1

Lawrence Berkeley National Laboratory Daihatsu Motor Co., Ltd. Ayako Kawase Toyota Central R&D Laboratories Shoich Shirai Kansai University Yoshinari Yamoto, Ryuichi Arakawa Tokyo Institute of Technology Toshikazu Takata

Electrochemical Reactions of Lithium-sulfur Batteries:

an Analytical Study Using the Organic Conversion Technique

Feb 12. 2016 in Faraday theatre

material, redox reactions could be monitored in real time in asimilar coin cell using synchrotron-based operando XANES(Figure 3 and Figure S4; see the Supporting Information for

additional details). Distortion was successfully minimized byconstraining the sulfur species to nanodimensions in thespheres (Figure S5, Supporting Information). Figure 3a showsthe operando XANES results during the first discharge at a C/5rate and for the second full cycle at C/10. First and seconddischarge capacities of ∼1200 and 1100 mAh·g−1 were achieved

at these slower rates that were utilized to minimize thecompositional change between consecutive spectra. Figure 3bcompares selected XANES spectra. The S K-edge step does notdecrease during discharge, suggesting that the solvatedpolysulfides are largely contained, reflecting the effective designof the nanospheres. For comparison, Figure 3c displays the exsitu spectra of the standard sulfur species. The strongabsorption peak observed at 2469.5 eV in the cathodecomposite is assigned to the high-intensity “white line” (1s→ 3p transition) of elemental sulfur along with a higher-energyfeature at 2476.8 eV15,16 (Figure 3c, top).The white line intensity is strongly affected by the

electrochemistry, owing to S−S bond cleavage. Contrary tosome previous XRD studies that did not detect crystalline α-S8after one full cycle,24,25 the XANES displays a spectrumcharacteristic of pure elemental sulfur at the end of charge(Figure 3c, top). At intermediate states of charge, a low-energypeak appears at 2467.7 eV. Comparison with the referencespectra (Figure 3c, middle) unequivocally shows that this low-energy feature is associated with linear polysulfides.14 Itsintensity increases with shortening of the polysulfide chain,opposite to the decrease in intensity of the white line. Thepositions of these two (white line and low-energy) bandsremain the same irrespective of the chain length, n, in Li2Sn (2 <n < 8), meaning that the polysulfides are primarily distinguishedby the ratio of their intensities. At the end of discharge, theintensity is minimal at the white line energy, in agreement withthe Li2S reference that exhibits two peaks at 2470.8 and 2473.7eV.16,26 The spectra at the discharged state do not exhibit anylow-energy intensity characteristic of linear polysulfides.Comparison with pure Li2S shows that some α-S8 remains inthe cathode composite (Figure 3c, bottom), which is consistentwith the electrochemistry that shows 75% capacity compared totheoretical results upon discharge. The reason for this isexplained below.Because the standards represent the sulfur species

successively formed by electrochemistry, operando spectracan be described with a linear combination fit (LCF).16

Considering the number of reference spectra, multiple solutionsexist, among which the LCF constructed from the minimumcombination of {α-S8, S6

2−, S42−, and S2−} gave the best fit and

agreement with the electrochemistry. The contribution of S22−

was clearly ruled out (see Figure S6, Supporting Informationfor full discussion and alternative fits). S3

2− and S82− are not

isolable due to complex equilibria in solution, as confirmedrecently by operando UV−vis spectrometry of Li−S catholytes;spectra could only be interpreted in terms of an averagestoichiometry of dissolved polysulfides, even though thetechnique should be sensitive to their chain length.11 The useof S4

2− and S62− here best represents the medium- and long-

chain polysulfide contributions, as shown by Figure S7(Supporting Information), which compares the compositionof the sulfur cathode upon cycling based on the XANES and onthe electrochemistry. The perfect accordance observed uponcharge proves that the area and depth probed are representativeof the overall cathode and that sulfur speciation is effectivelyachieved.For clarity, the following omits the first discharge at C/5 to

focus only on one full cycle at C/10 (see Figure S8, SupportingInformation). As noted above, the discharged cathode containsa fraction of unreacted elemental sulfur estimated at 20−25 wt% from the fit (Figure 3c, bottom). Figure 4a displays thecharge from Li2S to S8.

Figure 3. Sulfur K-edge XANES upon cycling and reference spectrashowing the following: (a) evolution of absorbance as a function of theelectrochemical cycling, at C/5 (first discharge) and then C/10; theends of the (dis)charges are highlighted in red; spectra were acquiredcontinuously to minimize the composition change between twospectra to 50 mAh·g−1sulfur (i.e., 3% of the total capacity); (b) selectedspectra during the cycling, labeled using the average composition LixSas estimated by the electrochemistry; (c) reference spectra forelemental sulfur (top), linear polysulfides (S2

2−, S42−, and S6

2−,middle) and Li2S (bottom), together with the initial, charged state(red line in the top panel) and discharge states (red lines in thebottom panel).

