Clarification of Solvent Effects on Discharge Products in Li-O2

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Clarification of Solvent Effects on Discharge Products in Li-O2 Batteries through a Titration Method Young Joo Lee, Won-Jin Kwak, Yang-Kook Sun, and Yun Jung Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14279 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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ACS Applied Materials & Interfaces

1

Clarification of Solvent Effects on Discharge

2

Products in Li-O2 Batteries through a Titration

3

Method

4

Young Joo Lee,†,‡ Won-Jin Kwak,†,‡ Yang-Kook Sun,*,† and Yun Jung Lee*,†

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†Department of Energy Engineering, Hanyang University, Seoul 04763, Republic of Korea

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KEYWORDS: Dimethyl Sulfoxide (DMSO), Tetraethylene glycol dimethyl ether (TEGDME),

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UV-Vis titration, Li2O2, LiOH

8

9

ABSTRACT. As a substitute for current lithium-ion batteries (LIBs), rechargeable Li-O2 batteries

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have attracted much attention due to their theoretically high energy density, but many challenges

11

continue to exist. For the development of this still problematic battery, understanding and

12

controlling the main discharge product (Li2O2, lithium peroxide) are of paramount importance.

13

Here, we comparatively analyzed the amount of Li2O2 in the cathodes discharged at various

14

discharge capacities and current densities.in DMSO and TEGDME solvents. The precise

15

assessment entailed revisiting and revising the UV-Vis titration analysis. The amount of Li2O2

16

electrochemically formed in DMSO was less than that formed in TEGDME at the same capacity

17

and even at a much higher full discharge capacity in DMSO than in TEGDME. Based on our

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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analytical experimental results, this unexpected result was ascribed to the presence of soluble

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LiO2-like intermediates that remained in the DMSO solvent and the chemical transformation of

3

Li2O2 to LiOH, both of which originated from the inherent properties of the DMSO solvent.

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1. INTRODUCTION

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Rechargeable lithium oxygen (Li-O2) batteries based on the overall reaction 2 Li++2 e-+ O2 ↔

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Li2O2 have been actively investigated for their high theoretical energy density (11,140 Wh kg -

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1

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roundtrip efficiency, poor rate capability, and poor cycle life. 1, 2, 4 To develop this prospective but

9

problematic battery, understanding and controlling the main discharge product (Li 2O2, lithium

10

peroxide) are pre-requisites. The poor decomposition of Li2O2 has been addressed employing

11

various catalysts either solid5-7 or liquid states.8 However, the actual processes in the Li-O2 cells

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behind the simple overall reaction are complicated and have not been fully unveiled yet.

1-3

Li).

Despite the potential for high energy, there are many challenges to overcome such as low

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Studies devoted to analyzing the mechanisms of formation and decomposition of the discharge

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products and side reactions have been reported. 9-11 The choice of electrolyte and its influence on

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the electrochemical processes inside the Li-O2 cells described above is of paramount importance,

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yet the effects of the solvents remain controversial. Among the candidate solvents, ethers and

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dimethyl sulfoxide (DMSO) have mostly been studied for use in Li-O2 batteries.12-14 The high

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donor number DMSO solvent demonstrated high discharge capacity through solution mediated

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Li2O2 formation, but appeared vulnerable to electrolyte decomposition. 10, 12 It is reported that

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DMSO is prone to be oxidized by electrochemical oxidation and superoxide anions to produce

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dimethyl sulfone (DMSO2).14, 15 Furthermore, the formation of LiOH besides Li 2O2 has been

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observed in carbon-based cathodes employing DMSO-based electrolytes, however, the relative

23

ratio of Li2O2 to LiOH differs in different publications.13, 15-17 In contrast, ethers exhibited low


2

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discharge capacity due to the surface mechanism for Li 2O2 formation, but have a longer cycle life

2

owing to their relatively high stability. 10, 18 For a comprehensive understanding of the effect of

3

ethers and DMSO solvent in Li-O2 batteries, it is of great importance to establish the relation

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between the solvents and the main discharge products, Li 2O2.

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Although it was reported that a larger amount of initial discharge capacity could be extracted in

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DMSO,12 there was no confirmation of how much Li 2O2 is formed at the same capacity or at the

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full capacity compared to ether. Clear information on the discharge products contributes to

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understanding the reaction mechanism and side reactions of Li-O2 batteries. The assessment of the

9

amount of Li2O2 formed necessitates the use of quantitative analyses such as differential

10

electrochemical mass spectrometry (DEMS), electrochemical quartz crystal microbalance

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(EQCM), and UV-Vis titration.15, 19-23 Among them, titration analysis using UV-Vis spectroscopy

12

is a relatively simple and effective method to measure the amount of Li 2O2 directly and selectively.

