The Role of Cesium Cation in Controlling Interphasial. Chemistry on Graphite Anode in Propylene Carbonate-Rich

Transcription

1 Supporting Information The Role of Cesium Cation in Controlling Interphasial Chemistry on Graphite Anode in Propylene Carbonate-Rich Electrolytes Hongfa Xiang,,# Donghai Mei, + Pengfei Yan, Priyanka Bhattacharya, Sarah D. Burton, Arthur von Wald Cresce, Ruiguo Cao, Mark H. Engelhard, Mark E. Bowden, Zihua Zhu, Bryant J. Polzin, Chong-Min Wang, Kang Xu, Ji-Guang Zhang, and Wu Xu*, Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99354, USA # School of Materials Science and Engineering, Hefei University of Technology, Hefei, Anhui 239, P. R. China + Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA 99354, USA Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354, USA Sensor and Electron Devices Directorate, U.S. Army Research Laboratory, Adelphi, MD 2783, USA Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 6439, USA * Corresponding author. wu.xu@pnnl.gov S-1

2 Table S1. Electrode information Electrode Graphite Anode NCA Cathode Components 92% MAG-1 graphite and 8% Kureha #C binder on 18 µm thick copper foil 86% Toda NCA, 4% SFG-6 conductive additive, 2% Super P Li, and 8% Solvay 513 binder on 2 µm thick aluminum foil Total calendered electrode Thickness (C+F) (µm) Calendered electrode porosity 41.7% 3.8% Total coating loading without foil (mg cm -2 ) Specific reversible capacity 341 ~1C (RT) (mah g -1 ) Reversible ~1C (RT) (mah cm -2 ) S-2

3 Table S2. Electrolyte formulations used in this work Electrolyte E1 Formulation 1. M LiPF 6 in EC-PC-EMC (5:2:3 by wt.) E1Cs E1 +.5 M CsPF 6 E1FEC E1 + 2wt% FEC E1VC E1 + 2wt% VC E2 1. M LiPF 6 in EC-EMC (3:7 by vol.) E2Cs E2 +.4 M CsPF 6 E3.5 M LiTFSI +.5 M CsTFSI in EC-PC-EMC (5:2:3 by wt.) E4.5 M LiTFSI +.5 M CsTFSI in EC-EMC (4:6 by wt.) E5.5 M LiTFSI +.5 M CsTFSI in PC E6 1. M LiTFSI in EC-EMC (4:6 by wt.) E7.95 M LiTFSI +.5 M CsTFSI in EC-PC-EMC (5:2:3 by wt.) E8.9 M LiTFSI +.1 M CsTFSI in EC-PC-EMC (5:2:3 by wt.) E9.8 M LiTFSI +.2 M CsTFSI in EC-PC-EMC (5:2:3 by wt.) E1.9 M LiTFSI +.1 M CsTFSI in EC-PC-EMC (3:2:5 by wt.) E11.9 M LiTFSI +.1 M CsTFSI in EC-PC-EMC (2:2:6 by wt.) E12.9 M LiTFSI +.1 M CsTFSI in EC-PC-EMC (1:2:7 by wt.) E13.6 M CsPF 6 in PC E14.6 M CsPF 6 in EC-PC (1:1 by mol.) E15 1. M CsTFSI in EC-PC-EMC (5:2:3 by wt.) E16 1. M LiTFSI in EC-PC-EMC (5:2:3 by wt.) E17 1. M LiPF 6 in EC-PC-EMC (2:1:7 by wt.) E17Cs E M CsPF 6 S-3

4 Table S3. Calculated results of d 2 in Figure S7 based on Bragg s equation. Cr Kα radiation = 2.291Å. Sample 2 Theta of (2) d 2 (Å) Pristine E E1Cs E1FEC E E E S-4

5 (A) 3 E13:.6M CsPF 6 /PC 25 Intensity (x1) Cs + (PC) Cs + (PC) (B) m/z E14:.6M CsPF 6 /PC+EC 2 Intensity (x1) Cs + (PC) Cs + (EC) 1 (PC) PC Cs+ (EC) 1 Cs + (PC) m/z Figure S1. ESI-MS results of (A) the.6 M CsPF 6 /PC electrolyte (E13) and (B) the.6 M CsPF 6 /EC-PC (1:1 by mol) electrolyte (E14). S-5

6 (A) -2 G (kj mol -1 ) Li + (EC) 1 (PC) n Li + (EC) 2 (PC) n Li + (EC) 3 (PC) n Coordination number of PC in solvates (B) -5 Cs + (EC) 1 (PC) n -4 Cs + (EC) 2 (PC) n Cs + (EC) 3 (PC) n G (kj mol -1 ) Coordination number of PC in solvates Figure S2. Solvation energy (kcal/mol) for (A) Li + -(sol) n=1-4 and (B) Cs + -(sol) m=1-4 (sol = EC and PC) in the liquid phase at the level of B3LYP/ G(d,p). S-6

