The synthesis and electrochemical performance of aniline based conducting polymers as cathode

materials for lithium ion batteries

Su-Ryeon Yun1, Yongku Kang2, Kwang-Sun Ryu1

1Department of Chemistry, University of Ulsan, Daehak-ro 93, Nam-gu, Ulsan, 680-749, Korea 2Korea Research Institute of Chemical Technology, Daejeon, 305-600, Korea

Corresponding-author: ryuks@ulsan.ac.kr

Abstract—The PDTDA and P(2-ATP), which have polyaniline main chain and bonds structure, were easily polymerization by chemical oxidation method. Although doped polyaniline was not soluble, doped PDTDA and P(2-ATP) were soluble. The chemical structure of dedoped and doped PDTDA and P(2-ATP) was characterized by FT-IR and XPS analysis. We studied the electrochemical performance such as the charge-discharge ability, discharge capacity, and cyclic voltammogram of them. The discharge capacities of PDTDA and P(2-ATP) after 50th cycle were ~38 and ~37mAh/g. The discharge capacities of PDTDA and P(2-ATP) as a function different current after initial stabilized step were measured. Recovering rates after high C-rate at 0.1C are ~92 % (PDADT) and ~50 %(P(2-ATP).

Keywords-conducting polymer; aniline; sulfur; lithium battery, cathode, capacity

I. INTRODUCTION

Disulfide compounds have been introduced as a new organic/polymeric cathode material in lithium ion batteries. (1,2) The series of compounds having –SH groups within the molecules are being considered as energy storage materials, whereby energy exchange occurs based on the reversible process because the cleavage and recombination of S-S bonds is expected to be easy. The reversible -S-S- oxidation-reduction reaction as expected for 2,5-dimercaptothiodazole and 1,3,5-trithiocyanuric acid played an important role in using polyaniline based composites as cathode materials in batteries. (3) According to previous literatures (4, 5), disulfide materials generally show reduction behavior with structural changes during oxidation-reduction processes. (6, 7)

In the process of obtaining better material for battery applications, different approaches have been tried to include –S-S- links. Among them, poly(2,2’-dithiodianiline) (PDTDA) and poly(2-aminothiophenol) (P(2-ATP)), a new material with polymer chains interconnected with S-S bonds, has been proposed by Naoi et al. (6, 7) They have shown that PDTDA and P(2-ATP) can be synthesized to from an electro-active thin film by cycling potential electro-polymerization method at the glassy carbon electrode surface. The free-volume of PDTDA and P(2-ATP) was increased than Pani structure by –S-S- links. Its solubility was expected that improved better than Pani.

In this work, 2,2’-dithiodianilne (DTDA) and 2-

aminothopheol (2-ATP) have been selected as a new monomer along with aniline for performing simple chemical synthetic method by oxidation of the monomers. The resulting polymer could have –S-S- bonds confined along the chains of polymer for soluble electrode. The optical spectro-electrochemical and thermal properties of the polymers were followed and compared with Pani to identify the modifications in the properties as a result of incorporation of –S-S- links in the backbone of Pani structure. Also, we have prepared polymeric electrode based on these disulfide compounds and investigated their electrochemical characteristics of lithium ion batteries.

II. EXPERIMENTALS

We used 2,2’-dithiodianiline (DTDA, 97%) and 2-aminothiophenol (2-ATP, 99 %) as monomer and ammonium persulfate (APS, 98 %) as initiator and 2-Naphthalenesulfonic acid (β-NSA), hydrochloric acid (HCl, 37 %) as dopant. All reagents were used as received from Aldrich.

A. Chemical synthesis of poly(2,2’-dithiodianiline) (PDTDA) and poly(2-aminothiophenol) P(2-ATP) DTDA monomer was dissolved in 1 M HCl solution at

80 under N2 atmosphere. 2-ATP monomer and β-NSA were dissolved in 1M HCl solution at 0 under N2atmosphere. Ammonium persulfate was used as redox initiator. The ammonium persulfate with 1M HCl solution was slowly added by using a dropping to the reactant solution with vigorous stirring. Then, the reaction mixture was agitated continuously for 24 h under N2 atmosphere at 80 (for DTDA) and 72 h under N2 atmosphere at 0 (for 2-ATP).The precipitated particles were filtered and washed ethanol, acetone, and distilled water to remove any impurities. Thefiltered product with HCl doped PDTDA or P(2-ATP) was dried under vacuum for 48 h. The filtered powder was reduced by excess 1 M NH4OH solution to prepare dedoped PDTDA or P(2-ATP). And then, dedoped PDTDA or P(2-ATP) wastreated again for doping by 1M HCl. The filtered product was dried.