13

22, 23

Based on this technique, the effects of solvent and salt on Li 2O2 have been reported.24-26

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Herein, we present our comparative analysis of the amount of discharge products formed at

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diverse current densities and capacities for DMSO and tetraethylene glycol dimethyl ether

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(TEGDME, tetraglyme ether) to evaluate the actual formation and transformation of Li 2O2. The

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quantitative titration analyses using UV-Vis revealed that the amount of electrochemically formed

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Li2O2 in cathodes with DMSO solvent was actually less than that of the system with TEGDME at

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the same capacities from 0.5 to 3.0 mAh. Even at the two times higher full discharge capacity of

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cells with DMSO, the amount of Li2O2 was lower, that is, approximately half the amount of Li 2O2

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in cells with TEGDME at much lower full discharge capacity. This unexpected result was ascribed

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to the presence of soluble LiO2-like species and transformation of Li2O2 to LiOH, both of which

23

originated from the inherent properties of DMSO. Although DMSO promotes the solution


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mediated synthesis of large toroidal Li2O2, which is usually regarded to result in higher discharge

2

capacity, it could lead to the formation of a lesser amount of Li2O2, which is detrimental to the

3

reversibility of the cells.

4 5

2. EXPERIMENTAL SECTION

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2.1. Materials. Commercial anhydrous Li2O2 powder (Sigma Aldrich) was stored inside a

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glovebox. Bis(trifluoromethane) sulfonamide (LiTFSI) lithium salt (99.95 %, Sigma Aldrich) was

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thoroughly dried in a vacuum oven before dissolving it in the electrolyte solvent. Residual H2O

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was removed from the solvent by treating tetraethylene glycol dimethyl ether (TEGDME, > 98%,

10

TCI) and dimethyl sulfoxide (DMSO > 99.8%, Junsei) with dried molecular sieves (molecular

11

sieves, 4 Ǻ, Sigma Aldrich) for at least 1 week in a glove box.

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2.2. Cell Fabrication and Electrochemical Test. The Li-O2 batteries were assembled within

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an Ar-filled glove box (O2 and H2O level < 0.1 ppm). The Li-O2 cell was assembled into a

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Swagelok cell type battery with a glass fiber separator (GF/D, Whatman), lithium foil (thickness,

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300 ㎛, Honjo), and an air cathode. The dried gas diffusion layer (35BC GDL, SGL group) was

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used as a freestanding carbon cathode. The electrolytes were 1 M LiTFSI in tetraethylene glycol

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dimethyl ether (TEGDME, > 98%, TCI) and dimethyl sulfoxide (DMSO, > 99.8%, Junsei). The

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water contents of the electrolytes were kept below 10 ppm, as determined by Mettler-Toledo Karl-

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Fischer titration without exposure to air. After the cell assembly, the cells were stabilized under

20

an O2 atmosphere at 1.5 bar Po2 for 3 h. Electrochemical tests were conducted using a galvanostat

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(Maccor series 4000) in a voltage window of 2.0– 4.5 V. To verify the existence of LiO2, Li was

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electrochemically supplied to the cathode without the presence of O 2 by discharging under Ar

23

atmosphere to determine whether the conversion of LiO2 to Li2O2 occurs. For this test, we changed


4

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the atmosphere from O2 gas to Ar gas after discharge of 0.25 mAh capacity. 5 Ar gas was allowed

2

to flow into the cell for 1 min with a rest time of 10 min, and the procedure was repeated five times

3

to completely remove dissolved oxygen gas. The pre-discharged cathode was further discharged

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in an Ar atmosphere to 2.0 V.

5

2.3. UV-Vis Titration Analysis. An ultraviolet-visible spectrometry (UV-Vis, V-650

6

spectrophotometer, Jasco) titration measurement was conducted to quantify the amount of Li2O2

7

in the discharged electrodes. The discharged electrodes were immersed in 3 mL of water-based

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titration solution, 2 % TiOSO4 dissolved in 1 M H2SO4. H2O2 or Li2O2 containing samples form a

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yellowish complex [Ti(O2)]2+ in the titration solution. Washing discharged electrodes with

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anhydrous acetonitrile before immersing into the titration solution was critical for precise

11

experimentation, especially for samples discharged with the DMSO solvent due to the

12

decomposition of H2O2 by DMSO. The yellowish titration solutions were diluted by a factor of 3

13

or 6 for UV-Vis absorption measurements. The yellowish complex [Ti(O2)]2+ shows an absorption

14

peak at λmax=405 nm on the UV-Vis absorption spectrum, and the peak intensity can be converted

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to the amount of Li2O2.27, 28 The possibility of a chemical reaction between DMSO and H2O2 in

16

the titration solution was also examined by carrying out a UV-Vis titration analysis with

17

commercial Li2O2 powder and H2O2 solution. Li2O2 powder was mixed with 3 mL of 2% TiOSO4

18

dissolved in 1 M H2SO4 solution. Li2O2 reacts with water in the titration solution to produce H2O2.

