Low Cost Metal Carbide Nanocrystals as Binding and. Electrocatalytic Sites for High Performance Li-S Batteries

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1 Supporting Information Low Cost Metal Carbide Nanocrystals as Binding and Electrocatalytic Sites for High Performance Li-S Batteries Fei Zhou 1,2, Zheng Li 3, Xuan Luo, 4 Tong Wu 5, Bin Jiang 4, Lei-Lei Lu 2, Hong-Bin Yao 1,2*, Markus Antonietti 6, Shu-Hong Yu 1,2,3,7* Experimental Sections MC-CNFs composite nanofibers fabrication. The CNFs and TiC-CNFs were synthesized as we described before. 1 To prepare W 2 C-CNFs (or Mo 2 C-CNFs), 100 mg CNFs were firstly washed with distilled water (DIW) and ethanol three times, respectively. Then, the CNFs was dispersed into 35 ml of DIW and stirred for 1 h to form a homogeneous suspension. Then, 0.23 mmol of sodium tungstate dehydrate (or sodium molybdate dehydrate) was added into the suspension under magnetic stirring and kept stirring for 0.5 h. Thereafter, the suspension was adjusted to be acidic with 50 µl of concentrated hydrochloric acid. Then the suspension was transferred into Teflon vessels and sealed and kept at 150 o C for 8 h. The precipitated products were collected by centrifugation and washed by DIW and ethanol for two times, respectively. The dried precursors were annealed under argon atmosphere at 1300 o C for 2 h. The argon flow rate is 150 ml min -1 with a heating rate of 5 o C min -1. Finally, the furnace was turned off and allowed to cool naturally to the room temperature and the annealed samples were collected.

2 Preparation of lithium polysulfide (Li 2 S 8 and Li 2 S 6 ). To prepare 5 M Li 2 S 8 solution, 0.56 g of sulfur and g Li 2 S were dissolved in 4 ml of 1,3-dioxolane (DOL, 2ml)/1,2-dimethoxyethane (DME, 2 ml) solution (1:1, v/v) mixed solvent. The obtained suspension was stirred and heated at 80 o C for 12 h to obtain red-brown Li 2 S 8 solution. For 5 M Li 2 S 6 preparation, we just change the mass of sulfur to 0.40 g and the other conditions kept the same. The adsorption of LiPS onto the MC. Before the adsorption test, all the metal carbides and acetylene black powder were dried under vacuum at 80 o C overnight. 100 µl of the as-prepared 5M of Li 2 S 6 solution was diluted into 8 ml of DOL (4 ml)/ DME (4 ml) solution (1:1, v/v) mixed solvent for the adsorption test. The acetylene black (Hefei Kejing Material Technology Co., Ltd.), W 2 C (Aladdin), Mo 2 C (Strem Chemicals), and TiC (Aladdin) powders were used as received. The Samples with different mass but same surface area were added into the above Li 2 S 6 solution under stirring. After that, the suspension was centrifuged and 25 µl of the supernatant was collected for the inductively coupled plasma atomic emission spectroscopy (ICP-OES) test for the sulfur content analysis. The precipitates were collected and dried for the XPS tests. The DFT calculation of binding energy between LiPS and MC. First principle density functional theory (DFT) calculations were performed with Vienna ab initio simulation package (VASP). 2, 3 To represent the metal carbide materials, Mo 2 C (0001), W 2 C (0001), and TiC(100) facets were modeled in terms of the slabs of 3 3

3 supercells, three metal-carbon bilayers with the 12 Å vacuum space in the vertical direction. Li 2 S 6 was considered as the prototype of adsorbates. The generalized gradient approximation (GGA) 4 was applied in describing the electron exchange and correlation energy using Perdew-Burke-Ernzerhof (PBE) functional 4 Projector augmented wave (PAW) method 5, 6 was used to describe the electron-ion interactions. The Kohn-Sham valence electronic wavefunction was expanded in a plane wave basis set with a cutoff at 400 ev. The Brillouin zone was sampled with a Monkhorst-Pack k-points grid mesh. The gaseous molecules were optimized in a periodic cubic box with the size of Å. The adsorbed structures were optimized with the upmost two metal-carbide bilayers relaxed and force convergence criterion of 10 mev/ Å. The binding energies (E b ) are calculated as E b =E total -E ads -E suf, where E total is the total energy of the system, E ads is the energy of the adsorbate in gas phase and E suf is the energy of the optimized clean surface slab. Multiple initial geometries of Li 2 S 6 on each metal carbide were considered during optimizations and the most stable adsorbed configurations were saved. Electrochemical tests. Electrochemical performance of Li-S batteries was evaluated with 2032 coin cells assembled in an argon-filled glove box with water and oxygen contents < 0.1 ppm. For preparing the working electrode, active material (W 2 C-CNFs, Mo 2 C-CNFs and TiC-CNFs), super-p acetylene black, and polyvinylidene fluoride (PVDF) were mixed at a weight ratio of 80:10:10 in the N-Methyl-2-pyrrolidone (NMP) solvent to form a uniform slurry, and then pasted on a Al foil. The foil was dried at 60 C under vacuum and then cut into the electrode with a diameter of 1 cm.

