On the incompatibility of lithium-O2 battery technology with CO2

Article abstract of DOI:10.1039/c7sc01230f

When solubilized in a hexacarboxamide cryptand anion receptor, the peroxide dianion reacts rapidly with CO₂ in polar aprotic organic media to produce hydroperoxycarbonate (HOOCO₂^-) and peroxydicarbonate (^-O₂COOCO₂^-). Peroxydicarbonate is subject to thermal fragmentation into two equivalents of the highly reactive carbonate radical anion, which promotes hydrogen atom abstraction reactions responsible for the oxidative degradation of organic solvents. The activation and conversion of the peroxide dianion by CO₂ is general. Exposure of solid lithium peroxide (Li₂O₂) to CO₂ in polar aprotic organic media results in aggressive oxidation. These findings indicate that CO₂ must not be introduced in conditions relevant to typical lithium-O₂ cell configurations, as production of HOOCO₂^- and ^-O₂COOCO₂^- during lithium-O₂ cell cycling will lead to cell degradation via oxidation of organic electrolytes and other vulnerable cell components.

Full text of DOI:10.1039/c7sc01230f

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On the incompatibility of lithium-O2 battery technol-
ogy with CO2†
Shiyu Zhang,a‡ Matthew J. Nava,a‡ Gary K. Chow,b Nazario Lopez,a,c Gang Wu,d David
R. Britt,b Daniel G. Nocera,∗,e and Christopher C. Cummins ∗,a
Peroxide dianion, solubilized in a hexacarboxamide cryptand anion receptor, reacts rapidly with
CO2 in polar aprotic organic media to furnish hydroperoxycarbonate (HOOCO2−) and peroxy-
dicarbonate (−O2COOCO2−). Peroxydicarbonate is subject to thermal fragmentation into two
equivalents of the highly reactive carbonate radical anion, which is active for promoting hydro-
gen atom abstraction reactions responsible for the oxidative degradation of organic solvents. The
activation and conversion of peroxide dianion by CO2 is general. Exposure of solid lithium perox-
ide (Li2O2) to CO2 in polar aprotic organic media results in aggressive oxidation. These findings
indicate that CO2 must not be introduced in conditions−relevant to typical lithium-O2 cell configu-
−
rations, as production of HOOCO2 and −O2COOCO2 during lithium-O2 cell cycling will lead to
cell degradation via oxidation of organic electrolytes and other vulnerable cell components.
recharge cycle of lithium-O2 batteries2 prompted us to consider
the possibility that carbonate formation may be a consequence
Introduction
of peroxide combination with carbon dioxide; this would likely
confer increased solubility and yield powerful oxidizers. To ad-
dress this topic, we utilize an anion-receptor solubilized form of
peroxide dianion12 to elucidate the molecular level details of its
reaction with carbon dioxide. As reported herein, we observe the
formation of strongly oxidizing peroxy(di)-carbonate intermedi-
ates and study their reaction with organic solvents to produce car-
bonate. In a complementary line of investigation, we show that
carbon dioxide activation of insoluble Li2O2 similarly engenders
solvent oxidation with the concomitant production of carbonate.
Our findings shed light on the identity and behavior of the hot ox-
idants generated upon the facile and quantitative combination of
Two-electron reduction of molecular oxygen to peroxide dianion
is an attractive cathode redox couple for developing recharge-
able lithium-O2 batteries.1 Lithium carbonate (Li2CO3) forma-
tion is deleterious to battery performance because it passivates
electrodes and causes a drastic reduction in roundtrip efficiency
of discharge-charge cycles.2,3 Carbonate formation is typically as-
cribed to oxidative degradation of organic electrolytes4–6 and car-
bon electrodes7 by superoxide8,9 and singlet oxygen.10 Although
peroxide is often considered to be a strong oxidant in aqueous
media, salts of its dianion (O22−) are poor oxidizers in organic
media due to their extremely low solubility and, for this reason,
the possible role of peroxide in furnishing carbonate is underap-
preciated.11 The presence of carbonate-derived CO2 during the
2−
O2 with CO2 via direct spectroscopic detection and exploratory
reaction chemistry.