B. Measurements and Fabrication ofcoin cellsThe Fourier transform infrared (FT-IR) spectra were

recorded using an FT-IR spectrometer (Varian 2000) in the

978-1-4673-1773-3/12/$31.00 ©2013 IEEE

400 to 4,000 cm-1 wavenumber range. The morphologies wereobserved by field emission-scanning electron microscopy (FE-SEM, Supra 40, Carl Zeiss Co., Ltd.). The coin type cell (2032) was fabricated for the charge-discharge test. Lithium foil was used for negative electrode and the doped polymer mixed with super-P and PVdF was used for positive electrode. The 1.15 M LiPF6 in EC/EMC/DMC (3/2/5, w/w) was used as electrolyte solution. The test cell was assembled in a dry box. The cell tested by using cyclic voltammetric measurement (WBCS3000, WonA Tech) for the Li//PDTDA and Li//P(2-ATP) cell from 2.0 to 4.5 V and 2.5 to 4.3 V with the scan rate of 0.05 mV/s, respectively. And charge/discharge cycler (WBCS3000, WonA Tech) in the voltage range from 2.5 to 4.0 V with the constant current density of 8 mA/g.

III. RESULTS AND DISCUSSION The wide scope of tuning different properties of Pani gave

us to explore the possibility of copolymerizing aniline with an aniline derivative having S-S link and -SH, dithiodianiline and aminothiophenol. The monomers were polymerized by oxidation polymerization. And then dedoped and doped states by base and acid solution were suggested in Scheme 1. Doped PDTDA and P(2-ATP) has S-S links in the backbone with Pani structure. Since the S-S links has increased free volume, the solubility of PDTDA and P(2-ATP) is expected.

A. FT-IR spectroscopy Fig. 1 represents the FT-IR spectra of the PDTDA and P(2-

ATP) prepared by doping and dedoping. Both polymers showed bonds characteristics of Pani and S-S bond. The characteristic bands of doped PDTDA and P(2-ATP) were similar. A absorption peaks due to the N-H stretching vibration of the amino groups is observed 3463and 3359 cm-1 in FT-IR spectrum of PDTDA. Also N-H stretching of P(2ATP) is observed 3466 and 3345 cm-1. Bands of doped PDTDA at 1478 and 1615 cm-1 are assigned to the non-symmetric aromatic ring stretching modes in Fig. 1(a). The higher frequency vibration arises from C=C stretching of quinoid rings whiles the lower frequency vibration from benzoid ring units. In Fig. 1(b) the bands of doped P(2-ATP) at 1490 and 1611 cm-1 can be attributed C=C stretching of quinoid and benzoid rings, respectively. The intensity of quinoid and benzoid ring was similar. But the intensity of quinoid ring was shorter than benzoid ring peaks in doped PDTDA and P(2-ATP). The occurrence of these two bands was expected that doped PDTDA and P(2-ATP). In addition, the bands at 1295 and 1264 cm-1 can be assigned to the C=N stretching of the doped PDTDA and P(2-ATP), respectively. For the PDTDA and P(2-ATP), bands at 759 and 749 cm-1 corresponding to C-S bending are additionally present (8).

B. XPS analysis XPS scans were taken for PDTDA and P(2-ATP) powder

prepared by chemical polymerization. The survey level XPS informs that PDTDA and P(2-ATP) contain C, N and S. In addition, percentage of atom-atom binding with doped and dedoped polymer was shown in Fig 2. In Fig 2, the dedoped PDTDA and P(2-ATP) posses three peaks corresponding to C-N/C=N, C-C/C-H and C-S bands.

Figure 1. FT-IR spectra of dedoping and doping; (a) PDTDA and (b) P(2-ATP).

And doped PDTDA and P(2-ATP) includes four peaks corresponding to C-N/C=N, C-C/C-H, C-S and C-N+/C=N+. The % L-G of C-C/C-H reduce from ~64.0 to ~23.1 % and C-N+/C=N+ increased from 0 to ~26.3 %. Like PDTDA, doped P(2-ATP) of %L-G of C-C/C-H reduce and C-N+/C=N+

increased. These results were shown from structure of dedoped and doped PDTDA and P(2-ATP) in scheme 1. As results, PDTDA and P(2-ATP) were doped by 1M HCl on Pani chain.

