Supporting Information

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1 Supporting Information Vapor-Phase Atomic-Controllable Growth of Amorphous Li 2 S for High-Performance Lithium-Sulfur Batteries Xiangbo Meng, David J. Comstock, Timothy T. Fister, and Jeffrey W. Elam * Energy Systems Division, and Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA *Corresponding author: jelam@anl.gov 1. In situ studies of Li 2 S ALD The timing s used for a majority of the in situ quartz crystal microbalance (QCM) measurements was determined to be optimal for the saturation growth of Li 2 S based on studies of the effects of varying the individual LTB and H 2 S exposures, as well as the corresponding purge times as shown in Figure SI-1 in Supporting Information (SI). Prior to the Li 2 S ALD on the QCM, an ALD Al 2 O 3 film was deposited on the QCM surface using alternating trimethylaluminum (TMA, 99%, Aldrich) and deionized H 2 O exposures with the timing sequence s to establish a uniform starting surface. During the first ~ 20 ALD Li 2 S cycles (0-400 s) on the Al 2 O 3 surface in Figure 1a, the Li 2 S ALD growth per cycle was smaller before gradually increasing and stabilizing to a constant growth per cycle. The region of increasing growth per cycle is magnified in Figure SI-2. Figure SI-3 shows the net mass change per cycle versus the number of Li 2 S ALD cycles deduced from the data in Figure 1a, exhibiting a constant value of ~ 17 ng.cm -2.cycle -1 after ~ 20 ALD cycles. The constant mass 1

2 change per cycle versus deposition temperature is shown explicitly in Figure SI-4. The mass gain is anomalously high during the first Li 2 S ALD cycle on the Al 2 O 3 surface in all cases, and this might result from tert-butoxy ligands that are not removed by the H 2 S exposures and remain on the Al 2 O 3 surface. We repeated the Li 2 S ALD at 225 o C using 100% H 2 S (see Figure SI-5) and the results were similar to Figure 1a, emphasizing that the surface reactions are fully saturated. Detailed measurements and analysis of the QCM step shape were performed to extract information regarding the mechanism for Li 2 S ALD. These measurements were recorded in the stable growth regime (i.e. after 1500 s of Li 2 S ALD). In contrast to the constant mass growth per cycle in the temperature range of o C (see Figure SI-4), the QCM mass ratio (i.e., R = m/m 1 as defined in the main text) decreased with temperature (Figure SI-6a). The fraction of -O t Bu ligands released into the gas phase as tert-butanol (HO t Bu) during the LTB exposures, x, is shown in Figure SI-6b, and this quantity also decreases with temperature. According to Eqns. 1a and 1b in the main text, the lower x values indicate a lower concentration of surface thiol (SH) species on the Li 2 S surface at the higher growth temperatures. 2. Characterization of ALD Li 2 S Films X-ray diffraction (XRD) measurements were performed on the ALD Li 2 S films prepared on fused silica substrates and the results of these measurements are shown in Figure SI-7 along with similar data recorded for bulk crystalline Li 2 S for comparison. It is evident that the ALD Li 2 S films are X-ray amorphous over the full range of growth temperatures o C 2

3 We discovered that the ALD Li 2 S films were highly air-sensitive and that longer air exposures had a pronounced effect on the thickness and morphology of the Li 2 S films (see Figure SI-8). A 60-s air exposure caused the film to become extremely rough, although the thickness remained at ~70 nm (see Figure SI-8a). However, air exposure for 4-h (Figure SI-8b) increased the film thickness dramatically by nearly 5 times to 320 nm. This increase in film thickness with air exposure was also verified by SE (Figure SI-8c). The most dramatic changes occur during the first ~10 min after which the thickness stabilized. In addition, the magnitude of the thickness increase after 1 day air exposure was somewhat dependent on the film thickness (Figure SI-8d). The thicker ALD Li 2 S films showed a greater change in relative thickness. One explanation for the changes observed in the ALD Li 2 S film thickness and morphology upon air exposure is reaction with atmospheric O 2, H 2 O, and CO 2. One possible reaction is: Li 2 S + 2 O 2 Li 2 SO 4. 1 Another potential route is: Li 2 S + H 2 O Li 2 O + H 2 S; Li 2 O + CO 2 Li 2 CO 3. Both reactions are expected to increase the film thickness due to the incorporation of additional atoms, and to roughen the surface from stress-induced cracking during the volume expansion. To protect the ALD Li 2 S film against reaction with the ambient we investigated a number of multilayer film structures where the ALD Li 2 S film was buried beneath other ALD films. Figure SI-9 shows SEM images of trilayers comprised of ALD Li 2 S/GaS x /Al 2 O 3 films deposited in a high-aspect-ratio silicon trench substrate, consisting of a layer of 600-cycle Li 2 S, a layer of 100-cycle GaS x, and a layer of 100-cycle Al 2 O 3 prepared at 150 C. The 3

