suppressing charging instabilities of Li-O 2 batteries

Transcription

1 Supporting information for Highly efficient Br /NO 3 dual-anion electrolyte for suppressing charging instabilities of Li-O 2 batteries Xing Xin, Kimihiko Ito, Yoshimi Kubo* GREEN, National Institute for Materials Science, 1-1 Namiki, Tsukuba , Japan Corresponding Author *Yoshimi Kubo KUBO.Yoshimi@nims.go.jp S-1

2 Methods 1. Electrolytes. High-purity tetraethylene glycol dimethyl ether (tetraglyme, TEGDME, Japan advanced chemicals, water content < 30 ppm) was used as received. Lithium trifluoromethanesulfonate (LiSO 3 CF 3, KISHIDA CHEMICAL Co., Ltd.), anhydrous LiI (Sigma-Aldrich), anhydrous LiBr (Sigma-Aldrich) and anhydrous LiNO 3 (KISHIDA CHEMICAL Co., Ltd.) were dried at 110 C for 12 h in a dry room (water content < 1 ppm) prior to use. Six electrolytes were prepared: 1. 1 M LiCF 3 SO 3 in tetraglyme; 2. 1 M LiCF 3 SO M LiI in tetraglyme; 3. 1 M LiCF 3 SO M LiBr in tetraglyme; 4. 1 M LiNO 3 in tetraglyme; 5. 1 M LiNO M LiI in tetraglyme; and 6. 1 M LiNO M LiBr in tetraglyme. The water content detected by Karl Fischer titration was approximately 50 ppm for LiCF 3 SO 3 -based electrolytes and 130 ppm for LiNO 3 -based electrolytes. 2. Electrochemical test. The electrochemical measurements were performed on test cells consisting of a Li metal anode (16 mm in diameter and 0.2 mm thick), separator (GF/A, Whatman) and carbon cathode (16 mm in diameter and 0.2 mm thick). Three types of carbon cathodes, RGO, KB and CNT were prepared. The carbon cathodes except for CNT was prepared by painting pastes of Ketjenblack (KB) (EC600JD, Lion) or reduced graphene oxide (RGO) on a carbon paper (TGP- H-060, Toray). The RGO powder was prepared according to the procedure reported in our previous research. 1 The RGO paste was prepared by mixing a 10 wt.% polyvinylidene fluoride (PVDF) binder in NMP. The KB paste was prepared by mixing a 10 wt.% polytetrafluoroethylene (PTFE) binder in water. The loading of KB or RGO on the carbon paper S-2

3 was 1 ± 0.2 mg/cm 2. The CNT cathode was a self-standing sheet (approximately 50-µm thick; approximately 5 mg/cm 2 ) of single-wall CNTs without any binder (Meijo Nano Carbon, Japan). RGO cathodes were used for the morphological observation of the discharge products because of the sheet-like structure. Binder-free CNT cathodes were used for performing DEMS measurements to avoid any effects arising from the binders. We employed KB cathodes in the cycling tests to demonstrate the effectiveness of the commercial material. Homemade split-type stainless-steel test cells with two valves for oxygen flow were used. The inner cell volume was approximately 2 ml (diameter = 25 mm; depth = 4 mm). In total, 250 µl of the electrolyte was added into the cell, which ensured a well-soaked but non-flooded cathode surface. The cells were assembled in an Ar-filled glove box and measured under a continuous oxygen flow of approximately 10 ml min Differential electrochemical mass spectrometry (DEMS). The DEMS measurement system was developed in house using a high-resolution MS (JMS- 700, JEOL). The electrochemical flow cell was specially designed using the same components (anode, separator and cathode) as described above. The gas evolution during charging was continuously measured by flowing He gas at a rate of 2 ml/min. 4. Characterization Powder XRD was conducted with a New D8 ADVANCE (Bruker) powder X-ray diffractometer (Cu Kα sources, 40 kv, 40 ma). A special gastight sample holder was used. The morphology, chemical composition and crystal orientation were analysed by a field emission scanning electron microscope (FE-SEM, JSM-7800F, JEOL) equipped with energy dispersive X- ray spectrometry (EDS, X-Max N 50, Oxford) and electron backscatter diffraction (EBSD, S-3

