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A flame-retardant electrolyte solution for dual-ion batteries Lei Zhang, Yuhao Huang, Hui Fan, and Hongyu Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01942 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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ACS Applied Energy Materials

A flame-retardant electrolyte solution for dual-ion batteries Lei Zhang †, ‡, Yuhao Huang †, ‡, Hui Fan§, ‖, 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 §

School of Materials Science and Engineering, Hunan University of Science and Technology,

Xiangtan 411201, China ‖ Hunan Provincial Key Laboratory of Advanced Materials for New Energy Storage and Conversion, Hunan University of Science and Technology, Xiangtan 411201, China * Corresponding author. Tel/Fax: 86-431-85262287 E-mail address: [email protected] (H. Wang). Abstract

The flammability of organic electrolyte solutions has safety risk during the large-scale application of energy storage devices. So it is essential to suppress the flammability of organic electrolyte solutions. In this paper, the flame-retardant electrolyte solution of 3M LiPF6-ethyl methyl carbonate (EMC)/trimethyl phosphate (TMP) (7:3 by vol.) has been applied in Li/graphite dual-ion battery. Both the initial-cycle discharge capacity (almost 100 mAh g-1) and capacity 1

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retention rate (92.3 % after 450 cycles) surpass those for the battery using 1M LiPF6-EMC. Besides, the burning times of the above electrolyte solutions are 6 s and 66 s, respectively. Conventional electrochemical tests, ex situ X-ray diffraction and nuclear magnetic resonance have been carried out to explore the influences of TMP on PF6– in EMC/TMP mixed solvents.

Key words: flame-retardant electrolyte solution; trimethyl phosphate; ethyl methyl carbonate; dual-ion batteries; anion-graphite intercalation compounds.

1. Introduction

Recently, dual-ion batteries (DIBs) have become a promising candidate for large-scale electric energy storage devices mainly because of the following advantages. At first, the utilization of graphite as the positive electrode material is economic and environmentally friendly. Secondly, graphite can reversibly intercalate anions at rather high potentials (near 5 V vs. Li/Li+), which can guarantee both the energy and power densities of DIBs. Moreover, the working mechanism of a DIB is based on the independent storage of anions and cations at the positive and negative electrodes, respectively (1). In theory, any electrode materials that reversibly accommodate suitable cations can be employed as the negative one in a DIB. Therefore, the choice of negative electrodes gets quite convenient and flexible. In contrast, satisfactorily handling electrolyte in a DIB is a not an easy task at all. Generally, there are two kinds of electrolyte systems frequently used in DIBs with high working voltages, including ionic liquids (2, 3) and conventional organic electrolyte solutions (4-7). In the terms of large-scale applications, the latter kind appears more competitive due to its low cost and facile 2


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preparation. In these cases, organic solvent molecules always co-intercalate into graphite together with anions (7-12), and then profoundly influence the performance of graphite positive electrode. Up to now, the electrolyte solutions of LiPF6 dissolving in EMC (ethyl methyl carbonate) has been found very compatible with graphite positive electrode (4, 5, 7). However, the flammability of organic electrolyte solutions will become a big obstacle for the practical applications of DIBs, in a situation similar to lithium-ion batteries (LIBs). To solve this problem, the addition of some flame-retardant reagents into the organic solutions may be an effective way, in which phosphate esters are usually used, such as trimethyl phosphate (TMP) (13-15) , triethyl phosphate (TEP) (16, 17), tributyl phosphate (TBP) and triphenyl phosphate (TPP) (18) , dimethyl methyl phosphate (DMMP) (19), diethyl ethylphosphonate (DEEP) (20) and so on (21, 22). It should be noted that high anti-oxidation ability is a vital prerequisite for organic solvents used in electrolyte solution for DIBs, otherwise organic solvents decompose drastically under the indispensable high potential before anion intercalation into graphite. Via quantum chemical calculation, TMP draws our attention in these phosphate esters because the HOMO value of TMP is more negative than that of EMC, indicating that anti-oxidation ability of TMP may be superior to some carbonates frequently used. The calculated values of HOMO and LUMO of different solvents are listed out in Table 1, in which DMC, DEC, PC, EC stand for dimethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, respectively.

