Intercalation into a Graphite Electrode - ACS Publications - American

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Synergetic effect of ethyl methyl carbonate and trimethyl phosphate on BF intercalation into graphite electrode 4-

Lei Zhang, Jiayu Li, Yuhao Huang, Dandan Zhu, and Hongyu Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00262 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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Synergetic effect of ethyl methyl carbonate and trimethyl phosphate on BF4 intercalation into graphite electrode

Lei Zhang †, ‡, Jiayu Li †, ‡, Yuhao Huang †, ‡, Dandan Zhu †, ‡, Hongyu Wang †, ‡, *

† State

Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China ‡

School of Applied Chemistry and Engineering, University of Science and Technology of China,

Hefei 230026, China * Corresponding author. Tel/Fax: 86-431-85262287 E-mail address: [email protected] (H. Wang).

ABSTRACT

LiBF4-ethyl methyl carbonate (EMC) based solutions has been not successfully employed in dual-ion batteries (DIBs) mainly on account of few solvated BF4– intercalation into graphite positive electrode. In this article, trimethyl phosphate (TMP) are introduced into 1M LiBF4-EMC solution as the electrolyte solutions for DIBs, thus revealing a synergetic effect in which the discharge capacity for the anion storage using 1M LiBF4-EMC/TMP (8:2 by vol.) (ca. 26.7 mAh

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g-1) is far superior to that for the batteries using 1M LiBF4-EMC (ca. 2.7 mAh g-1) or 1M LiBF4-TMP (1.1 mAh g-1). In this case, the effects of TMP on LiBF4 in 1M LiBF4-EMC/TMP electrolyte solutions are explored by conventional electrochemical tests, ex situ X-ray diffraction, in situ raman spectroscopy and nuclear magnetic resonance.

Key words: anion-graphite intercalation compounds; lithium tetrafluoroborate; trimethyl phosphate; ethyl methyl carbonate; dual-ion batteries; synergetic effect.

Introduction

In recent years, dual-ion batteries (DIBs) are getting more and more attractive in the field of electric energy storage devices. This tendency is based on the success in highly reversible storage of anions in graphite positive electrodes, which came true in this century

1-10,

although the

potential application of anion-graphite intercalation compounds in electric energy storage has been proposed near half a century ago

11-13.

In fact, the significant improvement in the performance of

graphite positive electrode can be majorly ascribed to the stringent control of graphite/electrolyte interface by a logical choice of non-aqueous electrolyte solutions with superior purity. There are roughly two kinds of electrolyte solutions appearing promising in DIBs, including ionic liquids 14-15

and traditional organic electrolyte solutions

4-5, 16-22.

6,

Both possess advantages and

shortcomings. The former may avoid the interference from organic solvents while the latter can save cost in large-scale applications. In our previous studies, we have stressed the influence of solvent on anion storage behavior in graphite electrode

5, 16-20.

All these studies actually included

at least one solvent with high permittivity, which may be an implicit prerequisite for reasonably explaining the solvation effect. Otherwise, the strength of ionic pair (anion-cation interactions)


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should not be ignored because it greatly affects the number of “free” anions in the electrolyte solutions, which plays a very important role in determining the electrochemical intercalation behavior of anions into graphite electrode. However, the problem with ion pair in DIBs has never been addressed, which will become more severe under the circumstance of concentrated electrolyte solutions composed of low-permittivity solvents and small-sized cations like Li+. The electrolyte solutions of LiBF4 dissolved into ethyl methyl carbonate (EMC) may be a good example for the above issue. EMC has proved to be a suitable solvent for PF6 intercalation into graphite

5, 17-18, 23.

