Supporting Information for

Transcription

1 Supporting Information for A New Class of Lithium and Sodium Rechargeable Batteries Based on Selenium as Positive Electrode Ali Abouimrane, 1,* Damien Dambournet, 1 Karena W. Chapman, 2 Peter J. Chupas, 2 Weng Wei, 1 Khalil Amine 1,* 1 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, USA. 2 X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, USA abouimrane@anl.gov; amine@anl.gov; S1

2 1) Materials and methods Preparation and characterization of the Carbon-Selenium Composite Composite was prepared by mixing selenium and multiwalled carbon nanotubes (Aldrich) in a 7:3 mass ratio. High-energy ball milled mixture (1h) leads to an amorphization of the selenium (Fig S1.A) The material was pressed into a pellet and heated at 260ºC for 12 h in air. The weight loss of < 1% was observed due to the low pressure vapor of Se (~10-2 torr at 260ºC). XRD pattern showed that the treatment allows recrystallization of the selenium. Although this procedure was shown to improve the lithium storage capacity (Figure S1.B), SEM study revealed that the composite was not homogeneous. Large crystals of selenium were indeed observed (Figure S2) showing that the impregnation procedure of Se within MWCNs was not optimum. Further optimization procedures with an emphasis on the impact of the carbon matrix nature on the electrochemical performance are currently underway. Figure S1. A. High energy x-ray diffraction powder patterns recorded for pristine Se, Se- C composite following high energy ball milling and heated at 260 C under air. B. The first discharge curve for pristine Se, Se-C high energy ball milled, and heated at 260 C. S2

3 Figure S2. SEM image of the Se-C composite electrode showing large crystals of selenium as confirmed by EDX analysis (not shown). The carbon selenium-sulfur composite was prepared with a slightly modified procedure. SeS2 and multiwall carbon nanotubes (Aldrich) were mixed with a 7:3 mass ratio, and treated for 15 minutes using a planetary ball miller. Finally, the mixture was heated at 160ºC for 12 hours under air. The weight loss induced by the thermal treatment was below 1.5%. Electrochemical Characterization Energy storage capacity was assessed using coin cells. The electrode was prepared by mixing 70% of active material, 10% carbon black, and 20% polyvinylidene fluoride (PVDF) binder previously dissolved in N-methyl-2-pyrrolidinone (NMP). The slurry was coated on an aluminum foil using a doctor blade. NMP was removed by drying the electrode at 75 C overnight. The cells were assembled in an argon-filled glovebox. Electrode materials were tested against metallic lithium or metallic sodium anode. A Celgard 2300 film was used as separator. The electrolytes consisted of lithium or sodium salts, i.e. LiPF6 (1.2M) or NaPF6 (1M), dissolved in 3:7 ethylene carbonate (EC)/ethyl methyl carbonate (EMC) solvents. The NaPF6 salt was purified by recrystallization in acetonitrile prior to use. S3

4 Cells were cycled using various potential windows ranging from 0.8V to 4.6V under different current densities. The capacity was calculated based on the active mass which corresponds to 50 wt% of the electrode. The electrical conductivity was measured on compressed pellets (diameter of ~ 10 mm) using the four-probe method. 2) Pair Distribution Function (PDF) Analysis For the PDF analysis, discharged and charged cells were dismantled inside a glove box. The cathode was scraped off the current collector, packed in a Kapton capillary, and sealed to prevent exposure to air. The PDF analysis were carried out based on X-ray scattering data measured at the 11-ID-B beamline of the Advanced Photon Source at Argonne National Laboratory. High energy X-rays (λ = Å) were used in combination with a large amorphous-silicon based area detector to collect data to high values of momentum transfer (Q ~ 22 Å -1 ). 1,2 Diffraction images were integrated within fit2d to obtain one-dimensional diffraction data. 3 Pair distribution functions, G(r), were extracted from the data within PDFgetX2, 4 after correcting for background and Compton scattering (Figures S-1 to S-4). The PDF data were refined with PDFgui software (Figures S-1 to S-3 and Table S-1). 5 S4

5 Figure S3. Pair distribution function (black) of elemental selenium, refined profile (red), and residual to the fit (blue). Structure refinement was performed using using Se trigonal structure leading to R w =14.9% Figure S4. Pair distribution function (black) of the fully discharged sample for Li-Se cell, refined profile (red), and residual to the fit (blue). Structure refinement was performed using Li 2 Se structure leading to R w = 23.6% G(r) (Å) r (Å) Na 2 Se Na a Se b (a:b ratio of 1:1) Se Figure S5. Evolution of the Se PDFs patterns during the first discharge redox reaction with sodium. S5

6 Figure S6. Pair distribution function (black) of the discharged sample for Na-Se cell, refined profile (red), and residual to the fit (blue). Structure refinement was performed using Na 2 Se structure leading to R w = 20.6% st discharge 2nd discharge 3nd discharge G(r) (Å -2 ) r(å) Figure S7. PDFs of samples recovered following the first, second, and third discharges of the Li-Se cell. A progressive broadening of features in the PDF suggests a reduction of the crystallinity (that is, decreasing particle size or increasing disorder). These structural changes may account for the evolution of the voltage profile. S6

7 Capacity (mahg -1 ) Li/(Se-CMP) Discharge Charge Discharge Charge Li/(Se-CNT) 301 mahg mahg Cycle number Figure S8. Cycling performance of Li / Se-CNT and Li / Se-CMP cells. Carbon nanotubes provide better performance than mesoporous carbon. 4.0 Na/Se-C Potential νs Na (V) st cycle 2 nd cycle 3 rd cycle Capacity (mahg -1 ) Figure S9. The voltage profile of Na/Se-C system showing the evolution of the voltage with cycling. S7

8 Table S1. Parameters obtained from the PDF refinements. Se A = Li A 2 Se A = Na a (Å) c (Å) V (Å 3 ) 4.343(3) 4.922(5) 80.40(11) (7) (4) 6.774(1) (8) Distances (Å) A-Se Se-Se (7) (3) (5) (5) (9) Space group, Z, Wyckoff and atomic positions P32, Z = 3 Se, 3a, (0.224(1), 0, 1/3) Fm-3m, Z = 4 Se, 4a, (0, 0, 0) X, 8c, (0.25, 0.25, 0.25) Table S2: Theoretical capacity of known selenium-sulfur phases. Material Se Se 5 S Se 5 S 2 Se 5 S 4 SeS Se 3 S 5 SeS 2 SeS 7 S Theoretical capacity (mahg -1 ) P.J. Chupas, X. Qiu, P.L. Lee, J.C. Hanson, C.P. Grey, and S.J.L. Billinge, J. Appl. Crystallogr. 36 (2003) P.J. Chupas, K.W. Chapman, and P.L. Lee, J. Appl. Crystallogr. 40 (2007) A. P. Hammersley, Fit2D Version Reference Manual Version 3.1, ESRF98HA01T, X. Qiu, J.W. Thompson, and S.J.L. Billinge, J. Appl. Crystallogr. 37 (2004) C. L. Farrow, P. J Chupas, J. W. Liu, D. Bryndin, E. S. Bozin, J. Bloch, T. Proffen, and S. J. L. Billinge, J. Phys.: Condens. Matter, 19 (2007) S8