Effects of Electrolyte Salts on the Performance of Li-O 2 Batteries

Transcription

1 Supporting Information Effects of Electrolyte Salts on the Performance of Li-O 2 Batteries Eduard Nasybulin, a Wu Xu, a * Mark H. Engelhard, b Zimin Nie, a Sarah D. Burton, b Lelia Cosimbescu, a Mark E. Gross, a Ji-Guang Zhang a ** a Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99354, United States b Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354, United States Corresponding authors * Wu Xu: wu.xu@pnnl.gov; Tel: ** Ji-Guang Zhang: jiguang.zhang@pnnl.gov; Tel:

2 Discharge performance in tetraglyme The properties for the studied electrolytes together with the measured discharge capacity values are provided in Table S1. Conductivity decreased in the order LiTFSI (2.65 ms cm -1 ) > LiBOB (2.43 ms cm -1 ) > LiPF 6 (2.11 ms cm -1 ) > LiClO 4 (1.72 ms cm -1 ) > LiBF 4 (1.06 ms cm -1 ) > LiTf (0.79 ms cm -1 ) > LiBr(0.27 ms cm -1 ). Therefore, even though there is some correlation between the discharge capacities and conductivity values, some of the electrolytes did not follow this trend. Specifically, LiTf and LiBr electrolytes delivered higher discharge capacities than one would expect from their conductivity values. This may be attributed to their lower viscosity and higher oxygen solubility compared to other electrolytes. At the same time, the LiBOB electrolyte showed one of the lowest discharge capacities despite its high conductivity value. Therefore, the variation of the discharge capacities for the different electrolyte salts is from the combined effects of electrolyte conductivity, viscosity, oxygen solubility and possibly other factors, such as the stability of the salt during Li-O 2 reaction. Table S1. Conductivity, viscosity and oxygen solubility data for the 1.0 M solutions of various salts in tetraglyme Electrolyte Conductivity, ms cm -1 Viscosity, cp Oxygen solubility, mg L -1 Capacity at 0.05 ma cm -2, Ah g -1 Capacity at 0.20 ma cm -2, Ah g -1 LiTFSI LiBOB LiPF LiClO LiBF LiTf LiBr

3 The discharge voltage profiles of the Li-O 2 batteries using the electrolytes with different lithium salts at a higher current density (0.20 ma cm -2 ) are shown in Fig. S1. The discharge voltage plateaus were about V lower than those recorded at 0.05 ma cm -2. The battery with LiTFSI electrolyte still showed notably high discharge voltage, apparently because it had the highest conductivity (2.65 ms cm -1 ). The discharge capacities of the batteries with LiTFSI, LiTf, LiPF 6 and LiBr electrolytes decreased by 30% or more depending on the lithium salt used when compared to those measured at 0.05 ma cm -2. This decrease can be attributed to the accelerated electrode-blocking behavior, i.e., Li 2 O 2 formed at a high discharge rate quickly covers the outside surface of the carbon electrode close to the gas-intake face of the cell and blocks the oxygen diffusion pathways; thus less oxygen can diffuse into the cell. However, the discharge capacity of the batteries with LiClO 4 electrolyte did not change, and the capacities of the batteries with LiBF 4 and LiBOB electrolytes even increased at higher discharge current rate. One possible reason is that the BF - 4 and BOB - anions participate in the oxygen reduction process. In addition, significant increase of the discharge capacities at the 2.0 V stage was observed. This can be a result of the relatively low conductivities of the tetraglyme-based electrolytes (compared to common carbonate-based electrolytes) and/or a poor current collector. The discharge capacities decreased in the order LiTFSI > LiPF 6 LiClO 4 LiTf > LiBOB LiBF 4 > LiBr. This order correlates well with the measured conductivity values (Table S1) and indicates that electrolyte conductivity starts to play a more important role at high current densities. 3

