Lithium/sulfur cell discharge mechanism â An original approach for ...
Lithium/sulfur cell discharge mechanism – An original approach for intermediate species identification Céline Barchasza,b,*, Florian Moltonc, Carole Dubocc, Jean-Claude Leprêtreb, Sébastien Patouxa and Fannie Alloinb * French Atomic Energy and Alternative Energy Agency (CEA) – Laboratory of Innovation
for New Energy Technologies and Nanomaterials (LITEN) – 17 rue des Martyrs, 38054 Grenoble Cedex 9, France * [email protected]
S-1
ABSTRACT
The lithium/sulfur battery is a promising electrochemical system that has a high theoretical capacity of 1675 mAh.g-1, but its discharge mechanism is well-known to be a complex multistep process. As the active material dissolves during cycling, this discharge mechanism was investigated through the electrolyte characterization. Using High Performance Liquid Chromatography, UV-visible absorption and Electron Spin Resonance spectroscopies, we investigated the electrolyte composition at different discharge potentials in a TEGDME-based electrolyte. In this study, we propose a possible mechanism for sulfur reduction consisting of three steps. Long polysulfide chains are produced during the first reduction step (2.4-2.2 V vs. Li+/Li), such as S82- and S62-, as evidenced by UV and HPLC data. S3•- radical can also be found in solution because of disproportionation reaction. S42- is produced during the second reduction step (2.15-2.1 V vs. Li+/Li), thus pointing out the gradual decrease of the polysulfide chain lengths. Finally, short polysulfide species, such as S32-, S22- and S2-, are produced at the end of the reduction process, i.e. between 2.1 and 1.9 V vs. Li+/Li. The precipitation of the poorly soluble and insulating short polysulfide compounds was evidenced, thus leading to the positive electrode passivation and explaining the early end of discharge.
S-2
Experimental section
Two-electrode coin-cell preparation - Elemental sulfur (Refined, -100mesh, Aldrich) was mixed with poly(vinylidene fluoride) (PVdF 1015, Solvay), carbon black (Super P®, Timcal) in N-methyl-2-pyrrolidinone (NMP, anhydrous, 99.5%, Aldrich). The binder ratio was kept constant to 10 wt%. The carbon material was kept to high contents, i.e. of 45 and 30 wt%. After homogenization, the slurry was coated onto a 20 µm thick aluminum current collector by doctor blade technique. The resulting cathode was dried at 55°C for 24h, then cut into Ø14 mm disks, and finally dried 24h under vacuum at room temperature. The coating thickness was about 100 µm that gives a 20 µm thick cathode after drying. The positive electrode area was 1.539 cm2 and the sulfur loading was about 2 mg.cm-2. The positive electrode was then assembled in an argon-filled glovebox into CR2032 coin cells. A lithium metal foil was used as a negative electrode and a Celgard 2400® foil as a separator. A non-woven Viledon® (polypropylene-based) separator foil was also added between the cathode foil and the Celgard® to store an excess of electrolyte on the cathode side. A liquid electrolyte was prepared by mixing tetraethylene glycol dimethyl ether (TEGDME, 99%, Aldrich) and 1,3dioxolane (DIOX, anhydrous, 99.8%, Aldrich) with a volume ratio of 50/50. Lithium bis(trifluoromethane sulfone)imide (LiTFSI, 99.95%, Aldrich) was used as a lithium salt and was dissolved at 1 mol.L-1 in the mixed solvents.
S-3
Chronoamperometry experimental set-up – A two-electrode set-up was used to carry out the chronoamperometry measurements in an argon-filled glovebox. Active sulfur material was introduced in the electrolyte through the dissolution of lithium polysulfides, which were synthesized by mixing elemental sulfur (Refined, -100mesh, Aldrich) and lithium metal in tetraethylene glycol dimethyl ether (TEGDME, 99%, Aldrich). Thus, a 0.01 mol.L-1 concentration equivalent Li2S8 was prepared and stirred for 48h. When all materials have been dissolved, a dark brown and viscous solution was obtained and mixed with lithium bis(trifluoromethane sulfone)imide (LiTFSI, 99.95%, Aldrich) in order to reach a 1 mol.L-1 lithium salt concentration. This polysulfide-containing electrolyte, so-called catholyte, was used as the working solution (5 mL in a 50 mL glass cell) and was continuously stirred during the measurements. A platinum foil was employed as a working electrode and a lithium metal foil as both counter and reference electrodes. Lithium sulfide (Li2S, 99%) was also purchased from Aldrich in the view to better understand its optical and HPLC responses.
