In Situ Formation of Stable Interfacial Coating for High Performance Lithium Metal Anodes

Transcription

1 In Situ Formation of Stable Interfacial Coating for High Performance Lithium Metal Anodes Haiping Wu 1, Yue Cao 1, Linxiao Geng 2 & Chao Wang 1* 1 Department of Chemistry, University of California Riverside, California 92521, USA. 2 Department of Chemical Engineering, University of California Riverside, California 92521, USA. Correspondence and requests for materials should be addressed to C. W. ( chaowang.chn@gmail.com) 1

2 Characterization of methyl viologen layer In order to study the methyl viologen layer, a cyclic voltammetry (CV) method was used to synthesize the layer in three electrode cell. The layer was synthesized by cycling between 0 and -1.2 V for 20 cycles. Ag/AgCl acted as reference electrode, platinum (Pt) wire was used as counter electrode and copper (Cu) foil was work electrode. Electrolyte was 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) with 1 wt% of methyl viologen (MV). Scan rate was 50 mv s -1. After reaction the morphology of Cu substrate was tested by scanning electron microscopy (SEM). The methyl viologen layer was further characterized by X-ray photoelectron spectroscopy (XPS) (Figure S2). Lithium deposition on Cu In order to investigate the lithium deposition process on Cu, the coin cells were discharged at 0.05 ma to 0 V. Then the SEM images of lithium on Cu were captured. The lithium nuclei in standard control ether electrolyte (DOL/DME/LiNO 3 ) are large spheres and some of them have cracks. The average size of the spheres is 1.33 µm (Figure S8). On the other hand, the lithium nucleation in electrolyte with methyl viologen have smaller size with average diameter of 0.45 µm. The difference of the lithium nucleation indicates the shield effect of methyl viologen coating and better control of lithium ion flux. The Cu surface after deposition of lithum was further studied with XPS. XPS results showed the typical carbon, nitrogen peak. The binding energies were calibrated with respected to the C1s peak at ev. For the sample in electrolyte with methyl viologen, the C1s peak at ev is attributed to C C and peaks located at 285.5eV, ev and ev are assigned to C N, C OR and C=O 1-3. The peak at ev is assigned to CF 3, which comes from LiTFSI. In addition, fluorine peaks are also observed, which should come from LiTFSI and the counter ions of the methyl viologen. LiTFSI decomposition can be evidenced by the F 1s signal which can be 2

3 described by two components, fluorine in LiF (684.9 ev) and in CF 3 groups (688.6 ev) 3. The difference of the control and electrolyte with MV is that the methyl viologen sample has C N, C=O and LiF peak. Electrochemical performance test The batteries performances were compared with different MV content, from 0.5 wt% to 8 wt% (Figure S10). With viologen, all the batteries can remain an efficiency of 97% after 200 cycles. The batteries with 8% MV show better performance compared with 3 wt% or 5 wt% MV. Higher percentage of salt can reduce the corrosion and suppress the formation of lithium dendrites. 4, 5 That might be the reason that MV-8 wt% has better cycling performance. The batteries can be tested at 4 ma cm -2. The efficiency of battery with 0.5 wt% MV can remain 97.0% after 166 cycles. The control coin cell without addition of MV has fluctuation after 135 cycles (Figure S13). The MV was added into electrolyte 1 M lithium hexafluorophosphate (LiPF 6 ) in ethylene carbonate (EC)/diethyl carbonate (DEC) (volume ratio 1:1). When cycling at 2 ma cm -2 with a capacity of 2 mah cm -2, the Columbic efficiency of original battery without methyl viologen drops to 85% after 34 cycles. With the addition of 0.5 wt% of MV, however, the batteries can now cycle to 60 maintaining a CE of 87.3% (Figure S15). SEM images also show that the methyl viologen coating can generate a much smoother lithium deposition and more stable SEI (Figure S15). After 1 cycle and 20 cycles of cycling, the batteries using standard EC/DEC/LiPF 6 electrolyte show large particles and clusters of Li dendrite on Cu substrate. The copper electrodes of coin cells using electrolyte added with MV, however, show flat and smooth morphologies (Figure S16). 3

4 Figure S1. CV of coin cells using electrolyte 1 M LiTFSI in DOL and DME without and with LiNO 3. Scan rate was 0.1 mv s -1. Figure S2. SEM image of (a) Cu surface and (b) (c) cross-section of Cu substrate after 20 cycles of CV test in 1 M LiTFSI in DOL and DME with 1 wt% of methyl viologen using three electrode cell. The thickness is about 210 nm. A bump can be observed after being irradiated by electron beam for a long time, indicating the formation of an organic coating layer. Figure S3. XPS spectra of (a) Cu substrate after 20 cycles CV test in 1 M LiTFSI in DOL and DME using three electrode cell; (b) Cu substrate after 20 cycles CV test in 1 M LiTFSI in DOL and DME with 1 wt% of methyl viologen using three electrode cell; (c) Pure methyl viologen powder. 4

5 Figure S4. Voltage profiles of the first discharge of coin cells at a current of 0.05 ma in ether electrolytes. Figure S5. Electrochemical impedance spectroscopy (EIS) of coin cells after different cycling test. Figure S6. Electrochemical impedance spectroscopy (EIS) of coin cells after 100 cycles tested under a current density of 2 ma cm -2 and a deposition capacity of 1 mah cm -2. The corresponding SEM images are showen in Figure 3. 5