4 Cell voltage / V (i) LiBF 4 (ii) LiBOB (iii) LiBr (iv) LiClO 4 (v) LiPF 6 (vi) LiTf (vii) LiTFSI iii ii i vi v iv vii Discharge capacity / Ah g -1 Fig. S1. Discharge voltage profiles for the Li-O 2 batteries with various electrolyte salts. The batteries were discharged at a current density of 0.20 ma cm -2. KB loading was 15 mg cm -2 and Ni mesh was used as current collector. 4

5 Discharge performance in BDG In addition to tetraglyme, BDG is another glyme-based solvent which leads to the formation of Li 2 O 2 as the major discharge product 1. LiBOB was not soluble in BDG, but the discharge performance of the BDG-based electrolytes with the other six lithium salts was tested at 0.05 ma cm -2 (Fig. S2). The discharge capacity decreased in the order of LiTFSI (3.0 Ah g -1 ) > LiPF 6 (2.6 Ah g -1 ) > LiClO 4 (1.8 Ah g -1 ) >> LiTf (0.5 Ah g -1 ) > LiBF 4 (0.4 Ah g -1 ) > LiBr (0.2 Ah g -1 ). In general, this trend is similar to what was observed in the tetraglyme-based electrolytes with the exception of LiTf and LiBr. By replacing tetraglyme with BDG, Li-O 2 batteries using the electrolyte with LiTFSI, LiPF 6, and LiClO 4 showed a ~20% increase in their discharge capacity, while the similar batteries using LiTf- and LiBr-based electrolytes showed significant drops. 5

6 Cell voltage / V (i) LiTFSI (ii) LiPF 6 (iii) LiClO 4 (iv) LiTf (v) LiBF 4 (vi) LiBr vi v iv iii ii i Discharge capacity / Ah g -1 Fig. S2. Discharge voltage profiles for the Li-O 2 batteries with 1.0 M solutions of various lithium salts in BDG. The batteries were discharged at 0.05 ma cm -2. KB carbon loading was 15 mg cm - 2. Ni mesh was used as the current collector. Conductivity, viscosity and oxygen solubility data for the BDG-based electrolytes are listed in Table S2. Conductivity values of all the BDG-based electrolytes were significantly lower than corresponding values of the tetraglyme-based electrolytes. Conductivity of LiTf and LiBr electrolytes decreased dramatically from and to and ms cm -1, respectively. This is the reason for the low discharge capacities observed in the cases of LiTf and LiBr in BDG-based electrolytes. Based on our observations, the conductivity of the glyme-based electrolyte should be at least several hundreds of μs cm -1 in order to be applied in a Li-air battery 6

7 at low discharge current densities, and higher conductivities are required for high discharge current densities. Table S2. Conductivity, viscosity and oxygen solubility data for the 1.0 M solutions of various salts in BDG Electrolyte Conductivity, ms cm -1 Viscosity, cp Oxygen solubility, Capacity at 0.05 ma cm -2, mg L -1 Ah g -1 LiTFSI LiPF LiClO LiBF LiTf LiBr LiBOB Not soluble Based on our observations, the conductivity of the glyme-based electrolyte should be at least several hundreds of μs cm -1 in order to be applied in a Li-air battery at low discharge current densities, and higher conductivities are required for high discharge current densities. 7

8 XPS spectra a O1s pure LiBOB c / a.u. pure LiClO 4 pure LiTf pure LiTFSI Binding energy / ev b O1s c / a.u. pristine KB/PEO Binding energy / ev Fig. S3. O1s spectra for a) the pure salts containing oxygen and b) pristine KB:PEO electrode. 8

9 C1s c / a.u. pure LiBOB Binding energy / ev Fig. S4. C1s XPS spectrum for pure LiBOB. 9

10 a ClO 4 - Cl2p Cl 2p 3/2 Cl 2p 1/2 c / a.u. chloride Cl 2p 3/2 Cl 2p 1/2 sweep 55 increasing number of sweeps sweep Binding energy / ev b ClO 4 - chloride Cl2p Cl 2p 1/2 Cl 2p 3/2 Cl 2p 1/2 Cl 2p 3/2 sweep 55 c / a.u. increasing number of sweeps sweep Binding energy / ev Fig. S5. Cl2p XPS spectrum for a) LiClO 4 and b) discharge products obtained in LiClO 4 electrolyte under prolonged exposure to X-ray beam. 10