A three-electrode set-up would have been more rigorous, in order to get ride of the ohmic drop and to fully control the cell polarization potential. However, when introducing a third (metallic lithium) electrode as a reference, the lithium metal was quickly passivated in contact with polysulfides and its potential was modified. Even in a separated glass compartment, involving a sintered-glass disk for ionic junction, the metallic lithium was passivated because of the fast lithium polysulfide diffusion from the bulk to this third electrode. As a consequence, this third electrode could not be used as a reference, and a two electrode set-up was preferred. Using the same metal foil for both reference and counter electrodes, this enables the metallic lithium electrode to remain stable in potential during the measurements. The electrode potential was indeed checked after each chronoamperometry measurements and proved to remain close to 0 +/- 0.01 V vs. Li+/Li. And as the current value was quite low at
S-4
the end of experiments (99%, Aldrich) and NaH2PO4 (>99%, Aldrich), and was quickly introduced at the same time as MeTf. Finally, 0.5 mL of extra eluent was added to the solutions, in order to avoid any precipitate. All these preparative treatments were carried out in an argon-filled glovebox, enabling dimethyl polysulfide characterizations to be performed in air and in presence of water. The corresponding dimethyl polysulfides solutions should allow to reflect the initial lithium polysulfide distribution of the catholyte samplings. However, despite the precautions that were taken to prevent the disproportionation reactions to occur (buffer addition), it was not sure whether the methylation was modifying or not the initial lithium polysulfide distribution. Although these data helped understanding the electrochemical behavior of the Li/S system, HPLC data were thus cautiously considered. 0.3 mL samples were finally prepared for X-band ESR spectra recording, which were carried out at 100 K on a Bruker EMX apparatus, equipped with an ER-4192 ST Bruker cavity and an ER-4131 VT. The quartz tubes were filled in the glovebox, then quickly dipped and stored in liquid nitrogen, as to prevent the disproportionation side reactions to further occur.
S-7
EQUATIONS •
1st step (~2.4 V vs. Li+/Li)
Electrochemical reaction:
S8 + 2 e- → S82- (slow)
Concurrent disproportionation:
S82- → S62- + ¼ S8 (fast)
Overall electrochemical reaction: S8 + 2 e- → S62- + ¼ S8 Disproportionation reactions:
S62- → 2 S3•2 S62- → S52- + S72-
Specific discharge capacity = 279 mAh.g-1 •
2nd step (~2.1 V vs. Li+/Li)
Electrochemical reaction:
2 S62- + 2 e- → 3 S42- (fast)
Concurrent disproportionation:
2 S3•- → S62- (slow)
Overall electrochemical reaction: 4 S3•- + 2 e- → 3 S42Disproportionation reactions:
2 S42- ↔ S32- + S62-
Specific discharge capacity = 140 mAh.g-1 •
3rd step (~2 V vs. Li+/Li)
Electrochemical reactions:
3 S42- + 2 e- → 4 S322 S32- + 2 e- → 3 S22S22- + 2 e- → 2 S2-
Or
S42- + 2 e- → 2 S22(S22- + S42- ↔ 2 S32-) S22- + 2 e- → 2 S2-
Overall electrochemical reaction: S42- + 6 e- → 4 S2Specific discharge capacity = 1256 mAh.g-1
S-8
0.4 0.3
5 µV.s-1 -1
10 µV.s
Current / mA
0.2
-1
20 µV.s
0.1 0 -0.1 -0.2 -0.3 -0.4 1.5
1.75
2
2.25
2.5
2.75
3
+
Voltage vs. Li /Li / V Figure S-1. Cyclic voltammograms obtained in a liquid electrolyte (LiTFSI 1 mol.L-1 + TEGDME/DIOX 50/50) for a sulfur electrode (S/C/binder at 45/45/10 wt%) using three different scan rates.
S-9
1.2 TEGDME Sulfur 10 mm cell
Absorbance / a.u.
1
0.8 0.6
0.4 0.2 0 250
300
350
400
Wavelength / nm Figure S-2. UV-visible absorption spectrum of elemental sulfur dispersed in TEGDME (10 mm quartz cell).
S-10
1
Wavelength / nm
Corresponding polysulfides
617
S3•-
560
S82-
470
S62-
450
S52-
420
S42-
350
S62- (?)
340
S32-
300
S62- (?)
280
S8
265
S22-
260
S62- (?)