6 Figure S7. (a) SEM image of Cu surface in the electrolyte 1 M LiTFSI DOL/DME with 5 wt% LiNO 3 after first discharge at 0.05 ma; (b-c) XPS spectra of solid-electrolyte interphase (SEI) layer shown in a; (d) SEM image of Cu surface in the electrolyte added with 0.5 wt% methyl viologen after first discharge at 0.05 ma; (e-f) XPS spectra of SEI layer shown in d. Cross-section SEM images of Cu after 10 cycles of activation and first deposition of lithium at current density of 2 ma cm -2 in (g) standard electrolyte and (h) standard control electrolyte added with 0.5 wt% of methyl viologen. Figure S8. Statistic analysis of size distributions of (a) lithium nuclei in control electrolyte (counted from Figure S7a) and (b) lithium nuclei in electrolyte with MV (counted from Figure S7d). 6

7 Figure S9. Statistic analysis of size distribution of (a) lithium clusters in control electrolyte (counted from Figure 1e) and (b) lithium clusters in electrolyte with methyl viologen (counted from Figure 1f). Figure S10. Cycling performance of batteries using electrolyte 1 M LiTFSI DOL/DME with the addition of different amount of MV(PF 6 ) 2 and 5 wt% LiNO 3. The current density was 1 ma cm -2 and deposition capacity was 1 mah cm -2. Figure S11. SEM images of SEI layers after cycling test using electrolyte with addition of (a) 5 wt% LiNO 3 and (b) 0.5 wt% MV(PF 6 ) 2 and 5 wt% LiNO 3.The current density was 1 ma cm -2 and deposition capacity was 1 mah cm -2. 7

8 Figure S12. (a) Surface of Cu in electrolyte without MV (b) surface of Cu in electrolyte with MV after first deposition of lithium in low magnification; SEM images of SEI layers in low magnification after cycling test using electrolyte (c) without MV (d) with MV. Figure S13. Cycling performance of batteries using standard DOL/DME/ LiTFSI/LiNO 3 electrolytes with the addition of 0.5 wt% MV. The current density was 4 ma cm -2 and corresponding deposition capacity was 1 mah cm -2. 8

9 Figure S14. The performances of coin cells test using electrolyte 1 M LiTFSI DOL/DME with the addition of 0.5 wt% MV(PF 6 ) 2 and 5 wt% LiNO 3 at different current density. Figure S15. (a) Cycling performance of coin cells using electrolyte 1 M LiPF 6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (volume ratio 1:1) with addition of 0.5 wt% MV(PF 6 ) 2 (red symbols). The current density was 2 ma cm -2 and deposition capacity was 2 mah cm -2. The black symbols represent data of the control samples. SEM images of the surface after cycling test in 1 M LiPF 6 EC/DEC (b) without MV(PF 6 ) 2 and (c) with addition of MV(PF 6 ) 2. 9

10 Figure S16. Top view SEM image of Cu substrate after 1 cycle test at a current density of 2 ma cm -2 in (a) 1 M LiPF 6 EC/DEC and (b) 1 M LiPF 6 EC/DEC with MV; Top view SEM image of Cu substrate after 20 cycles test in (c) 1 M LiPF 6 EC/DEC and (d) 1 M LiPF 6 EC/DEC with methyl viologen. Figure S17. Top view SEM image of Cu substrate after 1 cycle test at a current density of 2 ma cm -2 in (a) 1 M LiPF 6 VC/FEC/EC/DEC and (b) 1 M LiPF 6 VC/FEC/EC/DEC with MV. 10

11 Table S1 Comparison of different additives in Li metal batteries Electrolyte Additive Battery Current Cycling performance Ref. 5 M LiTFSI DME:DIO X (1:1, v:v) LiI 0.5 M (5 wt%) Li 2 S Electroly te Li C/5 Capacity retention 96% after 100 cycles 6 1 M LiTFSI DOL:DME (1:1, v:v) Li 2 S 8 (0.18M) LiNO 3 (5 wt%) Stainless steel foil Electrolyt e Li 2 ma cm -2 Efficiency, 1 mah cm % 400 cycles M LiPF 6 /PC CsPF 6 (0.05 M) Cu Electrolyte Li Li 4 Ti 5 O 12 Elec trolyte Li 0.2 ma cm -2 Efficiency, 1.25 C cm % 9 cycles 1 C Capacity retention 96% after 660 cycles 4 1 M LiPF 6 EC: DEC (1:1, v:v) Vinylene Carbonate (VC) (2 wt%) Ni Electrolyte Li 0.6 ma cm -2, 0.5 C cm -2 Efficiency 88% after 50 cycles 8 1 M LiClO 4 /PC Fluoroethylene carbonate (FEC) (5 wt%) Ni Electrolyte Li 0.5 ma cm -2, Efficiency 81% 0.3 C cm cycles 9 1 M LiTFSI DOL:DME (1:1, v:v) 1 M LiPF 6 EC: DEC (1:1, v:v) MV(PF 6 ) 2 (0.5 wt%) LiNO 3 (5 wt%) VC (1 vol%) FEC(10 vol%) MV(PF 6 ) 2 (0.5 wt%) Cu Electrolyte Li Cu Electrolyte Li 2 ma cm -2 Efficiency, 1 mah cm % after 400 cycles 2 ma cm -2, 2 mah cm -2 Efficiency 94.6% after 92 cycles This work 11

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