Results and Discussion
Reaction of O22−with CO2 using an anion receptor
a
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts
Avenue, Cambridge, MA 02139-4307, USA. Tel: +1 617 253 5332; E-mail: ccum-
mins@mit.edu
b
Despite the drastic and deleterious effect CO2 has upon the per-
formance of a cycling lithium-oxygen battery, our understanding
of the chemical entities responsible for this effect is poor and
based primarily upon computational studies or observation of
terminal reaction products.2,8 To examine the effect of CO2 on
the oxidative power of peroxide, an anion receptor complex13
of peroxide dianion, [O2⊂mBDCA-5t-H6]2− (1, Fig. 1),12 was
employed as a soluble source of peroxide dianion. The anion
receptor mBDCA-5t-H6 encapsulates peroxide dianion via six N–
H···O hydrogen bonds. Since its discovery, this cryptate has en-
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis,
CA 95616
c
Current Address: Centro de Investigaciones Químicas, IICBA, Universidad Autonoma
del Estado de Morelos, Avenida Universidad 1001, Colonia Chamilpa, C.P. 62209, Cuer-
navaca, Morelos Mexico
d
Department of Chemistry, Queen0s University, 90 Bader Lane, Kingston, Ontario
K7L3N6, Canada
e Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street,
Cambridge, MA 02138
† Electronic Supplementary Information (ESI) available: Full experimental, crystal-
lographic (CCDC 1512972) and spectroscopic data. See DOI: 10.1039/b000000x/
‡ These authors contributed equally.
1–7 | 1

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DOI: 10.1039/C7SC01230F
25 °C, sec
DMF
[CO3⊂mBDCA-5t-H6]2−+ [O]
[O2⊂mBDCA-5t-H6]2−+ CO2
�
�
O COOCO
2
�
O COOH
2
2
CO2
1
2
o
o
�
�
50 C
50 C
2
2-
2−
N
O
N
H
N
O
o
o
�
�
30 C DMF
�
d
DMF-
7
30 C
O
O
O
O
N
N
N
N
N
H
H
tBu
H
H
H
o
o
tBu
tBu
�
20 C
20 C
tBu
O
O
tBu
O
tBu
O
�
O
H
�10oC
O COOH
2
10oC
�
H
H
H
H
H
N
N
N
N
N
N
O
O
O
O
O
O
23oC
174
23oC
N
N
[O2⊂mBDCA-5t-H6]2−
[CO3⊂mBDCA-5t-H6]2−
170
166
162
158
154
350 300 250 200 150 100
�(17O)/ppm
50
�(13C)/ppm
1
2
Fig. 3 Variable temperature 13C NMR (left) and 17O NMR (right) analysis
of the reaction between 13CO2 and 1.
Fig. 1 Reaction scheme of peroxide cryptate 1 with CO2 and line drawing
of [O2⊂mBDCA-5t-H6]2− and [CO3⊂mBDCA-5t-H6]2−
.
abled exploration of the reactivity of peroxide dianion with small
molecules in polar organic media without the complicating in-
fluence of acidic protons.12,14 Despite being a simple molecule,
peroxide dianion has yielded rich and previously unknown chem-
istry, including a metal-free oxidation of carbon monoxide (CO)
generating carbonate, which is encapsulated by the anion recep-
tor as [CO3⊂mBDCA-5t-H6]2−(2, Fig. 1).14
sulfide oxidation.16,17 The salt [PPN][HOO13CO2] (PPN =
bis(triphenylphosphine)iminium) generated in situ from H2O2
and bicarbonate [PPN][H13CO3] (δ = 160.0 ppm)18–20 showed
a single 13C resonance at δ = 157.5 ppm, confirming the identity
−
of the minor intermediate as HOOCO2
.