4 Nordlys Nano, Oxford). Special holders were used to hold the electrodes after discharge to prevent contact with air. A cross-section polisher (IB-09020CP, JEOL) with a sample cooling system was used to prepare cross sections of the Li anode at approximately -150 C. XPS measurements were performed using a VersaProbe II Scanning XPS Microprobe (ULVAC-PHY). An Ar ion gun was used for depth-resolved analysis of the Li foil after cycling. Airtight sample transfer was realized by a special transfer vessel. The ICS-2100 ion chromatograph (Dionex) was used to determine the anion concentrations of the electrolyte from both the cathode and separator before and after cycling. The residual electrolytes in the carbon cathodes and separators were centrifuged and compared with the original electrolytes to determine the changes in the ion concentration. Liquid chromatography-mass spectrometry (LC/MS, Waters Corp.) was used to determine the side-reaction products (polymerized ethers) in electrolytes which were sampled from separator after cycling. The Li metal anodes were removed from the cell after the electrochemical tests, washed with tetraglyme several times in an Ar-filled glove box, and dried in vacuum. Airtight sample holders were used to transfer the samples for the XPS and SEM measurements. S-4

5 Figure S1. XRD patterns of KB cathodes (area of 2 cm 2 ) resulting from the same discharge capacity of 10 mah at a current of 0.1 ma. The XRD patterns for KB cathodes after discharge are nearly identical to those for RGO cathodes, shown in Figure 1b. All peaks are assigned to crystalline Li 2 O 2, although they are extremely weak and broad for the LiI-LiCF 3 SO 3 electrolyte. The absence of clear Li 2 O 2 peaks for the LiI-LiCF 3 SO 3 electrolyte can be attributed to the formation of amorphous-like Li 2 O 2. S-5

6 Figure S2. Galvanostatic discharge (at 0.1 ma to a cut-off voltage of 2.5 V) and subsequent charge profiles of Li-O 2 cells with RGO cathodes (area of 2 cm 2 ) in different electrolytes. S-6

7 Figure S3. SEM images of RGO cathodes after the first discharge-charge cycle, as shown in Figure S2, in different electrolytes with tetraglyme solvent: (a) 1 M LiCF3SO3; (b) 0.05 M LiI-1 M LiCF3SO3; (c) 0.05 LiBr-1 M LiCF3SO3; (d) 1 M LiNO3; (e) 0.05 M LiI-1 M LiNO3; (f) 0.05 M LiBr-1 M LiNO3; and (g) The pristine RGO cathodes. S-7

8 Figure S4. Galvanostatic discharge-charge voltage curves (up) and corresponding DEMS results of CO, CO 2, CH 3 O, and C 2 H 4 evolution during charging in different electrolytes with tetraglyme solvent: (a) 1 M LiCF 3 SO 3 ; (b) 0.05 M LiI-1 M LiCF 3 SO 3 ; (c) 0.05 M LiBr-1 M LiCF 3 SO 3 ; (d) 1 M LiNO 3 ; (e) 0.05 M LiI-1 M LiNO 3 ; and (f) 0.05 M LiBr-1 M LiNO 3. These measurements were performed simultaneously with the measurement of Figure 3 using CNT cathodes (area of 2 cm 2 ) at a current of 0.1 ma with a discharge capacity of 1.5 mah. CO spectra associated with CO 2 evolution at the end of charge are mostly due to the fragmentation of CO 2. S-8

9 Table S1. LC/MS analysis of the side-reaction products (polymerized ethers) in electrolytes sampled from separator after cycling. The cells were cycled five times using KB cathodes (area of 2 cm 2 ) at 0.1 ma with a fixed capacity of 2 mah. The table shows ratios of the total intensity of polymerized ethers (C 11 H 24 O 6, C 12 H 26 O 6, C 13 H 28 O 7, C 15 H 32 O 8, C 17 H 36 O 9, etc.) to that of the tetraglyme solvent (C 10 H 22 O 5 ). 1 M LiCF 3 SO 3 1 M LiNO 3 Without Li-halides 11.5 % 3.5 % With 0.05 M LiI 2.4 % 0.3 % With 0.05 M LiBr 8.3 % 0.9 % S-9

