Supplementary information
A structured three-dimensional polymer electrolyte with
enlarged active reaction zone for Li–O2 batteries
Nadège Bonnet-Mercier1, Raymond A. Wong1,2, Morgan L. Thomas1, Arghya Dutta1, Keisuke
Yamanaka3, Chihiro Yogi3, Toshiaki Ohta3, and Hye Ryung Byon1*
1
Byon Initiative Research Unit (IRU), RIKEN, Wako, Saitama 351-0198, Japan
2
Department of Energy Sciences, Tokyo Institute of Technology, 4259 Nagatsuta-cho,
Midori-ku, Yokohama 226-8502, Japan
3
Synchrotron Radiation Center, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
E-mail: hrbyon@riken.jp
1
Figure S1. Top-view (a–b) SEM and (c) AFM topography (left) and phase (right) images of
P(EO)20LiTFSI. The scale bars in (c) are 1 μm. The color bar on the right of AFM
topography image indicates the z-axis scale. For SEM images, the P(EO)20LiTFSI film was
coated with a thin layer of gold via sputtering. The AFM images of P(EO)20LiTFSI film, in
the absence of gold-sputtering preparation, show domains with hundreds of nanometers.
2
Figure S2. P(EO)20LiTFSI characterization. (a) Differential scanning calorimetry (DCS) of
P(EO)20LiTFSI (top solid line) and PEO-only (bottom dashed line) films. The melting
temperature of P(EO)20LiTFSI (~55 oC) is significantly lower than the PEO-only membrane
(~67 oC), indicating the contribution of Li salt for the phase transition of PEO. The inset
shows a glass transition (Tg) peak of the P(EO)20LiTFSI in a magnified region of –30 ~ –50
o
C. DCS was performed on a Q20 calorimeter from TA Instruments and temperature was
elevated from –80 to 150°C at a ramping rate of 10 °C min–1 under N2 atmosphere. (b)
Arrhenius plot of P(EO)20LiTFSI measured by electrochemical impedance spectroscopy
(EIS) using a stainless steel (SS) symmetric cell (SS/P(EO)20LiTFSI/SS). The orange dot
denotes an ionic conductivity of ~3.2  10–4 S cm–1 at 55 oC. The EIS (Biologic coupled with
EC-Lab software) measurements were carried out at 0.1 Hz to 1 MHz with an applied voltage
of 10 mV at a temperature range of 25–80°C. (c) Electrochemical impedance response with
respect to temperature using Li symmetric cell (Li/P(EO)20LiTFSI/Li). The interfacial
resistances between the P(EO)20LiTFSI and metallic Li are ~2302, 140, and 92  with
increasing temperature of 25, 55, and 60 °C, respectively. The interfacial resistance is
contributed to the sum of resistances of charge transfer and passivation film on the metallic
Li. The EIS measurements were carried out at 0.01 Hz to 1 MHz at a temperature range of
25–60°C.
3
Figure S3. Low-magnification SEM images of (a–b) CNT and (c–d) CNT/SPE surfaces.
Scale bars are (a, c) 50 μm and (b, d) 3 μm. The 3-D CNT network structures are retained for
the CNT/SPE.
4
Figure S4. Top-view AFM images of CNT/SPE with (a) topography and (b) phase modes.
The scale bars are 150 nm. The color bar on the right of topography image indicates the zaxis scale.
5
Figure S5. AFM topography images of CNT-only film surface. The color bar on the right of
AFM topography image indicates the z-axis scale.
6
Figure S6. The 1st-cycled discharge-recharge profiles of Li–O2 cells under O2 (blue) and Ar
(black) atmosphere at a current rate of 0.05 mA cm–2 and a temperature of 55 oC.
7
Figure S7. The 1st-cycled discharge-recharge profiles of Li–O2 cells at a current rate of 0.05
mA cm–2 with CNT/SPE P(EO)20LiTFSI (solid curve) and CNT/tetraglyme electrolyte
containing 0.5 M LiTFSI (dashed curve). The working temperature is 55 oC for CNT/SPE
and 25 oC for CNT/tetraglyme cells.
8
Figure S8. XANES spectra of Li, O and C–K edges with the fluorescence yield (FY) mode
for as prepared (black) 1DC (red) and 1RC CNT/SPE (blue), acquired at a current rate of
0.05 mA cm–2. The gray spectra are standard powders of lithium compounds. The nDC and
nRC denote n-times cycled electrodes at the end of discharge and recharge, respectively.
9
Figure S9. Thin film XRD patterns of 1DC CNT/SPE acquired at a discharge current rate of
0.01 (purple) and 0.05 mA cm–2 (red) with reference to as-prepared CNT/SPE (black) and
powders of LiOH (gray), and Li2O2 (gray). The newly formed pattern after 1DC at the lower
current rate of 0.01 mA cm–2 exhibits the dominant LiOH reflections (101 at 2 theta of 32.35o,
which is slightly shifted compared with the reference LiOH) and minor Li2O2 (e.g. very small
reflection of 101 at 34.85o) at the end of 1DC. These products are not clearly observed when
the 1DC is carried out at the higher discharge current rate of 0.05 mA cm–2. Thin film XRD
was performed using a Rigaku Smartlab with a Cu Kα source (λCu,Kα1 ≈ 1.542 Å) at a scan
rate of 0.5 degree min–1.