Moreover, 13C Gauge-Independent Atomic Orbital (GIAO) NMR
calculations of chemical shifts for potential candidates were per-
formed.15 From a range of potential chemical species (Fig. 4),
symmetric peroxydicarbonate (−O2COOCO2−) emerged as the
most plausible assignment for the major product at δ = 156.9
ppm, having the best match between observed and calculated
13C NMR chemical shift.15 In an effort to independently generate
−O2COOCO2−, an experiment was carried out in which excess
13CO2 was added to a frozen mixture of potassium tert-butoxide
and bis(trimethylsilyl) peroxide giving rise to a single new 13C
NMR resonance at δ = 155.5 ppm (−40 ◦C), tentatively support-
ing our assignment of the major 1 + CO2 product as symmetric
peroxydicarbonate; differences in the medium and reaction con-
ditions may account for the observed chemical shift difference
(155.5 ppm here versus 156.9 ppm, above). Similarly, superox-
ide (O2•−) has been documented to absorb two equivalents of
CO2, generating an unsymmetrical peroxydicarbonate (Fig. 4) as
a precipitate.21 In our hands, the low solubility of this unsymmet-
rical peroxydicarbonate material precluded its characterization by
solution 13C NMR studies under conditions we employed success-
◦
While the conversion of 1 to 2 under CO (1 atm, 40 C) takes
two hours to go to completion, exposing a dimethylformamide-
d7 (DMF-d7) solution of 1 to CO2 (1 atm, 25 ◦C) resulted
in essentially instantaneous formation of carbonate cryptate
[CO3⊂mBDCA-5t-H6]2− as indicated by 1H NMR spectroscopy.
Formation of O2 gas was not observed by gas chromatography
(GC) analysis of the reactor headspace gases,15 suggesting the
possibility of oxygen incorporation into the solvent molecules. To
probe the fate of the “missing oxygen atom” according to the
equation at the top of Figure 1, the reaction of CO2 and 1 was
next performed in the presence of oxygen atom acceptors. While
1 on its own is unreactive towards PPh3 and methoxythioanisole
at 25 ◦C, exposing a mixture of 1 and organic oxygen-atom ac-
ceptor to CO2 (1 atm, 25 ◦C) resulted in rapid formation of
triphenylphosphine oxide (90%, Fig. 2) or 1-(methylsulfinyl)-4-
methoxybenzene (61%, Fig. 2), respectively.
1 + CO2 + X
X = PPh3
MeO
2 + XO
XO = OPPh3
MeO
−
fully for in situ characterization of −O2COOCO2− and HOOCO2
;
90% yield
61% yield
this establishes that different oxidants are generated upon ad-
dition of CO2 to superoxide as compared with peroxide dianion
O
S
S
(Fig. 4).
Fig. 2 Addition of CO2 to 1 in the presence of an oxygen-atom acceptor.
−
Further support for the formation of HOOCO2
and
−O2COOCO2 upon interaction of CO2 with peroxide sources
was provided by variable temperature 17O NMR spectroscopy.
Due to the fast relaxation times of 17O nuclei, observation of
the 17O resonance for mid-size molecules such as 1-17O2 and 2-
CO17O2 is expected to be challenging in solution.22 Indeed, 17O
NMR measurements of independently prepared peroxide cryptate
1-17O2 and carbonate cryptate 2-CO17O2 (70%, 17O-enriched)
show no resonances between δ = −1100 and +1800 ppm (H2O
used as reference, δ = 0 ppm) in DMF. However, solid-state
17O NMR measurements for 1-17O2 and 2-CO17O2 were success-
−
Aiming to establish the chemical identity of the oxidant(s)
generated upon exposure of peroxide cryptate 1 to CO2, we
followed the reaction by variable temperature 13C NMR spec-
troscopy. A strong new signal at δ = 156.9 ppm, together
with one minor species resonating at δ = 157.4 ppm, was ob-
served at −50 ◦C (Fig. 3). We first consider peroxycarbon-
ate (−OOCO2−) and hydroperoxycarbonate (HOOCO2−, Fig. 3)
as candidates to correspond to the observed 13C NMR sig-
nals, since hydroperoxycarbonate is known to be active for
2 |
1–7

Page 3 of 7
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DOI: 10.1039/C7SC01230F
O
O
O
O
O
−
−
O
O
−
O
O
O
O−
O
O−
O
O−
O
O
H
O
O−
O
Symmetrical
Unsymmetrical
Peroxycarbonate
−O2COO−
Hydroperoxycarbonate
−O2COOH
Peroxydicarbonate Peroxydicarbonate
−
−
−
−
O2COOCO2
O2
O−
OOC(O)OCO2
O2
O
O
O
O
O
+CO2
+CO2
−
O
O2
O
O
O−
O
O−
O
O
O
O
O
O
−
+CO2
+CO2
O
O
2−
−
O−
O2
O
O
O
O−
O
Fig. 4 Possible intermediates during the conversion of 1 and CO2 to
2 (top) and formation of symmetric and unsymmetric peroxydicarbonate
(bottom).