10 Figure S5. Cycling stability of Li-O 2 cells at a current of 0.2 ma and fixed capacity of 1 mah using KB cathodes (area of 2 cm 2 ) under an O 2 atmosphere with different electrolyte systems with tetraglyme solvent: (a) 1 M LiCF 3 SO 3 ; (b) 0.05 M LiI-1 M LiCF 3 SO 3 ; (c) 0.05 M LiBr-1 M LiCF 3 SO 3 ; (d) 1 M LiNO 3 ; (e) 0.05 M LiI-1 M LiNO 3 ; and (f) 0.05 M LiBr-1 M LiNO 3. The red circles in (b) and (e) indicate plateaus in the second discharge voltage at ~3.7 V due to the reduction of I 2 to I - 3. S-10

11 Figure S6. Variation of the ion concentration in the electrolyte obtained from cathodes and separators before and after cycling. (a) CF 3 SO 3 - concentration in different electrolytes; (b) NO 3 - concentration in different electrolytes; (c) Variation of the I - concentration in the 0.05 M LiI-1 M LiCF 3 SO 3 electrolyte; (d) Variation of the I - concentration in the 0.05 M LiI-1 M LiNO 3 electrolyte; (e) Variation of the Br - concentration in the 0.05 M LiBr-1 M LiCF 3 SO 3 electrolyte; and (f) Variation of I - concentration in the 0.05 M LiBr-1 M LiNO 3 electrolyte. S-11

12 Figure S7. SEM images of the surface of the Li anode. (a) Fresh Li foil as received. Li foil anodes (area of 2 cm 2 ) after twenty cycles at 0.2 ma with a fixed capacity of 1 mah in different electrolytes with tetraglyme solvent: (b) 1 M LiCF 3 SO 3 ; (c) 0.05 M LiI-1 M LiCF 3 SO 3 ; (d) 0.05 M LiBr-1 M LiCF 3 SO 3 ; (e) 1 M LiNO 3 ; (f) 0.05 M LiI-1 M LiNO 3 ; and (g) 0.05 M LiBr-1 M LiNO 3, and corresponding EDS elemental mapping: (3) carbon (red), (4) oxygen (green). S-12

13 Figure S8. SEM images of the cross section of the Li foil. (a) Fresh Li foil as received. The Lifoil anodes (area of 2 cm 2 ) after twenty cycles at a current of 0.2 ma with a fixed capacity of 1 mah in different electrolytes with tetraglyme solvent: (b) 1 M LiCF 3 SO 3 ; (c) 1 M LiNO 3 ; and (d) 0.05 M LiBr-1 M LiNO 3, and corresponding EDS elemental mapping: (3) carbon (red), (4) oxygen (green). S-13

14 Figure S9. SEM images of the surfaces of Li foil anodes in different electrolytes with tetraglyme solvent after discharge ((a), (b) and (c)) and after full recharge ((d), (e) and (f)). (a) and (d) were tested in 1 M LiCF 3 SO 3 ; (b) and (e) were tested in 1 M LiNO 3 ; (c) and (f) were tested in 0.05 M LiBr-1 M LiNO 3. The Li foil anodes (area 2 cm 2 ) were discharged/recharged at a current of 0.1 ma with a fixed capacity of 4 mah, corresponding to the Li dissolution/deposition thickness of 10 µm. S-14

15 Figure S10. SEM images of the Li foil anodes in different electrolytes with tetraglyme solvent after discharge ((a) and (c)) and after full recharge ((b) and (d)). (a) and (b) were tested in 1 M LiNO 3 ; (c) and (d) were tested in 0.05 M LiBr-1 M LiNO 3. The Li foil anodes (area 2 cm 2 ) were discharged/recharged at a current of 0.1 ma with a fixed capacity of 4 mah, corresponding to the Li dissolution/deposition thickness of 10 µm. For the corresponding EDS elemental mapping, (1) shows carbon (red) and (2) shows oxygen (green). Reference (1) Xin, X.; Ito, K.; Kubo, Y. Graphene/Activated Carbon Composite Material for Oxygen Electrodes in Lithium-Oxygen Rechargeable Batteries. Carbon 2016, 99, S-15