activated carbon capacitors

Accepted Manuscript Title: Storage behavior of isomeric quaternary alkyl ammonium cations in graphite electrodes for graphite/activated carbon capacitors Authors: Jiayu Li, Cheng Zheng, Li Qi, Masaki Yoshio, Hongyu Wang PII: DOI: Reference:

S0013-4686(17)31589-X http://dx.doi.org/doi:10.1016/j.electacta.2017.07.148 EA 29965

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

28-2-2017 19-7-2017 24-7-2017

Please cite this article as: Jiayu Li, Cheng Zheng, Li Qi, Masaki Yoshio, Hongyu Wang, Storage behavior of isomeric quaternary alkyl ammonium cations in graphite electrodes for graphite/activated carbon capacitors, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.07.148 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Storage behavior of isomeric quaternary alkyl ammonium cations in graphite electrodes for graphite/activated carbon capacitors

Jiayu Li a,b, Cheng Zheng a, Li Qi a, Masaki Yoshio c, Hongyu Wang a, * a

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China

b

c

University of Science and Technology of China, Hefei 230026, China

Advanced Research Center, Saga University, 1341 Yoga-machi, Saga 840-0047,

Japan

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

Highlights 

1 Quaternary alkyl ammonium (QAA) cations as charge carriers in energy storage devices.



2 Three small isomeric QAA cations intercalate into graphite electrode in capacitors.



3 Isomeric effect on QAA storage can be studied by electrochemical in situ techniques.



4 Slim QAA cations are kinetically favorable for insertion into graphite electrode.

Abstract Quaternary alkyl ammonium（QAA） intercalated graphite compounds are promising as efficient and environmentally benign electrode materials in electric

energy storage devices. The intercalation and de-intercalation processes of three isomeric QAA cations have been investigated in graphite/activated carbon (AC) asymmetric capacitors based on propylene carbonate (PC) solutions. Some fundamental electrochemical techniques in conjunction with in situ XRD and in situ Raman spectroscopy have been employed to study the differences among them. Isopropyltrimethyl ammonium (iPTMA+) gives the most sluggish interaction with graphite. By contrast, propyltrimethyl and diethyldimethyl ammonium (PTMA+, DEDMA+) demonstrate quicker kinetic behaviors and higher rate capacity in capacitors. keywords： quaternary alkyl ammonium ; graphite intercalation compounds; isomeric effect; capacitors 1.Introduction Modern society calls for advanced electric energy storage devices which are safer, more robust and environmentally benign. Graphite has long been recognized as such an electrode material satisfying these requirements and played a vital role in the developments of many electrochemical technologies. In fact, its layered structure capable of accommodating many kinds of ions in a wide electrochemical window [14], constitutes the basis of charge storage through the formation of graphite intercalation compounds (GICs). One of the most famous examples may be Li-GICs that have already found successful applications as the negative electrode materials in commercial lithium-ion 2

batteries today. They have also been employed in lithium-ion capacitors, which were proved attractive for high power demands [5-8]. However, their utilizations in the electric energy storage devices used to suffer from the severe risk of lithium metal deposition at low temperatures or under high rate charge. Moreover, the fluent Li+ insertion/extraction processes in the capacitors calls for the formation of an efficient and stable solid electrolyte interface (SEI) film on the surface of graphite electrode [9]. Usually, this task is fulfilled with a pre-lithiation procedure by a sacrificial lithium metal electrode, which could potentially deteriorate the overall safety of a device [10]. These troublesome items actually will become inevitable among the GICs formed from metal cations like Na+ and K+. To avoid the above awkward situation stemming from the employment of metal cations, quaternary alkyl ammonium (QAA) has been proposed as the alternative charge carrier at the negative electrode side in new electric energy storage devices [11-13]. Such kind of cations is composed from multiple atoms free of metal. In contrast, a metal cation generally contains only one atom with a relatively simple, rigid and symmetrical structure. So it can be envisaged that the organic QAA cations are much more complex than the conventional metal cations from the viewpoint of chemical structures. Especially, their branched chains can demonstrate a variety of flexible conformations inside or outside the interlayer spaces of graphite. We have realized that some delicate aspects such as molecular weight and geometry might influence the storage behavior of these cations in graphite electrodes. Since many