Fig. 5 Experimental (black trace) and simulated (red trace) solid-state
17O NMR spectra of (a) 1-17O2, (b) 2-CO17O216O, and (c) product re-
sulting from treatment of solid 1-17O2 with CO2. All solid-state 17O NMR
experiments were performed on a Bruker Avance-600 (14.1 T) spectrom-
eter under the static condition. A Hahn-echo sequence was used for
recording the static spectra to eliminate the acoustic ringing from the
probe. A 4-mm Bruker MAS probe was used without sample spinning.
The effective 90◦ pulse was of duration 1.7 µs. High power 1H decou-
pling (70 kHz) was applied in all static experiments. A liquid H2O sample
was used for both RF power calibration and 17O chemical shift referenc-
ing (δ = 0 ppm).
ful, as reported previously in the case of 2-CO17O2,14 and in
the present work for 1-17O2, providing the benchmark 17O NMR
chemical shifts (δ = 260 ppm for 1-17O2 and 170 ppm for 2-
CO17O2) (Fi−g. 5). As seen in Fi−g. 3, 70% 17O-enriched samples
of HOOCO2 and −O2COOCO2 generated in DMF solution at
−78 ◦C from the reaction of 1-17O2 and 13CO2 resulted in a broad
17O NMR resonance at δ = 275.3 ppm, assigned as overlapping
−
signals of HOOCO2 and −O2COOCO2−. Upon gradual warming
◦
of the sample to −10 C, the intensity of the signal decayed; the
signal ultimately resolved into two peaks with equal intensities
at δ = 278.7 and 264.0 ppm, distinct from those observed for
1-17O2 and 2-CO17O2. The two peaks observed are attributed to
HOOCO2− which contains two−chemically inequivalent 17O atoms
(δ = 278.7 ppm for HOOCO2 and 264.0−ppm for HOOCO2−),
in contrast to the situation for −O2COOCO2 in which the peroxy
oxygen atoms are related by symmetry. The appearance of the
to OPPh3 (Fig. 6, OAT pathway). In contrast, addition of CO2 to
a solution of 1-18O2 in the presence of the hydrogen atom donor
9,10-dihydroanthracene (DHA) led to a statistical mixture of an-
thraquinone products with 16O and 18O incorporation.15 The ob-
served isotope scrambling is likely due to a sequence of H-atom
abstraction/radical recombination reactions. By analogy to the
behavior of organic peroxydicarbonates,26 symmetrical peroxy-
dicarbonate would be expected to undergo O–O bond homoly-
sis generating two equivalents of the reactive carbonate radical
CO3•− (Fig. 5, hydrogen atom transfer (HAT) pathway).27 Quan-
tum chemical calculations indicate that homolytic cleavage of the
−
relatively sharp 17O NMR signals assigned to HOOCO2 coupled
with the concurrent observation of monodeprotonated cryptand
)
([mBDCA-5t-H5]1− 14 by 1H NMR spectroscopy strongly suggests
that HOOCO2− is not strongly sequestered inside the anion recep-
tor. The observed 17O NMR chemical shifts are in accord with ex-
pectations arising from 17O NMR absolute shielding calculations
(Please see Table 1) and compare well with data for benchmark
organic compounds containing the peroxy functional group.23
−
O–O bond in −O2COOCO2 is only mildly endergonic (reaction
free energy +14 kcal/mol); this species thus has an unusually
weak O–O bond.
