% Co 3 O 4 /C (Co 3 O 4 /C) by
anodic polarization of Li 2 O 2 -filled positive electrodes in
1 M LiTFSI DME at 5 mV/s. Co 3 O 4 nanoparticles (NPs)
were synthesized as follows: A solution of 10 mL
dichlorobenzene and 10 mL of oleylamine was prepared at
room temperature (20°C) in a four-neck flask. The mixture
was heated to 160°C under Ar atmosphere with continuous
stirring. 400mg of Co 2 (CO) 8 in 4mL of dichlorobenzene
was quickly injected into the above solution and left for 1
hr. The solution was cooled down to room temperature,
and CoO NPs were precipitated by ethanol addition and
collected by centrifugation. The product was re-dispersed
in hexane and separated by ethanol addition and
centrifugation. This procedure was repeated 2 times. The
as-synthesized CoO NPs were transferred to a porcelain
boat and was annealed in a tube furnace at 500°C for 3 hrs
under O 2 which lead to Co 3 O 4 NPs. The heat-treated Co 3 O 4
NPs were loaded onto Vulcan Carbon (Co 3 O 4 : Carbon =
2:3 w/w) yielding 40 wt% Co 3 O 4 /C. The particle
dispersion of Co 3 O 4 NPs was uniform with average
particle size 10 nm, as shown in Figure V - 38a.
(a)
(b)
Figure V - 38: (a) TEM image of 40 wt% Co3O4/C (b) Li2O2 oxidation curves
of pure Carbon, Pt/C, and Co3O4/C in 1 M LiTFSI DME at 5 mV/s
Figure V - 38b shows the net Li 2 O 2 electro-oxidation
currents (carbon-mass normalized), which were obtained
by subtracting carbon-mass normalized currents of Li 2 O 2 ­
free electrodes from those filled with Li 2 O 2 . For C-Li 2 O 2
cells, net currents of 200 mA/g carbon were obtained at
4.05V Li , which agrees well with the literature (Bruce et al.,
JACS 2006, JPS 2007, Shao-Horn et al. ESSL, 2010). In
addition, net currents of 200 mA/g carbon were obtained at
3.70 V Li for Pt-Li 2 O 2 cells, which is consistent with our
previous study (Shao-Horn et al. ESSL, JACS, 2010). The
application of Pt NPs decreases the charge overpotential
by 350 mV compared to that of pure Carbon. Furthermore,
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