A stable cathode for the aprotic Li–O Supplementary ......All solvents had a final water content...

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Supplementary Materials

A stable cathode for the aprotic Li-O2 battery

Muhammed M. Ottakam Thotiyl1, Stefan A. Freunberger1,2, Zhangquan Peng1,3

Yuhui Chen1, Zheng Liu1 and Peter G. Bruce1*

1 School of Chemistry and EastChem, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16

9ST, UK

2 Current addresses: Institute for Chemistry and Technology of Materials, Graz University of Technology,

Stremayrgasse 9, 8010 Graz, Austria.

3 Current addresses: State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, 130022 Changchun, P.R. China.

Methods

HPLC grade dimethyl sulfoxide (DMSO) and tetraethyleneglycol dimethylether

(TEGDME) were distilled over a packed bed column under vacuum and transferred into an Ar

filled glove box without any exposure to air. DMSO was distilled with added sodium amide. The

distilled solvents were stored over activated type 4Å molecular sieves in an Ar-filled glove box.

All solvents had a final water content of <4 ppm, (determined using a Mettler-Toledo Karl-

Fischer titration). The molecular sieves (Aldrich) were first washed with acetone, dried in the

oven at 150 oC overnight then placed in a drying tube and further dried at 300 oC with a Büchi

oven under vacuum for 24 h. The drying tube with sieves was transferred into an Ar-filled glove

box without exposure to air. Battery grade LiClO4 (Aldrich 99.99%) and electrochemical grade

A stable cathode for the aprotic Li–O2 battery

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LiPF6 (Stella 99.99%) salts were used for preparing the electrolytes. Prior to use LiClO4 and

LiPF6 salts were dried under vacuum for 24 h at 160 and 120 °C, respectively, and then

transferred to an Ar filled glove box without any atmospheric contact.

The electrochemical cells used to investigate cycling were based on a Swagelok design.

Titanium carbide (Skyspring nanomaterials, 40 nm particle size, BET surface area 15 m2/g), β-

silicon carbide (Skyspring nanomaterials, 40 nm particle size, BET surface area 20 m2/g) and

titanium nitride nanoparticles (American elements, 20 nm particle size, BET surface area

22m2/g) were dried under vacuum at 200°C for 24 hours and then transferred without exposure

to air into an Ar filled glove box. The cathodes were fabricated by first making a slurry with

PTFE binder in the ratio 95:5 (m/m) using isopropanol. The slurry was then coated onto a

stainless steel mesh current collector. The electrodes were vacuum dried at 200°C for 24 hours

and then transferred to an Ar filled glove box without exposure to air. Typical loading of TiC

composite electrodes was 4mg/cm2 on a12 mm diameter stainless steel current collector.

LiFePO4 was used as the anode. The preparation of the composite electrode 95 %

(LiFePO4) 4 % (PTFE) and 1 % (Super P carbon), has been described previously (2-4). The

capacity of the LiFePO4 electrode is in 5-10 fold excess of the cathode. The cells were assembled

in an Ar-filled glove box. A glass fibre separator was impregnated with the electrolyte (0.5 M

LiClO4 in DMSO and 0.5 M LiPF6 in TEGDME) and placed between the cathode and LiFePO4

anode. Volumes of electrolyte used were 0.1 mL. The cell was placed in 1 atmosphere of O2. The

cell was gas tight except for the stainless steel window that exposed the cathode to the O2

atmosphere. Electrochemical measurements were conducted using a VMP3 electrochemical

workstation (Biologic). All potentials are reported vs. Li/Li+.

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The DEMS system was built in-house and employed a commercial quadrupole mass

spectrometer (Thermo Fischer) with a turbomolecular pump (Pfeiffer Vacuum). The setup was

based on a previously described design (1,2). Calibration, and quantification methods were as

described in detail previously (1,2).

Examination of electrodes involved first disassembling the cell, rinsing the cathode thrice

with dry acetonitrile and removing the acetonitrile under vacuum. Powder X-ray diffraction

(PXRD) was carried out using a Panalytical diffractometer operating in reflection mode with a

primary beam monochromator and position sensitive detector. Cu Kα1 radiation (λ=1.5406 Å)

was employed. The samples were contained in a low background Si substrate holder and then

sealed using a Kapton polyimide film. For solution phase 1H NMR analysis the previously rinsed

and dried electrodes were extracted with D2O and then the extracted solution examined on a

Bruker Avance II 400 spectrometer.

FTIR spectra were collected on a Nicolet 6700 spectrometer (Thermo Fisher Scientific)

in a N2-filled glove box. We prepared mechanical mixtures of Li2O2 with Li2CO3 and Li2O2 with

HCO2Li of varying ratios, collected their FTIR spectra, and constructed calibration curves (Fig

S2); from these curves, we determined the fractions of Li2CO3 in the FTIR spectra (Fig. 2) (1).

X-Ray photoelectron spectroscopy (XPS) data were collected on a Scienta ESCA-300 with a

rotating Al anode operating at 12KV x 150mA (1800W) power, an energy of 1486.6 eV and a

resolution of ~0.7eV. The energy scale was calibrated to the Au4f7/2 peak at 83.95eV.The

samples were mounted by adhesion to double sided, electrically conducting tape. Analysis of the

collected data was performed using the CasaXPS software. The spectral patterns are decomposed

into component signals using Gaussian-Lorenzian peak approximations and Shirley background

reduction.




