Effect of binder’s solvent on the electrochemical performance of electrodes for lithium-ion batteries and supercapacitors

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ScienceDirect Materials Today: Proceedings 6 (2019) 42–47

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3rd ISE SSRSEU 2018

Effect of binder’s solvent on the electrochemical performance of electrodes for lithium-ion batteries and supercapacitors O. Chernysh, V. Khomenko*, I. Makyeyeva and V. Barsukov Kyiv National University of Technologies and Design, 2 Nemyrovych-Danchenka str., Kyiv 01011, Ukraine

Abstract One of the most toxic materials used in the manufacture of electrodes for lithium-ion batteries and electrochemical supercapacitors is the N-methylpyrrolidone (NMP) solvent for the PVDF polymer. This work shows that the NMP solvent can be replaced by a safer solvent (DMSO), which considerably simplifies and improves battery technology. This provides opportunities to decrease the costs of the manufacture of lithium-ion batteries and supercapacitors. © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 3rd ISE Satellite Student Regional Symposium on Electrochemistry in Ukraine. Keywords: photovoltaic energy systems, batteries, supercapacitors, hybrid electrochemical capacitors, lithium-ion capacitors

1. Introduction The employment of an organic solvent for the PVDF polymer binder to produce electrodes is one of the challenges in manufacturing electrochemical supercapacitors and lithium-ion batteries. The toxic NMP solvent is commonly used in industry. The content of NMP in the composite for coating the electrode active materials onto the metallic current collector can amount to 50-70% [1]. Therefore, energy-consuming equipment is used to trap the solvent vapours and to purify them for further reuse. This makes the manufacture of electrodes more complex and considerably increases the costs [2].

* Corresponding author. Tel.: +38-067-646-2294; fax: +38-044-284-8266. E-mail address: [email protected]

2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of 3rd ISE Satellite Student Regional Symposium on Electrochemistry in Ukraine.

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A study was conducted to find out if it is feasible to replace NMP. Basically, acetone, dimethylacetamide, dimethylformamide and dimethylsulfoxide (DMSO) can be used to dissolve PVDF. DMSO is an important dipolar aprotic solvent. It is less toxic than other representatives of this solvent class such as dimethylformamide, dimethylacetamide and NMP. DMSO is widely used in different fields of chemistry as well as in manufacture of cosmetics and pharmaceuticals [3, 4]. Our study evaluated the feasibility of using DMSO in the production of electrodes for electrochemical supercapacitors and lithium-ion batteries. 2. Experimental The investigated specimens of electrodes were fabricated by a conventional technology, which is widely used in the production of electrochemical supercapacitors and lithium-ion batteries. The electrode mixture was doctorbladed onto a metallic current collector (foil of 20 µm thick). As the suspensions coated onto the metallic current collector were to have a certain density and viscosity, the density and viscosity values were determined for each electrode composition. The viscosity of solutions was controlled with a Fungilab rotary viscosimeter (Spain). Coating the electrodes was performed on special equipment of the Doctor Blade type. The basic part of the above equipment is an adjustable applicator (doctor blade), which is a thin sharpened steel plate for removing excess layer of the electrode composite. In coating the electrode by the doctor blade method, the suspension is loaded to the applicator, which, upon pulling, spreads the suspensions evenly over the metal foil. The thickness of the coating depends on the doctor blade velocity and the distance between its sharpened part and the foil surface. The quality of the coating depends to a large extent on the parameters of coating (applicator gap length, coating speed, viscosity of the slurry and the foil surface), and the composition of the suspensions. All the above parameters were optimized for each electrode composition of the electric double-layer capacitors (EDLC) [5]. After coating, the electrodes were air-dried for 20-30 minutes at 60-100 oC to remove the binder solvent. Final drying of the electrodes was carried out in a special vacuum oven at 100-120 oC for 8-12 hours. Then the electrodes were rolled and cut out for the CR2016 coin cell size. A 16-mm-diameter notch was used to fabricate the specimens of electrodes. Before assembling test cells in an argon-filled glovebox (MBraun, the USA), the electrodes were re-dried for 6 hours at 120 оС. Various electrochemical studies were carried out by using Computerized MSTAT 32 potentiostats (Arbin Corporation, the USA) and VMP3 potentiostats (Princeton Applied Researcher, Great Britain). The chemical composition of active electrode masses was determined with an X-Supreme 8000 X-ray fluorescence spectrometer from Oxford Instruments (Great Britain). 3. Results and Discussion Samples of PVDF can differ in molecular mass, sub-primary structure, other monomers being introduced into the backbone, the presence of surface active substances or plastifiers. The chemical nature of the polymer binder definitely affects its adhesion to substrate and therefore influences the electrochemical properties of the electrode. Three different types of PVDF and two types of solvents (NMP and DMSO) were chosen to fabricate the electrodes. Investigation of 5% PVDF solutions in the organic solvents was carried out. Table 1 shows viscosities of the PVDF solutions. Table 1. Viscosities of 5% PVDF solutions in different solvents PVDF