Table 1. The XPS peaks analysis of dedoping and doping state for PDTDA and P(2-ATP).

eV L-G (%) eV L-G (%) PDTDA dedoped P(2-ATP) dedoped

C-N/C=N 285.97 ~25.0 285.12 ~22.3 C/C-H 284.44 ~64.0 283.98 ~42.0

C-S 288.30 ~11.0 286.55 ~35.7 PDTDA doped P(2-ATP) doped

C-N/C=N 285.40 ~28.4 284.77 ~31.7 C-C/C-H 283.70 ~23.1 283.83 ~32.8

C-S 287.50 ~22.2 288.65 ~4.0 C-N+/C=N+ 284.47 ~26.3 284.88 ~31.7

Figure 2. XPS of dedoping and doping; (a) PDTDA and (b) P(2-ATP).

C. FE-SEM Fig. 3(a) and (b) are the scanning electron micrographs of

the doped PDTDA and P(2-ATP) powder by 1m HCl. Both PDTDA and P(2-ATP) powder has amorphous. But broadly PDTDA have globular shape than P(2-ATP).

Figure 3. SEM images of (a) PDTDA and (b) P(2-ATP).

D. Electrochemical performance Fig. 4 presents the cyclic voltammetric curves. These are

the curves of redox reaction which is stabilized after ~10cycles. There are two broad peaks, in which the upper one is corresponding to the oxidation (Li+ or PF6

- doping) and the lower one is for the reduction (Li+ or PF6

- dedoping). The doping and dedoping processes of Li+ or PF6

- ions are reversible and maintaining the peak shape uncharged with cycling. The oxidation peaks of PDTDA and P(2-ATP) are similar position at ~3.5V. Also, the reduction peaks of PDTDA and P(2-ATP) are similar position at ~3.2V. Because the similar redox potential of PDTDA and P(2-ATP) has same reaction with doping/dedoping on Pani chain in scheme 1.

Fig. 5 (a) and (b) show the charge/discharge curves for Li//PDTDA and Li//P(2-ATP) cell, respectively.

Figure 4. Cyclic voltammograms of (a) PDTDA and (b) P(2-ATP) (scan rate:

0.05 mV/s).

Scheme 1. The doping/dedoping reaction (charge/discharge) of (a) PDTDA

and (b) P(2-ATP). The shows that after 50 cycles the specific capacity for

Li//PDTDA and Li//P(2-ATP) cell. These are the curves of charge/discharge reaction which is stabilized after ~10 cycles. The specific discharge capacity of Li//PDTDA and Li//P(2-ATP) cell was similar as ~39 mAh/g at 50th cycle. The Scheme 1 shows that doping/dedoping reaction (charge/ discharge) of Pani main chain was same. Therefore, these materials have the possibility of using as an electrode material in flexible battery. Fig. 6(a) and (b) show the discharge capacity of Li//PDTDA and Li//P(2-ATP) cell as a function of different current, respectively.

Figure 5. Charge-discharge curve and capacity of (a) PDTDA and (b) P(2-

ATP) (c-rate: 0.1C, current voltage: 2.5-4.0).

Figure 6. Discharge capacity of a) PDTDA and (b) P(2-ATP) as a function of

different current. After charge/discharge at rates from 0.1 C to 5 C over a

total of 49 cycles, the cells run again at 0.1 C. In the high C rate, PDTDA and P(2-ATP) have low capacity than low C rate. But Li//PDTDA cell run again at 0.1 C attained the capacity of ~90 % as at the beginning while Li//P(2-ATP) cell has capacity of ~50 %. In scheme 1, doped PDTDA has S-S bond and also dedoped PDTDA. But structure of dedoped P(2-ATP) changed from S-S bond to -SH. This result demonstrates that the structure of PDTDA is stable and quite reversible than P(2-ATP) in lithium ion batteries. In this results, doped PDTDA and P(2-ATP) can be solubility by increasing free volume through S-S bond. Also, PDTDA and P(2-ATP) have the possibility as a polymer cathode material in lithium ion battery.

ACKNOWLEDGMENT This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea.

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