3

problems remained unsolved, a series of systematic investigations are required to uncover the mystery. In fact, a very interesting feature in this field can be ascribed to the isomeric effect, which comes from the organic cations with the same molecular formula but different atomic arrangements. Of course their discrepant chemical structures or configurations will lead to some considerable variations in the terms of many properties. However, such an issue has never been addressed at all in the community of GICs. In this study, we will explore how the isomeric effect works on the electrochemical intercalation/de-intercalation behaviors of QAA cations in graphite. Since the volumes of these organic cations are usually too large as compared with the ordinary metal cations, their intercalation levels into graphite are too limited to clearly identify the effect. Therefore, smaller organic cations are more favorable than bigger ones to acquire a higher storage capacity in graphite electrodes. Furthermore, the lower molecular weight an organic cation has, the less likelihood of divergent chemical structures there is. To elucidate the isomeric effect concisely, we picked up the three smallest isomeric QAA cations (isopropyltrimethyl ammonium, iPTMA+; diethyldimethyl ammonium, DEDMA+; and propyltrimethyl ammonium, PTMA+) in this preliminary attempt. Besides, graphite/activated carbon (AC) capacitors have been applied as a device evaluating intercalation processes into graphite negative electrodes [14]. With the help of in situ XRD and in situ Raman spectroscopy, combined with conventional electrochemical techniques to characterize these QAAGICs in the environment of capacitors, we could grasp comprehensive clues about the 4

different performance of these QAA isomers in the terms of electrochemical intercalation with graphite. 2.Experimental 2.1 Materials

Artificial graphite (KS6 from Timcal) served as the negative, whereas activated carbon (AC, PW15M13130 from Kureha) worked as the positive electrode materials, respectively, in the asymmetric capacitors. Their physical properties have been described in the past reports [14,15]. The electrolyte salts of QAA-BF4 were synthesized according to the Ue-Mori procedure [16] and their purities could be confirmed by 1H NMR (nuclear magnetic resonance) as shown in Fig. S1. BF4 was chosen as the anion in these electrolyte salts due to its thermal and anodic stability. PC was used as the solvent for the electrolyte solutions in the capacitors mainly because of its high permittivity and low toxicity. It was dried by molecular sieves until its water contamination content was below 10 ppm. 2.2 Measurements and Calculations The mass ratio of the active electrode material (AC or graphite) and the conductive binder of TAB (teflonized acetylene black) was 2:1. The mixture of active material and TAB was pressed on a piece of aluminum mesh (current collector) to fabricate an electrode.

5

Electrochemical measurements were conducted with coin or beaker cells. The weight ratio of the electrodes (AC/graphite) was kept at 1. A beaker cell comprised three electrodes soaking in an electrolyte solution. Besides the AC and graphite electrodes, excess amount of AC was used as additional quasi-reference electrode (QRE). Its validity has been testified before [17]. The conductivities of electrolyte solutions were measured by Conductivity Meter DDS-11A at 25

. Operations were

carried out in an Ar-filled dry glove box, inside which both the contents of H2O and O2 were less than 0.5 ppm. Unless otherwise specified, galvano-static chargedischarge tests were performed at the current density of 0.1 A/g between the cut off voltages of 0 and 3.5 V. The details of in situ XRD and in situ Raman measurements on graphite electrodes in the capacitors were similar to those described in our previous studies [18,19]. Quantum chemical calculation was performed to evaluate geometries and energy parameters of three cations (Spartan 10).The equilibrium conformers at ground state were fully optimized by the density functional theory (DFT) method, B3LYP functional and 6-311G* basis set were used for all the atoms. 3.Results and discussion For each QAA salt, some basic properties of the electrolyte solutions, including solubility and conductivity, need to be determined at first since they are closely connected with the overall performance of capacitors. All the three salts can be considerably dissolved in PC at room temperature. Their solubility values in PC are 6