Solvent

Viscosity of solution, cP

PVDF-1

NMP

1798

PVDF-1

DMSO

1825

PVDF-2

NMP

289

PVDF-2

DMSO

278

PVDF-3

NMP

885

PVDF-3

DMSO

893

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О. Chernysh et al./ Materials Today: Proceedings 6 (2019) 42–47

According to Table 1, the replacement of NMP by DMSO does not virtually affect the viscosity of solutions. The viscosity of solutions depends on the type of PVDF used. Therefore, DMSO can be employed in fabrication of electrodes by the doctor blade method without changing the composition of the suspension and additional adjustment of the equipment. In this study, different electrodes based on DMSO for lithium-ion batteries and electrochemical supercapacitors were fabricated. At first, obtaining quality coatings of the electrode masses on the current collector appeared to be impossible. The active layer coatings produced were uneven and had many defects (Fig.1).

а)

b)

Fig.1. Specimens of electrodes based on the solution of PVDF in DMSO. а) – high unevenness of the active layer over the surface of current collector, b) – the active layer has many defects (cavities and agglomerates)

Analysis of the causes of these defects points to a change in hydrophilicity of the suspension based on DMSO toward the metallic current collector. For quantitative evaluation, analysis of contact angles of PVDF solutions for different metal foil samples was carried out (Fig. 2).

Fig.2 Determination of edge contact angle: 1- surface of table; 2- material under study; 3- drop of working solution.

The edge contact angles of different current collectors with PVDF solutions were determined by the drop projection method [6]. A droplet of the working solution was deposited onto a horizontally positioned specimen of metal foil with a micropipette, and images were caught with a digital camera. Then the images were uploaded to a graphics editor, and the edge contact angle was measured. The measurements were repeated at least three times. The results are shown in Table 2. Table 2. Edge contact angles of PVDF solutions on different current collectors Type of current collector Type of PVDF

Solvent

Temperature, о С

Smooth Al foil

Matte Al foil

Etched Al foil

Copper foil

PVDF-1 PVDF-2 PVDF-3 PVDF-2 PVDF-3 PVDF-2 PVDF-3

NMP NMP NMP DMSO DMSO DMSO DMSO

40 40 40 40 40 20 20

38 48 42 54 52 60 56

36 38 38 42 45 46 48

19 18 20 22 24 25 27

50 58 57 67 71 74 76

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According to Table 2, the surface wetting of current collectors becomes worse when DMSO is used for dissolving PVDF. An increase in the edge contact angle is more pronounced at decreased temperatures and for smooth foils, especially copper foil. The pattern established can be explained by the fact that DMSO has a melting point of +18.45 0С and an ordered structure, which breaks down at temperatures of +40…+60 0С. Therefore, the process should be carried out in the +40…+60 0С range in order to provide the wetting of current collector surface with the suspension. Specimens of electrodes based on PVDF solutions in DMSO were fabricated on heating the suspension and current collector to +40 0С. In these conditions, a uniform layer of active mass is created (Fig.3). The structure of electrodes fabricated from DMSO solutions and the adhesion of the active layer are on a par with their analogues fabricated using NMP.

· Fig.3. Specimen of electrode based on PVDF solution in DMSO at a coating temperature of 40 0С

Electrochemical studies of LIB anodes based on the SLP-30 graphite (TIMCAL) and fabricated using DMSO and NMP were performed according to techniques disclosed in [7, 8]. The electrochemical studies showed that the specific capacity of both types of electrodes equals ~350 mАh·g-1 at a current density of 38 mA·g-1 (the 10-hour cycling mode С/10), the irreversible capacity loss at the first cycle being 35 mАh·g-1 (Fig. 4). Thus, no difference was revealed in the electrochemical characteristics of electrodes. E, V vs. Li

Q, mAh g-1 400

1,8

1

350

1,5

300

1,2

250

2

200

0,9

150 0,6

100

0,3

50 0

0 0

50

100

150

200

250

Q, mAh

g-1

300

350

400

Fig. 4 Charge-discharge curves for the SLP-30 graphite-based electrode coated onto the copper current collector from PVDF solution in DMSO.

0

10

20

30

40

50

60

70

Cycle Number

Fig. 5 Cycling of graphite anodes produced using PVDF solutions in NMP (curve 1) and DMSO (curve 2)

The admixtures exert a pronounced effect on the stability of anode characteristics in cycling. Fig. 5 shows results of a long-duration cycling of LIB anodes based on the SLP-30 graphite coated from PVDF solutions in different solvents. The long-duration cycling (~100 cycles) of the anodes showed that the electrode based on NMP was more stable than its analogue based on DMSO. According to Fig. 5, for the anode based on DMSO, a monotonous decrease in capacity is observed in cycling. Thus, the results of the study point to contamination of the anode from the DMSO solution.

