SUPPLEMENTARY INFORMATION MoS 2 /graphene Composite Paper For Sodium-Ion Battery Electrodes Lamuel David, Romil Bhandavat and Gurpreet Singh* Mechanical and Nuclear Engineering Department, Kansas State University, Manhattan, Kansas, 66506, United States *E-mail: gurpreet@ksu.edu. Tel.: +1-785-532-7085. Fax: +1-785-532-7057 1
(I) ζ POTENTIAL MEASUREMENT - STABILIZATION MECHANISM The ζ potential measured at varying solution ph with the same MoS 2 concentration can provide an understanding of the ph-dependent MoS 2 sheet stability in the solution. Hence, a separate solution of 1 mg ml -1 MoS 2 in superacid (top portion) was used for ζ potential measurements. Accordingly, the total potential energy (V T ) at the surface interaction of two adjacent MoS 2 sheets is the difference in repulsive potential energy (V DLVO 4Aε r ε o κζ 2 e κd ) and attractive van der Waals energy (V vdw Aπρ 2 C/ 2D 4 ). V DLVO for MoS 2 surfaces is determined using the measured ζ potential, surfactant concentration, and distance between two separated sheets, and V vdw is calculated using atomic density and surface energy. The total potential energy is then given by V T V DLVO V vdw (1) Substituting the expressions on the above equation we get the overall potential energy of two parallel, two-dimentional sheets as, V T 4Aε r ε o κζ 2 e κd Aπρ 2 C/ 2D 4 (2) where A is the area of MoS 2 sheet, ρ is the number of atoms per unit area, ε r and ε 0 are the relative permittivity of water (80.1 at 20 C) and the absolute permittivity (8.85 10 12 F m -1 ), respectively, ζ is the experimentally measured surface potential (36.3 mv), D is the distance of sheet separation, ρ 2 C is the estimated surface energy per unit area (approximately 2.06 10 38 J m -2 ), and κ is the double-layer thickness given as κ = 1/[ε r ε 0 kt/2e 2 n 0 ] 0.5 (3) (n 0 = 9.05 10 23 is the number of surfactant molecules per unit volume of solution; e = 1.6 10 19 C). 2
(II) SEM IMAGING SUPPLEMENTARY INFORMATION FIGURE S1. SEM images of (a) rgo and (b) 20MoS 2, (c) 40MoS 2 and (d) 60MoS 2 -raw paper and (e-h) their corresponding SEM cross-sectional images. The inserts in (a-c) are digital photographs demonstrating the flexibility of rgo and rgo/mos 2 paper shown in the corresponding SEM image. TEM images and SAED patterns (insert) of (i) rgo and (j-l) rgo/mos 2 composite corresponding to the SEM images above. 3
SUPPLEMENTARY INFORMATION FIGURE S2. High resolution SEM image of 60MoS 2 composite paper which shows flakes of exfoliated MoS 2 sheets overlapped with rgo sheets. 4
(III) ENERGY-DISPERSIVE X-RAY SPECTROSCOPY OF 60MoS 2 PAPER CROSS-SECTION SUPPLEMENTARY INFORMATION FIGURE S3. (a) SEM cross-sectional image of 60MoS 2 and EDX map of (b) carbon, (c) oxygen, (d) sulfur and (e) molybdenum. SUPPLEMENTARY TABLE S1. Elemental composition obtained by EDX analysis of cross-section of 60MoS 2 paper (obtained for Figure S3). Element Weight percentage (%) Atomic percentage (%) Carbon 53.46 79.78 Oxygen 6.96 7.79 Molybdenum 26.07 4.87 Sulfur 13.51 7.55 5
(IV) X-RAY PHOTOELECTRON SPECTROSCOPY SUPPLEMENTARY INFORMATION FIGURE S4. X-ray photoelectron spectroscopy plot of as-synthesized MoS 2 -SA powder and 60MoS 2 free-standing paper before (red) and after (blue) thermal reduction. 6
(V) RAMAN SPECTROSCOPY OF 60MoS 2 FREE-STANDING PAPER SUPPLEMENTARY INFORMATION FIGURE S5. Raman spectrum of 60MoS 2 freestanding paper showing peaks typical to both MoS 2 and rgo. 7
(VI) X-RAY DIFFRACTION OF 60MoS 2 FREE-STANDING PAPER SUPPLEMENTARY INFORMATION FIGURE S6. XRD of 60MoS 2 free-standing paper showing peaks typical to both MoS 2 (JCPDS #37-1492) and rgo (JCPDS #01-0646). 8
(VII) 1 ST AND 2 ND CYCLE ELECTROCHEMICAL DATA FOR rgo, 20MoS 2, 40MoS 2 and 60MoS 2 -raw SUPPLEMENTARY INFORMATION FIGURE S7. First and second cycle voltage profile of (a) rgo, (c) 20MoS 2, (e) 40MoS 2 and (g) 60MoS 2 -raw paper electrode along with their corresponding (b, d, f, h) differential capacity curves. 9