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On the other hand, an H2O2 solution of the same molecular concentration was added to the titration

20

solution. Then, 200 L of TEGDME and DMSO based electrolytes were injected to this yellow

21

solution.

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2.4. Material Charaterization. The discharged electrodes were characterized by high-

23

resolution X-ray diffraction (HR-XRD, SmartLab, Rigaku) with a Cu-Kα radiation source within


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2θ = 10.0°–70.0° with a step of 0.02° and scan rate of 1° min -1. To prevent transformation of the

2

discharge products by H2O or N2 in ambient atmosphere, the discharged electrodes were wrapped

3

with a polyimide tape with a broad XRD peak at 2θ = 20°. Before XRD analysis, the discharged

4

electrodes were washed in anhydrous acetonitrile (Sigma Aldrich) to remove residual electrolytes.

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The structure and morphologies of the Li2O2 powder and discharge products formed on the surface

6

of cathodes were analyzed by using scanning electron microscopy (SEM, Nova Nano SEM 450).

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Before SEM analysis, the discharged electrodes were washed in anhydrous acetonitrile to remove

8

residual electrolyte. Although the possible discharge products Li 2O2 and LiOH are insufficiently

9

electrically conductive for clear SEM imaging, we did not coat Pt at all to prohibit any damage of

10

the discharge products.

11 12

3. RESULTS AND DISCUSSION

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The amount of Li2O2 in the electrodes discharged in 1 M LiTFSI in TEGDME and DMSO was

14

assessed by a UV-Vis titration method as described in the experimental section. When the

15

discharged electrodes were immersed in the titration solution, the color of the solution changed

16

immediately to orange and yellow, as in Figure 1a and 1b, whereas the titration solutions without

17

Li2O2 remained as colorless clear solutions after the addition of electrolytes (1 M LiTFSI either in

18

TEGDME or DMSO) and a pristine electrode (Figure S1). As reported previously,23, 29 The Li2O2

19

formed on the electrode reacts with water contained in the titration solution according to the

20

reaction in equation 1, generating hydrogen peroxide (H2O2).

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Li2O2 + H2O → 2LiOH + H2O2

(1)

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When H2O2 is present in the titration solution, a color change to yellow or orange occurs

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depending on the amount of reaction products due to the formation of titanium peroxide complex


6

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from the following reaction, TiOSO4 + H2O2 + H2SO4 = H2[Ti(O2)(SO4)2] + H2O.27,

28

2

spectrophotometric method can be used to quantify the amount of H2O2 produced based on the

3

intensity of the signature peak at 405 nm. In the UV-Vis absorption spectra of the diluted titration

4

solution (dilution factor 3), the electrode discharged in TEGDME-based electrolyte had a peak

5

intensity of approximately 1.496 and that in DMSO exhibited a peak of approximately 0.788

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(Figure 1c and 1d). Surprisingly, the titration solution involving DMSO (Figure 1b) became

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colorless within a few minutes, whereas there was no obvious color change in the titration solution

8

with TEGDME in Figure 1a. Simultaneously, the intensity at 405 nm decreased considerably for

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DMSO (Figure 1d) unlike the unchanged peak intensity for TEGDME in Figure 1c. The

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remarkable color change indicates the disappearance of H2O2. Since the only difference between

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the two titration solutions is the type of solvent employed, we assumed that the DMSO remaining

12

in the discharged electrodes might reduce the amount of H2O2 produced in the titration solution,

13

whereas the remaining TEGDME is inert against the decomposition of H2O2.

This

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To clarify the possible reaction between DMSO and H2O2, fixed amounts of H2O2 liquid and

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commercial Li2O2 powder, respectively, were added to the sulfuric-based titration solution and,

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200 L of TEGDME and DMSO, respectively, was added subsequently. The H2O2-contained

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titration solutions showed a yellowish color and had intensities of 1.6 at λmax = 405 nm in Figure

18

2a, c. As stated previously, commercial Li2O2 reacts with water in titration solutions to form H2O2.

