Supporting Information for

Transcription

1 Supporting Information for Self-stabilized solid electrolyte interface on host-free Li metal anode towards high areal capacity and rate utilization Zhenglin Hu 1,3, Shu Zhang 1, Shanmu Dong*,1, Quan Li 2,3, Guanglei Cui*,1 and Liquan Chen 1,2 1 :Qingdao Industrial Energy Storage Technology Institute, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao , P. R. China 2 :Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing , P. R. China 3 :University of Chinese Academy of Sciences, Beijing , P. R. China. 1

2 1. Interfacial stability of lithium metal with different electrolytes Figure S1. Voltage profiles of Li metal plating/stripping in a Li Li symmetrical cell cycled under (a) current density of 1 ma cm -2 and areal capacity of 3 ma h cm -2 ; and (b) current density of 2.5 ma cm -2 and areal capacity of 5 ma h cm -2 with different electrolytes. 2

3 Figure S2. EIS spectra of Li Li cell in 1 M LiTFSI-VC electrolyte (A) before and (B) after the rate test. The impedance for Li Li cell before cycling was about 37 Ω and it decreased to about 30 Ω after the rate test due to the improved interface compatibility. EIS spectra proved that the Li Li cell did not get shortcircuited after the rate test for 50 cycles. 3

4 Figure S3. SEM images of lithium metal anodes in Li Li symmetric cells with different electrolytes: (a), (b) 1 M LiPF6-EC/DMC for 50 cycles; (c), (d) 1 M LiTFSI-VC for 1000 cycles. The current density is 5 ma cm-2 and the corresponding areal capacity is 10 ma h cm-2. 4

5 Figure S4. Voltage profiles of Li metal plating/stripping in Li Li symmetrical cells cycled under a current density of 5 ma cm -2 and an areal capacity of 10 ma h cm -2 with different electrolytes. 5

6 Figure S5. Voltage profiles of Li metal plating/stripping in a Li Li symmetrical cell cycled under a current density of 5 ma cm -2 and an areal capacity of 10 ma h cm -2 in commercial 1 M LiPF 6 -EC/DMC electrolyte with 1% VC as the additive. 6

7 2. Different morphology of lithium deposition on Cu current collector Figure S6. Top-view SEM images of Li metal after plating from (a-f) 1 M LiPF 6 -EC/DMC and (g-l) 1 M LiTFSI-VC at varied current densities: (a, d, g, j) 1 ma cm -2 ; (b, e, h, k)2.5 ma cm -2 ; (c, f, i, l) 5 ma cm -2 in Cu Li cells. (Areal capacity: 25 ma h cm -2 ) SEM images of Cu Li half cells reveal that lithium metal plating from 1 M LiPF 6 -EC/DMC solution displayed needle-like morphology. Abundant protuberance appeared on the the surface layer after the first plating process under different current densities and areal capacities. This deposition behavior may lead to dendritic lithium and severe side reactions. 7

8 Figure S7. The deposition morphology of lithium metal anode after plating on copper foils from 1 M LiTFSI-VC electrolyte in a Cu Li cell. The current density is 1 ma cm -2 for an areal capacity of 25 ma h cm -2. 8

9 Figure S8. The cross-view morphology of lithium metal after plating on copper foils in a Cu Li cell from different electrolytes: (a) 1 M LiPF 6 -EC/DMC and (b) 1 M LiTFSI-VC. The current density is 1 ma cm -2 for 25 ma h cm -2. 9

10 3. Excellent performance of the self-stabilized SEI skeleton Figure S9. The DMT Young`s modulus for (a) the SEI region in 1 M LiPF 6 -EC/DMC and (b) the selfstabilized SEI skeleton formed in 1 M LiTFSI-VC electrolyte after the initial several cycles in Cu Li half cells. 10

11 Figure S10. Thickness change of the SEI region for different cycles under a current density of 2.5 ma cm -2 and an areal capacity of 5 ma h cm -2 in different electrolytes. It is noted that the SEI region on copper foil after the first Li stripping in 1 M LiPF 6 -EC/DMC electrolyte was quite uneven with many protuberances and its thickness was immeasurable (the standard deviation would be too large to get a meaningful average thickness). However, the thickness of this SEI region increased sharply to 370 µm during the subsequent 50 cycles, which was assigned to the continuous side reactions and the accumulation of dead lithium inside. 11

12 Figure S11. Surface morphology of the SEI region after Li stripping from Cu foils in different electrolytes: (a)1 M LiPF 6 -EC/DMC and (b) 1 M LiTFSI-VC. 12

13 Figure S12. Voltage profile of Cu Li cell cycled in 1 M LiTFSI-VC electrolyte for 50 times under a current density of 2.5 ma cm -2 and an areal capacity of 5 ma h cm