4 Al 2 O 3 film was intended to serve as a gas diffusion barrier against moisture and O 2, 2-3 while the intermediate Ga 2 S 3 film was used to protect the Li 2 S from reacting with the H 2 O used as the oxygen source during the Al 2 O 3 ALD. The XPS depth profiling measurements shown in the main text (Figure 3a) revealed a Li:S ratio of 2:1 within the Li 2 S film and no detectable carbon, indicating minimal reaction with the environment to produce Li 2 O or Li 2 CO 3. A second multilayer strategy explored was an ALD ZnS/Li 2 S/ZnS trilayer film deposited on Si trench structures using 400 ALD ZnS cycles, 600 ALD Li 2 S cycles, and 400 ALD ZnS cycles at 150 o C. SEM images of this structure are shown in Figure SI-10. The abrupt boundaries between the amorphous Li 2 S and the crystalline ZnS are very apparent in these images. Moreover, the measured Li 2 S thickness of ~65 nm matches well the predicted value of 60 nm based on the number of ALD Li 2 S cycles performed. This is a good indication that the ZnS has protected the Li 2 S from air exposure which would have caused the Li 2 S film thickness to increase by up to 5x as demonstrated in Figure SI-8b. To confirm the Li/S ratio obtained from XPS, we performed X-ray fluorescence (XRF) measurements. The S XRF signals were calibrated using ALD ZnS films of known molar density prepared using alternating exposures to diethyl zinc and H 2 S. 4 It is not possible to see Li using our XRF instrument, but the Li content could be deduced from the QCM data by subtracting the known S contents as measured from XRF. The X-ray fluorescence measurements shown in Figure 3b were performed on nano-li 2 S films deposited on Si(100) and fused silica substrates over a range of temperatures between 150 and 225 C. 4

5 3. Electrochemical performance of ALD Li 2 S films To evaluate the electrochemical characteristics of the ALD-deposited amorphous Li 2 S nanofilms, we prepared ALD Li 2 S at a variety of deposition temperatures and thicknesses. In one experiment, we deposited the ALD Li 2 S on pre-fabricated MCMB graphite laminates. This capability distinguishes ALD from other coating methods, since the vapor phase precursors, atomically precise control, low temperature, and excellent conformality permit the uniform infiltration and coating of complex, porous structures that incorporate polymers (e.g. the PVDF binder). It is noteworthy that this ALD method is the only one that can directly be applied on pre-fabricated laminates to date. The 56-µm MCMB laminates mainly consist of MCMB particles, PVDF binder, and carbon fiber as conductive additives. The laminates are porous, as shown in Figure 4a in the main text and Figure SI-11. The ALD Li 2 S coatings on MCMB laminates were found to be uniform and conformal. At 200 o C, the MCMB laminates were coated by 700-cycle ALD Li 2 S, as shown in Figure 4b in the main text and Figure SI-12. Figure 4b shows the Li 2 S coating on the MCMB particles located on the top of the laminate while Figure SI-12 shows the Li 2 S coating on the MCMB particles near the laminate bottom (close to the Cu current collector). Combined, these images confirm that the Li 2 S ALD uniformly infiltrated the laminate layer. Figure SI-13 shows SEM images from a 360-cycle ALD Li 2 S at 300 o C and further confirms that ALD enables uniform and conformal nano-li 2 S coating in the range of o C. 5