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The resulting titration solutions turned yellow and the peak intensity at λmax = 405 nm was also

20

1.6, in Figure 2b, d. To these yellowish titration solutions containing H2O2 or Li2O2, TEGDME or

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DMSO solvent was added. After 12 h, there were neither color changes nor intensity decreases of

22

the solutions with added TEGDME in Figure 2a-b. On the other hand, the yellowish color of the


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solutions with H2O2 and Li2O2 had become colorless and the corresponding UV-Vis absorption

2

peak intensity decayed to almost zero in solutions with DMSO in Figure 2c-d. In Figure 2,

3

therefore, it appeared that DMSO induced the abatement of H2O2 in titration solutions.

4

Degradation of DMSO by H2O2 in the presence of alkali hydroxide (KOH) 30 and catalyst31 has

5

been reported; however, the exact reaction mechanism of DMSO with H2O2 is not clear present

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and is beyond the scope of this study. One clear observation is the interaction of DMSO with H2O2

7

in titration solutions in this study. To be noted in Figure 2 is the difference in time elapsed until

8

H2O2 completely disappeared from the time scale in Figure 1. This process took less than 5 minutes

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for electrochemically formed Li2O2 (Figure 1b, d), but approximately 12 h for commercial Li2O2

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(Figure 2c, d). A similar experimental result of accelerated DMSO oxidation by electrochemically

11

formed Li2O2 compared to commercial Li2O2 was reported in a previous study.13 This accelerated

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oxidation was ascribed to the presence of superoxide anions in the electrochemically generated

13

Li2O2 system, which are absent in commercial Li2O2. When mixed with KO2, which releases

14

superoxide anions, DMSO also responded rapidly to commercial Li2O2, which confirms that the

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superoxide anions promote oxidation of DMSO. In our results, the electrode discharged in DMSO-

16

based electrolyte might have dissolved oxygen radicals (O 2-) and solid32-34 or soluble LiO2-like

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species35 produced during the discharge process. In particular, it is highly probable that soluble

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superoxide species are present in DMSO with high DN.12 The superoxide anions quickly abstract

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protons from DMSO, and the resulting reactive dimsyl ions could decompose H2O2 more rapidly.

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To avoid the decomposition of H2O2 by residual solvents and obtain precise titration results,

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we washed the discharged electrodes (using TEGDME and DMSO-based electrolyte) with

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anhydrous acetonitrile (AN) to completely remove the residual solvents. For the electrodes

23

discharged with TEGDME, the peak intensity after thorough washing with AN (Figure 3c) was


8

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the same as the intensity before washing (Figure 1c), thus this washing process did not influence

2

the amount of Li2O2 in the discharged electrodes. More importantly, there were no changes in color

3

and UV-Vis peak intensity of titration solutions for electrodes discharged in DMSO (Figure 3b,

4

d). At this time, concerned that a small amount of Li 2O2 might be present in the GF/D separator,36

5

we immersed the separator in a titration solution after discharge. No Li2O2 was detected at the

6

separators in the Li-O2 cells using both electrolytes in Figure S2. Thus, titration of the electrodes

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after washing could ensure the precise and reliable titration analysis on the discharged electrodes.

8

The amount of Li2O2 on cathodes discharged at various capacities, current densities, and

9

electrolyte solvents was titrated, as shown in Figure 4, which shows the intensities of the

10

absorption peak at 405 nm vs. the discharge capacities at different current rates for the two solvents.

11

Photographs of the corresponding titration solutions with discharged cathodes are provided in

12

Figure S3. The intensity increases nearly linearly with the discharge capacity in both DMSO and

13

TEGDME. In addition, the variation in current rates did not change the amount of Li2O2 at the

14

same discharge capacities, except at a discharge capacity of 3 mAh, the highest tested; more Li2O2

15

was detected at relatively high current density, 0.1 mA, than at low current density, 0.05 mA. More

16

notably, the amount of Li2O2 was higher for all discharge capacities in TEGDME than in DMSO.

17

Since the discharge potential until a capacity of 3 mAh is above 2.7 V (Figure S4), it is unlikely

18

that decomposition of the carbon electrodes or organic solvents of the electrolyte occurred during

19

discharge.37,

20

discharge products such as LiO2 or LiO2-like products that do not form H2O2 upon reaction with

21

water.29

38

The difference might be partly due to the formation of chemically different

22

We conducted an electrochemical test that was described previously5 to check if LiO2-like species

23

are present in both DMSO and TEGDME systems. Figure 5a and b show the typical discharge


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profiles of cathodes discharged in DMSO and TEGDME with O2 gas at 0.05 mA for 10 h (0.5

2

mAh), respectively. For the comparative tests in Figure 5c and d, we changed the gas atmosphere

3

from O2 to Ar gas after discharging at 0.05 mA for 5 h (0.25 mAh) and then, the cells were further

4

discharge at 0.05 mA to 2.0 V with Ar gas. If LiO2-like species were present, the discharge process

5

would continue the electrochemical conversion reaction in equation (2) by LiO2-like species.