Table 1. The calculated values of HOMO and LUMO of different solvents. Solvent

HOMO / eV

LUMO / eV

TMP

7.93

0.846

TEP

7.81

0.772

TBP

7.76

0.848

3



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TPP

6.75

0.635

DMMP

7.84

0.694

DEEP

7.69

0.685

EMC

7.85

1.030

DMC DEC PC EC

–7.94 –7.84 –8.14 –8.24

0.993 0.974 0.621 0.547

Herein, the flame-retardant electrolyte solutions of LiPF6 in EMC/TMP mixed solvents are adopted in Li/graphite DIBs. Considering Li+ can be solvated by TMP in LIBs (23, 24), the effect of TMP on PF6 intercalation behavior from the mixed solvents of EMC and TMP are worthy to be explored in the DIBs. This study will pave an avenue towards a good-performance dual-ion battery with sufficient safety.

2. Experimental 2.1. Preparation of materials

The used natural graphite, of which the physical properties have been presented in our previous work (12), was from BTR New Energy Materials INC. Its average granularity was determined to be 131.49 m (Fig. S1). The graphite and TAB (teflonized acetylene black from Denka Co. Ltd.) were homogenously mixed at the weight ratio of 2:1. The composite which was made up of 10 mg graphite and 5 mg TAB was uniformly pressed on an aluminum mesh used as current collector. Electrolyte solutions were prepared by dissolving 1 M LiPF6 in the mixed solvents of EMC and TMP, in which EMC accounted for different volume proportions. Electrolyte solutions of different LiPF6 concentration in 70% EMC were also prepared. All the 4


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solvents were from TCI (Shanghai) and dried until the water content lower than 20 ppm. Batteries were assembled in a glove box filled with Ar atmosphere, where both contents of O2 and H2O were less than 0.5 ppm. The separator used in the dual-ion batteries was glass fiber filter from Advantec Toyo Kaisha, Ltd.

2.2. Relevant measurements and calculations

Galvano-static charge-discharge tests were conducted on coin cells (CR2032) with a current density of 100 mA g−1 from 3 V to 5.2 V (vs. Li/Li+) by a battery testing system (LAND, CT2001T). Cyclic voltammetric measurements of the graphite electrodes were operated by electrochemical workstation (Chenhua, CHI700D) in three-electrode cells in which Li was used as both the counter and reference electrodes. The potential range was from 2 to 5.2 V (vs. Li/Li+) and the scan rate was 2 mV s−1. The X-Ray Diffraction (XRD) experiments were carried out by a Rigaku MiniFlex600 X-ray diffractometer under the same test condition in the past studies (9, 25), using

graphite positive electrodes recovered from the Li/graphite cells under OCV, charged to

5.2 V and discharged back to 3 V. Nuclear magnetic resonance (NMR) spectroscopy of the electrolyte solutions was obtained from a BRUKER Advance III HD 500 using Si(CH3)4 and C6F6 and (NH4)2HPO4 ethanol solutions as the internal standard reagents for 1H NMR and

19F

NMR

and 31P NMR, respectively. In a flammability test (20, 26), a piece of dry and circular absorbent cotton (diameter: ca. 2.5 cm; mass: ca. 55 mg) soaked with 1 mL electrolyte solution was set horizontally on the steel mesh, and then was ignited by a common lighter. A timer was used to record burning time. Each test was repeated four times and the average value was adopted. Unless specified, all the measurements were carried out at the room temperature about 25 C. Quantum chemical calculations were performed to assess geometries and energy parameters of 5



phosphate esters and carbonates (Gaussian 09). Specific calculation conditions were same as our previous work (25).