10
Figure S10. 1H NMR spectra of 1DC (red), 1RC (blue), and 2DC (green) CNT/SPE,
acquired at a current rate of 0.05 mA cm–2, with reference to as-prepared CNT/SPE (black).
1
H chemical shift (δ) intensity of formate (HCO2D) at 8.46 ppm, referred to 0.05 wt% TMSP
in D2O, increases from 1DC to 2DC while that of acetate (CH3CO2D) at 1.92 ppm is very
small for all samples. The samples were prepared by immersing of the electrodes in a D2O
(Aldrich) mixed with 0.05 wt% 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt
(TMSP, Aldrich) for 2 days in the glove box. A total 600 μL of D2O/TMSP was collected for
the NMR measurement. 1H NMR spectroscopy analysis was performed using a 500 MHz
Varian NMR system.
11
Figure S11. XRD and XANES spectra of 1DC CNT/SPE with dried SiO2 nanofiller at 300
o
C acquired at a discharge current rate of 0.01 mA cm–2. (a) Thin film XRD pattern with as-
prepared SPE, LiOH, and Li2O2. The reflections of the as-prepared SPE are occasionally well
pronounced as shown here. (b) XANES spectra of O K edge with the FY (left) and TEY
(right) mode with LiOH, Li2CO3 and Li2O2. All data corroborate the primary 1DC product of
LiOH.
12
Figure S12. 1H NMR spectra of as-prepared, 1DC, 1RC, and 2DC CNT/SPE acquired at a
low current rate of 0.01 mA cm–2. Unlike Figure S11, a significant peak of methyl ether (OCH3) at a 1H chemical shift (δ) of ~3.4 ppm appears for 1DC and 2DC CNT/SPE. The
formate peak is also notable for 1RC and 2DC. Peaks at 1H chemical shift (δ) of ~1.3, ~3.0
and ~4.0 ppm could not be assigned at this time.
13
R
1
O
O
R
R
2
.
RH
R
1
. O R2
O
O2
R
.
1
O
O
RH
.O
R
R
2
O
½O2
peroxide
R
1
hydroperoxide
HO
(a)
formaldehyde
H2C=O
½O2
+
HCO2H
formic acid
R
2
O
O
R
H
O
O
.
O
(b)
1 alcohol
R
OH
+
O
1
O
R
O
R
2
(c)
1
R
O
+
R
OH
alcohol
O
2
1
O
ethanoate
ester
R
O
H
methanoate
ester
+
2
OH
alcohol
2
hemiacetal
oxyl radical
(d)
H
O
R
R
2
.
O
R
.
1
O
.
RH
R
.
R
+
R
RH
O
2
O
R
H
1
O
methyl
ether
H2O + HCO2H
Figure S13. Proposed mechanism of dissociation of PEO matrix in Li–O2 cell: oxidative
decomposition of PEO via a peroxide intermediate. This diagram shows a sub-set of possible
reaction pathways based on reports on the thermal oxidative decomposition of PEO, namely
(a) six-membered ring intramolecular rearrangement1-3, (b) “β scission”
4,5
, (c) C-O scission3
and (d) C-C scission3. Extrapolating this reaction scheme to the Li–O2 battery, the peroxide is
a key intermediate. The radical initiator, R, is assumed in this case to be the superoxide
radical6, or any other nucleophilic species formed during the decomposition. Further
oxidation of the depicted species then leads to increasing quantities of water and formic acid,
resulting in lithium hydroxide and lithium formate1. The presence of a methyl ether in the
NMR spectrum (Figure S12) after discharge can be attributed to reaction route (d) via the
hemiacetal oxyl radical. Moreover, the presence of formic acid in the first recharge and
second discharge can be attributed to reaction route (a) via the hydroperoxide.
14
Figure S14. Linear sweep voltammetry (LSV) of CNT/SPE and CNT in 0.5 M LiTFSI in
tetraglyme at a sweep rate of 1 mV s–1 under Ar atmosphere. The working temperature is 55
o
C for CNT/SPE and 25 oC for CNT/tetraglyme cells. In the CNT/SPE cell, the CNT/SPE
was sandwiched between metallic Li disc and SS current collector. The CNT/tetraglyme cell
was comprised of metallic Li, Celgard and glass fiber separators, CNT electrode and SS
current collector.
15
Figure S15. In situ OEMS result of 1st (dashed lines) and 2nd (solid lines) recharge of Li–O2
cell with CNT/tetraglyme (0.5 M LiTFSI in tetraglyme) at a current rate of 0.2 mA cm–2 and
25 oC.
16
Figure S16. High-magnification AFM images of as-prepared (left column), 1DC (middle
column) and 1RC (right column) CNT/SPE surfaces with topography (top row) and phase
(bottom row) modes. The color bars on the right of topography images indicate the z-axis
scale. The scale bars are 200 nm.
17
Figure S17. Top-view SEM images of (a) as-prepared, (b) 1DC, and (c) 1RC CNT/SPE. The
scale bars are 1 μm.
18
Figure S18. The 2nd (dashed curves) and 3rd (dot curves) cycled discharge-recharge profiles
of CNT/SPE Li–O2 cell at a current density of 0.05 mA cm–2 and 55 °C. The inset shows an
optical image of disassembled CNT/SPE after the cell failure.
19
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20