6

LiO2 + Li+ + e- → Li2O2

7

There is a clear difference in the discharge profiles of the DMSO and TEMDGE systems in

8

Figure 5c and d, respectively. In DMSO, the discharge process lasted about 0.06 mAh in Ar

9

atmosphere. However, the cell voltage immediately dropped to 2.0 V in TEGDME. To verify the

10

conversion of LiO2-like species to Li2O2, we also conducted UV-vis titration experiments on the

11

electrode after discharging for 0.25 mAh under O2 and the following discharging to 2.0 V under

12

Ar (Figure S5). In Figure S5a, the color of titration solution after the additional discharging became

13

more yellowish compared to discharging for 0.25 mAh under O2. Also, the corresponding titration

14

result measured (Figure S5b) shows the increase of Li 2O2, which could confirm the conversion of

15

LiO2-like species to Li2O2. Since LiO2 is not detected in the titration analysis, and, additionally,

16

we washed out the LiO2 dissolved in DMSO, the amount of Li 2O2 formed at the same discharge

17

capacity could be lower than when discharged in TEGDME.

(2)

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We further examined the amount of Li2O2 in the fully discharged states. Figure 6a presents the

19

full discharge and charge profiles of Li-O2 batteries using DMSO and TEGDME. The Li-O2 cell

20

with DMSO-based electrolyte exhibited discharge capacity of approximately 12.67 mAh, which

21

is nearly double the discharge capacity of approximately 5.63 mAh with the TEGDME-based

22

electrolyte. This is consistent with reports of high discharge capacities for high DN DMSO solvent,

23

which was ascribed to the formation of Li2O2 in solutions. The high capacity of DMSO, thus, was


10

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1

attributed to the formation of more Li2O2. In titration analysis, however, the amount of Li2O2 in

2

the fully discharged electrode with DMSO was about half the amount with TEGDME. Despite the

3

low discharge capacity of about half, the TEGDME-based electrolyte produced more Li2O2. This

4

titration result is surprising since it is quite contrary to expectation. The formation of LiO2 in the

5

DMSO system as indicated in Figure 5c can be one explanation. Apart from the existence of LiO2,

6

the formation of LiOH due to possible reaction of DMSO with Li2O214, 15 and/or LiO213 can be

7

another reason for the smaller amount of Li 2O2 in the cell using DMSO-based electrolyte.

8

Resolving the discrepancy between the discharge capacity and the amount of Li2O2 would require

9

the discharge products to be identified; therefore, characterization of the discharge products was

10

performed for the discharge processes under different conditions.

11

In Figure 7, we compare the discharge products of limited and unlimited capacity (full discharge

12

capacity). The discharged electrodes at 0.05 mA for 60 h (3 mAh capacity) in TEGDME and

13

DMSO showed only clear peaks of Li2O2 at 2θ = 33º and 35º without LiOH in Figure 7a. On the

14

other hand, electrodes fully discharged to 2.0 V in DMSO exhibited additional sharp peaks

15

corresponding to crystalline LiOH. It indicates the transformation of Li2O2 (LiO2) to LiOH during

16

the full discharge process in DMSO that allows sufficient time for the reaction. The fully

17

discharged electrode in the TEGDME system also showed small broad LiOH peaks, but compared

18

to the DMSO system, they are negligible. Minor LiOH formation should come from water

19

exposure during the long discharge of the full discharge process (about 113 h). The SEM images

20

of the corresponding discharged electrodes confirm the different morphologies of the discharge

21

products produced in TEGDME- and DMSO-based electrolytes (Figure 7b-e). When the cathode

22

was discharged to 3.0 mAh in TEGDME, the morphology of discharge products was a mixture of

23

film-like and flat toroidal shapes (Figure 7b). When fully discharged to 2.0 V, the discharge


11


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products were film-like Li2O2, closely packed on the cathode surface (Figure 7d). Meanwhile, the

2

discharge products in DMSO assumed the thick toroidal shape, shown in Figure 7c, which is

3

approximately spherical, at a discharge capacity of 3 mAh. When the electrode was fully

4

discharged to 2.0 V in DMSO, the morphology of the discharge products changed to flake-like, as

5

in Figure 7e, which is the signature shape of LiOH. It can be interpreted as the transformation to