3. Results and discussion

The influence of TMP introduction into a dual-ion battery at first can be perceived from the ionic conductivity of electrolyte solutions (Fig. S2). It is worth noting that the relationship between ionic conductivity of 1M LiPF6-EMC/TMP solutions and TMP volume portion demonstrates a symmetrical, inverse “V” shape. This may reflect the delicate balance between the viscosity and permittivity of solvents. The former factor relates to the mobility of ions while the latter one determines the numbers of “free” ions. Furthermore, the thermal stability of the solutions becomes considerably improved (Fig. S3).

Fig. 1. Initial galvanostatic charge-discharge curves of graphite electrodes in the electrolyte solutions of 1M LiPF6-EMC/TMP.

On the other hand, TMP addition will exert great impact on the performance of graphite/electrolyte interface. Fig. 1 plots the 1st-cycle galvanostatic charge-discharge curves of 6


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graphite electrodes in the electrolyte solutions of 1M LiPF6 dissolved in EMC/TMP mixtures. Each curve roughly consists of a drastically sloping line and some voltage plateaus. The former section corresponds to the charge storage at the electric double-layers of graphite/electrolyte interface, which account for a very small portion of capacity because of the tiny surface area of graphite. The latter section can be ascribed to the stage transformation of PF6–-graphite intercalation compounds (1, 7). It is evident that TMP addition in a solution will shorten or even erase some voltage plateaus. Meanwhile, the voltage polarization between charge and discharge processes becomes large. The inflection point from the sloping line to voltage plateaus in a charge curve shifts upside (from ca. 4.4 to 5.0 V (vs. Li/Li+). This means that the beginning of anion intercalation into the graphite layers gets more difficult. Accordingly, the discharge capacity intensively decreases when the TMP content is more than 30 % in the 1M LiPF6-EMC/TMP. So it can be speculated that TMP suppresses PF6– intercalation into graphite layers. This phenomenon can be witnessed in the cases of EC and SL as well (7, 11, 27). Therefore, the trend in the polarization and capacity changes can be explained by Nernst equation. The decrease in EMC-solvated PF6– (electrochemically active) content with the addition of TMP into the solution should shift the standard intercalation/de-intercalation potential to higher values. Then the potential range for anion intercalation shrinks and then limits the anion uptake.

7



Fig. 2. Cyclic voltammograms of graphite electrodes in the electrolyte solutions of 1M LiPF6-EMC/TMP at the first cycle.

The insights gained from the galvanostatic charge-discharge tests may be further verified by cyclic voltammetric studies. Fig. 2 plots cyclic voltammograms of graphite electrodes in 1M LiPF6-EMC/TMP solutions. As can be seen, the onset potential of the anodic peak was determined at 4.46 V (vs. Li/Li+) in EMC, gradually moves to 4.96 V (vs. Li/Li+) in EMC/TMP (6:4 by vol.) and finally fade away (in TMP), possibly because of the greater effort needed for the anion to strip off its solvent shell before co-intercalation with solvents (28). And the number of anodic and cathodic peaks decreases or even disappears when more TMP is introduced in a solution, which means less intercalation/de-intercalation courses takes place. Moreover, the area surrounded by the curve shrinks along with the rise in TMP proportion, may reflect that less amounts of anions participate in the intercalation/de-intercalation process. In the TMP solvent, PF6– hardly intercalates in graphite. All these facts agree with the results of Fig. 1. Based on the above results, it is confirmed that TMP prevents PF6– from intercalating into graphite positive electrode. 8


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Furthermore, TMP addition into LiPF6-EMC/TMP solutions delays PF6– intercalation into graphite, which implies that TMP competes with EMC to solvate PF6 and exert a negative influence on the anion storage.

Fig. 3. Relationship between the initial discharge capacity of graphite positive electrode and TMP content in EMC/TMP mixtures.

