Monitoring Reactions in Li-O Cells by EQCM

Monitoring Reactions in Li-O2 Cells by EQCM
gold electrodes (but porous carbons can also be used)
• The EQCM response of mass and current changes
in Triglyme and DMSO solutions shows a
relatively reasonable fitting between them.
Gold dissolution?
PC
Triglyme
DMSO
Mass
Current
• In the carbonate based solution, the mass change
is not reversible and the mass continue to
accumulate even during the anodic scan.
Li-O2 cell design
Technical University in Munich
IBM/BIU
Al-mesh
Air-electrode
Separator
Li-foil
b)
Spacer
Conical spring
Meini et al., Electrochem. Solid-State Lett., 15, A45-A48 (2012)
Developed monolithic electrodes comprising activated carbon micro-fibers (ACM)
The solution volume – electrode’s surface area ratio is small (as in real systems)
Consequently, if side reactions occur, they are well seen.
Polyether solutions exhibit
reversible behavior
charged
4.3V
Pristine ACM electrode
Discharge
2V
Triglyme/LiTFSI
charged
4.3V
EC-DMC/LiTFSI
Cycling of ACF electrodes at a current
density of 0.2 mA/cm2
• The decomposition products of the electrolyte solution (mostly due to oxidation , up to
4.4V) are being reduced in the next discharge processes.
Adding catalysis:
Preparation of monolithic activated
carbon micro-fibers electrodes
decorated with α-MnO2
nanoparticles.
MnO2 allotropes are known to be
effective catalysts for oxygen
reduction
Etacheri, V.; Sharon, D.; Garsuch, A.; Afri, M.; Frimer, A. a.; Aurbach, D. Journal of Materials
Chemistry A 2013, 1, 5021–5030.
α-MnO2 nanoparticles were
electrodeposited on the ACM surface
• The coating of the nano
MnO2 particles was
done by electro
deposition in aqueous
solutions accompanied
by washes and annealing
treatment.
500 nm
X-ray diffraction studies confirmed the existence of αtype MnO2 (peaks at 26°, 29.4°, 39° and 43°).
Raman bands at 300, 360, 575 and 649 cm -1 are
characteristic of MnO2 octahedra.
Discharge
2V
charged
3.75 V
2 µm
Triglyme/LiTFSI
Pristine electrode
α-MnO2 decorated ACM electrodes during cycling
• The EDX spectra of discharged ACM/α-MnO2 electrodes demonstrated the formation of
oxygen rich compounds and the images of electrodes charged to 3.75 V were identical to
that of pristine electrodes. The catalyst particles remained in place.
ACM α-MnO2 electrodes can reduce the
oxidation potential
4.3 V
ACM
ACM α-MnO2
The over-potential dropped to in more then 0.5 V by introducing MnO2 catalyst
XRD: Li2O2 is the single crystalline
product formed in triglyme/LiTFSI
Charged
Discharged
Pristine
XRD patterns of the ACM electrode after being discharged to 2.0 V in triglyme/LiTFSI (1 M)
under oxygen atmosphere, which exhibit peaks characteristic of crystalline Li2O2.
662
588
Intensity (Arb.Units)
875
FT-IR
1370
NMR
1626
So why the promising poly-ethers aren't
good enough for Li-oxygen systems?
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
• From these post-mortem analysis of Li-O2 triglyme LiTFSI (1M) cells after first
discharge ,we can observe lithium oxides formation and a verity of side products that
can be assigned to solvent fragments, formed during oxygen reduction .
Product analysis of ORR in triglyme/LiTFSI solutions by matrixassisted laser desorption/ionization
(MALDI) mass spectrometry
Solvent decomposition fragments
Salt fragments
The carbon cloth electrodes can be used as the matrix for the products formed herein, so
no additional matrix was necessary as in ordinary MALDI measurements. The mode of
ionization was set for positive voltage, and the resulting fragments are positively charged.
Proposed degradation mechanism of polyether
solvents during oxygen reduction
(in the presence of Li ions)
Li O O Li
b
a
a
CH3OCH2CH2OCH2
 


b
CH3OO Li
2
+
Li2O2
HCO2Li
7
6b
H
HB
d

OCH 2CH2OCH2
  3
c
Li O
H 2C O
:B
CH3O Li
1
d
H3CCO 2Li
O
5
Li2O2
CH3CHO
6a
c
O Li
Li2O2
LiOCO2Li

