conversion reactions

Transcription

1 Supporting information SnO 2 model electrode cycled in Li-ion battery reveals the formation of Li 2 SnO 3 and Li 8 SnO 6 phases through conversion reactions Giulio Ferraresi, Claire Villevieille, Izabela Czekaj, Michael Horisberger, Petr Novák, Mario El Kazzi,* Paul Scherrer Institute, Electrochemical Energy Storage Section, CH-5232 Villigen PSI, Switzerland Cracow University of Technology, Faculty of Chemical Engineering and Technology, Cracow, Poland Paul Scherrer Institute, Laboratory for Scientific Developments and Novel Materials, CH-5232 Villigen PSI, Switzerland Corresponding Author * Mario El Kazzi: mario.el-kazzi@psi.ch S-1

2 Note 1. XPS is considered as powerful technique to elucidate surface processes, it is also well known that it suffers from some limitations especially when measurements are performed on conventional Li-ion battery electrodes due to the complexity of the electrode chemistry. Such electrodes make the chemical shift analysis of core levels and the assignment of the species very challenging. The complexity comes from the variety of mixed conductive/non-conductive materials together with high surface roughness and porosity, which all together lead to a non-uniform charge distribution on the surface causing an irregular core level peaks shift. 1-2 Recently, a model based on buried electrical potential between the solid electrolyte interphase (SEI) and the active materials was proposed to explain the origin of the binding energy (BE) shift. However, their conclusions are still questionable and they do not propose a flawless methodology to overcome the shift correction limitation. Table S1. Volumes of different Sn, Li-Sn, Sn-O and Li-Sn-O compounds (2 unit cells/volume) Structure Volume (2 u.c./ Å 3 ) SnO Å 3 SnO Å 3 Sn Å 3 Li 2 SnO Å 3 Li 8 SnO Å 3 LiSn Å 3 Li 17 Sn Å 3 S-2

3 Intensity [counts] XPS on Sn thin film 100.0k SnO x 80.0k 60.0k 40.0k 20.0k 0.0 Sn Binding energy [ev] Figure S1. XPS Sn 3d 5/2 spectra of 30 nm Sn thin film deposited on Cu foil With the aim to have a reference value for the binding energy position of metallic Sn, a 30 nm Sn thin film was deposited by DC sputtering on copper foil. The sample measured by XPS shows two main contributions in the Sn3d 5/2 energy level: at ev ( = 1.00 ev) related to Sn 0 and at ev ( = 1.9 ev) related to tin oxide (SnO, SnO 2 ). The latter has a wider compared to the SnO 2 thin film ( = 1.5 ev), as the surface oxide layer consists of several sub-oxides formed upon natural oxidation of the metallic Sn film in air. Cyclic voltammetry measurement of Sn vs. SnO 2 S-3

4 Current [a.u.] Current [a.u.] Current [a.u.] a) II I SnO 2 Sn Cycle Potential [V] vs. Li + /Li b) II I SnO 2 Sn Cycle Potential [V] vs. Li + /Li c) II I SnO 2 Sn Cycle Potential [V] vs. Li + /Li Figure S2. CV plots of Sn (dashed line) vs. SnO 2 (solid line): a) 1 st cycle; b) 3 rd cycle; c) 5 th cycle By comparing the CV curves of 20 nm of SnO 2 and 30 nm of Sn films, we can identify two main processes evidence in the region I (0.8 V 0.5 V) and region II (1.2 V 2.0 V). Peak evolution in the region I shows the changes in the reaction mechanism for the SnO 2 film along cycling. Initially, the SnO 2 broad peak converts into minor sharp peaks (after 2 nd cycle already). The latter ones are matching the one of Sn film indicating that the back conversion to SnO 2 is not completely reversible. Such evolution confirms the transition from a pronounced conversion/alloy reactions mechanism towards a simple alloy reaction mechanism. This is supported also by the changes in the region II, S-4

5 Intensity [cps] where the broad peak observed in the 1 st cycle, associated to the re-oxidation of Sn 0 into SnO 2, fades away after every cycle. XPS F1s and Li1s core spectra 50k 25k OCP F 1s 0.4k 0. Li 1s 50k 25k 2.0 V 0.4k k 100k 1.4 V LiF LiF 50k 25k 1.0 V 6k 3k 0.81 V 4k 0.45 V 6k 3k 5 mv 6k 3k 0.9 V 20k 10k 1.5 V 6k 3k 2.0 V Binding energy [ev] Figure S3. XPS spectra of Li 1s and F 1s at different potentials S-5