Then the introduction of TMP into DIBs becomes a paradox to simultaneously meet the needs of both high capacity and remarkable flame-retardant ability. It was once proposed that at least 62 vol.% of TMP in the electrolyte solutions of LiPF6-EMC/TMP could completely eradiate the flammability (14). However, in the case of DIBs, the TMP dose must be restrained strictly taking account of its side effect on the anion intercalation. Fig. 3 helps determine the maximal dose of TMP under this circumstance. It presents the relationship between the initial discharge capacity of graphite positive electrode and EMC contents in the mixed solutions of 1M LiPF6-EMC/TMP. Since the discharge capacity starts to drop down abruptly at the TMP content of 30 vol. %, this value was selected as the upper limit for TMP addition.

9



Fig. 4. Initial galvanostatic charge-discharge curves of graphite electrodes in the electrolyte solutions of 1M LiPF6-EMC, 1M and 3M LiPF6-EMC/TMP (7:3 by vol.). Nevertheless, the discharge capacity delivered by graphite electrode is less than 80 mAh g1 in the electrolyte solution of 1M LiPF6-EMC/TMP (7:3 by vol.). To enlarge the capacity, some strategies might be attempted such as elevating the charge cut-off voltage of a cell or pre-treating graphite. Alternatively, another one could be adopted with a minimal modification on the electrolyte itself. That is to increase PF6 concentration while fixing the EMC/TMP proportion of 7:3. Nearly one decade ago, it has already been realized that in the electric energy storage devices based on the dual-ion charge mechanism, like EDLCs or dual-carbon cells, an adequate concentration of electrolyte salts must be guaranteed to suffice the storage ability of electrode materials (29, 30). In theory, 0.2 ml electrolyte solution (volume of a coin cell) with the concentration of 1 mol L1 amounts to the capacity of ca. 5.36 mAh (0.21/100026801, where 26801 mAh is equivalent to the Faraday constant) if all the ions in the electrolyte solution could be accumulated at both the electrodes of a cell. The above value is much higher than the capacity (less than 0.8 mAh) delivered by a graphite electrode with the mass of 10 mg in the cell. So 1M electrolyte solution provides enough anions for 10 mg graphite to accommodate in a coin cell. In 10


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contrast, the utilization of highly concentrated electrolyte solutions (several mol L1) has achieved remarkable improvement in the performance of some electric energy storage devices (31-43). This was generally associated with the diet of ion solvation. Recently, similar beneficial effect on dual-ion batteries has also been found (2, 6, 44). And it does work in this study. Fig. 4 mainly compares 1st-cycle galvanostatic charge-discharge curves of graphite electrodes in the electrolyte solutions of 1M LiPF6-EMC and 3M LiPF6-EMC/TMP (7:3 by vol.). It is of interest to note that both the charge and discharge curves corresponding to these two electrolyte solutions almost overlaps. Obviously, the electrolyte concentration of 3M in the mixed solvent makes the graphite electrode regain the capacity value which can be delivered in 1M LiPF6-EMC. At other proportions of EMC/TMP, the increase in the LiPF6 concentration also elevates the capacity values remarkably (Fig. S4).

Fig. 5. Cycle performance of Li/graphite cells using the electrolyte solutions of 1M LiPF6-EMC and 3M LiPF6-EMC/TMP (7:3 by vol.), and their respective burning times.

Moreover, the cycle performance of graphite electrode in the concentrated solution has compared with that in 1M LiPF6-EMC as shown in Fig. 5. It seems that both series of capacity values along with cycles are quite near. In the inset of this figure, the burning time of electrolyte 11



solution of 3M LiPF6-EMC/TMP is far less than 1M LiPF6-EMC (6 vs. 66 s), indicating the much more significant flame-retardant ability of the former one. In terms of capacity, cycle-ability and flame-retardant ability, 3M LiPF6-EMC/TMP (7:3 by vol.) may be more suitable than 1M LiPF6EMC as an electrolyte solution in Li/graphite DIBs.

