Electrochimica Acta 51 (2006) 2589–2593
Benzene-based polyorganodisulfide cathode materials for secondary lithium batteries Shi-Ren Deng a , Ling-Bo Kong a , Gao-Qiang Hu a , Tao Wu b , Dan Li b , Yun-Hong Zhou a , Zao-Ying Li a,∗ a b
Department of Chemistry, Wuhan University, Wuhan 430072, PR China Department of Chemistry, Shantou University, Shantou 515063, PR China
Received 27 May 2005; received in revised form 19 July 2005; accepted 30 July 2005 Available online 8 September 2005
Abstract A novel kind of benzene-based organodisulfide, 5,8-dihydro-1H,4H-2,3,6,7-tetrathia-anthracene (DTTA) and its homopolymer, poly(5,8dihydro-1H,4H-2,3,6,7-tetrathia-anthracene) (PDTTA) were designed, synthesized and presented as a cathode material for high-energy secondary lithium batteries. The polyorganodisulfides were characterized by FT-IR, XPS and elemental analysis. The designed system has many advantages, such as high theoretical charge density (ca. 471 mAh/g), fast redox process and enhanced cycling stability, which are due to the intramolecular electrocatalytic effect of polyphenyl chain and the intramolecular cleavage–recombination of the S S bond, respectively. The cyclic voltammetry test results proved the effect of this kind of electrocatalysis, and the charge–discharge experimental results showed that the polymer has the specific capacity of 422 mAh/g at 2nd cycle and 170 mAh/g at the 44th cycle. © 2005 Elsevier Ltd. All rights reserved. Keywords: Secondary lithium battery; Organodisulfide; Conjugated polymer; Cathode material; Synthesis
1. Introduction A series of organodisulfide polymers [1–3] having two mercapto groups within the molecules are being considered as a new energy storage materials in lithium batteries because of their high specific energy, whereby energy exchange occurs based on the reversible polymerization– depolymerization process (2S− ↔ S S). These materials have much higher theoretical capacities than that of conventional battery materials such as LiCoO2 , LiNiO2 , LiMn2 O4 , etc. Particularly, 2,5-dimercapto-1,3,4-thiadiazole (DMcT)  with a theoretical specific capacity as high as 362 Ah/kg is one of materials, which have been widely studied. However, the slow redox reaction at room temperature results in its low practical capacity, at the same time, the depolymerized monomer will dissolve in electrolyte and escape from the cathode electrode, leading to a low cycling stability [5,6]. ∗
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To solve these disadvantages, many approaches [7–9] have been adopted. According to our previous studies , the most promising method was to combine the S S bond to the backbone of some kind polymer, so that the polymer can act as the electrocatalyst to enhance the redox process of disulfides. On the other hand, the depolymerized “monomer” after discharge was in fact a polymer, which will not lose in the electrolyte, thus could provide an improved recombination efficiency of S S bond to enhance the cycling stability of this cathode material. In this paper, we report the synthesis and electrochemical properties of a new benzene-based polyorganodisulfide derivative, poly(5,8-dihydro-1H,4H-2,3,6,7-tetrathiaanthracene) (PDTTA), which has two six-membered cycles containing two groups of disulfide bonds as shown in Scheme 1. The designed system has many advantages . Firstly, the high disulfide density makes it has a high theoretic specific capacity of 471 Ah/kg. Then, PDTTA is expected to show enhanced redox process of the S S bond by its intramolecular electrocatalysis of polyphenyl. The
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2.2.1. Synthesis of 1,2,4,5-tetrabromo-methylbenzene (2) The compound 2 was bromized from the starting material (1) with N-bromosuccinimide (NBS) in CCl4 solvent at the present of benzoyl peroxide (BPO), just described as the literature . Scheme 1. Charge–discharge process of PDTTA.
most promising advantage is that the cleavage and recombination of the S S bond could occur intramolecularly, so the recombination chance of the S S bond is much higher than the intermolecular ones, leading to the improvement of the cyclic ability [10,11]. Preliminary results for the basic electrochemical performance of PDTTA are discussed as follows.