But its permittivity (ca. 2.96) is too low to block off the strong attractions

between Li+ and BF4–, then few solvated-BF4– can be liberated to take part in the intercalation into graphite. However, LiBF4 has some charming virtues as compared with other lithium electrolyte salts, such as small size, relatively simple chemical structure

24,

fairly high anodic stability

25-26,

and considerable humidity resistance. These benefits of LiBF4 always challenged the dominant status of LiPF6 in practical applications. Herein, how to separate the Li+-BF4– pairs becomes the bottleneck for utilizing LiBF4-EMC-based solutions in DIBs. Generally speaking, the addition of a co-solvent with higher permittivity may be an effective and facile way to split off the robust ionic pairs. There are many organic solvents can satisfy this criteria. In this study, we picked up trimethyl phosphate (TMP) considering its merits such as anodic stability, permittivity and flame-retardant ability (burning times of different electrolyte solutions as shown in Figure S1). Moreover, TMP has a high donor number, which implies that it preferentially coordinates with Li+ (Table 1). Herein, we will show that both TMP and EMC can suppress BF4 intercalation into graphite to different extents. But EMC/TMP mixtures may facilitate the anion intercalation. This synergetic solvent effect will be explored as well.


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Experimental Section Materials Natural graphite flakes (B.E.T specific surface area: ca. 62.85 m2 g-1; average granularity: ca. 131.49 μm; tap density: ca. 0.605 g cm-3)

23, 27

purified by Kansai Coke & Chem. Co. Ltd. was

employed as the positive electrode. The fabrication method of graphite positive electrode was the same as those in our previous work 23. The negative electrode material was lithium chip (purity: ≥ 99.9%; diameter: ca. 7.8 mm; thickness: ca. 0.56 mm, China Energy Lithium Co. Ltd.). Electrolyte solutions were prepared by dissolving 1M LiBF4 in the mixed solvents of EMC and TMP with different volume proportions. Glass fiber filter (Advantec Toyo Kaisha, Ltd) was used as the separator. After graphite electrodes and separator were dried at 130 C under vacuum for 3 hours, cells were assembled in a glove box filled with pure argon atmosphere where O2 and H2O contents were less than 0.5 ppm. Relevant measurements The tap density of graphite flakes was calculated after vibrating 2000 times in measuring cylinder by tap density tester (FZS-4, Ningbo, China, Hai shuriko Instrument Co. Ltd). The viscosity of the electrolyte solutions was measured via a digital viscometer (LDV-2+Pro, Shanghai, China, Nirun Intelligent Technology Co., Ltd.). The conductivity of electrolyte solutions was measured by a conductivity meter (DDS-11A, Shanghai REX Instrument Co. Ltd.). Galvanostatic charge-discharge tests (current density: 100 mA g−1, voltage range: 3 V to 5 V, unless specified otherwise) were conducted on coin cells (CR2032) by a battery testing system (CT2001T, Wuhan, China, LANDt Co. Ltd.). In situ Raman spectroscopy calibrated by Si wafer were recorded by a LabRam HR800 spectrometer using a HeNe laser (633 nm) as the excitation


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source, at the same time the mentioned battery testing system was used to control the cell voltage. Unless otherwise specified, the X-ray diffraction (ex situ XRD) measurements were carried out on graphite positive electrodes disassembled from the DIBs charged to 5 V under the same test condition as our previous work 20 by a Rigaku MiniFlex600 X-ray diffractometer. Electrochemical impedance spectroscopy (EIS) measurements were performed at the potential amplitude of 5 mV in the frequency range from 100 kHz to 10 mHz using PARSTAT-2273 advanced electrochemical system. Nyquist plots were recorded using symmetric cells consisting of two uniform graphite positive electrodes recovered from the Li/graphite coin cells charged to 5.0 V using the same electrolyte solution. Nuclear magnetic resonance (NMR) spectroscopy of the electrolyte solutions of 1M LiBF4-EMC/TMP was obtained from a BRUKER Advance III HD 500 using Si(CH3)4, C6F6 , and LiClO4 ethanol solutions as the internal standard reagents for 1H,

19F,

and 7Li NMR,

respectively. All the measurements were performed at room temperature near 22 C. Result and discussion Table 1 Parameters of the physical properties of selected solvents. Solvent

EMC

TMP

a

2.32 b

Viscosity (25 C)/ cP Donor number

0.65

6.5 c

23.0 d

Relative dielectric constant (25 C)