Fig. 6. ex situ XRD patterns of graphite electrodes during the galvanostatic charge-discharge of Li/graphite batteries using the electrolyte solutions of (a) 1M LiPF6-EMC/TMP (7:3 by vol.), (b) 3M LiPF6-EMC/TMP (7:3 by vol.), and (c) 1M LiPF6-EMC, under OCV, being charged to 5.2 V and discharged to 3.0 V.

To shed more insights into the charge storage mechanism of solvated anions in the graphite 12


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electrode, ex situ X-ray diffraction (XRD) has been performed. As Fig. 6 shows, when the cell voltage reached 5.2 V, the (002) peak of original graphite (26.5o) nearly faded away and new diffraction peaks of anion-graphite intercalation compounds (AGICs) appeared in response, which reveals that PF6– can intercalate into graphite positive electrode in LiPF6-EMC/TMP solutions. While the cells were discharged to 3 V, the (002) peak of graphite (26.5o) arose again in the absence of AGICs diffraction peaks, manifesting a reversible storage of PF6–. From the ex situ XRD patterns of graphite electrodes from the cells charged to 5.2 V, the inter-gallery height values of these AGICs can be calculated. These values are listed out in the Table 2. It is definite for us that the inter-gallery height values of identical anion in different solvent are distinct, so the similar inter-gallery height values of these AGICs in the above discriminate electrolyte solutions may imply that PF6– intercalation into the graphite layers from the mixed solvent is the same as that from pure EMC (7, 11, 45). It is probable TMP just acts as a flame-retardant solvent and does not intercalate deeply into the graphite electrode together with PF6–. However, in situ Raman spectroscopic measurements on the graphite electrode in the solution of LiPF6-TMP exhibits clear clues of anion intercalation at the surface (Fig. S5). Furthermore, the less change of AGICs stage number in the three electrolyte solutions may indicate that the anion intercalation extents are very close. Combined with the appearance of inflection in the charge curves of Fig.1, more TMP competes with EMC to solvate PF6, resulting in greater effort needed for PF6 co-intercalation with EMC into graphite and the less mount of PF6– storage capacity (a higher stage number).

13



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Table 2. Parameters of the anion-graphite intercalation compounds in Fig. 6. Electrolyte solutions

Iterative unit distance / nm

Stage number

Intercalated gallery height / nm

1M LiPF6-EMC/TMP (7:3 by vol.)

n1

1.12

2.03

0.774

3M LiPF6-EMC/TMP (7:3 by vol.)

n1

1.10

1.97

0.774

1M LiPF6-EMC

n1 n2

0.78 1.08

1.01 1.97

0.773 0.775

Fig. 7. Chemical shifts of 19F (a) and 31P (b) NMR peaks versus TMP content in 1M LiPF6 -EMC/TMP solutions.

Since the performance of graphite positive electrode can be controlled by compositions of the electrolyte solutions, it is necessary to explore the interaction between solvents and PF6– in the solutions. Nuclear magnetic resonance (NMR) is one of the powerful techniques that can directly characterize the delicate interactions between different substances in solution. As shown in the 19F and

31P

NMR spectra for the electrolyte solutions of 1M LiPF6 dissolved in EMC/TMP mixed

solvents (Fig. S6), there are two 19F peaks with the same intensity reside in adjacent locations (Fig. S6 (a)), which originates from two types of tiny different micro-environments of 19F in PF6– with a structure of octahedron (46). On the other hand, as shown in the internal graph of Fig. S6 (b),

14


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there are two types of signals representing

31P

in PF6– and in TMP, respectively. Due to the

influence of the connected magnetic six nucleus of 19F, the 31P NMR peak for PF6– actually splits into seven sub-signals, corresponding with the “(n+1)” rule in NMR. So the signals of PF6– at higher magnetic field can be easily differentiated from that of TMP molecule at lower filed. Fig. 7 (a) and (b) display the chemical shifts of