6 Using the ALD Li 2 S coated MCMB laminates, we investigated their electrochemical characteristics in Li-S batteries including the effect of different film ALD Li 2 S thicknesses. In Figure 4d of the main text, we illustrated the charge-discharge characteristics of the 360-cycle ALD Li 2 S deposited at 300 o C on MCMBs in the potential window of V at a current density of 55 ma/g (C/20), exhibiting the typical electrochemical behavior of Li 2 S as 1, 5-6 reported previously. Figure SI-14 show the electrochemical characteristics of the 700-cycle ALD Li 2 S deposited at 200 o C on MCMBs. Figure SI-14a illustrates the first three charge-discharge profiles and reveals that they are similar to the ones in Figure 4d. At the same current density as used in Figure 4d, however, the sustained capacity of this 700-cycle ALD Li 2 S is much lower, amounting to ~400 mah/g at 30 th cycle (see Figure SI-14b). This indicates that film thickness has big influence on the Li 2 S cycling performance. We believe that the insulating nature of Li 2 S made the electrochemical performance very sensitive to thickness change, for a thicker film increases the resistive path length for both electrons and ions. To this end, ALD is apparently an important tool to precisely controlling Li 2 S film deposition to achieve an optimal film thickness to harvest the best electrochemical performance. Noticeably, the 700-cycle ALD Li 2 S film achieved a Coulombic efficiency of ~100% beginning with the 7 th cycle. Figure SI-15a and SI-15b illustrate the dq/dv curves of the first two charge-discharge cycles for the 360-cycle ALD Li 2 S (Figure 4d) and 700-cycle ALD Li 2 S (see Figure SI-14a). These dq/dv profiles have similar oxidation and reduction peaks, consistent with previous studies. 5, 7 6

7 References 1. Han, K.; Shen, J.; Hayner, C. M.; Ye, H.; Kung, M. C.; Kung, H. H., Li 2 S-reduced Graphene Oxide Nanocomposites as Cathode Material for Lithium Sulfur Batteries. J. Power Sources 251, Carcia, P. F.; McLean, R. S.; Reilly, M. H.; Groner, M. D.; George, S. M., Ca Test of Al 2 O 3 Gas Diffusion Barriers Grown by Atomic Layer Deposition on Polymers. Appl. Phys. Lett. 2006, 89, Groner, M. D.; George, S. M.; McLean, R. S.; Carcia, P. F., Gas Diffusion Barriers on Polymers Using Al 2 O 3 Atomic Layer Deposition. Appl. Phys. Lett. 2006, 88, Bakke, J. R.; King, J. S.; Jung, H. J.; Sinclair, R.; Bent, S. F., Atomic Layer Deposition of ZnS via in situ Production of H 2 S. Thin Solid Films 2010, 518, Yang, Z. C.; Guo, J. C.; Das, S. K.; Yu, Y. C.; Zhou, Z. H.; Abruna, H. D.; Archer, L. A., In Situ Synthesis of Lithium Sulfide-Carbon Composites as Cathode Materials for Rechargeable Lithium Batteries. J. Mater. Chem. A 2013, 1, Zhang, S. S., Liquid Electrolyte Lithium/Sulfur Battery: Fundamental Chemistry, Problems, and Solutions. J. Power Sources 2013, 231, Cai, K. P.; Song, M. K.; Cairns, E. J.; Zhang, Y. G., Nanostructured Li 2 S-C Composites as Cathode Material for High-Energy Lithium/Sulfur Batteries. Nano Lett. 2012, 12, Saulys, D.; Joshkin, V.; Khoudiakov, M.; Kuech, T. F.; Ellis, A. B.; Oktyabrsky, S. R.; McCaughan, L., An Examination of the Surface Decomposition Chemistry of Lithium Niobate Precursors under High Vacuum Conditions. J. Cryst. Growth 2000, 217, Hamalainen, J.; Holopainen, J.; Munnik, F.; Hatanpaa, T.; Heikkila, M.; Ritala, M.; Leskela, M., Lithium Phosphate Thin Films Grown by Atomic Layer Deposition. J. Electrochem. Soc. 2012, 159, A259-A263. 7