6 Table S2. Binding energy () and full-width-half-maximum () for Sn3d 5/2, C1s and O1s core levels. SnO 2 /Li 2 SnO 3 Li 8 SnO 6 Sn 0 / Li x Sn Sn3d 5/2 Pristine OCP V V V V V mv V V V S-6

7 C-H C-O C=O CO 3 C1s Pristine OCP V V V V V mv V V V S-7

8 O-Sn Li 2 CO 3 C=O Li 2 O O1s Pristine OCP V V V V V mv V V V Figure S4. Absolute intensity of the core levels Sn3d 5/2, O1s, C1s, F1s and Li1s, estimated from the area under the peaks recorded at selected potentials, S-8

9 Intensity [cps] Fluorine-free electrolyte To confirm the presence of SnF x species, the SnO 2 thin film was cycled in a fluorine free electrolyte (1 M LiClO 4 DMC: EC, 1:1 so-called LC30). The Sn3d 5/2 peak (Figure S5) measured on electrode stopped at 5 mv displays only two components at ev and ev associated to Li 8 SnO 6 and Li x Sn, respectively. However, the component at ev is not visible indicating that the component is F-based. Thus, the presence of SnF x species can result from the reaction between the PF - 6 salt and Sn mv 1 st Cycle LP mv 1 st Cycle LC Binding energy [ev] Figure S5. XPS Sn3d 5/2 spectra of 20 nm SnO 2 film lithiated at 5 mv in LP30 or LC30 electrolyte. S-9

10 Tilted angle % Sn 4+ /Sn / / / /27 Table S3. Angle resolved of the Sn atomic percentage between the Li 2 SnO 3 and Sn 0 components referring to Figure 6. DFT calculations Figure S6. Crystal structure of various Sn, Sn-O, Li-Sn and Li-Sn-O compounds The models of all Li-Sn-O compounds are presented in Figure S6 and include Sn atom as a central atom surrounded by at least two shells of neighbors. All investigated Li-Sn-O systems have triclinic structure; however, they are differing not only in bond length but also in the number of neighbors. S-10

11 1. Metallic Sn: central Sn atoms has 4 neighbors (bond length 2.88 Å); 2. SnO: central Sn atoms has 4 oxygen neighbors (bond length 2.26 Å) and 8 Sn neighbors (bond length 3.86 Å); 3. SnO 2 : central Sn atoms has 6 oxygen neighbors (bond length 2.09 Å), 2 Sn neighbors (bond length 3.24Å) and 8 Sn neighbors (bond length 3.78Å); 4. Li 2 SnO 3 : central Sn atoms has 4 oxygen neighbors (bond length 2.02 Å), 2 oxygen neighbors (bond length 2.31 Å), 2 Li neighbors (bond length 2.89 Å) and 4 Li neighbors (bond length 3.26 Å); 5. Li 8 SnO 6 : central Sn atoms has 6 oxygen neighbors (bond length 2.12 Å), 6 Li neighbors (bond length 2.75 Å) and 6 Li neighbors (bond length 3.20 Å); 6. LiSn: central Sn atoms has 2 Sn neighbors (bond length 3.22 Å), 1 Sn neighbors (bond length 2.92 Å), 1 Sn neighbors (bond length 3.68 Å), 4 Li neighbors (bond length 3.01 Å), 2 Li neighbors (bond length 3.37 Å) and 2 Li neighbors (bond lengt 3.09 Å); 7. Li 17 Sn 4 : central Sn atoms has 2 Li neighbors (bond length 2.88 Å), 2 Li neighbors (bond length 2.80 Å), and 5 Li neighbors (bond length 2.99 Å). Note 2. The program package StoBe is a modified version of the DFT-LCGTO program package DeMon, originally developed by A. St.-Amant and D. Salahub (University of Montreal), with extensions by L. G. M. Pettersson and K. Hermann. S-11

12 Operando XRD Figure S7. Contour plot representation of an operando XRD measurement of a 100 nm SnO 2 thin film deposited on glassy carbon cycled vs. Li metal with LP30 electrolyte. REFERENCES [1] Oswald, S.; Hoffmann, M.; Zier, M., Peak position differences observed during XPS sputter depth profiling of the SEI on lithiated and delithiated carbon-based anode material for Li-ion batteries. Appl. Surf. Sci. 2017, 401, [2] Maibach, J.; Lindgren, F.; Eriksson, H.; Edström, K.; Hahlin, M., Electric Potential Gradient at the Buried Interface between Lithium-Ion Battery Electrodes and the SEI Observed Using Photoelectron Spectroscopy. J. Phys. Chem. Lett. 2016, 7 (10), S-12