2.5 M (mol dm-3) for PTMA-BF4, 1.5 M for DEDMA-BF4 and 1 M for iPTMA-BF4, respectively. A high solubility is advantageous for the plentiful supply of charge carriers in the graphite/AC capacitors, especially at the end of charge process. However, a super high concentration of electrolyte salt dissolved in PC will make the solution become very viscous and then decrease the ionic conductivity. Fig. 1 illustrates how the conductivity of electrolyte solution changes with the addition of an electrolyte salt into PC. Generally, the conductivity climbs up concavely as the salt concentration rises. However, in the case of PTMA-BF4 dissolved in PC, the conductivity of the solution reaches a maximum value near 2 M and declines as the concentration further increases. In the following electrochemical tests, we set 1 M as the universal concentration standard since all these 1 M electrolyte solutions demonstrate similar conductivities. So the difference in the ionic transport of these cations in the solution phases could be minimized and neglected. Then we could pay attention to charge transfer across the graphite/solution interface or diffusion of cations inside graphite electrode through the performance of total capacitors. As a routine menu, long cycling tests have always been carried out to confirm the electrochemical stability of charge storage. Fig. 2 depicts the long cycle performance of graphite/AC capacitors using different electrolytes. The capacitor using iPTMA-BF4 displays lower discharge capacity compared with DEDMA-BF4 or PTMA-BF4 as a whole. Along with the slow attenuation in hundreds of cycles, their capacity retention gradually approached. Accelerated ageing based on floating test has been applied to estimate their electrochemical stablility at different high voltages. The 7

central step consists of five galvano-static cycles and a floating at maximum voltage for 2 h [20]. It was repeated 60 times, i.e. for a total floating time of 120 h. The floating measurements of graphite/AC capacitors using different electrolytes were conducted at maximum voltages of 3.5V, 3.3V and 3.1 V, as shown in Fig. 3, Fig. S2(a) and Fig. S2(b), respectively. Overall, capacitors containing iPTMA+ display inferior discharge capacity in any case. Those involved PTMA+ perform better at first but last for a short term. Capacitors with DEDMA+ are high-capacity and durable, especially at lower cut-off voltages. A coin cell may not be straightforward enough to reflect the changes at both electrodes，so we addressed this issue by three-electrode beaker cells, in which an AC-QRE was introduced besides the working electrode of graphite and its counter electrode of AC. Potential values of both the working and counter electrodes versus the AC-QRE were separately recorded from disparate circuits. This technique was proved effective in the research on electric double-layer capacitors [21]. We mainly focus on the first cycle because the electrochemical activation of electrode materials in this stage is vital for its performance [11]. The total potential curves of capacitors and separate electrodes during the initial charge-discharge cycle are shown in Fig. 4. It is convinced that the interaction happened on graphite negative electrodes makes the main differences among total capacitors. While the potential curves of AC electrodes are mostly overlapped, indicate the same process at the positive electrode side (adsorption and desorption of BF-4 ). The potential curve of graphite electrode using iPTMA-BF4 exhibits a more obvious rebound at the 8

beginning of discharge, which may be characteristic of a loose contact of iPTMA+ with graphite [22]. To shed more light on this process，we applied in situ XRD measurements to determine the crystal structure changes of graphite electrodes. Fig. 5(a) and Fig. 5(b) compare the XRD patterns of graphite electrodes in graphite/AC capacitors during charge and discharge processes. Original graphite has the (002) diffraction peak at 26.5 °. As the cell voltage climbs up to a higher value, the intensity of (002) peak fades and new diffraction peaks emerge gradually besides it, as a result of intercalation of cations. Conversely, these reflections disappear almost completely upon de-intercalation, the (002) diffraction peak returns at its initial position after discharging [4]. Based on the new diffraction peaks at 3.5 V, we calculated some key parameters of GICs according to the method in previous studies [23,24]. As listed in Table 1, all of them form high-stage GICs due to the minor size, conform well with Lerner’s report [25]. The intercalated gallery heights (IGH) of GICs obtained from all the three cations are about 0.93nm, essentially indistinguishable. A more sensible difference here actually lies in the voltage at which new phases start to emerge. As illustrated in Fig. 5(a), the (002) peak starts to split at about 2.3 V in the case of DEDMA-BF4, the earliest of all. But the same change happens in the case of iPTMA+ and PTMA+ until the voltage is higher than 2.7 V. The intercalating pace of PTMA+ overtakes iPTMA+ above 3.1 V. Accordingly, in the process of discharge (shown in Fig. 5(b)), the diffraction peaks due to GIC disappear quickly for DEDMA+, at around 2.9V. This transition is put off until 2.3 V for iPTMA+ and 9