2. Experimental 2.1. Measurements FT-IR spectra were recorded on a Shimadzu Infred spectrophotometer with KBr pellets. XPS tests were carried out on a KRATOS XSAM800 instrument with the magnesium source Mg K␣ 1253.6 eV operated at 12.5 kV and 16 mA. The electroconductivity of the polymers was measured at room temperature on pressed pellets by two-probe method. The element analyses were measured with a Finnigan FLASH 1112 SERIES EA. Raman spectra were recorded on a NEXUS670 spectrophotometer. NMR spectra were obtained on Varian Mercury-VX 300 spectrometers. Mass spectra were obtained on a Thermo Finnigan TRACE MS PLUS. The crystal data were collected with a Bruker AXS SMART 1000 CCD. Cyclic voltammetric measurements were recorded on CHI600 by using a three-electrode single-compartment electrochemical test cell working electrode of DTTA or PDTTA adhering on Al foil, counter electrode of lithium (Li) and reference electrode of a Li/Li+ . The charge–discharge tests were performed on Arbin BT2000 at a current density of 30 mA/g. The lithium batteries were fabricated with the composite containing PDTTA or DTTA (40 wt%), acetylene carbon (40 wt%) and poly(tetrafluoroethylene) (20 wt%) as the cathode, 1 M LiClO4 /EC–DMC (1:1, v/v) solution as the electrolyte and lithium as the anode. 2.2. Materials The synthesis for target compound poly(5,8-dihydro1H,4H-2,3,6,7-tetrathia-anthracene) is achieved in three steps starting from 1,2,4,5-tetramethyl-benzene (1) as outlined in Scheme 2. The new monomer, 5,8-dihydro1H,4H-2,3,6,7-tetrathia-anthracene (DTTA) was synthesized via a similar method described as the literature . All other chemicals were of reagent grade and used as received.
2.2.2. Synthesis of 5,8-dihydro-1H,4H2,3,6,7-tetrathia-anthracene The compound 2 (2.0 g, 4.4 mmol) and sodium thiosulfate (4.4 g, 17.7 mmol) were dissolved in 120 mL of EtOH/H2 O (1:1). The mixture was refluxed with stirring until the solution was homogeneous. The iodine was then added in small portions until the color remained. Then, a sodium bisulfite solution was added to remove the excess of iodine. After filtration, the precipitate was washed with water and then dried in vacuum at 80 ◦ C, then extracted by 100 mL of boiling CHCl3 . Evaporation of solvent left a yellow solid, which was purified using silica gel chromatography (chloroform/petroleum ether, 10:3) to give 0.66 g (57%) of DTTA as a white solid: mp 215 ◦ C (dec.); 1 H NMR (CDCl3 ) δ 6.82 (s, 2H, ph-H), 4.00 (s, 8H, ph-CH2 ); 13 C NMR (CDCl3 ) δ 129.1, 128.9, 31.9; FT-IR (KBr) 3004, 2923, 2890, 1504, 1400, 915; Raman 514 (νS S ) cm−1 ; EI–MS m/z 258[M]+ ; anal. for C10 H10 S4 —calcd: C, 46.47; H, 3.90; S, 49.63; found: C, 47.06; H, 3.88; S, 49.06. 2.2.3. Synthesis of poly(5,8-dihydro1H,4H-2,3,6,7-tetrathia-anthracene) DTTA was polymerized simply by oxidative coupling reaction to PDTTA by FeCl3 in chloroform at room temperature for 48 h. After reaction, the chloroform was filtered off; FeCl3 was removed by washing with dilute hydrochloric acid then water till pH 7. The fine yellow polymer was dried at 60 ◦ C in oven to constant weight.