6

LiOH through the reaction between Li2O2 or LiO2-like species and DMSO under electrochemical

7

conditions. Briefly, the reaction could occur through multi-steps: (1) the presence of Li2O2 or LiO2-

8

like species would facilitate proton abstraction from DMSO, resulting in the formation of DMSO -

9

(dimsyl ion) and a free proton, (2) a Li+ ion in Li2O2 or LiO2-like species couples with DMSO- and

10

be exchanged with the free proton, forming LiOOH, and then (3) LiOOH reacts with another

11

DMSO, which generates LiOH and DMSO.13-15 However, the reaction chemistry is still under

12

debates and the further studies are required. The lower intensity of DMSO in the UV-Vis titration

13

in Figure 6b could be mainly due to LiOH formation since LiO2, which may exist in the discharge

14

products at low limited capacity, might transform to Li2O2 at this high discharge capacity. We

15

concluded that large discharge capacities in particular solvents do not necessarily mean a larger

16

amount of Li2O2.

17

Based on the aforementioned results, we summarized the origin of the low amount of Li2O2 in

18

electrodes discharged in DMSO compared to TEGDME as follows, based on the reaction

19

mechanism and side reactions during discharge (Scheme 1).

20

(1) DMSO with a high donor number produces discharge products through solution growth

21

during which soluble LiO2-like species are formed in the solvent. Unlike Li2O2, LiO2-like

22

species do not generate H2O2 upon reaction with H2O. Therefore, the amount of LiO2-like

23

species, especially those that are soluble, cannot be measured by UV-Vis titration.


12

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1

(2) Moreover, the transformation of Li2O2 to LiOH occurred in the electrochemical

2

environment. This transformation is more obvious during full discharge as confirmed in

3

Figure 7 which allows sufficient time for reaction, although the transformation is occurring

4

consistently during the cell operation. The growth in solution enabled the cathodes

5

discharged in DMSO to reach higher full discharge capacity; however, the severe side

6

reactions with DMSO eventually resulted in a smaller amount of Li2O2.

7 8

4. CONCLUSION

9

This study revisited and revised the titration-based analytical method with the aim of precisely

10

quantifying the amount of Li2O2 in the discharged electrodes, especially when discharged in

11

DMSO. The revised reliable UV-Vis titration measurement indicated that the actual amount of

12

electrochemically formed Li2O2 on the cathodes can be less when we use DMSO rather than

13

TEGDME as the solvent for the electrolyte. Based on our analytical results, we pointed out two

14

factors that caused these unexpected outcomes. The first is the existence of soluble LiO2-like

15

intermediates that remain in the DMSO solvent, and the other is the chemical transformation of

16

Li2O2 to LiOH. We trust that the methodology and results in this study provide a good indicator

17

for the formation and deterioration of Li2O2 and that our results lead to further insight into the

18

evaluation of a suitable solvent for Li-O2 cells.

19 20


13


(a)

(b) After 5 min

After 5 min

Discharged cathode using 1 M LiTFSI in DMSO

Discharged cathode using 1 M LiTFSI in TEGDME

(c)

(d)

2.1 1.8 1.5 1.2 0.9 0.6 0.3 0.0

400

500

600

700

Wavelength (nm)

1

2.1

TEGDME 0.1 mA - 15 hr discharge 1.496 * 3 = 4.488 after 5min 1.488 * 3 = 4.464

Absorbance (a.u.)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 29

800

DMSO 0.1 mA - 15 hr discharge 0.788 * 3 = 2.364 after 5min there is no peak of H2O2

1.8 1.5 1.2 0.9 0.6 0.3 0.0

400

500

600

700

800

Wavelength (nm)

2

Figure 1. Titration of the discharged electrodes (before electrode washing): (a-b) photograph of

3

titration solutions with discharged electrodes (left) immediately after the addition, and (right) 5

4

min after the addition. Added cathodes discharged using (a) TEGDME and (b) DMSO. UV-Vis

5

absorption spectra of titration solutions with the cathodes discharged in (c) TEGDME and (d)

6

DMSO.

7


14

Before adding TEGDME 12 h after addition TEGDME 200 uL

1.6 1.2

After 12 h

0.8 0.4 0.0 400

(c) 2.0

500

600

700

Wavelength (nm)

1.2 0.8 0.4 0.0 500

600

700

1.2

After 12 h

0.8 0.4 0.0

(d) 2.0

After 12 h

Wavelength (nm)

Before adding TEGDME 12 h after addition TEGDME 200 uL

400

Before adding DMSO 12 h after addition DMSO 200 uL

1.6

Li2O2 in Titration solution

1.6

800

H2O2 in Titration solution

400

1

(b) 2.0

H2O2 in Titration solution

Absorbance (a.u.)