(VIII) GALVANOSTATIC INTERMITTENT TITRATION STUDIES: rgo and 60MoS 2 ELECTRODES Galvanostatic intermittent titration technique or GITT experiment was performed for the two electrode specimens: rgo and 60MoS 2 in their second cycle. In the GITT experiment, charge was inserted (or withdrawn) by applying a current pulse of 25 ma g -1 for 15 min, followed by a 4 h of relaxation between pulses during which the change in potential with time was measured. Magnitude of voltage increased during the relaxation period in the insertion half but it decreased in the extraction half of the cycle. The open circuit voltage (OCV) at the end of each relaxation is considered as the thermodynamic equilibrium potential. Further, the reaction resistance for rgo and MoS 2 electrode specimen was calculated for both the discharge and charge half of the GITT experiment by simply calculating the ratio of overpotential to the current density at each pulse. The data is summarized in Fig. S8. 10
SUPPLEMENTARY FIGURE S8. Galvanostatic titration cycling data (solid line: transient voltage profile, symbol: equilibrium OCV) for (a) rgo electrode, and (b) 60 MoS2 electrode. Reaction resistance during Na-insertion in (c) rgo and (d) 60 MoS2. Reaction resistance for Na-extraction from (e) rgo and (f) 60 MoS2 electrodes. 11
(IX) 1 ST AND 2 ND CYCLE ELECTROCHEMICAL DATA FOR 90% MoS 2 SUPPLEMENTARY INFORMATION FIGURE S9. (a) First and second cycle voltage profile of 90% MoS 2 composite paper and (b) its differential capacity curves. 12
(X) DIGITAL AND SEM IMAGES OF ALL ANODES AFTER ELECTROCHEMICAL TESTING SUPPLEMENTARY INFORMATION FIGURE S10. Post electrochemical analysis of the cycled paper electrodes: (a through e) Digital camera images of rgo, 20MoS 2, 40MoS 2, 60MoS 2 and 60MoS 2 -raw composite anodes, respectively. (f through j): Low magnification SEM images, and (k through o): High magnification SEM images of the dissembled electrode surface. 13
(XI) ENERGY-DISPERSIVE X-RAY SPECTROSCOPY OF 60MoS 2 AFTER ELECTROCHEMICAL TESTING SUPPLEMENTARY INFORMATION FIGURE S11. (a) SEM cross-sectional inmage of 60MoS 2 and EDX map of (b) carbon, (c) oxygen, (d) sodium, (e) sulfur and (f) molybdenum. SUPPLEMENTARY INFORMATION TABLE S2. Elemental composition obtained by EDX analysis of top surface of 60MoS 2 paper after electrochemical cycling (obtained from Figure S11). Element Weight percentage (%) Atomic percentage (%) Carbon 11.83 24.06 Oxygen 27.43 41.87 Molybdenum 34.91 8.89 Sulfur 7.52 5.73 Sodium 18.31 19.46 14
(XII) X-RAY PHOTOELECTRON SPECTROSCOPY (XPS) OF 60MoS 2 AFTER ELECTROCHEMICAL TESTING (IN THE SODIATED STATE) SUPPLEMENTARY FIGURE S12. (a) X-ray photoelectron survey spectrum of MoS 2 -rgo composite paper anode before and after discharge cycle shows additional Na peaks. (b,c) High resolution elemental analysis showing characteristic Mo metal and S-Na peaks due to the Na/MoS 2 conversion reaction at the end of discharge cycle. Reference: NIST XPS Database. 15
SUPPLEMENTARY INFORMATION TABLE S3. Comparison of properties of rgo, 20MoS 2, 40MoS 2 and 60MoS 2 free-standing composite paper electrode. Specimen Average Conductivity S cm -1 1 st cycle charge capacity mah g -1 1 st cycle efficiency % Charge capacity at 20 th cycle mah g -1 rgo 2.97 81.5 10.2 70.5 20MoS 2 1.86 139 40.2 123 40MoS 2 1.17 263 66.4 171.8 60MoS 2 0.47 338 72.2 218.2 16
SUPPLEMENTARY INFORMATION FIGURE S13. Schematic representation showing the predicted mechanism for Na insertion and extraction into an idealized MoS 2 /rgo free-standing composite paper electrode. rgo provides the mechanical/ structural stability and high electrical conductivity network to the TMDC undergoing insertion and conversion reactions with Na-ions. 17