Li OCH 2CH2O Li
4
CO2 + LiOLi

10
The lithium cation is a hard electrophile which is expected to bond strongly to
the hard Lewis base oxygen anions. This in turn helps to convert the alkoxy
groups into better leaving groups and facilitates nucleophilic attacks by Li2O2.
Stability of DMSO during oxygen reduction in the
presence of Li ions.
• ACM electrodes at 0.05 mA/cm2 in
oxygen atmosphere in DMSO/ LiPF6 (1M).
• Interestingly, when using cell s with high
surface carbon cathodes under pure
oxygen atmosphere, the oxidation of
DMSO occurs at low potentials: in the
range 4.2 - 4.3V.
• Evidence of dimethyl sulfone formation
by anodic oxidation of DMSO above 4.2 V
(Li/Li+) was confirmed by Calvo et . Al ⃰.
⃰ Mozhzhukhina, N.; Méndez De Leo, L. P.; Calvo, E. J. J. Phys. Chem. C 2013.
DMSO2 formation
Blank experiment- charging in oxygen
XRD patterns of ACM electrodes after ORR and OER in DMSO/ LiPF6 solutions
Polarized to 4.1 V
Polarized to 2 V
Pristine
The diffraction patterns of electrodes charged to 4.1V indicate the decomposition
of the Li2O2 particles; however, we still observe spots of LiOH. This can be
attributed to the limitation of low oxidation potential, as LiOH particles are hard to
oxidize ⃰
⃰ Meini, S.; Tsiouvaras, N.; Schwenke, K. U.; Piana, M.; Beyer, H.; Lange, L.; Gasteiger, H. a Phys. Chem. Chem. Phys. 2013, 15, 11478.
Electrodes cycled in DMSO/LiTFSI (1M)
Pristine
Discharge to 2 V
Discharge to 4.1V
sulfur was detected on the cathode surface after ORR after
cycling. This indicated significant DMSO decomposition.
The high oxygen content on discharged cathodes
indicates the formation of oxygen rich compounds
like Li2O2 and LiOH. Small peaks of fluorine and
phosphorus are attributed to the known
decomposition products of LiPF6.
The high concentration (8%) of sulfur on the cathode
Carbon
Oxygen
Oxygen
4.1 V
surface indicates solvent decomposition.
Element
Pristine
Reductiontreatment
Discharged
Charged
C
O
F
P
S
84%
16%
97.32%
2.68%
40.90%
45.69%
4.62%
0.55%
8.02%
62.76%
21.8%
9.67%
2.39%
3.31
Oxygen
Sulfur
2V
Oxidation of DMSO occurs in the course of electrochemical
reduction of oxygen, via reactions of the oxides (super-oxide amd
peroxide) with solvent molecules.
Polarized to 2 V
(c)
O
O
S
H3C
(b)
CH3
FT-IR
O
S
H3C
CH3
(a)
Surface and solution analyses of discharge cells that contained DMSO LiPF6 solution
indicated that DMSO decomposes during oxygen reduction (via reactions with oxide
radicals).
The surface of cycled electrodes shows
complex sulfur XPS peaks
Charged
• The sulfoxide groups (>S=O) from the
DMSO are absent as their XPS peaks should
appear in lower binding energy of around
164 to 166 eV.
• We attribute the sulfur XPS spectra to
SO4-2
sulfur at high oxidation states, such as sulfite
(SO32–) and/or sulfate (SO42–) groups.
Both groups should exhibit sulfur XPS
peaks around 168.0–170.1 eV.*
⃰
Lindberg, B. J.Molecular Spectroscopy by Means of ESCA II. Sulfur
Compounds. Correlation of Electron Binding Energy with Structure. Phys.
Scr. 1970, 1, 286–298.
Discharged
SO3-2
Degradation mechanism of DMSO
during ORR
O H
O
H3C S CH2
..
+
LiO2
H3C ..
S CH2
Li+ + 1e-
..
Li
HOO
+
O
+
HOO .
(1)
Li2O2
Li
H3C ..
S CH2
O Li
O
H3C ..
S CH3
H3C
DMSO
HO
(2)
HOO Li
O H
H3C S CH2
+
Dimsyl Anion
DMSO
HOO .
Li
..
S
CH3
O
+
HOO Li
(3)
O
H3C S CH3 + LiOH
(4)
O
DMSO2
Tetrahedral
intermediate
•
The lifetime of LiO2 should be very short, but it can be extended due to the high
Guttman donor number of DMSO. The latter reduces the acidity of the Li cation, and
thereby helps to extend the stability of LiO2 before disproportionating to Li2O2.
Amide based solvents
O
H3C
O
N
CH3
CH3
Dimethylacetamide (DMA)
H
N
CH3
CH3
Dimethylformamide (DMF)
1) Walker, W.; Giordani, V.; Uddin, J.; Bryantsev, V. S.; Chase, G. V; Addison, D. A. J. Am. Chem. Soc.
2013, 135, 2076–9.
2) Chen, Y.; Freunberger, S. a; Peng, Z.; Bardé, F.; Bruce, P. G. J. Am. Chem. Soc. 2012, 134, 7952–7.

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