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Details of carbonate/carboxylate analysis, may be found elsewhere (2). Briefly the

cathodes at different states of discharge and charge were removed and washed in acetonitrile in

an Ar filled glove box and then dried under vacuum. The cathodes were then treated with acid to

decompose the inorganic carbon (Li2CO3) and Fenton’s reagent to decompose the organic carbon

(Li carboxylates). The CO2 evolved in each case was measured by mass spectrometry.

REFERENCES

1) Peng, Z.; Freunberger, S. A.; Chen, Y.; Bruce, P. G. Science 2012, 337, 563.

2) Ottakam Thotiyl, M. M.; Peng, Z.; Freunberger, S. A.; Bruce, P. G. J. Am. Chem. Soc.

2013, 135, 494.

3) Chen, Y; Freunberger, S; Peng, Z; Barde, F; Bruce, P. J. Am. Chem. Soc. 2012, 134, 7952.

4) Bryantsev, V; Uddin, J; Giordani, V; Walker, W; Addison, D; Chase, G. J. Electrochem. Soc. 2013, 160, A160.





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Figure S1: Galvanostatic charge-discharge cycles recorded for a TiC cathode in 0.5 M LiClO4 in DMSO at a geometric current density of 0.5 mA/cm2. The slight differences noted in the curves at the end of discharge and charge are not due to side-reactions, the extent of which are the same as at 1 mA/cm2. We anticipate that such differences are due to mechanical changes in the electrode associated with the greater quantity of Li2O2 at 0.5 mA/cm2 compared with 1 mA/cm2. In order to compare directly the cycling in DMSO, shown here, with that in TEGDME (Fig. 1c) under the same conditions of current density and capacity, it was necessary to limit the capacity in Fig. S1 to a similar value of that in Fig. 1c.





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Figure S2: FTIR calibration curves obtained by mixing known amounts of (a) Li2CO3 and Li2O2

and (b) HCO2Li and Li2O2 with different ratios.

a b





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Figure S3: 1H NMR data obtained by washing the TiC cathodes with D2O at the end of discharge and charge in DMSO and TEGDME based electrolytes.





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Figure S4: 1H NMR data obtained by washing a nanoporous Au cathode with D2O at the end of 1st discharge in DMSO based electrolyte.





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Figure S5: In-situ DEMS at a TiC cathode during (a) discharge and (b) charge in 0.5 M LiClO4-DMSO. Linear potential scans at 0.05 mV/s were used. Discharge was carried out in Ar:O2 (5:95 v/v) gas mixture and charge was carried out in pure Ar atmosphere. The charge to mass balance presented in Table 1 was obtained by holding the charging potential at 4 V until oxygen evolution ceased.





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Figure S6: In-situ DEMS at a TiC cathode during (a) discharge and (b) charge in 0.5 M LiPF6 -TEGDME. Linear potential scans at 0.05 mV/s were used. Discharge was carried out in Ar:O2 (5:95 v/v) gas mixture and charge was carried out in pure Ar atmosphere. The charge to mass balance presented in Table 2 was obtained by holding the charging potential at 4.1 V until oxygen evolution ceased.





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Figure S7: CO2 flux from in-situ DEMS data while charging a TiC cathode without prior discharge, in an Ar:O2 gas mixture (5:95 v/v) in 0.5 M LiPF6 in TEGDME. .





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Fig. S8. (a) Galvanostatic discharge and charge for a SiC cathode in 0.5M LiClO4 in DMSO, geometric current density 0.1 mA/cm2. (b) FTIR spectra of the SiC cathode showing formation of Li2O and Li2O2 on discharge. The Li2O is not oxidized up to 4.25 V on charge.

Fig. S9. In-situ DEMS of a SiC cathode during the 1st discharge-charge cycle in 0.5 M LiClO4 in DMSO. Potential scan rate 0.05 mV/s between 2 and 4.2 V. Current is expressed as a flux of electrons. Discharge-charge was carried out under an Ar:O2 (5:95 v/v) gas mixture.





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Fig. S10. Galvanostatic discharge and charge for a TiN electrode in 0.5M LiClO4 in DMSO at a geometric current density of 0.1 mA/cm2. As polarization on charge is immediate, the charging curve is coincident with the y-axis.





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Fig. S11. XPS spectra of cycled (a) TiN (Ti 2p), and (b) SiC (Si 2p). The spectra remain invariant on cycling. The unlabeled higher binding energy peaks correspond to the Ti 2p½ signals.





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Fig. S12. SEM of TiC composite electrode (a) before, (b) after 1st discharge and (c) after 100th discharge in 0.5 M LiCLO4 in DMSO at 0.5 mAcm-2.





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Fig. S13. Comparison of (a) specific capacity and (b) specific energy of the cathode (and therefore the whole cell for a Li-O2 cell with no excess Li) containing respectively carbon, TiC and Au cathodes, as a function of cathode porosity, based on the total weight of the electrode including Li2O2. Note that a given porosity on the x-axis corresponds to filling 80% of that particular pore volume, as 20% of the space is left for the electrolyte.

a b





A stable cathode for the aprotic Li–O Supplementary ......All solvents had a final water content...

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