7 Li + -EC Li + -(EC) 2 Li + -(EC) 3 Li + -(EC) 4 Li + -PC Li + -(PC) 2 Li + -(PC) 3 Li + -(PC) 4 Figure S3. Optimized structures of Li + -(EC) m and Li + -(PC) n, where 1 m,n 4. S-7

8 Li + -(EC) 1 (PC) 1 Li + -(EC) 1 (PC) 2 Li + -(EC) 1 (PC) 3 Li + -(EC) 2 (PC) 1 Li + -(EC) 2 (PC) 2 Li + -(EC) 3 (PC) 1 Figure S4. Optimized structures of Li + -(EC) a (PC) b where 1 a,b 3 and 2 a + b 4. S-8

9 Cs + -(EC) 2 Cs + -EC Cs + -(EC) 3 Cs + -(EC) 4 Cs + -PC Cs + -(PC) 2 Cs + -(PC) 3 Cs + -(PC) 4 Figure S5. Optimized structures of Cs + -(EC) m and Cs + -(PC) n, where 1 m,n 4. S-9

10 Cs + -(EC) 1 (PC) 1 Cs + -(EC) 1 (PC) 2 Cs + -(EC) 1 (PC) 3 Cs + -(EC) 2 (PC) 1 Cs + -(EC) 2 (PC) 2 Cs + -(EC) 3 (PC) 1 Figure S6. Optimized structures of Cs + -(EC) a (PC) b where 1 a,b 3 and 2 a + b 4. S-1

11 (2) Intensity (a.u.) E4:.5M CsTFSI in EC-EMC (11) (4) (1) (12) (13) (11) E3:.5M CsTFSI in EC-PC-EMC E9:.2M CsTFSI in EC-PC-EMC Theta (deg.) Figure S7. Micro-XRD of the graphite electrodes charged to.3 V in various electrolytes. The parameters of (2) peaks were calculated based on Bragg s equation with Cr Kα radiation = 2.291Å and the results are listed in Table S3. The parameters of the (2) peaks of various graphite samples are almost the same as the pristine graphite. Even in the electrolytes with a high concentration of Cs + (.5 M), the graphite bulk has not changed significantly. No any peak related to graphite intercalated compounds can be detected. The result indicates that both Li + and Cs + have not intercalated into the graphene layers when the charge cutoff voltage is.3 V. All these results suggest that the main difference between various graphite electrodes is on their surface, and the different electrochemical properties between various electrolytes are derived from their interfacial chemistries on the graphite electrode. S-11

12 Figure S8. SEM (A) and TEM (B) images show the graphite exfoliation in the E1 electrolyte. Compared with the pristine electrode, many cracks (yellow arrows in Figure S8A) are easily seen in the graphite electrode, which is a result of graphite exfoliation. In Figure S8B, the yellow arrow indicates that two graphene layers are exfoliated from the graphite bulk. S-12

13 Intensity (A.U.) E1 E1FEC Li + 2 E1Cs Depth (nm) Figure S9. ToF-SIMS plots of the graphite electrodes charged to.3 V in electrolytes E1, E1Cs and E1FEC. ToF-SIMS is more sensitive for Li detection than EDX and XPS. Here, more Li 2 + ions are detected in the charged graphite electrodes in E1 and E1FEC than that in E1Cs. This result strongly supports the point that there is a lower content of Li compounds formed, thus, a much thinner SEI layer is covered on the graphite electrode in E1Cs. S-13

14 2 Capacity (mah g -1 ) E1 E1Cs E1FEC Cycle number Figure S1. Cycling performance of the Li NCA half cells using various electrolytes. The first two formation cycles were performed at.1 C and then all cells were cycled at C/3 for charge and discharge. The solid symbols denote curves for charge capacity and the open symbols discharge capacity. All the electrolytes with or without Cs + exhibit excellent cycling stability and their capacities are quite close, especially for the electrolytes with either.5 M CsPF 6 or 2wt% FEC additives. This means that those electrolytes have a good anodic stability with the NCA cathode. It also implies that the cell performances of the studied electrolytes in graphite NCA full cells will be mainly determined by their compatibility with the graphite anode, especially the properties of the SEI film on the anode. S-14

15 5 Voltage (V vs. Li/Li + ) E1 E1Cs E1FEC CE: 3% 84% 8% Capacity (mah g -1 ) Figure S11. Initial charge-discharge voltage profiles of the graphite NCA full cells with the E1, E1Cs and E1FEC electrolytes, performed at room temperature at the current rate of C/2. What is very consistent with the initial voltage profiles of the Li graphite half cells is the low Coulombic efficiency (~3%) of the baseline electrolyte, owing to the serious reduction decomposition of PC and graphite exfoliation. Therefore, the graphite NCA full cell with the baseline electrolyte only delivers a low capacity of less than 5 mah g -1 after the formation cycles shown in Figure 8A. After additives (CsPF 6 or FEC) were added, the reduction decomposition of PC was successfully suppressed by forming a stable SEI film. A high discharge capacity of 172 mah g -1 can be achieved during the initial formation cycle, along with a high Coulombic efficiency (>8%). S-15