•−
Homolytic cleavage of the O–O bond and generation of CO3
Mechanism of CO2/peroxide driven oxidation
appears to be favorable for two reasons: (i) repulsion of the
negative charge due to poor solvation in organic solvents result-
ing in Coulombic explosion28 and (ii) resonance stabilization of
the unpaired electron of carbonate radical anion over the car-
bonate π system. Carbonate radical has been generated previ-
ously via laser photolysis of aqueous persulfate in the presence
of bicarbonate.29 Carbonate radical has been implicated in gua-
nine oxidation27 and is also believed to be formed upon treat-
ment of peroxynitrite (ONOO−) with CO2, in that case gener-
ating nitrogen dioxide as a byproduct.27,30,31 Furthermore, in
manganese-catalyzed oxidation of amino acids by H2O2, forma-
Having thereby established the identity of the active oxidants gen-
2−
−
and CO2 as HOOCO2 and
erated from combination of O2
−O2COOCO2−, we next turned our attention to the mechanism
of CO2/peroxide driven oxidation. The reaction of 18O-labeled
1 and CO2 was performed in the presence of an oxidizable sub-
strate. Exposure of a mixture of 1-18O2 and PPh3 to CO2 furnished
18OPPh3 as the oxidized product based on GCMS analysis.15 The
obtained 18O isotope labeling data preclude the possibility of O–O
bond cleavage prior to the oxygen atom transfer (OAT) reaction,
as such a process would yield isotopic scrambling and result in
a mixture of 16OPPh3 and 18OPPh3. Therefore, H18O2CO2–with
its peroxy unit intact as it was derived from peroxide dianion–is
implicated as the active species for the OAT conversion of PPh3
−
tion of reactive oxygen species occurred only when HCO3 buffer
was used.32,33 Carbonate radicals generated in lithium-oxygen
batteries can then engage in HAT reactions with solvents contain-
1–7 | 3

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Table 1 Experimental solid-state 17O NMR and ADF computational results on [K2(DMF)3][17O2⊂mBDCA-5t-H6], [K2(DMF)3][C17O3⊂mBDCA-5t-H6],
and related compounds.
Compound
δ
iso/ppma
260
308
170
223
221
180
182
227
δ
11/ppm
335
388
266
335
398
—
δ
22/ppm
335
388
194
222
398
—
δ33/ppm CQ/MHz ηQ/MHz
1
Exp
ADF
Exp
110
148
50
−16.6
−17.5
7.5
0.0
0.000
0.7
2
ADF
112
−133
—
7.03
0.95
2−
O2
Exp
−18.66
−16.31
−16.81
−18.66
0.000
0.687
0.969
0.00
H2O2
Expb
ADF
Expc
383
352
211
352
−48
Li2O2
−23
[a] The uncertainties in experimental data are: δiso ꢀ 2 ppm; δiiꢀ 10 ppm; CQ ꢀ 0.2 MHz; ηQ ꢀ 0.1. [b] See reference.24 [c] See reference.25
(b)
(a)
ing weak C-H bonds, driven by the high O-H bond strength (BDE
107 kcal/mol) of the bicarbonate that is formed.27 Accordingly,
∼
=
we suggest that for stability under lithium-O2 cell cycling condi-
tions, an organic solvent/electrolyte should have no C-H bonds of
∼
BDE 107 kcal/mol or less.
