Achieving Low Overpotential Lithium–Oxygen Batteries by Exploiting a

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Achieving Low Overpotential Lithium-Oxygen Battery by Exploiting a New Electrolyte Based on N,N'-Dimethylpropyleneurea Ruliang Liu, Yu Lei, Wei Yu, Haifan Wang, Lei Qin, Da Han, Wei Yang, Dong Zhou, Yan-Bing He, Dengyun Zhai, Baohua Li, and Feiyu Kang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00581 • Publication Date (Web): 05 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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Achieving Low Overpotential Lithium-Oxygen Battery by Exploiting a New Electrolyte Based on N,N'-Dimethylpropyleneurea Ruliang Liu† Yu Lei† Wei Yu†‡ Haifan Wang†‡ Lei Qin†‡ Da Han† Wei Yang† Dong Zhou†‡ Yanbing He† Dengyun Zhai* † Baohua Li*† and Feiyu Kang†‡

† National Local Joint Engineering Laboratory of Carbon functional materials Graduate School at Shenzhen Tsinghua University Shenzhen, 518055 (P. R. China)

‡ Department of Materials Science and Engineering Tsinghua University Beijing, 100084 (P. R. China)

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ABSTRACT:

Recently lithium-oxygen (Li-O2) battery attracts much interest due to its ultrahigh theoretical energy density. But its potential application is limited by instable electrolyte system, low round-trip efficiency and poor cyclic performance. In this study, we present a new electrolyte based on N, N'-Dimethylpropyleneurea (DMPU) applied for Li-O2 battery. This electrolyte possessing high ionic conductivity achieves a low discharge/charge voltage gap of 0.6 V, which would be mainly owing to the possible one-electron transfer charge mechanism. The introduction of the antioxidant butylatedhydroxytoluene (BHT) as additive stabilizes the superoxide radical by chemical adsorption and improves the cyclic performance remarkably. Thus this new electrolyte system may be one of the candidates for Li-O2 batteries.

TOC GRAPHICS

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With growing demand for energy through the world, the rechargeable metal– oxygen battery is regarded as a promising energy storage device in the future since its theoretic energy density is far larger than that of conventional lithium-ion battery1-3. Lithium–oxygen (Li-O2) battery is the typical one of the metal–oxygen batteries. The concept of Li–O2 system as primary battery was proposed by Galbraithin et al in 1976 for the first time.4 After in the late 1990s Abraham et al reported the rechargeable nonaqueous Li–O2 cell,5 there was a growing interest in developing this promising system for energy storage devices.6-13 However, some key issues need to be addressed in the practical development of Li–O2 battery, including the poor reversibility14-16, the low round-trip efficiency17,

18

and the relatively instability of commonly used

nonaqueous electrolytes19, 20. Of all the issues, the most important is to exploit a stable electrolyte for Li-O2 system. An ideal electrolyte should have low volatility, high oxygen solubility, good ionic conductivity and especially be resistant against oxygen reduction species, i.e., the peroxide (O22-) and the superoxide radical (O2-)21, 22. In the recent years, various solvent/salt combinations have been investigated, but there seem to be no electrolytes that meet the above requirements perfectly. Dimethylacetamide (DMA)23, dimethylformamide (DMF)24 and dimethyl sulfoxide (DMSO)25 have recently been explored as the solvents in Li-O2 batteries, and shown to be quasi-stable in combination with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt. A trimethyl-silylsubstitutionalglyme (1NM3) was proposed by zhang et al and applied in Li-O2 battery.26 However the subsequent study demonstrated that 1NM3 was not stable during cycle and proposed some inconclusive decomposition 3 ACS Paragon Plus Environment

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mechanism.27 Recently, Adams et al28 reported that a new ether-based solvent, which is named as 2,3-dimethyl-2,3-dimethoxybutane (DMDMB). DMDMB possesses a hydrophobic backbone and Lewis basic ethereal oxygen, and improved the cyclic stability of Li–O2 battery greatly. In addition, the use of ionic liquid-based electrolytes in Li-O2 cells has been discussed. For example, a novel Li–O2 battery exploiting PYR14TFSI− LiTFSI as ionic liquid-based electrolyte was reported by Elia et al29 and Xie et al30, achieving a very low charge overpotential, although the cyclability still needed to be improved.

Here, we first report a new nonaqueous electrolyte system based on N,N'-Dimethylpropyleneurea (DMPU) as solvent and Butylatedhydroxytoluene (BHT) as anti-peroxy radical additive, as shown in Figure S1 in the Supporting Information. In this electrolyte system, DMPU is a cyclic urea with low volatility (boiling point = 246 oC) and strong polarity, and affords a stable LiTFSI/DMPU salt complex; BHT is a typical antioxidant which can chemically absorb O2- generated during cycle and alleviate the decomposition of solvent DMPU. Finally the electrolyte composing of DMPU and BHT achieves the low charge overpotential and stable cycling performance. The electrolyte based on tetraethylene glycol dimethyl ether (TEGDME) as the reference is discussed as well.

