ARTICLES
NPG electrode, was purchased from Noris Blattgold GmbH, Germany. Super P
carbon black was obtained from Timcal.
removed at the end of charge shown in Figs 2 and 3, FTIR and in-situ
SERS data are shown in Fig. 4. The FTIR data were collected on cath-
odes that were removed from the cells immediately at the end of dis-
charge and charge, and the SERS data were collected in situ. The data
demonstrate that in the presence of TTF the discharge product on
each cycle is Li2O2 and that this is oxidized completely at the end
of each charge. The previous study of LiClO4 in DMSO at a NPG elec-
trode without TTF showed very small quantities of Li2CO3 and
HCO2Li (ref. 24). Interestingly, there is little or no evidence of
The preparation of the NPG electrode was as described in previous papers24,34,42
.
The NPG electrode was prepared by dealloying white gold sheet by floating it on a
bath of concentrated nitric acid for 30 minutes. This process resulted in a
freestanding film of NPG. The NPG was dried by heating under vacuum at 150 8C
overnight. The NPG electrode was mounted on a stainless steel mesh. The active area
of the NPG cathode was 50 m2 g21, as determined by potential scanning between 0.2
and 1.5 V versus Ag/AgCl in 0.5 M H2SO4 according to an established procedure43
Partially charged LiFePO4 was used as the anode. The preparation of the
composite electrode 95% LiFePO4, 4% PTFE and 1% Super P carbon is described
.
these species in the presence of TTF; it may be that the more-efficient elsewhere34. The capacity of the LiFePO4 electrode is 1–10 mAh per piece. The cells
were assembled in an Ar-filled glovebox. A glass-fibre separator was impregnated
oxidation of Li2O2 by the redox mediator reduces further the already
small degree of electrolyte decomposition. Taken together, the elec-
trochemistry, DEMS, FTIR and SERS data show that a Li–O2 cell con-
with the electrolyte (1 M LiClO4 in DMSO with and without TTF) and placed
between the NPG cathode and the LiFePO4 anode. Volumes of electrolyte ranged
from 0.04 to 0.4 ml, and 10 mM TTF were employed to give 0.4 and 4 mmol TTF,
taining a redox mediator, such as TTF, can be cycled 100 times at
rates that are impossible in the absence of TTF.
respectively. The maximum TTF solubility was greater than 1 M. The NPG
electrodes used ranged from 0.15 to 5 mg cm22. The cell was placed in 1 atm O2
(purity N6.0). Electrochemical measurements were conducted using a VMP3
electrochemical workstation (Biologic).
The process of mediating oxidation and the reason for the
enhanced oxidation kinetics may be summarized as follows. Solid
insulating Li2O2 particles within the pores of a porous electrode,
but not in close contact with the surfaces of the pore wall, suffer
from hindered charge transport during oxidation, which in the
absence of TTF leads to severe polarization on charging, even at
modest rates, as shown in Fig. 2. The TTF molecules, however,
being dissolved in the electrolyte, easily undergo oxidation at the
surfaces of the pore wall; they can then diffuse the short distance
from these surfaces to Li2O2 particles within the pores, where they
oxidize the Li2O2 particles. The overall effect is a much-reduced
polarization because of the facile TTF oxidation and faster (chemi-
cal) oxidation of the Li2O2 particles within the pores than is the case
with direct electrochemical oxidation.
Cyclic voltammetry was carried out on the redox mediators in a three-electrode
cell and with a platinum counter electrode separated by a sintered glass frit. A 5 mM
solution of each redox mediator in 0.1 M TBAClO4 (tetrabutylammonium
perchlorate) in DMSO was used as the electrolyte. The gold working electrode was
polished carefully prior to use. After each measurement, the silver wire reference
electrode was calibrated by 5 mM FC in 0.1 M TBAClO4 in DMSO. The redox
potential obtained against FC was converted into the Li/Liþ scale. The cell was
assembled in an Ar-filled glovebox. Electrochemical measurements were made
under Ar and O2; the gases were bubbled through the electrolyte for ten minutes
prior to the measurements.
To investigate the ability of the redox mediators to oxidize Li2O2, each redox
mediator was dissolved in 0.1 M LiClO4 in DMSO to a concentration of 5 mM, and
then subjected to oxidation such that the quantity of oxidized mediator was
equivalent to 0.03 mAh of charge. The solution was injected into a vial with excess
Li2O2, and the vial was connected to the mass spectrometer with PEEK tubing.
The flow of argon carrier gas was 0.1 ml min21
.