The Journal of Physical Chemistry Letters Letter

dx.doi.org/10.1021/jz401763d | J. Phys. Chem. Lett. 2013, 4, 3227−32323229

2

〈UV-Vis〉

Challenge

Absence of reference spectra for isolated polysulfide

M. Patel, et al.ChemSusChem. 6, 1177−1181 (2013).

[Introduction]

・Low utilization of S・Cycle degradation

Problem of Li-S

・Insulating property・Dissolution-deposition cycling

Factor

Cell reaction is unclear

〈LC-Mass〉 High reactivity of polysulfide ⇒Decomposed during the injection and ionization process

Insufficient sensitivity 〈33S NMR〉

A substantially different method of analysis is required to address shortcomings and to obtain detailed reaction mechanism

Previous studies

※ M. Cuisinier, et. al. J. Phys. Chem. Lett. 4, 3227 (2013)

〈S-K edge XAS〉

〈IR、Raman〉

Other methods

3

Abstract

[Introduction]

In Organic / Environmental chemistry

※1 T. Takata, el. al. Inorg. chem. 42, 3712 (2003). ※2A. Kamyshny, Jr. et. al. Environ. Sci. Technol. 41, 2395 (2007)

Polysulfide can be derivatized to organic polysulfide and identified by 1H NMR※1 and HPLC※2

1.5

1.7

1.9

2.1

2.3

2.5

2.7

0 500 1000 1500 2000 2500Capacity / mAh/g(S)

Volta

ge /

V vs

Li/L

i+

Li S S S

S Li

LC/MS

Making polysulfide unreactive via derivatization

⇒Reflecting the cell reaction Sulfur cell reaction

S S S

S

LC/MS

33S NMR 1H NMR

Stabilized during the ionization process

1H NMR is applicable by addition of proton

Conventional

The organic conversion technique was applied to lithium polysulfide produced in electrochemical reactions in cells.

UV-Vis UV-Vis Theoretical calculations for peak assignments

This study

Cl

4

Methods

Cell assembling Cathode Sulfur, Carbon, Binder

Electrolyte 1.5M LiTFSI/DME:DOL(1:1) Separator PE Separator

Anode Li foil

Measurement Method CC

Current Rate 0.8 [mA/cm2] Voltage range 1.5～2.7[V]

CDCl3

NMR

Li S S S Li S

LC/MS

THF

LC

Mobile phase A: 5mM CH3COONH4 aq　 B: CH3OH 　A:B=1:9

Column ODS 5um Flow rate 200 uL/min Detector 254 nm UV

MS Ion mode positive

Vaporizer temperature 250 ℃

1H NMR Solvent CDCl3

【Sampling】

[Experimental]

S S S

S

1H NMR calculation software Gaussian03 method GIAO b3lyp/6-311+g(2d,p)

Extraction

【Analysis】

< a few seconds

Dimethoxyethane (DME) & Benzyl chloride

5

1.5

1.7

1.9

2.1

2.3

2.5

2.7

0 500 1000 1500 2000 2500Capacity / mAh/g(S)

Volta

ge /

V vs

Li/L

i+

1.5

1.7

1.9

2.1

2.3

2.5

2.7

0 500 1000 1500 2000 2500Capacity / mAh/g(S)

Volta

ge /

V vs

Li/L

i+

Discharge-charge profile

Discharge process Charge process

[Result & Discussion]

(2-1)

(3-4)

(4)

(5)

(6-1)

(6-2)

(7) (8) (9)

(2-2) (3-3) (3-2)

(1)

(3-1)

(6-3)

1st plateau

2nd plateau 2nd plateau 1st plateau

Transition state Transition state

Sampling at 15 points during discharge-charge process（1st cycle)

6

15 20 25 30

LC/MS R

elat

ive

Inte

nsity

/ a.

u.

(1)

(2-1)

(2-2)

(3-1)

(3-4)

(4)

(5)

Bz2S2

Bz2S

S8

(3-3)

(3-2)

Bz2S

(1)

(2-1)

(2-2)

(3-1)

[Result & Discussion]

0 5 10 15

×1 Bz2S2

0 5 10 15

S8

295 296 297 298 299 300 m/z

0

20

40

60

80

100 0

20

40

60

80

100 R

elat

ive

Abun

danc

e 296.0590

297.0622 298.0547

299.0580

296.0596

297.0629 298.0554

299.0587

[Bz2S3+NH4]+

RT: 6.38-6.98 min 2.86E5

327 328 329 330 331 332 m/z

0

20

40

60

80

100 0

20

40

60

80

100 328.0309

330.0266 329.0341

331.0300

328.0317

330.0275 329.0350 331.0308

[Bz2S4+NH4]+

RT: 7.86-8.44 min

Theoretical isotope pattern C14H14S4N1H4

1.85E5

Theoretical isotope pattern C14H14S3N1H4

×20 Bz2S7 Bz2S8 Bz2S6

Bz2S4

Bz2S3 Bz2S5 All LC peak can be

determined by MS

Intermediate was identified successfully

7

LC/MS ～ Intermediate ～ [Result & Discussion]

All possible species produced by decomposition of S8

including Li2S7 and Li2S5 (undetected previously) were detected.