16 (A) Discharge capacity (mah g -1 ) (B) Discharge capacity (mah g -1 ) RT E2 E1VC E1Cs E1Cs E1VC E Cycle number 6 o C E1Cs E1VC E Cycle number Figure S12. Cycling performances of graphite NCA full cells containing three electrolytes, E1Cs, E1VC (i.e. 1 M LiPF 6 in EC-PC-EMC (5:2:3 by wt.) with 2 wt% VC) and PC-free conventional electrolyte E2 (i.e. 1 M LiPF 6 in EC-EMC (3:7 by vol.) at (A) room temperature and (B) elevated temperature (6 C). Figure S12A shows that the cycling stability of the graphite NCA full cells with the E1Cs electrolyte is comparable to that of the conventional electrolyte E2 at room temperature. However, the cycling stability of the E1VC electrolyte is obviously worse than E1Cs and E2 at room temperature, and also worse than the E1FEC electrolyte (Figure 8A). At elevated temperature, as shown in Figure S12B, the conventional electrolyte E2 exhibit a good cycling stability at initial 4 cycles, but after subsequent cycling, distinct capacity fading occurs. For the E1VC electrolyte, the cycling stability at elevated temperature is not as good as the E1Cs electrolyte, and seems to be slightly better than the E1FEC electrolyte (Figure 8B). S-16

17 (A) 5 Voltage (V vs. Li/Li + ) E17 CE: E17 79% E17Cs 84% E17Cs (B) Capacity (mah g -1 ) Capacity (mah g -1 ) E17Cs E17 E17 E17Cs Cycle number Figure S13. Comparison of cell performance of graphite NCA full cells containing the electrolytes of 1 M LiPF 6 in EC-PC-EMC (2:1:7 by wt.) with (E17Cs) and without (E17).4 M CsPF 6 additive. (A) Initial charge/discharge voltage profiles at C/2 rate and (B) cycling stability at elevated temperature (6 C) with charging and discharging at a C/2 rate after two formation cycles at room temperature. During the initial formation cycle (Figure S9A), the E17Cs electrolyte has the higher Coulombic efficiency of 84% than the corresponding E17 electrolyte (79%), which does not contain Cs +, as the result of the improved SEI film built by Cs +. Also, the cell using the E17Cs electrolyte delivers a higher reversible capacity of 164 mah g -1 while that of the E17 electrolyte is only 153 mah g -1. As shown in Figure S9B, the Cs + additive obviously improves the cycling performance at elevated temperatures in terms of reversible capacity and capacity retention during cycling. S-17

18 (A) 4.2 E17Cs Voltage (V vs. Li/Li + ) o C -3 o C -2 o C o C RT (B) Discharge capacity (mah g -1 ) E2 Voltage (V vs. Li/Li + ) o C -3 o C -2 o C o C RT Discharge capacity (mah g -1 ) Figure S14. Comparison of low-temperature performance of graphite NCA full cells containing two electrolytes. (A) E17Cs: 1 M LiPF 6 in EC-PC-EMC (2:1:7 by wt.) with.4 M CsPF 6 additive and (B) Non-PC based conventional electrolyte E2: 1 M LiPF 6 in EC-EMC (3:7 by wt.). Cells were charged at a C/5 rate at room temperature, kept for 3 hours at selected low-temperature, and then discharged at a C/5 rate at the low-temperature. The graphite NCA full cells with this Cs + -containing electrolyte maintained a higher discharge capacity at same temperature conditions than the conventional electrolyte without the Cs + additive, especially at temperatures below 2 C. As for the Cs + -containing electrolyte, the discharge capacities and the corresponding capacity retention compared to the room temperature capacity of the graphite NCA full cells at 3 o C and 4 o C are 126 mah g -1 (75%) and 13 mah g -1 (61%), respectively. However, the conventional electrolyte experienced drastic capacity fading. Its capacity and capacity retention are only 91 mah g -1 and 64% at -3 C, but dropped to 25 mah g -1 and 18% at 4 C. S-18

19 Figure S15. Curves created from electrochemical impedance spectroscopy analysis of Li graphite cells using the E1, E1Cs and E1FEC electrolytes at the initial charge potential of.3 V. The results were fitted by using an equivalent circuit (inset), in which R s, R SEI, R ct, and Z W stand for resistances of the electrolyte solution, the SEI film, the charge transfer, and the Warburg diffusion, respectively. The fitted impedance results show the E1Cs electrolyte has the lowest R SEI (66.8 ohm) and R ct (2.7 ohm), compared with the E1 (R SEI ohm, R ct 43.6 ohm) and the E1FEC (R SEI 9.4 ohm, R ct 35.6 ohm) electrolytes. This result suggests that the ultrathin SEI layer formed in the E1Cs electrolyte has the lowest SEI charge transfer impedance. S-19