Supporting information to. Cell using Operando X-ray Absorption Spectroscopy

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1 Supporting information to Tracking the Chemical and Structural Evolution of the TiS 2 Electrode in the Lithium-Ion Cell using Operando X-ray Absorption Spectroscopy Liang Zhang, 1 Dan Sun, 2 Jun Kang, 3 Hsiao-Tsu Wang, 4 Shang-Hsien Hsieh, 4 Way-Faung Pong, 4 Hans A. Bechtel, 1 Jun Feng, 1 Lin-Wang Wang, 3 Elton J. Cairns, 2,5 Jinghua Guo 1,6* 1. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States 2. Energy Storage and Distributed Resources Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States 3. Material Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 4. Department of Physics, Tamkang University, Tamsui, 251, Taiwan 5. Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, United States 6. Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States * jguo@lbl.gov 1

2 Figure S1. Cyclic voltammograms of TiS 2 electrode using Li/Li + as a reference at a scan rate of 0.05 mv/s with a discharge cutoff voltage of 1.4 V (a) and 0.05 V (b). Figure S2. Operando XAS of TiS 2 electrodes in the voltage range of V at a specific current of 0.05 A/g. Operando Ti K-edge (a) and sulfur K-edge (c) XAS spectra for the first discharge process. Operando Ti K-edge (b) and sulfur K-edge (d) XAS spectra for the first charge process. 2

3 Figure S3. Operando XAS of TiS 2 electrodes for the second cycle in the voltage range of V. (a) The voltage profile for the operando XAS measurement. Operando sulfur K-edge (b) and Ti K-edge (d) XAS spectra for the second discharge process. Operando sulfur K-edge (c) and Ti K-edge (e) XAS spectra for the second charge process. Figure S4. Fitted linear relationship between the photon energy and oxidation state of Ti at different states of charge for the first cycle in the voltage range of V. 3

4 Figure S5. Intensity evolution of T 2g and e g states in the operando S K-edge XAS spectra as a function of specific capacity for the first cycle in the voltage range of V. Figure S6. Operando XAS of TiS 2 electrodes in the voltage range of V at a specific current of 0.05 A/g. Operando Ti K-edge (a) and sulfur K-edge (c) XAS spectra for the first discharge process. Operando Ti K-edge (b) and sulfur K-edge (d) XAS spectra for the first charge process. 4

5 Figure S7. Fitted linear relationship between the photon energy and oxidation state of Ti at different states of charge for the first cycle in the voltage range of V. Figure S8. Li 2 S adsorbed on Ti (001) surface. The binding energy is calculated by Eb=E(Li 2 S+Ti)- E(Li2S)-E(Ti), where E(Li 2 S+Ti), E(Li 2 S), and E(Ti) are the total energies of the Li 2 S adsorbed Ti (001) surface, the Li 2 S molecule, and the bare Ti (001) surface, respectively. A 2 2 supercell of the Ti (001) surface is used, and there are 8 Ti atomic layers. All the layers are allowed to relax, and it is found that the bottom layer shows negligible atomic displacement compared to the bulk Ti. The reference state of Li 2 S is an isolated molecule in vacuum. The calculation indicates that the interaction between Li 2 S and Ti is quite strong, with a binding energy of ev, leading to a significant change in the oxidation state of Ti. 5

6 Figure S9. Intensity evolution of T 2g and e g states in the operando S K-edge XAS spectra as a function of specific capacity for the first cycle in the voltage range of V. Figure S10. Operando XAS of TiS 2 electrodes for the second cycle in the voltage range of V. (a) The voltage profile for the operando XAS measurement. Operando S K-edge (b) and Ti K-edge (d) XAS spectra for the second discharge process. Operando S K-edge (c) and Ti K-edge (e) XAS spectra for the second charge process. 6

7 Figure S11. Ex situ sulfur K-edge XAS of TiS 2 electrodes at different states of charge as labelled. All the electrodes are in an equilibrium state due to the enough relaxation time. The electronic structure is TiS 2 is recovered after 1 and 10 cycles with a voltage range of 1.4 and 3.0 V, indicating the high reversibility of the lithium interaction reaction. In contrast, the electronic structure of TiS 2 is recovered after 1 cycle with a voltage range of 0.05 and 3.0 V, indicating the reversible conversion reaction of the first cycle. However, after 10 cycles, the electronic structure of TiS 2 is not recovered due to the transformation of TiS 2 to Ti-O related compounds. Figure S12. Comparison of the formation energies of different reaction paths. The formation energy of intercalation reaction (Li+TiS 2 ->LiTiS 2 ) is calculated by Eb=E(LiTiS 2 )-E(TiS 2 )-E(Li), where E(LiTiS 2 ), 7

8 E(TiS 2 ), and E(Li) are the total energies of LiTiS 2 ( ev), TiS 2 ( ev), and bulk Li ( ev), respectively. The formation energy of the conversion reaction (LiTiS 2 +3Li->2Li 2 S+Ti) is calculated by Eb=E(2Li 2 S)+E(Ti)-E(LiTiS 2 )-E(3Li), where E(2LiTiS 2 ), E(Ti), E(LiTiS 2 ), and E(3Li) are the total energies of 2Li 2 S ( ev), bulk Ti ( ev), LiTiS 2 ( ev), and bulk 3Li ( ev), respectively. In this case, the formation energies of the intercalation reaction and the conversion reaction can be directly compared. The reaction paths here means the two possibilities for the reaction between Li 2 S and Ti during the charge process: one is the formation of TiS 2, which means the conversion reaction is reversible; the other is the conversion from Li 2 S to sulfur, which indicates that the conversion reaction is not reversible. Our experimental results indicate that the conversion reaction is partially reversible in the first charge process. The calculation results indicate that the absolute value of the formation energy for the conversion reaction of TiS 2 is smaller than that of Li 2 S/sulfur, suggesting that the that the formed Li 2 S is more thermodynamically favorable to convert back to TiS 2 rather than elemental sulfur during the charge process. Figure S13. Ex situ Raman spectra of TiS 2 electrodes at different states of charge as labelled. The characteristic peaks of TiS 2 are significantly degraded after discharge to 1.4 V and instead two pronounced peaks centered at approximately 1350 and 1580 cm -1 are observed, which are assigned to the D band and G band of carbonaceous components in the SEI layer, respectively. The intensity of both peaks gets suppressed after charging back to 3.0 V because of the decomposition of the carbonates in the SEI layer. The same phenomenon is also observed for the TiS 2 electrodes cycled between 0.05 and 3.0 V. The Raman results are in good agreement with the O K-edge XAS results. 8

9 Figure S14. Ex situ Ti K-edge phase-corrected k 2 -weighted EXAFS χ(k) spectra of TiS 2 electrodes at different states of charge with a discharge cut-off voltage of 1.4 V (a) and 0.05 V (b), respectively. Figure S15. Ex situ XRD spectra of pristine TiS 2 and TiS 2 discharged to 0.05 V. The results clearly indicate that the characteristic diffraction pattern of TiS 2 totally vanishes and instead the corresponding pattern of Li 2 S is developed. 9