1 mm cell
Absorbance / a.u.
0.8
0.6
0.4
0.2
0 200
300
400
500
600
700
800
Wavelength / nm
Figure S-3. On the left side, UV-visible absorption spectra obtained for a solution of lithium polysulfides (10-3 mol.L-1 equivalent Li2S8 dissolved in TEGDME). The measurements were carried out during one day each 10 minutes, so as to detect the disproportionation reactions. The black arrows represent the bands’ evolution over time. On the right side, the possible UV band attribution supported by experimental and literature data.1-2-3
S-11
8 7
Dimethyl polysulfides
2
6
1.5
5 3 4 8
3
1
Intensity / a.u.
Intensity / a.u.
2
0.5
0
4 2
Li2S8 & S8
-0.5
1
5 6
-1 60
7
0
65
70
75
80
85
90
Retention time / min
10
20
30
40
50
60
70
80
Retention time / min
Figure S-4. Chromatogram obtained for a solution of lithium polysulfides (after species methylation, solution of 10-2 mol.L-1 equivalent Li2S8 dissolved in TEGDME).
S-12
7
1 10
2
a
b 6
8 10 Peak integration / a.u.
Log (Retention time)
1.8
1.6
1.4
1.2
6
6 10
6
4 10
6
2 10
1
0
0.8 1
2
3
4
5
6
7
8
0
9
n of Li S
2 n
0.005 0.01 0.015 0.02 0.025 -1 Lithium polysulfide concentration / mol.L
Figure S-5. On the left side (a), linear relation obtained when the logarithm of the retention time is plotted versus the number of sulfur atoms in the polysulfide chain. On the right side (b), linear relation obtained when the peak integration is plotted versus the overall sulfur concentration.
S-13
50 S
2
S
3
% of sulfur compounds
40
S
4
S
5
S
30
6
20
10
0 0.001
0.01
0.1
1
Overall sulfur concentration / mol.L-1 Figure S-6. Polysulfide distribution depending on the overall sulfur concentration.
S-14
REFERENCES (1) Leghié, P.; Levillain, E.; Lelieur, J.P.; Lorriaux, A.; New J. Chem., 1996, 20, 1121-1130 (2) Li, Y.; Zhan, H.; Liu, S.; Huang, K.; Zhou, Y.; J. Power Sources, 2010, 195, 2945-2949 (3) Heatley, N.G.; Page, E.J.; Anal. Chem., 1952, 24, 1854
S-15
for New Energy Technologies and Nanomaterials (LITEN) – 17 rue des Martyrs, 38054 Grenoble Cedex 9, France * [email protected]
S-1
ABSTRACT
The lithium/sulfur battery is a promising electrochemical system that has a high theoretical capacity of 1675 mAh.g-1, but its discharge mechanism is well-known to be a complex multistep process. As the active material dissolves during cycling, this discharge mechanism was investigated through the electrolyte characterization. Using High Performance Liquid Chromatography, UV-visible absorption and Electron Spin Resonance spectroscopies, we investigated the electrolyte composition at different discharge potentials in a TEGDME-based electrolyte. In this study, we propose a possible mechanism for sulfur reduction consisting of three steps. Long polysulfide chains are produced during the first reduction step (2.4-2.2 V vs. Li+/Li), such as S82- and S62-, as evidenced by UV and HPLC data. S3•- radical can also be found in solution because of disproportionation reaction. S42- is produced during the second reduction step (2.15-2.1 V vs. Li+/Li), thus pointing out the gradual decrease of the polysulfide chain lengths. Finally, short polysulfide species, such as S32-, S22- and S2-, are produced at the end of the reduction process, i.e. between 2.1 and 1.9 V vs. Li+/Li. The precipitation of the poorly soluble and insulating short polysulfide compounds was evidenced, thus leading to the positive electrode passivation and explaining the early end of discharge.
