A stable cathode for the aprotic Li O 2 battery

Transcription

1 A stable cathode for the aprotic Li O 2 battery Muhammed M. Ottakam Thotiyl 1, Stefan A. Freunberger 1,2, Zhangquan Peng 1,3 Yuhui Chen 1, Zheng Liu 1 and Peter G. Bruce 1* 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, 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 o C overnight then placed in a drying tube and further dried at 300 o C 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 NATURE MATERIALS 1

2 LiPF 6 (Stella 99.99%) salts were used for preparing the electrolytes. Prior to use LiClO4 and LiPF 6 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 m 2 /g), β- silicon carbide (Skyspring nanomaterials, 40 nm particle size, BET surface area 20 m 2 /g) and titanium nitride nanoparticles (American elements, 20 nm particle size, BET surface area 22m 2 /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/cm 2 on a12 mm diameter stainless steel current collector. LiFePO 4 was used as the anode. The preparation of the composite electrode 95 % (LiFePO 4 ) 4 % (PTFE) and 1 % (Super P carbon), has been described previously (2-4). The capacity of the LiFePO 4 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 LiClO 4 in DMSO and 0.5 M LiPF 6 in TEGDME) and placed between the cathode and LiFePO 4 anode. Volumes of electrolyte used were 0.1 ml. The cell was placed in 1 atmosphere of O 2. The cell was gas tight except for the stainless steel window that exposed the cathode to the O 2 atmosphere. Electrochemical measurements were conducted using a VMP3 electrochemical workstation (Biologic). All potentials are reported vs. Li/Li +. 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION 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= 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 1 H NMR analysis the previously rinsed and dried electrodes were extracted with D 2 O 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 N 2 -filled glove box. We prepared mechanical mixtures of Li 2 O 2 with Li 2 CO 3 and Li 2 O 2 with HCO 2 Li of varying ratios, collected their FTIR spectra, and constructed calibration curves (Fig S2); from these curves, we determined the fractions of Li 2 CO 3 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 ev and a resolution of ~0.7eV. The energy scale was calibrated to the Au4f 7/2 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. NATURE MATERIALS 3

4 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 (Li 2 CO 3 ) and Fenton s reagent to decompose the organic carbon (Li carboxylates). The CO 2 evolved in each case was measured by mass spectrometry. REFERENCES 1) Peng, Z.; Freunberger, S. A.; Chen, Y.; Bruce, P. G. Science 2012, 337, ) Ottakam Thotiyl, M. M.; Peng, Z.; Freunberger, S. A.; Bruce, P. G. J. Am. Chem. Soc. 2013, 135, ) Chen, Y; Freunberger, S; Peng, Z; Barde, F; Bruce, P. J. Am. Chem. Soc. 2012, 134, ) Bryantsev, V; Uddin, J; Giordani, V; Walker, W; Addison, D; Chase, G. J. Electrochem. Soc. 2013, 160, A NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION Figure S1: Galvanostatic charge-discharge cycles recorded for a TiC cathode in 0.5 M LiClO 4 in DMSO at a geometric current density of 0.5 ma/cm 2. 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/cm 2. We anticipate that such differences are due to mechanical changes in the electrode associated with the greater quantity of Li 2O 2 at 0.5 ma/cm 2 compared with 1 ma/cm 2. 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. NATURE MATERIALS 5

6 a b Figure S2: FTIR calibration curves obtained by mixing known amounts of (a) Li2CO3 and Li 2 O 2 and (b) HCO 2 Li and Li 2 O 2 with different ratios. 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION Figure S3: 1 H NMR data obtained by washing the TiC cathodes with D 2 O at the end of discharge and charge in DMSO and TEGDME based electrolytes. NATURE MATERIALS 7

8 Figure S4: 1 H NMR data obtained by washing a nanoporous Au cathode with D 2 O at the end of 1 st discharge in DMSO based electrolyte. 8 NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION Figure S5: In-situ DEMS at a TiC cathode during (a) discharge and (b) charge in 0.5 M LiClO 4 - DMSO. Linear potential scans at 0.05 mv/s were used. Discharge was carried out in Ar:O 2 (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. NATURE MATERIALS 9

10 Figure S6: In-situ DEMS at a TiC cathode during (a) discharge and (b) charge in 0.5 M LiPF 6 - TEGDME. Linear potential scans at 0.05 mv/s were used. Discharge was carried out in Ar:O 2 (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. 10 NATURE MATERIALS

11 SUPPLEMENTARY INFORMATION Figure S7: CO 2 flux from in-situ DEMS data while charging a TiC cathode without prior discharge, in an Ar:O 2 gas mixture (5:95 v/v) in 0.5 M LiPF 6 in TEGDME.. NATURE MATERIALS 11

12 Fig. S8. (a) Galvanostatic discharge and charge for a SiC cathode in 0.5M LiClO 4 in DMSO, geometric current density 0.1 ma/cm 2. (b) FTIR spectra of the SiC cathode showing formation of Li 2O and Li 2O 2 on discharge. The Li 2O is not oxidized up to 4.25 V on charge. Fig. S9. In-situ DEMS of a SiC cathode during the 1 st discharge-charge cycle in 0.5 M LiClO 4 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:O 2 (5:95 v/v) gas mixture. 12 NATURE MATERIALS

13 SUPPLEMENTARY INFORMATION Fig. S10. Galvanostatic discharge and charge for a TiN electrode in 0.5M LiClO 4 in DMSO at a geometric current density of 0.1 ma/cm 2. As polarization on charge is immediate, the charging curve is coincident with the y-axis. NATURE MATERIALS 13

14 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. 14 NATURE MATERIALS

15 SUPPLEMENTARY INFORMATION Fig. S12. SEM of TiC composite electrode (a) before, (b) after 1 st discharge and (c) after 100 th discharge in 0.5 M LiCLO 4 in DMSO at 0.5 macm -2. NATURE MATERIALS 15

16 a b Fig. S13. Comparison of (a) specific capacity and (b) specific energy of the cathode (and therefore the whole cell for a Li-O 2 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 Li 2 O 2. 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. 16 NATURE MATERIALS