11 Br3d c / a.u. pure LiBr LiBr Binding energy / ev Fig. S6. Br3d XPS spectra for pure LiBr and discharge products obtained in LiBr electrolyte. 11

12 Cycling efficiency Charging efficiency / % LiBF 4 LiBOB LiClO 4 LiPF 6 LiTf LiTFSI Cycle number Fig. S7. Variation of charge/discharge efficiency with cycling for Li-O 2 batteries with various electrolytes at 0.05 ma cm -2 current density. KB loading was 1 mg cm -2 and the current collector was carbon paper. 12

13 Shallow cycling in LiTf/tetraglyme electrolyte Furthermore, shallow cycling or capacity-limited cycling for the batteries with LiTFSIand LiTf-based electrolytes was tested. The discharge capacity was limited to 1000 mah g -1. Typically, both electrolytes could be cycled for about 20 cycles. Evolution of the dischargecharge voltage profiles with cycling for LiTf electrolyte is shown in Fig. S8. The discharge voltage showed a flat plateau at 2.68 V for the first 10 cycles and then a slight decrease was observed during the discharge for the following 10 cycles or so. The voltage dropped significantly during the discharge step in the last 3 5 cycles. As for the charging counterpart, the charging voltage showed a plateau at 4.15 V for the first cycle only. The voltage profiles were different for the second and following cycles without distinctive plateaus. Importantly, 1000 mah g -1 capacity could be recovered only in the first 10 charging steps. After that, the charging voltage had reached the 4.5 V cutoff limit before 1000 mah g -1 capacity was achieved. The continuous increase of charging voltage with cycling is a result of the accumulation of insulating side products generated during the discharge steps as discussed earlier. McCloskey et al. have recently provided theoretical evidence that even a monolayer of carbonates on a carbon surface decreases exchange current density by 1 2 orders of magnitude because of its high interfacial resistance 2. Increase in charging voltage raises the additional problem of anodic stability of the glyme-based electrolytes. Therefore, we were not able to reproduce successful 100 cycles reported recently by Jung et al. 3 although all the experimental conditions for cycling test were the same as described in their work. 13

14 4.5 vi v iv iii ii i Voltage / V (i) 1 st cycle (ii) 5 th cycle (iii) 10 th cycle (iv) 15 th cycle (v) 20 th cycle (vi) 25 th cycle i,ii,iii iv v vi Capacity / Ah g -1 Fig. S8. Shallow cycling (1000 mah g -1 ) with LiTf electrolyte at 0.05 ma cm -2 current density. KB loading was 1 mg cm -2 and the current collector was carbon paper. 14

15 References (1) Xu, W.; Hu, J.; Engelhard, M. H.; Towne, S. A.; Hardy, J. S.; Xiao, J.; Feng, J.; Hu, M. Y.; Zhang, J.; Ding, F.; Gross, M. E.; Zhang, J.-G. The Stability of Organic Solvents and Carbon Electrode in Nonaqueous Li-O 2 Batteries. J. Power Sources 2012, 215, (2) McCloskey, B. D.; Speidel, A.; Scheffler, R.; Miller, D. C.; Viswanathan, V.; Hummelshøj, J. S.; Nørskov, J. K.; Luntz, A. C. Twin Problems of Interfacial Carbonate Formation in Nonaqueous Li O 2 Batteries. J. Phys. Chem. Lett. 2012, 3, (3) Jung, H.-G.; Hassoun, J.; Park, J.-B.; Sun, Y.-K.; Scrosati, B. An Improved Highperformance Lithium Air Battery. Nature Chem. 2012, 4,