19F

in LiPF6 and

31P

NMR peaks in TMP for the

electrolyte solution of 1M LiPF6 in EMC/TMP mixed solvents, respectively. The comparison between Fig. 7 (a) and Fig. 7 (b) demonstrates that as the TMP proportion increases, both the 19F NMR peaks of 19F in PF6– and the 31P NMR peak of 31P in TMP shift to the low magnetic field. Moreover, once the volume percentage of TMP in EMC/TMP exceeds 40 %, the shifts of 19F NMR and 31P NMR become less obvious than those at lower TMP contents. The above phenomenon may be explained as follows: (a) PF6– is a strong Lewis alkaline which is easily bonded to the phosphorus center polarized by the oxygen atom with high electronegativity in TMP. Increasing the TMP content in 1M LiPF6-EMC/TMP means that more polarized electropositive phosphorus centers of TMP subject to the bondage with F from PF6–. On the other side, the more TMP in a solution, the more targets to that a considerable amount of PF6– can transfer electrons, and the less electrons each phosphorus center of TMP can accept from PF6–. Thus it results in both the shifts of 19F NMR and 31P NMR. (b) When the content of TMP in 1M LiPF6-EMC/TMP reaches an extremum, the number of phosphorus centers in TMP bonded with F from PF6– hardly changes, shift a little bit.

15


19F

NMR peaks just


Fig. 8. Chemical shifts of 19F (a), 31P (b) and 1H (c) NMR peaks versus LiPF6 concentration in LiPF6-TMP solutions.

In order to support the above inference, different concentrations LiPF6 in pure TMP were measured by NMR as well. Fig. S7 demonstrates the 19F (a), 31P (b), and 1H (c) NMR spectra of the LiPF6-TMP electrolyte solutions. In Fig. 8 (a),

19F

NMR peaks shift to the lower magnetic

field just a little bit with the rise in LiPF6 concentration, which also can be explained by the nearly 16


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saturated numbers of phosphorus centers in TMP bonded with F in PF6–. Fig. 8 (b) implies that increasing the concentration of LiPF6 in TMP is to raise the average number of the electrons that each phosphorus center of TMP can accept from PF6–. Moreover, from Fig. 8 (c), the 1H NMR peaks shift to higher magnetic field gradually with the rise in LiPF6 concentration, which may indicate that more CH---F hydrogen bonds are formed as the molar ratio of LiPF6/TMP in solutions increases (47). It is worth mentioning that the 1H NMR peaks look like two kinds of H, actually only one in the large coordinate range , which can be explained by the different rotational gesture of H in CH3 belonged to TMP, of which this phenomenon is quite common in 1H NMR. Comparing the shifts of 31P in Fig. 8 (a) and 1H NMR in Fig. 8 (c), 1H NMR shifts more obviously than 31P NMR, which may be related to the number and location of H and P in a TMP molecule.

4. Conclusion

3M LiPF6-EMC/TMP (7:3 by vol.) was proposed as a flame-retardant electrolyte solution in Li/graphite DIBs. Graphite electrode in this solution delivers a capacity value (ca. 100 mAh g1) close to that for the electrolyte solution of 1M LiPF6-EMC. In addition, the burning time of the former solution get shortened more than 90 % as compared to the latter solution, indicating the improved safety. As for the cycle-ability of graphite electrode in the former solution, the discharge capacity still maintains 91.72 mAh g1 after 450 cycles, which is also superior to that for the latter solution. The results of conventional electrochemical tests and ex situ XRD demonstrated that TMP competes with EMC to solvate PF6– and TMP suppresses PF6– intercalation into graphite. NMR results revealed that the interaction sites between TMP and PF6– are H, P in TMP and F in PF6–. TMP content in the solution bears a direct relationship with the chemical shifts of these interaction sites. 17



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Acknowledgments

This work was financially supported by National Natural Science Foundation of China (21673222) and Doctoral Scientific Research Foundation of Hunan University of Science and Technology (E51875).

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