2.96 a

20.6 b

Density (25 C) / g cm3

1.20 a

1.97 b

a

Reference 28 b Reference 29 c Reference 30 d Reference 31 In our previous studies, the suppressive effect of solvents on PF6 intercalation into graphite electrode has been witnessed in the cases of ethylene carbonate (EC) and sulfolane (SL) 5, 18, 32-33. This phenomenon was roughly ascribed to the solvation of anions at present. In fact, to clarify its


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mechanism details is not an easy task at all. Nearly all the solvents dissolving electrolyte salts will involve the solvation effect. But only a part of the solvent-anion combinations can experience a significant obstacle during intercalation into graphite electrode. The hard entrance of solvated anion into the interlayer spaces between the adjacent graphene layers in graphite must depend upon at least two prerequisites, the strong solvent-anion binding and the huge configuration of solvated anion. On the other hand, it was found that BF4 can intercalate much more fluently than PF6 in the presence of EC and SL. This can be explained by the apparently smaller size of BF4. Since both BF4 and PF6 may interact with the solvent molecules through the H---F hydrogen bonds (H from the solvent and F from the anions)

34,

it is difficult to discriminate their

solvent-anion binding strengths right now. On the contrary, both TMP and EMC take effect in preventing BF4 intercalation into graphite electrode, which adds more complexity and ambiguity into the issue. Figure 1 compares the 1st-cycle charge-discharge curves of graphite electrodes in 1M LiBF4-EMC/TMP solutions. Graphite electrodes deliver very small capacity values in both solutions from the pure solvents. In these charge-discharge curves, similar sharp slopes occupy most of the lengths. They come from the charge storage at the graphite/electrolyte interface forming electric double-layers and contribute very small values of capacity because of the tiny graphite surface area. At higher potentials, a bending from the slopes can be observed in the charge curves, which may develop into a plateau standing for the anion intercalation process. The charge curve for EMC has a more prominent bending tendency than that of TMP. As a result, the discharge curve for EMC exhibits a inflexion shape while that for TMP is just a short slope. The difference between the influences of EMC and TMP on BF4 storage in graphite electrodes will become more evident if the charge


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cut-off voltage of Li/graphite coin cells was elevated (Figure S2). Capacity values increase in accordance with the rise of the charge cut-off voltage. The bending tendency becomes more obvious and the potential plateaus grow longer. In the ex situ XRD patterns of graphite electrodes from the cells charged to 5.2 V, the signs of BF4 intercalation can be perceived, whether clearly or not. In the case of EMC, the split of the original (002) diffraction peak of graphite and the emergence of a new peak at a lower diffraction angle provide a decisive evidence for the anion intercalation. In contrast, the diffraction peak of graphite electrode appears broader at the end of charge in contact with the TMP-based solution. A closer observation on its asymmetric shape can discern a shoulder peak at a lower diffraction angle besides the original (002) graphite peak. Although the above results prove that BF4 can intercalate into graphite electrode from the LiBF4-TMP or -EMC solutions, this electrochemical process is extremely sluggish, especially in the TMP solution.

Figure 1. Initial galvanostatic charge-discharge curves of graphite electrodes in the electrolyte solutions of 1M LiBF4-EMC/TMP.


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When a ternary mixture of 1M LiBF4-EMC/TMP was applied in the DIBs, it reveals an enhancement of the anion storage. Generally, the potential plateau gets lengthened in the EMC/TMP solutions as compared with the neat EMC or TMP solutions, which fact means that the mixed solvents facilitate more BF4– intercalation into graphite electrode. Furthermore, the potential plateau of that employing 1M LiBF4-EMC/TMP (8:2 by vol.) begins to appear at about 4.84 V, lower than the 1M LiBF4-EMC or -TMP (-EMC: 4.93 V; -TMP:4.96 V), which illustrates that less effort is spent for the solvated-anion beginning intercalation into the graphite sheet from this mixed solution. The results gained from galvanostatic charge-discharge test can be verified by voltammetric measurement as shown in Figure S3. This cooperation between EMC and TMP is of interest in terms of the charge transfer through the interface between graphite electrode and electrolyte solution. To gain a more comprehensive understanding, this problem should be investigated from both sides of the interface. As for graphite electrodes, in situ Raman spectra and ex situ XRD patterns were tested to characterize their structure changes near the surface and inside the bulk, respectively.