Absorbance (a.u.)

(a) 2.0

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


800

Absorbance (a.u.)

Page 15 of 29

500

600

700

Wavelength (nm)

800

Li2O2 in Titration solution Before adding DMSO After 12 h adding DMSO 200 uL

1.6 1.2

After 12 h

0.8 0.4 0.0 400

500

600

700

Wavelength (nm)

800

2

Figure 2. Change of titration solutions with H2O2 or commercial Li2O2 before and after adding

3

solvents: before and after adding (a-b) TEGDME and (c-d) DMSO solvent. Titration solutions

4

with (a, c) H2O2 and (b, d) Li2O2. (Inset) photographs of solutions with H2O2 or commercial Li2O2

5

(left) before and (right) 12 h after solvent addition.

6


15


(a)

(b)

Discharged cathode using 1 M LiTFSI in DMSO with washing before titration No color change (same amount of H2O2)

Discharged cathode using 1 M LiTFSI in TEGDME with washing before titration No color change (same amount of H2O2)

(d)

2.1

TEGDME 0.1 mA - 15 hr discharge ( wash) 1.496 * 3 = 4.488 after 5 min 1.488 * 3 = 4.464

1.8 1.5 1.2 0.9 0.6 0.3 0.0

400

500

600

700

Wavelength (nm)

1

800

2.1

Absorbance (a.u.)

(c) Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 29

DMSO 0.1 mA - 15 hr discharge ( wash) 0.778 * 3 = 2.334 after 5 min 0.768 * 3 = 2.304

1.8 1.5 1.2 0.9 0.6 0.3 0.0

400

500

600

700

800

Wavelength (nm)

2

Figure 3. Titration of the discharged electrodes after thoroughly washing with AN: (a-b)

3

photograph of titration solutions with discharged electrodes (left) immediately after the addition,

4

and (right) 5 min after the addition. Added cathodes discharged using (a) TEGDME and (b) DMSO.

5

UV-Vis absorption spectra of titration solutions with the cathodes discharged in (c) TEGDME and

6

(d) DMSO.

7


16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity at 405 nm (Arb, a.u.)

Page 17 of 29

10 8 6 4 2 0 0.0

1

Discharged cathode _ 1 M LiTFSI in TEGDME at 0.05 mA Discharged cathode _ 1 M LiTFSI in TEGDME at 0.1 mA Discharged cathode _ 1 M LiTFSI in DMSO at 0.05 mA Discharged cathode _ 1 M LiTFSI in DMSO at 0.1 mA

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Capacity / mAh

2

Figure 4. UV-Vis absorption intensities at 405 nm of titration solutions with electrodes discharged

3

with different solvents, rates, and capacities.

4 5


17


Voltage (V)

4.5

(b) 5.0

1M LiTFSI in DMSO 0.05 mA - 10 h with O2 atmosphere

4.5

Voltage (V)

(a) 5.0 4.0 3.5 3.0 2.5 2.0

1M LiTFSI in TEGDME 0.05 mA - 10 h with O2 atmosphere

4.0 3.5 3.0 2.5 2.0

0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

Capacity (mAh)

4.5 4.0 3.5

(d) 5.0

1M LiTFSI in DMSO Initial Discharge 0.05 mA - 5 h with O2 atmosphere

4.5

Following discharge 0.05 mA - 5 h with Ar atmosphere

3.0 2.5

0.3

0.4

0.5

4.0 3.5

1M LiTFSI in TEGDME Initial Discharge 0.05 mA - 5 h with O2 atmosphere Following discharge 0.05 mA - 5 h with Ar atmosphere

3.0 2.5 2.0

2.0 0.0

1

0.2

Capacity (mAh)

Voltage (V)

(c) 5.0 Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

0.1

0.2

0.3

0.4

0.5

0.0

0.1

Capacity (mAh)

0.2

0.3

0.4

0.5

Capacity (mAh)

2

Figure 5. Voltage profiles of Li-O2 batteries during discharge: using (a, c) 1 M LiTFSI in

3

TEGDME and (b, d) 1 M LiTFSI in DMSO. (a, b) discharged to 0.5 mAh at O2 atmosphere and

4

(c,d) discharged to 0.25 mAh at O2 atmosphere initially then further discharging at Ar atmosphere.