S-2
Experimental section
Two-electrode coin-cell preparation - Elemental sulfur (Refined, -100mesh, Aldrich) was mixed with poly(vinylidene fluoride) (PVdF 1015, Solvay), carbon black (Super P®, Timcal) in N-methyl-2-pyrrolidinone (NMP, anhydrous, 99.5%, Aldrich). The binder ratio was kept constant to 10 wt%. The carbon material was kept to high contents, i.e. of 45 and 30 wt%. After homogenization, the slurry was coated onto a 20 µm thick aluminum current collector by doctor blade technique. The resulting cathode was dried at 55°C for 24h, then cut into Ø14 mm disks, and finally dried 24h under vacuum at room temperature. The coating thickness was about 100 µm that gives a 20 µm thick cathode after drying. The positive electrode area was 1.539 cm2 and the sulfur loading was about 2 mg.cm-2. The positive electrode was then assembled in an argon-filled glovebox into CR2032 coin cells. A lithium metal foil was used as a negative electrode and a Celgard 2400® foil as a separator. A non-woven Viledon® (polypropylene-based) separator foil was also added between the cathode foil and the Celgard® to store an excess of electrolyte on the cathode side. A liquid electrolyte was prepared by mixing tetraethylene glycol dimethyl ether (TEGDME, 99%, Aldrich) and 1,3dioxolane (DIOX, anhydrous, 99.8%, Aldrich) with a volume ratio of 50/50. Lithium bis(trifluoromethane sulfone)imide (LiTFSI, 99.95%, Aldrich) was used as a lithium salt and was dissolved at 1 mol.L-1 in the mixed solvents.
S-3
Chronoamperometry experimental set-up – A two-electrode set-up was used to carry out the chronoamperometry measurements in an argon-filled glovebox. Active sulfur material was introduced in the electrolyte through the dissolution of lithium polysulfides, which were synthesized by mixing elemental sulfur (Refined, -100mesh, Aldrich) and lithium metal in tetraethylene glycol dimethyl ether (TEGDME, 99%, Aldrich). Thus, a 0.01 mol.L-1 concentration equivalent Li2S8 was prepared and stirred for 48h. When all materials have been dissolved, a dark brown and viscous solution was obtained and mixed with lithium bis(trifluoromethane sulfone)imide (LiTFSI, 99.95%, Aldrich) in order to reach a 1 mol.L-1 lithium salt concentration. This polysulfide-containing electrolyte, so-called catholyte, was used as the working solution (5 mL in a 50 mL glass cell) and was continuously stirred during the measurements. A platinum foil was employed as a working electrode and a lithium metal foil as both counter and reference electrodes. Lithium sulfide (Li2S, 99%) was also purchased from Aldrich in the view to better understand its optical and HPLC responses.
A three-electrode set-up would have been more rigorous, in order to get ride of the ohmic drop and to fully control the cell polarization potential. However, when introducing a third (metallic lithium) electrode as a reference, the lithium metal was quickly passivated in contact with polysulfides and its potential was modified. Even in a separated glass compartment, involving a sintered-glass disk for ionic junction, the metallic lithium was passivated because of the fast lithium polysulfide diffusion from the bulk to this third electrode. As a consequence, this third electrode could not be used as a reference, and a two electrode set-up was preferred. Using the same metal foil for both reference and counter electrodes, this enables the metallic lithium electrode to remain stable in potential during the measurements. The electrode potential was indeed checked after each chronoamperometry measurements and proved to remain close to 0 +/- 0.01 V vs. Li+/Li. And as the current value was quite low at
S-4
the end of experiments (99%, Aldrich) and NaH2PO4 (>99%, Aldrich), and was quickly introduced at the same time as MeTf. Finally, 0.5 mL of extra eluent was added to the solutions, in order to avoid any precipitate. All these preparative treatments were carried out in an argon-filled glovebox, enabling dimethyl polysulfide characterizations to be performed in air and in presence of water. The corresponding dimethyl polysulfides solutions should allow to reflect the initial lithium polysulfide distribution of the catholyte samplings. However, despite the precautions that were taken to prevent the disproportionation reactions to occur (buffer addition), it was not sure whether the methylation was modifying or not the initial lithium polysulfide distribution. Although these data helped understanding the electrochemical behavior of the Li/S system, HPLC data were thus cautiously considered. 0.3 mL samples were finally prepared for X-band ESR spectra recording, which were carried out at 100 K on a Bruker EMX apparatus, equipped with an ER-4192 ST Bruker cavity and an ER-4131 VT. The quartz tubes were filled in the glovebox, then quickly dipped and stored in liquid nitrogen, as to prevent the disproportionation side reactions to further occur.