14 Figure S13. AFM images of the electronic conductivity and surface morphology for the SEI region after lithium plating/striping in 1 M LiPF 6 -EC/DMC under a current density of 2.5 ma cm -2 and an areal capacity of 5 ma h cm -2 for 50 cycles. It was worth mentioning that SEI region formed in 1 M LiTFSI-VC showed homogeneous distribution of response current than that induced in 1 M LiPF 6 -EC/DMC, which reflects its uniform components on surface, facilitating compact and smooth deposition of lithium metal from the initial stage of Li plating. Smooth and flat surface of this SEI skeleton in 1 M LiTFSI-VC was observed with a fluctuation less than 270 nm in a scanning area of 5µm 5µm, while the SEI region in 1 M LiPF 6 -EC/DMC electrolyte was uneven (fluctuation nearly 850 nm). 14

15 Figure S14. Nyquist plots of bare Li electrodes with (a) 1 M LiPF 6 -EC/DMC and (b) 1 M LiTFSI-VC electrolytes in Li Li symmetric cells measured at open-circuit potential for different storage time. The static stability of SEI layers induced in different electrolytes was tested by VPM-300 electrochemical workstation in Li Li symmetrical cells. The as-assembled cell with 1 M LiPF 6 -EC/DMC electrolyte exhibited higher impedance value than that in 1 M LiTFSI/VC electrolyte. Meanwhile, the impedance value for cell in 1 M LiPF 6 -EC/DMC was unstable during the whole testing process for 7 days, increasing from around 160 Ω to about 330 Ω. However, it was relative stable around 40 Ω for cell in 1 M LiTFSI-VC electrolyte, which proved the stable SEI layer with high ionic conductivity in 1 M LiTFSI-VC electrolyte. 15

16 Figure S15. Nyquist plots and the corresponding fitting lines of Li Li symmetric cells in (a) LiPF 6 - EC/DMC and (b) 1 M LiTFSI-VC electrolyte after the first cycle under a current density of 2.5 ma cm -2 with an areal capacity of 5 ma h cm

17 Figure S16. The impedance variation of Li Li symmetric cell with the extension of charging time from 30 minutes to 240 minutes in 1 M LiTFSI-VC electrolyte. 17

18 4. Investigation of interfacial components and stability Figure S17. XPS spectra of C 1s, F 1s, N 1s and Li 1s for SEI skeleton fetched from Cu Li half cells after 1st cycle, 10th cycles, 20th cycles and 50th cycles with 1 M LiTFSI-VC electrolyte. 18

19 Figure S18. XPS spectra of C 1s, F 1s, P 2p and Li 1s for the SEI region fetched from Cu Li half cells after 1 st cycle, 10 th cycles, 20 th cycles and 50 th cycles with 1 M LiPF 6 -EC/DMC electrolyte. The cell was cycled under 2.5 macm -2 for 5 ma h cm -2 in each half cycle. F 1s, P 2p and Li 1s spectra of the SEI region in 1 M LiPF 6 -EC/DMC electrolyte reflected the instability of SEI layer after different cycles. The amoumt of element F increased drastically after 20 cycles and a new peak around 687 ev was observed after 50 cycles. For P 2p spectrum, a new peak arount 137 ev appeared after 20 cycles as well. Li 1s image also revealed the formation of new reaction products after 20 cycles according to the shift of the main peak. These findings were assigned to the destruction of in-situ SEI region, resulting in severe reactions between lithium metal and 1 M LiPF 6 -EC/DMC electrolyte further. 19

20 Figure S19. FTIR spectra of lithium metal anode harvested from Cu Li half cell after 50 cycles in 1 M LiTFSI-VC electrolyte (red line) and the pure VC solvent (black line). The cell was cycled at 2.5 macm -2 for 5 ma h cm -2 in each half cycle. 20

21 Figure S20. SIMS spectra of element (a) Li, (b) C, (c) F and (d) O for the SEI regions in Cu Li half cells with 1 M LiPF 6 -EC/DMC (Orange line) and the SEI skeleton in 1 M LiTFSI-VC (Cyan line) electrolytes after 50 cycles. The etching time is 600s. All cells were cycled under 2.5 macm -2 for 5 ma h cm -2 in each half cycle. 21

22 Figure S21. Volage curves of LiFePO4 Li cells for different cycles in (a) 1 M LiPF6-EC/DMC and (b) 1 M LiTFSI-VC electrolytes from 2.5V to 4.0V. SEM images of lithium metal anodes fetched from LiFePO4 Li cells for (c, d) 500 cycles in 1 M LiPF6-EC/DMC electrolyte and (e, f) 5000 cycles in 1 M LiTFSI-VC electrolytes. (Current density: 1.5 ma cm-2) LiFePO4 Li cell in 1 M LiPF6-EC/DMC electrolyte exhibited dramatic capacity fading with a large polarization voltage merely cycled for 500 times. Loose and porous surface layer was observed with a thickness about 150 µm. 22

23 Figure S22. Cycling performance and Coulombic efficiency of LiCoO 2 Li cell with 1 M LiTFSI-VC electrolyte cycled for 100 cycles from 3.0 V to 4.2 V. Experiment result shows that LiCoO 2 Li cell with 1 M LiTFSI-VC electrolyte exhibits high capacity retention and Coulombic efficiency during the whole test process for 100 cycles. Figure S23. Volage curves of LiCoO 2 Li cells for different cycles in 1 M LiTFSI-VC electrolytes from 3.0 V to 4.2 V. 23