Facile preparation method for polymer and exfoliated graphite composites and their application as conduction-promoting materials

Abstract

This paper introduces a new carbon material with high electrical conductivity and dispersibility, as well as its application in lithium-ion batteries. This new carbon material is characterized by an edge-exfoliated structure with grafted polymer and restacking ability through removal of the grafted polymer.

---

Conduction promoting agents (CPAs) have become increasingly important as fillers that provide electrical conduction for insulating polymer materials, and also as conductive path materials for power sources such as rechargeable batteries and fuel cells. Conventional carbon CPAs include materials such as carbon black^1,2 and carbon fibre,³ as well as graphite.⁴ Electrical conductivity is manifested by the displacement of π electrons in the fused benzene ring.^5,6

When carbon black is used as CPA, the electrical conductivity rapidly increases as the critical filling factor increases and reaches a constant value thereafter. This is because conductive paths are formed when carbon black (primary particles) agglutinates and forms a continuous phase. Ketjen black^4,7,8 and acetylene black^4,9–11 are well-known carbon blacks, but there are some essential problems when used as CPA: (1) they have low compatibility and poor dispersibility with polymer materials, binders, electrical conduction materials, etc. (2) Dispersibility can be improved by down-sizing, but it lowers electrical conduction of the primary particles. (3) Down-sizing increases dispersibility but makes it difficult to form conductive paths because of reduction of π-conjugate area.

As one of the successful improvement of graphites, Park et al. reported edge-tethered graphite prepared by Friedel–Crafts acylation reactions to introduce fluorobenzoyl and poly(ether ketone) groups.¹² Here, we introduce the new and facile preparation method for an improved carbon CPA having high dispersibility and electrical conductivity, without down-sizing the carbon raw material, to overcome the drawbacks. This new carbon material (Fig. 1) is an extremely promising material as a CPA, as it promotes the formation of conductive paths efficiently by compositing. This paper also provides a report on the results obtained for the application of this material as CPA used as a cathode in lithium-ion batteries.

Fig. 1 Schematic illustration on polymer and edge-exfoliated graphite composite (polymer-EExG) and restacking of EExG by removal of the incorporated polymer.

In this paper, the developed carbon material is named as the polymer and edge-exfoliated graphite composite and abbreviated as polymer-EExG. The polymer-EExG is obtained by incorporating polymers in graphite. The method is characterised by compositing, which is performed using polymer radicals that are generated by thermal decomposition of polymer materials, and with the radical trapping function at the edge portion of the graphite. Two types of polymer graphite composites were fabricated in this study. The representative synthesis methods of polymer-EExG are briefly described below (see a more detailed description in the ESI†): expanded graphite (PF8) and azodicarbonamide (ADCA) are used as a thermal decomposition foaming agent, as well as polypropylene glycol (PPG) is mixed with tetrahydrofuran (THF). This mixture was exposed to ultrasonic irradiation. The obtained mixture was then shaped using the solvent casting method and heated stepwise: at 80 °C for 2 h, 110 °C for 60 min, 150 °C for 60 min, and 230 °C for 120 min. The ADCA was thermally decomposed and foamed using this heat treatment process. Finally, the thermal decomposition of the PPG was promoted by treating at 450 °C for 30 min to fabricate polymer-EExG, which is abbreviated as P-EExG.

G-EExG was prepared with polyglycidyl methacrylate (GMA) instead of PPG according to the similar procedure of P-EExG (the detailed preparation are provided in ESI†).

It was confirmed that the polymer-EExG was well-dispersed in organic solvents such as THF.

The structure of polymer-EExG was investigated using the thermogravimetric analysis (TGA), Brunauer–Emmett–Teller analysis (BET), X-ray diffraction analysis (XRD), IR and microscopic Raman spectroscopies, as well as scanning electron microscopy (SEM) (see ESI† for details).