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Figure 2. in situ Raman spectra of graphite positive electrodes from the Li/graphite DIBs using single solution or mixed solutions in the first cycle.

in situ Raman spectroscopy has been confirmed a very sensitive tool to detect the anion intercalation into graphite electrode

19-20, 33, 35-36.

As shown in Figure 2, a typical spectrum of

graphite consists of two main bands observed at near 1330 and 1580 cm1 known as the D- and G-bands, respectively. The former band is assigned to A1g mode related to the disorders or defects in graphite crystal structure, in addition, it behaves like breath when anion embedded or extracted into/from the graphite. The latter band is the graphite characteristic peak related to the lattice vibration, and it splits into a double peak of E2g2 (i) (about 1578-1582 cm1) and E2g2 (b) (about 1597-1601 cm 1) when anion intercalates into the graphite layers 20, 35, 37. Conversely, the E2g2 (b) peak attenuation together with the G-band recovery reflects the anion de-intercalation process from graphite layers. Figure 2 (a) and (b) can evidence the shallow insertion of BF4– into graphite


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electrode from 1M LiBF4-EMC and -TMP at the cut-off voltage near 5.0 V. In contrast, the earlier appearance of G-bands cleavage and the delayed attenuation of the E2g2 (b) peak in Figure 2 (c) may prove that anion intercalation from the mixed solvents is far easier and more intense than those from the pure solvents. At the same time, the finally increasing intensity of the D-bands may illustrate that the anion storage process destroys the original surface states of the graphite.

Figure 3. ex situ XRD patterns of graphite electrodes during the galvanostatic charge-discharge of Li/graphite batteries using the electrolyte solutions of 1M LiBF4-EMC/TMP.

To shed more insights into the solvated anions storage in the graphite electrode, ex situ XRD was carried out to explore the crystal structure changes of graphite electrodes recovered from DIBs charged to 5.0 V. As Figure 3 shows, when TMP content ranges from 10 to 50 vol. % in EMC/TMP mixtures, the charged graphite electrodes lost their original (002) diffraction peak at 26.5o. Instead, new diffraction peaks of anion-graphite intercalation compounds (AGICs) aroused. This apparent change in crystal lattice of graphite definitely reveals the deep intercalation of BF4– into graphite electrode

19-20, 33, 38-39.

On the contrary, the original (002) diffraction peak of


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graphite almost keeps intact in the electrolyte solutions with very low or high contents of TMP, illustrating very difficult intercalation of solvated BF4– into graphite from these solutions. Table 2 Parameters of the anion-graphite intercalation compounds in Figure S2 and Figure 3. Electrolyte solutions

Iterative unit distance / nm

Stage number

Intercalated gallery height / nm

1M LiBF4-EMC/TMP (5:5 by vol.)

2.66

6.67

0.767


2.27

5.49

0.764


2.14

5.09

0.769


1.60

3.48

0.764


2.07

4.87

0.769

1M [email protected] V

2.49

6.16

0.763

From these ex situ XRD patterns, the inter-gallery height values of the AGICs were calculated 40

and listed in Table 2. In our previous studies, we have found that for certain anions including

BF4–, ClO4 and PF6, the inter-gallery height value of an AGIC is principally characteristic of the solvents co-intercalated

5, 16-18, 20.

The AGICs obtained from these electrolyte solutions

demonstrate very near inter-gallery height values, which means that EMC-solvated BF4– intercalates into graphite.

Figure 4. Nyquist plots of the interfaces between graphite positive electrodes charged to 5.0 V


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and different electrolyte solutions.

To accurately evaluate the interfacial properties between the graphite positive electrode and the electrolyte solutions, electrochemical impedance spectroscopy (EIS) measurements were carried out on symmetrical batteries consisting of two same graphite positive electrodes charged to 5.0 V (vs. Li/Li+). As shown in Figure 4, the solution resistance (R Ω in Table 3) of the three electrolyte solutions, which can be represented by the horizontal ordinate value of the starting point of semicircle, are ranked as follows: 1M LiBF4-EMC/TMP (3:7 by vol.)

Intercalation into a Graphite Electrode - ACS Publications - American

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