PTMA+. But the (002) peak has the most complete recovery after extracting PTMA+, which means less destructive effect on the pristine crystal structure of graphite [4]. The storage response of DEDMA+ seems the most sensitive towards electrochemical control, in contrast, iPTMA+ appears the tardiest. XRD tests seem unable to explain the comparative charge capacity since the insertion of iPTMA+ falls behind others. We need a more comprehensive method to study it. Here, Raman spectrometry can provide abundant information about the vibration both on the surface and in the bulk of materials, which is competent for the task [26,27]. A typical spectrum of graphitic materials comprises two main bands observed at about 1330 and 1580 cm−1 known as the D- and G-bands, respectively. The D band is assigned to A1g mode, which comes from some disorders and defects in graphite crystal structure. It behaves like breathing when ions embedded or extracted. The G-band is the characteristic peak of graphite ascribed to the lattice vibration. It will split into a double peak of E2g2(i) (1578-1582 cm−1) and E2g2(b) (1597-1601 cm−1) if any ions intercalate into the spaces between the graphene layers. Similarly, the E2g2(b) peak attenuates with the recovery of G-band means a deintercalation process [26]. Fig. 6 compares the in situ Raman spectra of graphite negative electrodes in the graphite/AC capacitors using different salts. Generally, the D-band fades away with the cations insertion and reappears in the process of removal. It is remarkable that the capacitor using iPTMA+ gives a stronger D-band after discharging, a signal of more cracks on the surface of graphite [28]. While, it is milder for the other two, especially 10

in PTMA+ . The corresponding graphite electrodes may suffer from less damage and it shows good agreement with XRD results. The intensity ratio of E2g2(i) to E2g2(b) is a key parameter, which is proportional to the stage number of GICs [27]. The Ii/Ib reveals a special insertion process between iPTMA+ and PTMA+.As a whole, PTMA+ has a narrow lead over iPTMA+ in action ,whether from Raman or XRD results. But as an exception, iPTMA+ takes the lead briefly at 3.2 V according to the Ii/Ib. We attributed it to the more shallow but less deep-seated insertion of iPTMA+ here, which can be detected by Raman rather than XRD. It makes the approximate initial charge capacity all of three. Besides that, iPTMA+ also appears the slowest behavior of de-intercalation and obvious split peaks can be seen when discharging to 2.0 V. To be more accurate, Fig. 7 shows the variation of Ii/Ib with the potential of graphite electrodes in graphite/AC capacitors. As the intercalation occurs, the Ii/Ib drops down sharply, particularly in DEDMA+ .The decrease pace is ranked as follow: DEDMA+>PTMA+≥iPTMA+. With respect to the de-intercalation process, the plot for iPTMA+ has a gentler slope than others, the increased pace of Ii/Ib value is ranked as follow: PTMA+≥DEDMA+>iPTMA+. In general, iPTMA+ gives sluggish interaction with graphite, especially in the deep region. The de-intercalation of iPTMA+ seems slow and unsmooth, which can cause more defects to the graphitic structure. Since the above disagreement roots in kinetics, a test of rate capability can distinguish and justify it well. Fig. 8 shows the rate capability of graphite/AC 11

capacitors during galvano-static charge-discharge at various current densities. Two stages were picked up from the capacitors under long cycling tests, after 200 cycles or 1000 cycles. PTMA+ and DEDMA+ have better rate capability in respective stages. However, capacitors with iPTMA-BF4 continue the poor performance all along due to its tardiness. We attempt to dig the nature of this isomeric effect. It was taken for granted that the stability or size of cations may lead to the disagreement. As a reference, Table 2 lists related energy and geometric parameters of three cations according to the computational calculation. They have matched energy values of the lowest unoccupied molecular orbital (LUMO)，which reflect the ability against cathodic reduction. From the viewpoint of geometry, three cations have almost the same van der Waals volume values but some nuances in shape. The minimal distance (dmin) means the most slender part of a cation at equilibrium state, as shown in Fig. 9. The dmin can be ranked as DEDMA+
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However, increasing the voltage, negative electrodes with lower potential attract a growing number of cations crowd into graphitic layers，the interaction can not limit to a line and leads the graphite expand to accommodate more. Presumably, it may involve the mechanical flexibility of graphene, the company of solvent molecules, the transformation of QAA and their entry side by side. Such complicated factors fill the gap among single cations, the IGH of GICs increased and converged to about 0.93 nm as in situ XRD noted. This interesting phenomenon is still under investigation. Unluckily, analogous QAA-GICs are highly air sensitive and difficult to characterize isolated, they need to be discovered with more advanced technologies. According to all of the results, we can conclude that three isomeric cations have tiny differences in many respects but the isomeric effect becomes obvious when they have electrochemical intercalation with graphite. One possible explanation for these findings is their unique configurations and subtle conformational transitions during intercalation and deintercalation process. So they make the dissimilarity of velocity and the pace rank appears inconformity between the two processes. 4.Conclusion The storage behavior of isomeric quaternary alkyl ammonium cations in graphite electrodes was studied from PC-based solutions. Some basic electrochemical methods verified that QAA–GICs make the different performance among capacitors. in situ XRD and in situ Raman results proved it is mainly caused by the gap of kinetics. iPTMA+ moves slowly in graphite electrodes compared with DEDMA+ and PTMA+, and this gap presents more significant in their de-intercalation process. The coarse 13