Polymer-EExGs were firstly confirmed that they contained polymers using infrared spectroscopies. The typical absorption bands were observed near 1730 cm⁻¹ (ester bonds) in G-EExG, and near 2900 cm⁻¹ (stretching vibration) in P-EExG, as shown in Fig. S1 and S2 (ESI†).

A TGA analysis gave us further significant information on the structures of polymer-EExG. As shown in Fig. 2, a significant weight reduction was observed for both types of EExG in the temperature range of 600 to 800 °C. This temperature range was lower than the terminal temperature of decomposition (700 to 900 °C) for the graphite raw material. However, this is assumed to be related to the intrusion (grafting) of polymers in graphite.

Fig. 2 Thermogravimetric analysis of (a) P-EExG and (b) G-EExG.

The XRD patterns of the graphite raw material, as well as P-EExG and G-EExG, provided the evidence that supported the reduction in crystallization (graphite structure) because of compositing with polymers, the progression of partial exfoliation, as shown in Fig. S3 (ESI†).

On the other hand, a reduction in weight that is assumed to be polymer components was confirmed in the temperature range from 400 to 600 °C. This temperature range was higher than the decomposition temperature of PPG and GMA (Fig. 2). However, this suggests that the polymers are intruding between graphite layers (Fig. 1). This presumption was supported by the fact that hardly any precipitation of polymers was confirmed outside the graphite during the SEM observation of P-EExG and G-EExG (Fig. 3).

Fig. 3 SEM images of (a) PF8, (b) P-EExG and (c) G-EExG.

Calculations were performed to derive the composition ratio of graphite and the grafted polymer, based on the above-mentioned assumptions and by referring to Fig. 2. The weight ratio of P-EExG was 25 wt% and G-EExG was 65wt%. This difference is considered to have increases due to the difference in the decomposition mechanisms of the PPG and GMA: PPG decomposes from the end of polymer chains¹³ and produces radicals, while a large amount of radicals are produced by the main chain of the GMA.¹⁴

The polymer radicals generated by decomposition of the ADCA and polymers reacted with the edge portions of the graphite. The defoliation progressed at the same time, and that is presumed to have resulted in grafting of the polymers.

However, the unreacted polymers that existed on the outside of graphite were removed by thermal decomposition during the final heat treatment process and are assumed to have formed a composite structure with polymers existing between the graphite layers that were partially exfoliated (Fig. 1).

The process of exfoliation of graphite in a low oxygen concentration condition is another attractive aspect of this heat treatment process,^15,16 because no graphene oxides are produced, and therefore, no reduction process is required.

Polymer-EExG using the micro-Raman spectra showed lesser amount of graphite structures collapsing due to oxidation in comparison with Ketjen black and acetylene black as shown in Fig. S1 (ESI†).^17,18

Another attractive aspect of our polymer graphite composite includes baking the material in the temperature range from 450 to 600 °C. Not only it is possible to remove the grafted polymer from the composite but also it increases the 2θ peak intensity as shown in Fig. 4. This is also supported by the electro-conductivity change. The P-EExG has lower electro-conductivity (130–320 S cm⁻¹ in the powder-pressed sheet) than the starting material PF8 (1100–1600 S cm⁻¹ in the powder-pressed sheet) because of both exfoliated structures and polymer incorporation. However, the heat treatment recovered the electro-conductivity up to 70% of PF8. Therefore, the exfoliated portions of polymer-EExG are considered to have restacked when the polymer components were removed. Such restoration of a graphite structure from rebonding of graphite foils that have once been exfoliated is extremely rare. This characteristic can be exploited to exhibit a superior performance that exceeds that of conventional graphite CPA.

Fig. 4 X-ray diffraction patterns of PF8 raw material and P-EExG after heat treatment at given temperatures for 30 min.

The introduction rate (residual ratio) of polymers in the composite was adjusted easily by varying the composition ratio of graphite and polymers, oxygen concentration and baking time, as shown in Fig. 5. The preparation conditions are summarized in Table S1.† This process was also accompanied by adjusting the surface area of the resultant composites.