=
To experimentally confirm the generation of CO3•−, 1 was
treated with CO2 in the presence of the spin trap 5-tert-
butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO) with reac-
tion monitoring by EPR spectroscopy.34 Upon addition of the
CO2, signals for the hydroxyl adduct [BMPO–OH]• together with
small quantities (ca. 5%) of an unidentified spin-trap adduct sus-
pected to be [BMPO–OCO2]•−, were observed within seconds
−
(c)
Boc
Boc
O−
H
Boc
Boc
OCO2
H
Boc
CO3
−CO2
N+
O−
N
O
N
O
(Fig. 7). Formation of [BMPO–OH]• is proposed to occur via
•−
H+
rapid reaction between chemically generated CO3
and BMPO
yielding initially [BMPO–OCO2]•−, followed by decarboxylation
and protonation; the proton source under these conditions could
be the anion receptor mBDCA-5t-H6 (Fig. 7C). This sequence is
directly along the lines proposed for the related spin trap DMPO
under exposure to carbonate radical.35,36 On longer timescales,
[BMPO–OH]• was further oxidized to [BMPO–O]• and other
unidentified decomposition products.34
[O]
OH
H
O
N
O
N
O
Fig. 7 (a) X-band EPR spectra of: pristine BMPO in DMF (red), BMPO+1
without adding CO2 in DMF (purple), and exposure of BMPO+1 to CO2
in DMF (black). (b) Simulation of the EPR spectra of BMPO+1+CO2 in
DMF by linear combination of the contribution from: [BMPO–OCO2]•−
(green), [BMPO–OH]• (red), and [BMPO–O]• (yellow). (c) Formation of
[BMPO–O]• from [BMPO–OH]• and an oxidant “[O]”
O−
CO2
−O
1
2
−O O−
+
—
O2
OER pathway
Activation of solid Li2O2 with CO2 in aprotic organic media
To examine the effect of CO2 on the oxidative power of Li2O2
under conditions relevant to the charging of lithium-air cells,
commercially available solid Li2O2 was exposed to CO2 (1 atm,
25 ◦C, 48 h) in 1,2-dimethoxyethane (DME). In contrast to the
results from control experiments carried out similarly but in the
absence of CO2, substantial amounts of methyl methoxyacetate
were identified among the products of DME oxidation (Fig. 8).
Approximately 51% of the Li2O2 had been consumed, and
quantitative conversion of consumed Li2O2 to Li2CO3 (based
upon lithium) was observed by 13C NMR spectroscopy and total
inorganic carbonate (TIC) analysis.15 The consumed peroxide
must generate an oxidizing equivalent; 74% was identified as
evolved O2 and 15% as methyl methoxyacetate (Fig. 8), with
the balance unidentified. We also introduced solid Li2O2 into
neat DMSO under a CO2 atmosphere (1 atm, 25 ◦C, 48 h), given
the reported use of DMSO in lithium-O2 cells.37 More than 90%
O
Regenerating CO2
CO2
HAT pathway
O
O
O
O
O−
CO2
−O
via CO3
O−
1
−
—
+
2 CO2
HCO3
O
O
−O
1
1
1
2
—2 R2CH2
2−
—2 R2C=O + —CO3
H+
S
SO
O
O
O−
O−
OAT pathway
HO
HO
O
Fig. 6 Proposed mechanistic pathways for CO2-mediated solvent de-
composition in lithium-O2 batteries. OER is an “oxygen evolving reac-
tion”, HAT is a “hydrogen atom transfer” oxidative process involving hy-
drogen atom abstraction by carbonate radical anion, and OAT is “oxygen
atom transfer” to a substrate, S.
4 |
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of the Li2O2 consumed participated in the conversion of DMSO
iments suggest that the oxidation of 4-oxo-TEMP is most likely
due to CO2/peroxide-derived oxidants as opposed to singlet oxy-
gen formation or at a minimum suggest that 4-oxo-TEMP is not a
selective probe for singlet oxygen in Li-O2 cells under conditions
of CO2 availability.
to DMSO2 (Fig. 8). Viewed in the context of cycling lithium-O2
cells, the rate of CO2-induced solvent decomposition in bona
fide lithium-air cells is perhaps lower than that observed in the
current study due to the difference in CO2 partial pressures.
Nonetheless, considering the low cycling rate and long cycling
time of a typical lithium-air battery,38 our findings underscore
that extensive oxidative degradation of the electrolyte in a cell
will occur during cell cycling even when a small amount of CO2
is introduced or otherwise generated in the system.39 During
cell cycling, CO2 is generated at the surface of lithium peroxide
impregnated carbon electrodes,39 leading us to speculate that the
proposed chemistry (Fig. 8) should be expected to occur as well
on a polarized electrode/electrolyte interface. It should be noted
that while commercial Li2O2 was used in the present study, it is
conceivable that the varied morphologies of electrochemically
generated Li2O2 may react with CO2 at different rates. Due to
the preponderance of conditions which result in varied Li2O2
crystallinity, size and surface structure,40,41 commercial Li2O2
was chosen as an ideal benchmark reactant with CO2.