S-7
EQUATIONS •
1st step (~2.4 V vs. Li+/Li)
Electrochemical reaction:
S8 + 2 e- → S82- (slow)
Concurrent disproportionation:
S82- → S62- + ¼ S8 (fast)
Overall electrochemical reaction: S8 + 2 e- → S62- + ¼ S8 Disproportionation reactions:
S62- → 2 S3•2 S62- → S52- + S72-
Specific discharge capacity = 279 mAh.g-1 •
2nd step (~2.1 V vs. Li+/Li)
Electrochemical reaction:
2 S62- + 2 e- → 3 S42- (fast)
Concurrent disproportionation:
2 S3•- → S62- (slow)
Overall electrochemical reaction: 4 S3•- + 2 e- → 3 S42Disproportionation reactions:
2 S42- ↔ S32- + S62-
Specific discharge capacity = 140 mAh.g-1 •
3rd step (~2 V vs. Li+/Li)
Electrochemical reactions:
3 S42- + 2 e- → 4 S322 S32- + 2 e- → 3 S22S22- + 2 e- → 2 S2-
Or
S42- + 2 e- → 2 S22(S22- + S42- ↔ 2 S32-) S22- + 2 e- → 2 S2-
Overall electrochemical reaction: S42- + 6 e- → 4 S2Specific discharge capacity = 1256 mAh.g-1
S-8
0.4 0.3
5 µV.s-1 -1
10 µV.s
Current / mA
0.2
-1
20 µV.s
0.1 0 -0.1 -0.2 -0.3 -0.4 1.5
1.75
2
2.25
2.5
2.75
3
+
Voltage vs. Li /Li / V Figure S-1. Cyclic voltammograms obtained in a liquid electrolyte (LiTFSI 1 mol.L-1 + TEGDME/DIOX 50/50) for a sulfur electrode (S/C/binder at 45/45/10 wt%) using three different scan rates.
S-9
1.2 TEGDME Sulfur 10 mm cell
Absorbance / a.u.
1
0.8 0.6
0.4 0.2 0 250
300
350
400
Wavelength / nm Figure S-2. UV-visible absorption spectrum of elemental sulfur dispersed in TEGDME (10 mm quartz cell).
S-10
1
Wavelength / nm
Corresponding polysulfides
617
S3•-
560
S82-
470
S62-
450
S52-
420
S42-
350
S62- (?)
340
S32-
300
S62- (?)
280
S8
265
S22-
260
S62- (?)
1 mm cell
Absorbance / a.u.
0.8
0.6
0.4
0.2
0 200
300
400
500
600
700
800
Wavelength / nm
Figure S-3. On the left side, UV-visible absorption spectra obtained for a solution of lithium polysulfides (10-3 mol.L-1 equivalent Li2S8 dissolved in TEGDME). The measurements were carried out during one day each 10 minutes, so as to detect the disproportionation reactions. The black arrows represent the bands’ evolution over time. On the right side, the possible UV band attribution supported by experimental and literature data.1-2-3
S-11
8 7
Dimethyl polysulfides
2
6
1.5
5 3 4 8
3
1
Intensity / a.u.
Intensity / a.u.
2
0.5
0
4 2
Li2S8 & S8
-0.5
1
5 6
-1 60
7
0
65
70
75
80
85
90
Retention time / min
10
20
30
40
50
60
70
80
Retention time / min
Figure S-4. Chromatogram obtained for a solution of lithium polysulfides (after species methylation, solution of 10-2 mol.L-1 equivalent Li2S8 dissolved in TEGDME).
S-12
7
1 10
2
a
b 6
8 10 Peak integration / a.u.
Log (Retention time)
1.8
1.6
1.4
1.2
6
6 10
6
4 10
6
2 10
1
0
0.8 1
2
3
4
5
6
7
8
0
9
n of Li S
2 n
0.005 0.01 0.015 0.02 0.025 -1 Lithium polysulfide concentration / mol.L
Figure S-5. On the left side (a), linear relation obtained when the logarithm of the retention time is plotted versus the number of sulfur atoms in the polysulfide chain. On the right side (b), linear relation obtained when the peak integration is plotted versus the overall sulfur concentration.
S-13
50 S
2
S
3
% of sulfur compounds
40
S
4
S
5
S
30
6
20
10
0 0.001
0.01
0.1
1
Overall sulfur concentration / mol.L-1 Figure S-6. Polysulfide distribution depending on the overall sulfur concentration.
S-14
REFERENCES (1) Leghié, P.; Levillain, E.; Lelieur, J.P.; Lorriaux, A.; New J. Chem., 1996, 20, 1121-1130 (2) Li, Y.; Zhan, H.; Liu, S.; Huang, K.; Zhou, Y.; J. Power Sources, 2010, 195, 2945-2949 (3) Heatley, N.G.; Page, E.J.; Anal. Chem., 1952, 24, 1854
S-15