5 6


18

Page 19 of 29

(a) 5.0

Voltage (V)

4.5

1M LiTFSI TEGDME 1M LiTFSI DMSO Full discharge to 2.0 V and recharge

4.0 3.5 3.0 2.5 2.0 0

2

4

6

8

10

12

14

Capacity (mAh) (b)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0

Full discharge to 2.0 V Dillution: 1/6 1M LiTFSI TEGDME Intensity: 1.87*6 = 11.22 1M LiTFSI DMSO Intensity: 0.85*6 = 5.1

1.6 1.2

DMSO

0.8 0.4 0.0 400

1

TEGDME

500

600

700

800

Wavelength (nm)

2

Figure 6. (a) Full discharge and charge voltage profiles of Li-O2 batteries using TEGDME and

3

DMSO based electrolytes. The cells were discharged at 0.05 mA. (b) UV-Vis spectra of titration

4

solutions with fully discharged electrodes using TEGDME- and DMSO-based electrolytes.

5 6 7 8


19


(a) Discharged cathode_TEGDME 0.05mA-60h Discharged cathode_TEGDME 0.05mA-Full Discharged cathode_DMSO 0.05mA-60h Discharged cathode_DMSO 0.05mA-Full

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

Li 2O2 LiOH

30

(b)

1 μm

(d)

1

1 μm

35

40

45

2-theta / degree

50

(c)

1 μm

(e)

1 μm

2

Figure 7. (a) XRD data for discharged cathode with TEGDME or DMSO based electrolytes with

3

limited and unlimited capacities. SEM data for discharged cathodes with (b) 1 M LiTFSI in

4

TEGDME and (c) 1 M LiTFSI in DMSO at 0.05 mA for 60 h. SEM data for fully discharged to

5

2.0 V cathode with (d) 1 M LiTFSI in TEGDME and (e) 1 M LiTFSI in DMSO at 0.05 mA.

6


20

Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TEGDME

O2(sol)

Decomposition of solvent and binder

Surface Mechanism

O2˙- (sol)

Li+ Side reaction of water impurity with Li2O2

LiO2* Film and toroidal Li2O2

O2(sol)

+ e-

Solution Mechanism

Electrode Surface

DMSO

Li+ O2(sol)

LiO2 (sol) + e-

O2˙- (sol)

Decomposition of DMSO :H abstraction Side reaction of ‘Dimsyl Anion’ with Li2O2

Large spherical Li2O2

1

Electrode Surface

2

Scheme 1. Schematic representation of reaction mechanism to understand the reason for the

3

discrepancy in the amount of Li2O2 in different electrolytic solvents.

4 5 6 7 8 9 10 11 12


21


Page 22 of 29

1

 ASSOCIATED CONTENT

2

The Supporting Information is available free of charge on the ACS Publications website at DOI:.

3

Photograph of titration solutions with added solvents and pristine electrodes and with

4

separators from the discharged cells. Photograph of the titration solutions with cathodes

5

discharged under various conditions in Figure 4 and corresponding discharge profiles.

6

Photographs of titration solution and titration results after discharging under O 2 for 0.25 mAh

7

and additional discharging under Ar. (PDF)

8

 AUTHOR INFORMATION

9

Corresponding Author

10

* E-mail: [email protected] ; [email protected]

11

Notes

12

The authors declare no completing financial interest

13

Author Contributions

14

‡These authors contributed equally to this work.

15

 ACKNOWLEDGMENT

16

This work was supported by the Human Resources Development program (No. 20154010200840)

17

of the Korean Institute of Energy Technology Evaluation and Planning (KETEP) via a grant

18

funded by the Ministry of Trade, Industry, and Energy of the Korean government. This research

19

was also supported by the Basic Science Research Program through the National Research

20

Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (No.

21

NRF-2014R1A2A1A11049801).


22

Page 23 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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3 4 5 6 7 8


28

Page 29 of 29

1

Tables of Contents Graphic

2.1

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.8 1.5

0.6

Li 2O2

1.5

1 M LiTFSI TEGDME 1.5 mAh discharge Intensity: 1.488 * 3 = 4.464 1 M LiTFSI DMSO 1.5 mAh discharge Intensity: 0.768 * 3 = 2.304

0.9

DMSO

1.8

Through solution-mediated mechanism of DMSO

1.2

0.3 0.0

2

2.1

LiO2 -like product

LiOH

1M LiTFSI TEGDME Full discharge Intensity: 1.87*6 = 11.22 1M LiTFSI DMSO Full discharge Intensity: 0.85*6 = 5.1

1.2 0.9 0.6 0.3

400

500

600

700

800

Wavelength (nm)

0.0

400

500

600

700

800

Wavelength (nm)

3 4


29

Clarification of Solvent Effects on Discharge Products in Li-O2

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