Ionic conductivity is a key property for the application of electrolytes in energy storage devices. Figure 1a presents the temperature dependence of ionic conductivity for 1 M LiTFSI/DMPU and LiTFSI/TEGDME at a temperature range from -20 to 90

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o

C. It is worth noting that for two electrolyte samples, the plots of log σ versus T-1/2

exhibit a non-linear relationship, which can be well described by Vogel–Tamman– Fulcher (VTF) empirical equation below31: σ =σ oT −1/ 2 exp(−

Ea ) R (T − To )

(1)

where Ea is the activation energy, σo is the pre-exponential factor, To is a parameter correlated to the glass transition temperature, and R is the ideal gas constant. As seen from the VTF fitting-parameters listed in Table S1, the value of Ea obtained by fitting using the VTF equation (8.74 ×10-3 eV) is quite lower than LiTFSI/TEGDME as electrolyte (2.59 ×10-2 eV), signifying that the ion mobility presents great difference in different solvents, since Ea is considered to be the barrier for ionic conduction. Furthermore, it is observed from Figure 2b and Table S1 that the ionic conductivity of the DMPU reaches a high value of 2.54 × 10-3 S cm−1 at 25 oC, much higher than that of LiTFSI/TEGDME electrolyte (6.47 × 10-4 S cm−1 at 25 oC). The high ionic conductivity of DMPU would contribute to reduce the ohmic polarization in Li-O2 battery system.

The stability of DMPU in which the commercial Li2O2 or the metal Li foil was immersed was also tested (Figure S2 in the Supporting Information). The 1H NMR spectra show that the DMPU is considerably stable after Li2O2 or the metal Li foil is immersed for 48 hours.

The electrochemical stability window of LiTFSI/DMPU is examined with cyclic voltammetry (CV) using a three-electrode cell, as shown in Figure 1b. Under 5 ACS Paragon Plus Environment

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Ar atmosphere, the electrolyte is very stable between 2.0 and 4.0 V (vs Li+/Li) since there are no redox peaks in the scanning voltage range. While the cell is filled with O2, a couple of redox peaks appear, indicating that the following reaction occur reversibly: 2Li + O2 ↔ Li2O2. Thus, in the next galvanostatic discharge/charge test, the lower and upper cutoff voltage of LiTFSI/DMPU is 2.3 and 4.0 (vs Li+/Li), respectively.

Figure 1. a) The ionic conductivity of 1 M LiTFSI/DMPU, 1 M LiTFSI/TEGDME and 1 M LiTFSI/DMPU + 4 mM BHT as a function of temperature. The plots represent the experimental data and the solid lines represent VTF fitting results. b) Cyclic voltammograms of LiTFSI/DMPU electrolyte on a glassy carbon at 100 mV s-1 under Ar and O2 atmosphere. c) The full discharge curves for LiTFSI/DMPU and LiTFSI/TEGDME at current densities of 100 µA cm-2. d) The first galvanostatic discharge-charge curves for the LiTFSI/DMPU and LiTFSI/TEGDME electrolytes at current densities of 100 µA cm-2. Li-O2 cells using the carbon nanotubes (CNTs) as the cathode material were assembled in 1 M LiTFSI/DMPU and 1 M LiTFSI/TEGDME, respectively. Figure 1c shows the full discharge curves in two electrolytes. The discharge capacity in 6 ACS Paragon Plus Environment

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LiTFSI/DMPU is 7140 mAh g-1, much larger than the capacity of 3200 mAh g-1 in LiTFSI/TEGDME. The discharged product is characterized by XRD and Raman measurements, as shown in Figure 2a and 2b. In XRD patterns of TEGDME and DMPU there are four peaks around 32.9, 35.0, 40.6 and 58.7 o which are identified as the 100, 101, 102 and 110 peaks of Li2O2 according to standard XRD histogram of Li2O2 (JCPDS No. 09-0355). Raman spectrum of DMPU also shows a hump around 790 cm-1 associated with the presence of Li2O219,

32, 33

. Figure 2c shows that the

discharged product in LiTFSI/DMPU are toroidal particles, similar with Li2O2 in LiTFSI/TEGDME9, 11, 12.