80

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О. Chernysh et al./ Materials Today: Proceedings 6 (2019) 42–47

Electrochemical tests of the specimens of electrodes for electrochemical supercapacitors fabricated using PVDF dissolved in DMSO yielded results which were on a level with similar electrodes based on the conventional NMP binder (Table 2). Table 3. Internal resistance of EC * with electrodes produced using different solvents of PVDF Specific capacitance of activated carbon , F·g-1

Internal resistance** of EDLC, Ohm

Type of PVDF

Solvent

PVDF-1

DMSO

79

0.30

0.92

PVDF-2

DMSO

74

0.27

0.71

PVDF-1

NMP

72

0.27

0.59

PVDF-2

NMP

76

0.29

0.78

ESR

EDR,

*ЕC based on the Norit Supra 50 activated carbon and 1.5 М ТЕАBF4 electrolyte in acetonitrile, electrode area 2 cm2 ** Testing methodology and procedures described in [9]

However, the study revealed unstable electrical characteristics of electrochemical supercapacitors with electrodes fabricated using DMSO. Fig. 6 shows change in capacity of a CR2016 coin supercapacitor based on electrodes fabricated using PVDF solutions in NMP (1) and DMSO (2).

C (F)

0,4

1

0,3

0,2

2 0,1

0,0 0

50

100

150

200

250

Cycle number Fig.6. Change in capacity of EDLC based on electrodes fabricated using NMP (1) and DMSO (2). Cycling voltage -3 V; Electrolyte: 1 М ТЕАBF4 solution in acetonitrile

For DMSO, the EDLC capacity decreases in its galvanostatic cycling. Comparing the results of cycling (curve 1 and 2 in Fig.3), a better stability is achieved for NMP at a cycling voltage of 3V. Results of X-ray fluorescence analysis indicate the presence of sulfur-containing products in the electrodes fabricated using DMSO (Fig.7). DMSO must have been adsorbed on the surface of active material during fabrication. On heating, DMSO breaks down by the reaction: 2(CH3)2SO  (CH3)2SO2 + (CH3)2S. As a result, the sulfur-containing products remain in the pores of the active material. These products cannot be removed completely at the temperature of vacuum drying. Finding their way into the EDLC electrolyte, these sulfurcontaining compounds cause side reactions, which results in self-discharge of the EDLC and limitation of its working voltage to 1.5 V (compared to 2.7-3.0) V. It is possible to remove the sulfur-containing products from the electrode by additional washing in ethanol. After washing the electrodes and drying them thoroughly, additional Xray fluorescence analysis was performed (Fig. 7). According to the XRFA (Fig. 8), there were no sulfur-containing products revealed in the electrodes, which proves that they can be removed by additional washing in ethanol. This makes the fabrication of electrodes somewhat more complicated but the use of DMSO becomes feasible.

О. Chernysh et al./ Materials Today: Proceedings 6 (2019) 42–47

Fig. 7. X-ray fluorescence spectra of Кα line of sulfur obtained for electrodes based on PVDF-2 (3) and PVDF-1 (2) dissolved in DMSO and PVDF-2 in NMP (1)

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Fig. 8. X-ray fluorescence spectra of Кα line of sulfur obtained for electrodes based on PVDF-2 in DMSO before (1) and after (2) washing the electrodes in ethanol

Our study established the absence of degradation processes in galvanostatic cycling of electrochemical supercapacitors with electrodes fabricated using DMSO and additional washing of the electrodes in ethanol. Conclusions The results of our study show that NMP for the PVDF polymer can be replaced by the safer DMSO solvent in the manufacture of electrodes for lithium-ion batteries and electrochemical supercapacitors. The process of coating the active material suspension based on MSO must be carried out at 40-60 0С. However, heating the electrodes causes their contamination with sulfur compounds due to break-down of DMSO. The sulfur compounds, in their turn, cause side reactions, decreasing the life of current sources. The study suggests additional washing of the electrodes in ethanol. The fabrication of electrodes based on DMSO which involves additional treatment with ethanol can improve one of the main steps of creating a safe, eco-friendly and cheap power source. Acknowledgement Authors acknowledge the European Commission for the financial support of these researches in the framework of FP7 “Energy Caps” project, as well as SOLVAY (Brussels), YUNASKO-Ukraine LLC and all other project participants for efficient collaboration. References [1] B. Bitsch, J. Dittmann, M. Schmitt et. al., Journal of Power Sources 265 (2014) 81-90. [2] P. A. Nelson, D. J. Santini, J. Barnes, World Electric Vehicle Journal 3 (2009) 0457-0469. [3] R. Vignes, American Chemical Society Annual Meeting (2000) 20. [4] Yu. N. Kukushkin, Soros Education Journal 9 (1997) 54-59. [5] B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. Kluwer Academic Press/ Plenum Publishers, New York 1999. [6] G. Bracco, B. Holst (eds.), Surface Science Techniques, Springer Series in Surface Sciences 51, Berlin Heidelberg 2013, pp. 3-35. [7] V. Khomenko, E. Raymundo-Pinero, F. Beguin, J. Power Sources 177 (2008) 643-651. [8] V. Barsukov, F. Langouche, V. Khomenko, et al., Journal of Solid State Electrochemistry 19 (2015) 2723-2732. [9] S. Zhao, F. Wu, L. Yang, L. Gao, A. Burke, Electrochemistry Communications 12 (2010) 242-245.