Critical Role of Redox Mediator in Suppressing Charging Instabilities of Lithium-Oxygen Batteries

Transcription

1 Supporting information Critical Role of Redox Mediator in Suppressing Charging Instabilities of Lithium-Oxygen Batteries Zhuojian Liang and Yi-Chun Lu * Electrochemical Energy and Interfaces Laboratory, Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China yichunlu@mae.cuhk.edu.hk Figure S1. Cyclic Voltammetry of LiBr at 25 mv/s on the Ketjenblack electrode, in 50 mm LiBr - 1 M LiTFSI in diglyme electrolyte, with a piece of Li foil acting as both the anode and the reference electrode. S1

2 Figure S2. FTIR analysis results of 1M LiTFSI in diglyme, 10 mm LiBr - 1M LiTFSI in diglyme and 10 mm LiBr - 1M LiTFSI in diglyme after cycling between V vs. Li/Li + at 1 mv/s. Figure S2 and Figure S3 show the results of Fourier transform infrared spectroscopy (FTIR) and electrospray ionization mass spectrometry (ESI-MS) of the pristine electrolyte (1M LiTFSI in diglyme), as prepared 10 mm LiBr - 1M LiTFSI in diglyme and 10 mm LiBr - 1M LiTFSI in diglyme after cycling between V vs. Li/Li + (at 1 mv/s). No difference can be identified by FTIR in the region of C-Br bond for all samples and no byproducts arising from reaction between the Brcontaining species and the diglyme such as C 6 H 13 O 3 Br (m/z = for [C 6 H 13 O 3 Br+Li] + ) can be identified by ESI-MS for all three samples. The peaks at m/z = 141, 157 and 275 are attributed to [C 6 H 14 O 3 +Li] +, [C 6 H 14 O 5 +Li] + (C 6 H 14 O 5 is diglyme peroxide, which is the autoxidation product of diglyme) and [2C 6 H 14 O 3 +Li] +, respectively. S2

3 Figure S3. ESI-MS analysis results of: (a) 1M LiTFSI in diglyme; (b) 10 mm LiBr - 1M LiTFSI in diglyme and (c) 10 mm LiBr - 1M LiTFSI in diglyme after cycling between V vs. Li/Li +. S3

4 Figure S4. Oxygen pressure reduction monitored by pressure transducer during discharge with and without 10 mm LiBr at 1000 ma/g to 1000 mah/g. Figure S5. X-ray diffraction patterns of Li-O 2 positive electrodes after discharge with and without 10 mm LiBr at 1000 ma/g to 1000 mah/g. S4

5 Figure S6. Voltage profiles of the lithium anode of Li-O 2 cells upon cycling at 1000 ma/g (= 800 ua absolute current) to 1000 mah/g with and without 10 mm LiBr. A third electric connection point was incorporated at the center of the anode side of the original cell, on which a piece of Φ3 mm Li was added as an independent reference electrode. S5

6 Figure S7. Voltage and gas evolution profiles of the first charge with 10 mm LiI after discharged to 1000 mah/g at 1000 ma/g. Panel (c) shows statistics based on at least 3 replications. S6

7 Figure S8. Mass spectrometer signals at 26, 27 and 28 amu during charging. S7

8 Figure S9. Photos of the cell components in an assembled OEMS cell with (a) 60 ul, (b) 100 ul and 120 ul electrolyte on both sides. The cell design incorporates a quartz spacer and a shell with observing holes to enable both excellent hematic sealing and convenient visual inspection of the cell components in the assembled state. S8

9 Figure S10. Voltage and gas evolution profiles (without H 2 magnification) during charging with 10mM LiBr at (a) 500 ma/g, (b) 1000 ma/g and (c) 2000 ma/g after discharged to 500 mah/g at 1000 ma/g. S9

10 Figure S11. Voltage and gas evolution profiles of the 5 th charge without and with 10 mm LiBr after discharged to 1000 mah/g at 1000 ma/g. S10

11 Figure S12. Voltage and gas evolution profiles during charging without LiBr at (a) 500 ma/g, (b) 1000 ma/g and (c) 2000 ma/g after discharged to 500 mah/g at 1000 ma/g. The profiles of charging with 10 mm LiBr were overlaid for comparison. S11

12 Supplementary discussion 1: The capacity of oxygen reduction reaction is 0.8 mah. The full capacity of the redox mediator in this study (60ul of 10mM LiBr) is: 10 x 10-3 (mol Br - /L) * 60 * 10-6 (L) * 1 mole (e - /mol Br - ) * (C/mole e - ) * 1/3.6 (mah/c) = mah, which is only about 2% of the capacity associated with oxygen reduction reaction. Supplementary discussion 2: Thermodynamic data used in the calculation of the reversible potential of Reaction 6(a): Compound G f (kj/mol) Reference Li 2 CO CO LiO The thermodynamic data of LiO 2 at standard condition is not available in CRC handbook. Here we use the G value provided in ref 2. Accordingly, the Gibbs free energy of Reaction (6a) in the spontaneous direction (reduction): G = 4 ( kj/mol) 4 ( kj/mol) 2 ( kj/mol) = kj/mol The reversible potential at standard condition is: E 0 = G kj = ( ) C/mol = V nf mol In addition, the environment in the cell can be quite deviated from the standard condition. To estimate the effect of this deviation, we assume a CO 2 partial pressure of 1 mbar and a LiO 2 concentration of 1 µm, the reaction potential is estimated using the Nernst equation to be: E = E 0 + RT zf ln[co 2] 4 [LiO 2 ] 2 = V J/(K mol) K C/mol S12 ln[10 3 ] 4 [10 6 ] 2 = V Thus, the range of the reversible potential of Reaction (6a) is estimated to be V. Supplementary discussion 3: The O 2 -evolution efficiency after 3.5 V is estimated using: O 2 Evolution Efficiency = The definitions of the terms are: Q Discharge : the discharge capacity (i.e. 500 mah/g); Q Discharge OER 3.5 V to End of Charge Q 3.5 V to End of Charge ORR Q 3.5 V to End of Charge : the charge capacity between 3.5 V to the nominal end of charge (500 mah/g); ORR: the amount of O2 consumed during discharge; OER 3.5 V to End of Charge : the integrated amount of O2 evolved between 3.5 V to the nominal end of charge (500 mah/g).

13 OER 3.5 V to End of Charge : the integrated amount of O 2 evolved between 3.5 V to nominal end of charge (500 mah/g). References: (1) Lide, D. R. CRC Handbook of Chemistry and Physics, 85th Edition. CRC Press: 2004 (2) Laoire, C. O.; Mukerjee, S.; Abraham, K. M.; Plichta, E. J.; Hendrickson, M. A. J. Phys. Chem. C 2010, 114, S13