1st

pla

teau

2nd

plat

eau

2nd

plat

eau

1st

pla

teau

Dis

char

ge

Char

ge

S8 Li2S 8 Li2S 7 Li2S 6 Li2S 5 Li2S 4 Li2S 3 Li2S 2 Li2S

- 456 424 392 360 328 296 264 -

12.6 25.2 18.6 14 10.6 8 6.5 5.1 4.61 184 0 0 0 0 0 0 0 02-1 103 6 10 27 109 86 68 4 142-2 32 6 11 33 135 112 97 8 83-1 14 4 7 27 129 126 144 21 153-2 4 1 3 15 80 116 242 101 253-3 0 0 0 0 0 9 264 163 233-4 0 0 0 0 0 8 100 392 294 0 0 0 0 1 1 1 494 1345 0 0 0 0 0 0 2 348 4896-1 0 0 0 0 0 1 1 253 5056-2 0 0 0 0 2 1 0 328 1716-3 0 0 1 3 31 68 216 169 307 2 1 2 11 73 104 231 61 208 35 5 10 29 115 100 83 5 119 44 7 14 36 134 108 80 6 4

m /z

([Bz2S x+NH4]+)

Retention tim e/m in.

Relative amount at observation points

8 Bz2S6

Rel

ativ

e Am

ount

1 2-1 3-1 4 5 6-1 7 8 9 3-4 6-3 3-3

S8 Bz2S6

Bz2S5

Bz2S3

Bz2S2 Bz2S

Bz2S6

S8

Bz2S3

Bz2S4 Bz2S4

Bz2S5

Discharge process Charge process

1.5

1.7

1.9

2.1

2.3

2.5

2.7

0 500 1000 1500 2000 2500Capacity [mAh/g(S)]

Volta

ge [V

vs

Li/L

i+] (2-1)

(3-4)

(4)

(5)

(6-1)

(6-2)

(7) (8) (9)

(3-1)

(2-2) (3-3)(3-2)

(1)2nd plateau

1st plateau2nd plateau 1st plateau

LC/MS [Result & Discussion]

Shuttle reactions Longer polysulfide (> S6)cause shuttle reactions

S22- + 2e- → 2S2-

S32- + 2e- → S2

2- + S2-

S82- + 2e- → S6

2- + S22-

S82- + 2e- → S5

2- + S32-

S82- + 2e- → 2S4

2-

S72- + 2e- → S4

2- + S32-

S62- + 2e- → 2S3

2-

S52- + 2e- → S3

2- + S22-

S42- + 2e- → 2S2

2-

2nd stage

2nd stage (point 2-1 ～ 3-3)

4th stage

4th stage (point 3-4 ～ 5)

1st stage

1st stage (point 1 ～ 3-1)

S8 + 2e- → S82-

S8 + 4e- → S72- + S2-

S8 + 4e- → S62- + S2

2-

S8 + 4e- → S52- + S3

2-

S8 + 4e- → 2S42-

3rd stage

3rd stage (point 3-1 ～ 4)

9 3.94.04.14.24.3

Chemical shift / ppm

1H NMR

(1)

(2-1)

(3-1)

(4)

(5)

(2-2)

(3-2)

(3-3)

(3-4)

[Result & Discussion]

Bz2S2

Bz2S

Bz2S2/ Bz2S

Experimental

Sam pling point

Relative amount

Bz2S3

Bz2S2 /Bz2S

Bz2S4 Bz2S5

1 2-1 3-1 4 3-4 3-3 5 3.53.63.7Chemical shift / ppm

×3 ×1 Calculation model

Bz2S5 Bz2S4 Bz2S3

Calculation

Calculated relative chem. shift

Bz2S3 Bz2S7

Bz2S6 Bz2S8 Bz2S4

Bz2S5

10

1.5

1.7

1.9

2.1

2.3

2.5

2.7

0 500 1000Capacity / mAh/g(S)

Volta

ge /

V vs

Li/L

i+Discharge process

1st stage

2nd stage

3rd stage

4th stage

S8 ⇒ (Li2S8, Li2S7, Li2S6,) Li2S5, Li2S4, Li2S3, (Li2S2, Li2S)

Li2S8, Li2S7, Li2S6, Li2S5, Li2S4 ⇒ Li2S3, (Li2S2, Li2S)

Li2S3 ⇒ Li2S2, Li2S

Li2S2 ⇒ Li2S

(1) (3-1)

(2-1) (3-3)

(3-1) (4)

(3-4) (5)

Summary [Conclusion]

　We hope this work will be applied to various studies and accelerate the development of Li-S battery

Future hope 1st plateau 2nd plateau

Transition state

(2-1)

(3-4)

(4)

(5)

(2-2) (3-3) (3-2)

(1) (3-1)

A.Kawase, S. Shirai, T. Yamoto, R. Arakawa and T. Toshikazu, Phys.Chem.Chem.Phys., 2014, 16, 9344

Thank you for your attention!