Fig. 5 Correlation diagram of residual polymer and specific surface area of polymer-EExG preparing in various conditions. (a) An open circle indicates the G-EExG prepared by the standard conditions. G-EExGs prepared under the other conditions are indicated as solid circles for different oxygen concentrations, open triangles for different GMA amounts and solid triangles for different ADCA amounts. (b) An open circle indicates the P-EExG prepared by the standard condition. P-EExGs prepared under the other conditions are indicated as solid circles for different oxygen concentrations and open squares for different heating treatment times. The preparation conditions were summarized in Table S1 (ESI†).

Fig. 5a shows the correlation between BET and the residual polymer quantity of G-EExG when the compounding ratio of graphite and GMA was changed from 1 : 20 to 1 : 5, the compounding ratio of graphite and ADCA was changed from 1 : 0 to 1 : 5, and the oxygen concentration was changed from 20 to 5%. The maximum value practically reached 350 m² g⁻¹, and this value was equivalent to about 15 times that of the expanded graphite raw material. In case of P-EExG, the residual polymer quantity could be adjusted by varying the baking condition as shown in Fig. 5b.

In this paper, the fabrication and performance of cathode materials for lithium batteries is described, as an application that utilises high electric conductivity and dispersibility and of the polymer-EExG. The fabrication procedure of cathode materials for lithium batteries and performance evaluation were carried out by the methods described in ESI.† Ketjen black was also used in this study to compare the fabricated cathodes with G-EExG.

A summary of the fabricating conditions and the performance of the cathodes for the fabricated lithium electrodes are provided in Table 1. The measurement of electrostatic capacity was performed while repeating charging and discharging of the assembled lithium-ion rechargeable batteries to evaluate the cycle characteristics of these lithium-ion batteries. Fig. 6 shows the comparison of cycling performance of charge–discharge process in the lithium batteries fabricated with G-EExG and with Ketjen black.

Table 1 Preparation condition of various lithium electrodes and their performances^a

	Conduction promoting agent
	G-EExG	P-EExG	Ketjen black
a Active material: LiCoO2; 92 wt%, binder resin: polyvinylidene fluoride; 3 wt%; G-EExG, P-EExG and Ketjen black as conduction promoting agents were used with 5 wt%.
Resistivity of composite powders (Ω cm)	0.18	0.03	0.80
Volume resistivity of electrode sheet (Ω cm)	1.50	1.00	7.30
Interfacial resistance between electrode sheet and current collector (Ω cm^2)	0.50	0.47	0.36
Electrode sheet density after roll pressing (g cm^−3)	2.9	3.1	3.0
Exfoliation of current collector and active material after roll pressing	Not found	Not found	Partially exfoliated

---

Fig. 6 Cycling performance of charge–discharge process in lithium batteries fabricated with G-EExG and Ketjen black. The charging and discharging processes were carried out by the procedures described in ESI.†

The volume resistivity in the powder-pressed sheet obtained using P-EExG and G-EExG as CPA for the cathode active material (LiCoO₂) was revealed to be extremely low in comparison with the conventional Ketjen black as shown in Table 1. Furthermore, although the interface resistance value with the electric collector of the electrode sheet, which was a composite of the cathode (LiCoO₂) and the binder (PVDF) as well as the electrode sheet density were same as those of Ketjen black, the volume resistivity of electrode sheets made with P-EExG and G-EExG were low. In the fabricated electrode, no agglutination of EExG was also confirmed in SEM observation. These results indicate that EExG is well-dispersed and the electrical conduction network is formed efficiently.

Furthermore, EExG is assumed to maintain the internal resistance value of the electrode sheets low, since it is a non-oxide graphite material and has little defect with the sp²-hybridized crystal structure. In addition, it is considered to sustain a high level of electrical conduction.