4 According to the cross sectional SEM images, the sulfur cathode thickness is ~ 20 µm, ~15 µm and ~ 18 µm for W 2 C NPs-CNFs, Mo 2 C NPs-CNFs and TiC NPs-CNFs, respectively (Figure S6). For assembling the cell, a lithium foil was used as the counter electrode, and a polyethylene separator from Celgard was used. The electrolyte is 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, Sigma-Aldrich) and 2 wt-% LiNO 3 in 1,3-dioxolane/1,2-dimethoxyethane (1:1, v/v) (Sigma-Aldrich) µl of above-synthesized 5M Li 2 S 8 (2 mg equivalent mass loading of sulfur) was uniformly drop cast onto the electrode slice as the catholyte. CV measurements were performed on a CHI-760D electrochemical station with a scan rate of 0.05 mv s -1 in the voltage range of V (vs. Li + /Li). Electrochemical impedance spectroscopy Nyquist plots were also recorded by CHI-760D electrochemical station between 100 mhz and 100 khz. Galvanostatic discharge-charge (GDC) cycling of the assembled cell was carried out using a Land multichannel electrochemical testing system in the voltage range of V (vs. Li + /Li). The specific capacity is calculated based on the mass of sulfur. Details of symmetrical cells tests could be found in Note S1. To evaluate whether the electrochemical catalytic effect of MC NPs works in solid sulfur cathode, we tested the electrochemical performance of sulfur/w 2 C NPs-CNFs hybrid cathode coupled with planar Li metal anode using PEO-LiTFSI solid polymer electrolyte. For comparison, the sulfur/cnfs cathode was also tested. Firstly, sulfur was incorporated with the matrix materials by melting diffusion at 155 o C for 12 h. Then, 50 mg electrode material (S/W 2 C NPs-CNFs and S/CNFs) together with 15 mg PEO-LiTFSI

5 (EO/TFSI=18) and 5 mg Ketjen black power were dispersed in acetonitrile and stirred for 24 h to get a homogeneous slurry. The slurry was uniformly coated onto Al-Carbon foil and then cut into the round electrode piece with ~2 mg sulfur mass loading. The cells were assembled in 2032 coin cell to evaluate the electrochemical performance. The testing temperature was 60 o C and the current rate is 0.05 C (1C = 1672 ma/g). The voltage range is 2.8 V-1.5V. Characterizations. Scanning electron microscope (SEM, JEOL-6700F), transmission electron microscope (TEM, Hitachi H-7650) were employed to visualize the morphologies, sizes, structures, and elemental compositions of the products. The nitrogen absorption/desorption isotherms were obtained at 77 K on an ASAP 2020 accelerated surface area and porosimetry instrument (Micromeritics). Powder X-ray diffraction (PXRD) patterns were carried out on a Philips X Pert PRO SUPER X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) analysis was conducted on an ESCALAB MK II X-ray photoelectron spectrometer using nonmonochromatized Al-Mg KR radiation exciting source. The content of element sulfur was measured by Plasma Quad 3 (America) coupled plasma mass spectrometry.

6 Figure S1. TEM images of the as-synthesized (a) CNFs, (b) CNFs-WO 3, (c) CNFs-MoO 3 and CNFs-TiO 2 (Scale bar = 200 nm). Figure S2. The TGA curves of MC NPs-CNFs analyzed in air with 10 o C/min heating rate. Figure S3. The nitrogen adsorption/desorption curves of MC NPs-CNFs.