The DEMS system was built in-house and employed a commercial quadrupole
mass spectrometer (Thermo Fischer) with a turbomolecular pump (Pfeiffer
Vacuum). The set-up was based on a previously described design24,34. Calibration
Conclusions
By incorporating a redox-mediating molecule (here illustrated with
TTF) in the electrolyte of the rechargeable non-aqueous Li–O2
battery, we demonstrate that it is possible to recharge such a
battery at current densities (1 mA cm22) that are impossible
without the redox mediator in the same cell. The TTF molecule is
oxidized to TTFþ at the positive electrode and then, in turn, oxidizes
the insulating solid Li2O2 (which formed in the porous cathode
during the previous discharge); in the process, TTFþ is reduced
back to TTF. By using a molecular electron–hole transfer agent a
far more effective oxidation of Li2O2 is possible than can be achieved
in its absence. The reversible formation of Li2O2 is an essential
process at the cathode in such rechargeable cells. Cells that
contain TTF were cycled 100 times with complete reversibility of
Li2O2 formation/decomposition on each cycle. Although TTFþ
could diffuse to the anode, the distance is much greater than that
between Li2O2 and the cathode surface. Also, it is probable that,
as for the aqueous Li–O2 cells, the lithium anode will have to be pro-
tected by an ionically conducting but electronically insulating mem-
brane (such as Ohara, LISICON, glass ceramic) to avoid reduction
of O2 by the anode. Such protection would also eliminate any
internal reduction of the redox mediator. The ability of a redox
mediator to oxidize Li2O2 efficiently and promote recharging, and
hence cycling, of the non-aqueous Li–O2 battery addresses one of
the important challenges that face the rechargeable non-aqueous
Li–O2 cell.
and quantification methods were as described in detail previously24,34
.
FTIR spectra were collected on a Nicolet 6700 spectrometer (Thermo Fisher
Scientific) using an attenuated total reflectance unit in a N2-filled glovebox24,34,42. For
the in-situ SERS the gold working electrode was placed behind a 1 mm thick
sapphire window using a spectroelectrochemical cell described previously24,42
.
Electrochemical measurements were carried out using a CHI600C electrochemical
workstation (CHI instruments). A near infrared diode laser (Toptica Photonics) at
l ¼ 785 nm (maximum power 300 mW) was used. The laser beam was coupled to
the excitation part of the Raman probe (Emvision LLC) after passing through a line
filter. The collection part of the probe was then coupled to the spectrometer (Jobin
Yvon iHr 550) using an F-number matcher. The F-number of the spectrometer was
6.4, and that of the collection fibre was 2.27. Hence a pair of lenses with focal lengths
16 and 8 mm, respectively, was used in the F-number matcher, between which an
edge filter (SEMROCK) was inserted to filter out 785 nm photons. The spectrometer
was equipped with a 300 lines mm21 grating and a liquid nitrogen cooled CCD
camera (CCD-20483512-FIVS-3LS).
Received 5 November 2012; accepted 28 March 2013;
published online 12 May 2013
References
1. Abraham, K. M. & Jiang, Z. A polymer electrolyte-based rechargeable
lithium/oxygen battery. J. Electrochem. Soc. 143, 1–5 (1996).
2. Bruce, P. G., Freunberger, S. A., Hardwick, L. J. & Tarascon, J-M. Li–O2 and Li–S
batteries with high energy storage. Nature Mater. 11, 19–29 (2012).
3. Shao, Y. Y. et al. Electrocatalysts for nonaqueous lithium–air batteries: status,
challenges, and perspective. ACS Catal. 2, 844–857 (2012).
4. Girishkumar, G., McCloskey, B., Luntz, A. C., Swanson, S. & Wilcke, W.
Lithium–air battery: promise and challenges. J. Phys. Chem. Lett. 1,
2193–2203 (2010).
Methods
DMSO was distilled with NaNH2 under vacuum, and then further dried for several
days over activated molecular sieves (type 4 Å). The molecular sieves (Aldrich) were
first washed with acetone, dried in the oven at 150 8C for 12 hours and then placed in
a drying tube and further dried at 300 8C with a Bu¨chi oven under vacuum for 24
hours. The drying tube with sieves was transferred into an Ar-filled glovebox without
exposure to air. The final water content of the DMSO after drying was ,4 ppm
(determined using a Mettler Toledo Karl Fischer titration apparatus). Battery-grade
lithium perchlorate (LiClO4) was dried at 160 8C under vacuum for 24 hours prior to
use. 12-Carat white gold leaf (Au:Ag 50:50, m/m), the precursor used to prepare the
5. Scrosati, B., Hassoun, J. & Sun, Y. K. Lithium-ion batteries. A look into the
future. Energy Environ. Sci. 4, 3287–3295 (2011).
6. Christensen, J. et al. A critical review of Li/air batteries. J. Electrochem. Soc. 159,
R1–R30 (2012).
7. Chen, Z., Choi, J. Y., Wang, H. J., Li, H. & Chen, Z. W. Highly durable and active
non-precious air cathode catalyst for zinc air battery. J. Power Sources 196,
3673–3677 (2011).
8. Black, R., Adams, B. & Nazar, L. F. Non-aqueous and hybrid Li–O2 batteries.
Adv. Energy Mater. 2, 801–815 (2012).
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