Conclusions

This paper described the new fabrication method of the polymer and edge-exfoliated graphite composites. Our method can be summarized with several advantages: (1) since our graphite composites have partially-exfoliated structures in the edge area but also amorphous polymer is incorporated there, the dispersibility in organic solvents is much improved to increase the processability. (2) Since the polymer incorporation is carried out by thermal decomposition of polymer without any special reaction, our method can be easily applied for various polymers. Therefore, this versatility will expand the diversity in modified graphites. (3) The incorporated polymer is removable by a simple heating process, which regenerates the restacked structure. This phenomenon is significantly advantageous when EExG is used as a conduction-promoting agent.

In addition, this study focused on this characteristic of this substance and applied it to conduction promoting agents to the cathode of lithium-ion rechargeable batteries. This resulted in the improved initial electrostatic capacity of cathode active material and a superior performance of the cycle characteristic. Therefore, this carbon material can be used for applications in a variety of fields, as a CPA for cathodes or anodes of lithium-ion batteries or for electric double layer capacitors by improving the specific surface area.

Notes and references

1. W. G. Wang, X. Wang, L. Y. Tian, Y. L. Wang and S. H. Ye, J. Mater. Chem. A, 2014, 2, 4316–4323.
2. X. Duan, Y. Han, L. Huang, Y. Li and Y. Chen, J. Mater. Chem. A, 2014, 3, 8015–8021.
3. Z. Lin, D. Sun, Q. Huang, J. Yang, M. W. Barsoum and X. Yan, J. Mater. Chem. A, 2015, 3, 14096–14100.
4. S. Kuroda, N. Tobori, M. Sakuraba and Y. Sato, J. Power Sources, 2003, 119–121, 924–928.
5. P. R. Wallace, Phys. Rev., 1947, 71, 622.
6. D. D. L. Chung, J. Mater. Sci., 2002, 37, 1475–1489.
7. K. T. Chung, A. Sabo and A. P. Pica, J. Appl. Phys., 1982, 53, 6867–6879.
8. G. He, C. J. Hart, X. Liang, A. Garsuch and L. F. Nazar, ACS Appl. Mater. Interfaces, 2014, 6, 10917–10923.
9. C. Giet, US Pat., 4,279,880, 1981.
10. A. Merzouki and N. Haddaoui, ISRN Polym. Sci., 2012, 2012, 493065.
11. G. Liu, H. Zheng, V. S. Battaglia, A. S. Simens, A. M. Minor and X. Song, ECS Trans., 2007, 14, 45–56.
12. J. S. Park, M. H. Lee, I. Y. Jeon, H. S. Park, J. B. Baek and H. K. Song, ACS Nano, 2012, 6, 10770–10775.
13. L. X. Song, X. Q. Guo, F. Y. Du and L. Bai, Polym. Degrad. Stabil., 2010, 95, 508–515.
14. T. Kashiwagi and A. Inabi, Polym. Degrad. Stabil., 1989, 26, 161–184; K. Pielichowski and J. Njuguna, Thermal Degradation of Polymeric Materials, Rapra Technology Limited, Shawbury, 2005.
15. W. Lv, D. M. Tang, Y. B. He, C. H. You, Z. Q. Shi, X. C. Chen, C. M. Chen, P. X. Hou, C. Liu and Q. H. Yang, ACS Nano, 2009, 3, 3730–3736.
16. Y. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach and R. S. Ruoff, Science, 2011, 332, 1537–1541.
17. M. Nakamizo, R. Kammereck and P. L. Walker, Carbon, 1974, 12, 259–267.
18. X. He, W. Li and H. Li, Vacuum, 1994, 45, 977–980.

---

Footnote

† Electronic supplementary information (ESI) available: Preparation procedure of P-EExG and G-EExG, fabrication and evaluation of lithium batteries, FT-IR, Raman and XRD spectra, SEM images. See DOI: 10.1039/c6ra00619a

---