7 Figure S4. N 2 adsorption/desorption isotherms of commercial (a) W 2 C, (b) Mo 2 C and (c) TiC commercial particles, respectively. Figure S5. The S 2p spectrums of the metal carbides after the Li 2 S 6 adsorption test. (a) S 2p of W 2 C; (b) S 2p of Mo 2 C; (c) S 2p of TiC. Figure S6. The cross sectional images of MC NPs-CNFs cathodes. (a) W 2 C NPs-CNFs; (b) Mo 2 C NPs-CNFs; (c) TiC NPs-CNFs. The scale bars in all images are 10 µm.

8 Figure S7. Onset potential for Li-S redox reactions. Differential CV curves of (a) CNFs-W 2 C, (b) CNFs-Mo 2 C, (c) CNFs-TiC and (d) CNFs. The baseline voltage is defined as the variation on current density is the smallest, namely di/dv = 0.

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10 Figure S8. Corresponding onset potentials of redox peak of (a) W 2 C NPs-CNFs, (b) Mo 2 C NPs-CNFs, (c) TiC NPs-CNFs and (d) CNFs. The onset potential is determined when the current density goes 10 µa cm -2 beyond the corresponding baseline current density. The baseline voltage are the same as in Figure S7. Figure S9. Cycling performance and voltage profiles of Li 2 S 8 /W 2 C NPs-CNFs using electrolyte without LiNO 3 additive.

11 Figure S10. Cycling performance of PS/W 2 C-CNFs, PS/Mo 2 C-CNFs, PS/TiC-CNFs and PS/CNFs over 100 cycles at 0.2C with high sulfur mass loading (3.57 mg/cm 2 ). Figure S11. The galvanostatic current discharge capacity and Coulombic efficiency of S/MC NPs-CNFs and S/CNFs vs. cycling numbers.

12 Figure S12. Electrochemical performance of S/W 2 C NPs-CNFs and S/CNFs cathodes based on PEO-LiTFSI solid polymer electrolytes at 60 o C. The current density is 0.05 C. (1C = 1672 ma/g). (a) Cycling performance of S/W 2 C NPs-CNFs and S/CNFs. (b) Voltage-capacity profiles of S/W 2 C-CNFs and S/CNFs in the initial two cycles. Figure S13. EDX mapping results of S/W 2 C NPs-CNFs discharged to 1.8 V. (a) High-Angle Annular Dark-Field (HAADF) STEM image of S/W 2 C NPs-CNFs; (b) Carbon; (c) Sulfur; (d) Tungsten. The scale bar is 250 nm in all images. Figure S14. (a-c) PXRD results of W 2 C NPs-CNFs, Mo 2 C NPs-CNFs, and TiC NPs-CNFs before and after electrochemical tests, respectively. Note S1. The calculation progress of the adsorption quantities. The Li 2 S 6 mole amount before adsorption test is: n (Li 2 S 6 ) before = c(li 2 S 6 ) V(Tran)

13 After adsorption tests: n (Li 2 S 6 ) after = m (Sulfur) M Sulfur 6 V(ICP) V(Tran) V(Total) Herein, m(sulfur) is calculated according to the ICP test results. V(ICP) = ml, V(Tran) = 0.1 ml, V(Total) = 8.1 ml, M(Sulfur) = 32 g/mol. The total surface area of all the samples is 0.5 m 2. Note S2. Symmetrical cell assembly and measurements for catalytic analysis. The MC NPs-CNFs electrodes were punched into round disks and used as identical working and counter electrodes. 40 µl electrolyte containing 0.5 M Li 2 S 6 and 1M LiTFSI in DOL/DME (1:1, v/v) was added to assemble the cells. For the CV tests, the symmetrical cells were performed at scan rate of 50 mv s -1 between 0.7 V and -0.7 V upon the open circuit voltage. The slight difference in electrodes mass is uniformized for comparability. Table S1. The BET surface area of commercial MC NPs and acetylene black power. W 2 C Mo 2 C TiC AB powder BET area (m 2 /g) References 1. Zhou, F.; Song, L. T.; Lu, L. L.; Yao, H. B.; Yu, S. H. ChemNanoMat 2016, 2, (10), Kresse, G.; Furthmüller, J. Phys. Rev. B. 1996, 54, (16), Kresse, G.; Furthmuller, J. Comp. Mater. Sci. 1996, 6, (1), Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, (18),

14 5. Blöchl, P. E. Phys. Rev. B 1994, 50, Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59,