In addition, to further confirm the reversibility of the electrochemical process based on LiTFSI/DMPU electrolyte, XRD and SEM characterization of the cathode were investigated at the specific capacity of 2000 mAh g-1 in the first cycle. As can be seen from Figure S3, XRD result shows the Li2O2 on the discharge cathode is produced. With the increases of the recharge capacity, the intenstity of Li2O2 peaks gradually becomes weak until it disappears completely. The SEM images also show the evolution of the toroidal particles during cycle.

Figure 1d shows the first galvanostatic discharge/charge profiles at 100 µA cm-2 with a controlled capacity of 500 mAh g-1. On the charge process, the voltage profile of TEGDME reaches quickly above 4.0 V, consistent with the previous studies.34, 35 In contrast, the voltage plateau of DMPU is as low as around 3.4 V and the corresponding discharge/charge voltage gap is reduced to 0.7 V, and such a low

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charge plateau is also one of the lowest in the recent reports on the organic electrolytes applied in Li-O2 batteries23-25, 36, 37. The low charge voltage plateau of DMPU is probably attributed to the following reasons. Firstly, the high ionic conductivity of DMPU would facilitate the mass transportation for oxygen reduction/evolution reaction (ORR/OER) processes.38 Another important reason would be the charge mechanism we propose in the next discussion.

Figure 2. a) The XRD characterization of discharged product using 1 M LiTFSI/DMPU, 1 M LiTFSI/TEGDME and 1 M LiTFSI/DMPU/BHT as electrolytes, respectively; b) Raman spectra and c, d) SEM images of discharged product in 1 M LiTFSI/DMPU and 1 M LiTFSI/DMPU/BHT as electrolyte, respectively. Figure 3a shows the first galvanostatic discharge/charge profiles at different current densities in LiTFSI/DMPU. The discharge/charge voltage gap is less than 1.0 V even when the current density increases to 200 µA cm-2. It is worth noting that there is an interesting phenomenon during charge. A potential barrier appears at the 8 ACS Paragon Plus Environment

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beginning of the first charge at all different current densities and it disappears in the next cycles (Figure 3c). There is no clear linear relationship between the potential barrier and the current density, suggesting that the barrier would be probably driven by thermodynamics. In Figure 3b, the cell after the first cycle is refilled with the fresh LiTFSI/DMPU electrolyte and the potential barrier appears again (see Supporting Information for experimental details). Furthermore, Figure S4 in the Supporting Information shows that the potential barrier even appears in fresh DMPU system when the discharge product Li2O2 is produced in TEGDME-based electrolyte. Thus, it suggests that the appearance of the potential barrier is closely correlative with the fresh LiTFSI/DMPU electrolyte.

Figure 3. (a) The first galvanostatic discharge-charge curves in LiTFSI/DMPU electrolyte at various current densities. (b) The first galvanostatic discharge curves before and after refilled with the fresh LiTFSI/DMPU electrolyte. (c) The galvanostatic discharge-charge cycles in LiTFSI/DMPU electrolyte and d) LiTFSI/DMPU/BHT electrolyte.

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Aiming at the above discussion, we are encouraged to propose a possible charge mechanism in DMPU system as shown in Figure 4. Based on Pearson’s hard soft acid base(HSAB) theory, hard acids prefer hard bases and soft bases prefer soft acids.39 A solvent’s basicity is usually characterized by its donor number (DN). Li+ is hard Lewis acid and the DN of DMPU is as high as 34.40 Thus in DMPU-based electrolyte a Li+ is solvated by a few solvent molecules and form solvent separated ion pairs, just like Li salt dissolved in dimethyl sulfoxide (DMSO)41-43. And the strength of the coordination bonds in Li+-(DMPU)n formed with the solvent also leads to the remarkable decreasing of electrolyte’s acidity.44 O2- is a moderately soft base41, and therefore it has a relatively high affinity for the soft acid Li+ in DMPU-based electrolyte, that is, O2- would be stable in the DMPU-based electrolyte. At the beginning of charging, Reaction I (Equation 1) first occurs and trace of O2- dissolves in electrolyte, as shown in Figure 4. Then the O2- with Li+ through Reaction II (Equation 2) disproportionates to form Li2O2 and release O2. These two cyclic reactions occur continuously until the Li2O2 decompose completely. The whole charge process only involves one-electron transfer and that would mainly lead to the low charge voltage plateau in DMPU-based electrolyte. Recently Wang et al30 also reported that one-electron process for OER achieved low charge potential in Li-O2 batteries. For Reaction I, actually it is the delithiation process which is not energetically favorable and thus the energy barrier for Li desorption along with the solvation of O2- would be required.45 That is probably why the potential barrier appears for the first OER process. The solvated O2- as the intermediate in the first 10 ACS Paragon Plus Environment