8 Figures Figure SI-1. Effects of precursor dosing and purging on Li 2 S ALD as measured by QCM: (a) effect of LTB and H 2 S dosing times; (b) effect of LTB and H 2 S purging times. 8

9 Figure SI-2. Region of increasing growth per cycle for Li 2 S ALD on ALD Al 2 O 3 surface at four different deposition temperatures as measured by QCM. This temperature range was selected because the LTB bubbler temperature was 140 ºC, and above 300 ºC, LTB is known to decompose and the growth is expected to become non-self-limiting chemical vapor deposition (CVD)

10 Figure SI-3. Net mass changes per cycle for Li 2 S ALD performed at 150, 175, 200, 225, and 300 ºC, as measured by QCM. 10

11 Figure SI-4. Mass growth rates with temperature, as measured by QCM. 11

12 Figure SI-5. QCM measurements during Li 2 S ALD on ALD Al 2 O 3 surface using LTB and 100% H 2 S at 225 o C: (a) 100 ALD cycles showing nucleation and constant growth regimes (b) enlarged view of three consecutive ALD Li 2 S cycles in the regime of constant growth per cycle. 12

13 Figure SI-6. Temperature dependence for: (a) QCM mass ratio, R; and (b) fraction of ligands released during LTB exposure, x. 13

14 Figure SI-7. XRD patterns of ALD Li 2 S films deposited at different temperatures. 14

15 Figure SI-8. SEM images of ALD Li 2 S films deposited on trenched Si substrates at 150 o C: (a) exposed to air for 1 min; (b) exposed to air for 4 hours. (c) In situ measurements of thickness changes for ALD Li 2 S films deposited at different temperatures with extended exposure to air. (d) Thickness changes of ALD Li 2 S films deposited at 225 o C before and after one-day air exposure for different ALD Li 2 S cycles. 15

16 Figure SI-9. SEM images of the trilayers of ALD Li 2 S/GaS x /Al 2 O 3 film deposited in a high-aspect-ratio silicon trench substrate, consisting a layer of 600-cycle Li 2 S, a layer of 100-cycle GaS x, and a layer of 100-cycle Al 2 O 3 at 150 C: (a,c) lower magnification image showing entire trench and (b,d) higher magnification images showing (b) the top and (d) the bottom of the trench. 16

17 Figure SI-10. SEM images ALD ZnS/Li 2 S/ZnS trilayer film deposited on Si trench structures using 400 ALD ZnS cycles, 600 ALD Li 2 S cycles, and 400 ALD ZnS cycles at 150 o C: (a) low magnification and (b) high magnification. 17

18 Figure SI-11. SEM images of bare MCMB graphite laminates: (a) low magnification top-view and (b) cross-sectional images, (c,d) high magnification, top-view images. 18

19 Figure SI-12. SEM images of 700-cycle ALD Li 2 S films on MCMB particles near Cu foils, deposited at 200 o C: (a,b ) low magnification and (c,d) high magnification. 19

20 Figure SI-13. SEM images of 360-cycle ALD Li 2 S films on MCMB particles deposited at 300 o C: (a,b ) low magnification and (c,d) high magnification. 20

21 Figure SI-14. Electrochemical characteristics of Li 2 S films deposited on MCMB particles using 700 ALD Li 2 S cycles at 200 o C: (a) charge-discharge profiles in the first three cycles (inset: first three cycles versus time) with a voltage window of V and a current density of 55 ma/g and (b) cyclability and Coulombic efficiency over 30 cycles. 21

22 Figure SI-15. dq/dv profiles of ALD Li 2 S films during the first two charge-discharge cycles with a voltage window of V and a current density of 55 ma/g: (a) 360 ALD Li 2 S cycles at 300 o C; and (b) 700 ALD Li 2 S cycles at 200 o C 22