Conclusions
While previous studies on lithium-O2 batteries have attributed
the low cycling number and capacity fading to singlet oxygen10
and superoxide,4–7 it is now clear that CO2/peroxide-derived oxi-
dants are responsible for carbonate formation by way of the active
−
•−
oxidants HOOCO2 and CO3 via −O2COOCO2−. Since proto-
typical lithium-air cells (ether electrolyte, carbon cathode) lose
5-7% of their capacity to parasitic CO2 formation per complete
cycle39 and have a typical cycling number of ca. 50, the result-
ing CO2/peroxide dianion-derived oxidants are expected to cause
organic electrolyte degradation. This oxidative degradation may
occur both during discharge through reaction of peroxide dianion
with CO2 and during recharge through electrochemical oxidation
of carbonate generating initially carbonate radical (CO3•−). It
has been established that recharge of a lithium-O2 battery regen-
erates CO2 from Li2CO3; however, the mechanism and product(s)
identification of electrochemical Li2CO3 degradation have been
unclear.2 Our studies provide evidence for a mechanistic path-
A recent publication reported that the secondary amine
2,2,6,6-tetramethyl-4-piperidone (4-oxo-TEMP), when present in
a charging lithium-air cell (> 3.5 V vs. Li+/Li), was converted to
the oxyl amine radical 4-oxo-TEMPO, as confirmed by EPR spec-
troscopy. 4-oxo-TEMP has been used in the past as a trap for
singlet oxygen, leading the authors to propose that singlet oxy-
gen was responsible for the observed conversion.10 An alternative
explanation for the production of 4-oxo-TEMPO involves the ox-
idants generated by activating Li2O2 with CO2, considering that
the onset potential of CO2 formation in a typical lithium-air cell is
also 3.5 V.42 Accordingly, we found that exposing Li2O2 to CO2 (1
atm, 25 ◦C, 24 h) in the same solvent and electrolyte as described
in the literature, but without the application of an electrode po-
tential, also resulted in the formation of 4-oxo-TEMPO. Of the
oxidizing equivalents generated during transformation of Li2O2
to Li2CO3, ca. 15% were incorporated into the 4-oxo-TEMPO re-
action product, based on EPR spin quantification. These exper-
way by which carbonate radical anions when generated engage
∼
in C-H abstraction from the solvent (C-H bond 107 kcal/mol,
=
or less)27 and lead to solvent degradation and reformation of
CO2 (Fig 6). The regenerated CO2 sets in motion a decomposi-
tion cycle; therefore if even a small percentage of the total Li2O2
is converted to CO2, extensive oxidative degradation of the elec-
trolyte in a cell will occur over the course of many cycles. If CO2
cannot be excluded from these systems, then it is critical that the
electrolyte and other cell components be invulnerable to reactive
CO2/peroxide-derived oxidants if the full potential of recharge-
able lithium-O2 battery systems is to be realized.
Acknowledgments
This publication is based on work funded by the Robert Bosch
Company and National Science Foundation under CHE-1305124.
The EPR spectroscopy in this study was funded by the National
Science Foundation under CHE-1305124. We thank Carl Brozek,
Ioana Knopf, Wesley Transue and Chong Liu for assistance with
instrumentation. We thank Prof. Yogesh Surendranath (MIT) for
helpful discussion.
74%
O2
MeO
Li2O2(s) + CO2
O
OMe
Li2CO3(s) + [O]
15%
11%
O
O
+ H2O
excess
?
O
S
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Table of Contents Entry
Peroxide dianion reacts with CO2 in polar aprotic organic media to afford hydroperoxycarbonate and carbonate radical anion. These highly
reactive species, if formed in lithium-O2 cell, can lead to cell degradation via oxidation of electrolytes and electrode.
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