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cycle would continue to exist and play a role in the next cycles, and thus the potential barrier disappear. The similar experimental phenomenon was also reported on lithium-sulphur batteries.46

Figure 4. Schematic illustration of the Li2O2 decomposition in 1 M LiTFSI/DMPU. Reaction I: Li2O2 → 2Li+ +O2-(sol) + e-

(2)

Reaction II: Li+ +O2-(sol) → 1/2 Li2O2 +1/2 O2↑

(3)

1

H NMR result indicates that DMPU is stable after the first full discharge, as

show in Figure 5. After the first charge the 1H NMR result shows that the decomposition of DMPU begins to occur, which is probably because DMPU is not stable in the presence of O2- during charge47. And the DMPU decomposition also makes the voltage rise rapidly at the end of the charge (Figure 3a and b). With the increasing of the cycle number the degree of DMPU decomposition become serious (Figure 5), leading to the capacity fading from the fifth cycle (Figure 3c). Based on the 1H NMR result, we propose the possible decomposition process of DMPU. The 11 ACS Paragon Plus Environment

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decomposition of DMPU begins with hydrogen abstraction from the CH2-N site by O2-; therefore, the ring-opening reaction would occur under strong oxidizing agents existing in solution. The main products of decomposition of DMPU are possibly propanedioic acidlithium (OCCH2CO δ=2.91) and 1,3-dimethylurea (NCH3δ=2.8).

Figure 5. The 1H-NMR spectra of LiTFSI/DMPU and LiTFSI/DMPU/BHT electrolyte deposited on separator after different cycles: a) pristine DMPU, b) full discharge for LiTFSI/DMPU, c) first cycle for LiTFSI/DMPU, d) 10th cycle for LiTFSI/DMPU, e) full discharge for LiTFSI/DMPU/BHT, and f) 20th cycle for LiTFSI/DMPU/BHT. The D2O is selected as solvent. To improve the cyclability of LiTFSI/DMPU electrolyte system, a kind of additive BHT48 which is a common antioxidant is introduced, expected to stabilize the O2- by chemical adsorption, as shown in Figure 4. An encounter complex was formed esasily between BHT and O2- in electrolyte; this adsorption reaction is reversible, and the collapse of the complex may produce ground or oxidation state of BHT. Figure 3d shows the galvanostatic discharge/charge profiles when 4 mM BHT is added into 1 M LiTFSI/DMPU. The first discharge/charge voltage gap can be controlled below 0.8 V, and the cycle performance improves greatly (20 cycles). The potential barrier at the 12 ACS Paragon Plus Environment

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beginning of the first charge still appear, probably attributed to the formation of soluble O2-. At the end of the charge, the voltage does not rise rapidly, but keeps climbing slightly. And 1H NMR result shows that there is only about 0.3% DMPU appearing decomposition after 20th cycle. Although the adding of BHT in LiTFSI/DMPU electrolyte decrease the ion conductivity (Figure 1a), the charge voltage is still below 3.8 V after 20th cycle.

In summary, a new electrolyte based on DMPU as the solvent and BHT as the additive was developed for Li-O2 batteries, achieving reducing low charge overpotential significantly. That would be probably attributed to high ionic conductivity of LiTFSI/DMPU electrolyte and the possible charge mechanism based on the one-electron transfer in LiTFSI/DMPU electrolyte. During the first OER process the soluble O2- may be formed as the intermediate through one-electron transfer in electrolyte, and then it decompose by disproportionation reaction. Meanwhile, the presence of O2- also prompts the decomposition of DMPU, leading the quick capacity fading of Li-O2 cells. A kind of antioxidant BHT is introduced into the electrolyte, stabilizes the soluble O2- effectively by chemical adsorption and avoids the attack of O2- against DMPU solvent, and eventually improves the cyclability remarkably. The promising results are expected to give more insight into the OER process of Li-O2 batteries and courage us to further investigate the DMPU-based electrolyte system.

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■ASSOCIATED CONTENT

Supporting Information

Experimental methods, extra H NMR spectrogram, SEM images, XRD and Li− O2 cell performance

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (D. Z.). *E-mail: [email protected] (B. L.).

Present Address † National Local Joint Engineering Laboratory of Carbon functional materials Graduate School at Shenzhen Tsinghua University Shenzhen, 518055 (P. R. China)

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT

This work was supported by Shenzhen Engineering Laboratory for the Next Generation Power and Energy Storage Batteries, China Postdoctoral Science Foundation

(No.2015M580092).

Shenzhen

Technical

Plan

Project

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(JCYJ20130402145002382,

ZDSYS20140509172959981

and

JCYJ20140417115840246).

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