interaction between iPTMA+ and graphite brings new defects in graphite and inferior rate capability in capacitors. Acknowledgments This work was financially supported by National Natural Science Foundation of China (21673222 and 21173203). References 1. J.R. Dahn, J.A. Seel, Energy and capacity projections for practical dual-graphite cells, J. Electrochem. Soc. 147 (2000) 899-901. 2. J.A. Read, A.V. Cresce, M.H. Erwin, K. Xu, Dual-graphite chemistry enabled by a high voltage electrolyte, Energy Environ. Sci. 7 (2014) 617-620. 3. H. Zheng, K. Jiang, T. Abe, Z. Ogumi, Electrochemical intercalation of lithium into a natural graphite anode in quaternary ammonium-based ionic liquid electrolytes, Carbon 44 (2006) 203-210. 4. P.W. Ruch, M. Hahn, F. Rosciano, M. Holzapfel, H. Kaiser, W. Scheifelea, B. Schmittb, P. Nováka, R. Kötza,A. Wokaun, In situ X-ray diffraction of the intercalation of (C2H5) 4 N+ and BF 4− into graphite from acetonitrile and propylene carbonate based supercapacitor electrolytes, Electrochimi. Acta 53 (2007) 10741082. 5. J.H. Kim, J.S. Kim, Y.G. Lim, J.G. Lee, Effect of carbon types on the electrochemical properties of negative electrodes for Li-ion capacitors, J. Power Sources 196 (2011) 10490-10495. 14

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Fig. 1. Conductivity vs. concentration of iPTMA/DEDMA/PTMA-BF4 in PC at 25 °C.

18

Fig. 2.Cycling performance of graphite/AC capacitors using different salts ( iPTMA/DEDMA/PTMA-BF4).

19

Fig. 3. Effect of floating on graphite/AC capacitors using different salts (iPTMA/DEDMA/PTMA-BF4) at 3.5V.

20

Fig. 4. Potential curves of graphite negative electrodes and AC positive electrodes vs. AC-QRE in the capacitors using 1 M iPTMA/DEDMA/PTMA-BF4 in PC, respectively.

21

[a]

[b]

Fig. 5. in situ XRD patterns of graphite negative electrodes in capacitors containing different cations during the initial cycle, including (a) charge and (b) discharge processes.

22

Fig. 6. in situ Raman spectra of graphite negative electrodes in the AC/graphite capacitors containing different cations in the initial cycle.

23

Fig. 7. The variation of Ii/Ib with the potential of graphite negative electrodes in different capacitors.

24

[a]

[b]

Fig. 8. Discharge capacity at various current densities in graphite/AC capacitors (a) after 200 cycles; and (b) after 1000 cycles.

25

Fig. 9. dmin sketches of the different cations at their equilibrium states.

Table 1 Parameters of the QAA-GICs at 3.5 V in Fig. 5(b) Cation type

Basal repeat length /nm

Stage No.

Intercalated gallery height/nm

iPTMA

1.711

3.33

0.929

DEDMA

2.322

5.14

0.934

PTMA

1.758

3.47

0.930

26

Table 2 Quantum chemical calculation results of different cations

Cation type

LUMO/eV

Volume/nm3

dmin/nm

iPTMA

-3.55

0.1400

0.3495

DEDMA

-3.53

0.1404

0.3146

PTMA

-3.69

0.1408

0.3219

27