3. Results and discussion 3.1. Characterization Detecting the compound’s single-crystal structure is the most convinced and direct method to confirm a novel compound’s molecular structure. We successfully grew the single-crystal of the DTTA from CHCl3 /C2 H5 OH and submitted to X-ray crystallography. The ORTEP drawing of DTTA is shown in Fig. 1, from which we can see clearly that the molecular has a non-planar structure. Complete crystallographic data have been deposited at the Cambridge Crystallographic Data Centre under the reference number CCDC 261412. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: +44 1223 336033 or e-mail: [email protected]
]. The 1 H NMR and 13 C NMR spectra of DTTA are assigned as the following. In the 1 H NMR spectrum, the protons of the four methylene groups and the benzene ring are both single
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Scheme 2. The synthetic route of PDTTA.
Fig. 1. ORTEP drawing of the structure of DTTA. CCDC reference number 261412.
peaks at 4.00 and 6.82 ppm, respectively. In the 13 C NMR spectrum, the signal of 31.9 ppm is clearly identified as the methylene carbons. The two signals of the phenyl carbons are detected at 129.1 and 128.9 ppm, respectively. Unfortunately, because the PDTTA was almost insoluble in any of the known solvent, we could not get its NMR data, the attempt of getting its solid-state 13 C NMR spectra was also failed. The monomer of DTTA and PDTTA has been characterized by FT-IR spectroscopy as shown in Fig. 2(a and b), respectively. It can be clearly observed that the absorption
bands of the polymer obviously become broader than that of the monomer, which is the characterization of the polymer compound. PDTTA has some similar vibration bands to that of DTTA, the strong absorptions at 2919 and 1426 cm−1 are attributed to the C H stretching band and the C H bending vibration of the methylene, the weak absorption at 717 cm−1 is assigned to the C S stretching band. The spectrum 2(b) also shows other bands of secondary intensity, which are known to be characteristic for poly(para)phenylene [14,15]: the band at 1618, 1610 and 1496 cm−1 is attributed to the skeletal vibrations of benzenoid ring of the polymer, the C C vibrations of the poly(para)phenylene appears at 1100 and 1062 cm−1 . All the evidences indicate that the polymer has the similar chain structure to that of poly(para)phenylene. In order to confirm that the disulfide groups were well protected from further oxidation, the XPS spectra of DTTA and PDTTA have been carried out to prove the preservation of S S bonds after chemical polymerization. Both spectra of the monomer and the polymer show S2p peaks at 163.7 eV, which indicate the S S bonds were protected from further oxidation to sulfoxide or sulfone groups . Raman spectroscopy is also a powerful method to detect the existence of the disulfide bonds, but we failed to get the polymer’s Raman spectrum due to the serious interference of the polymer’s high fluorescent effect. The results of element analyses show the hydrogen content of polymer (3.35%) is lower than its monomer (3.90%), but a little higher than its theoretic content (3.14%),which reveals the existence of the oligomer, maybe attributed to the trimer and tetramer. We also detected the polymer’s conductivity. After doped in iodine steam, the as-synthesized PDTTA showed relatively low electrical conductivity of ca. 10−12 S/cm measured with two-point technique , which may be due to the influence of the bulky condensed aliphatic heterocyclic substitute or the existence of oligomer in the product . 3.2. The cyclic voltammetry test results
Fig. 2. FT-IR spectra of: (a) DTTA and (b) PDTTA (pellet by KBr).
The electrochemical properties of the DTTA monomer and PDTTA were studied by cyclic voltammetry. Fig. 3(a) shows the cyclic voltammogram for DTTA monomer adhered on Al foil in 1.0 M LiClO4 , EC/DMC (1:1, v/v). The sharp
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Fig. 3. Cyclic voltammogram of: DTTA (a) and PDTTA (b) adhered on Al foil in 1.0 M LiClO4 , EC/DMC (1:1, v/v) at room temperature. Scan rate: 0.1 mV/s.
cathodic peak at ca. 2.0 V is due to the cleavage of the S S bond. The broad anodic peaks observed at ca. 3.1 V (versus Li/Li+ ) are attributed to the recombination of S S bond. The typical large anodic peak at ca. 3.8 V corresponds to the formation of the benzene radical cation. From the data above, it can be obtained that the potential peak gap of the redox of S S bond is about 1.1 V. Then, the PDTTA adhered on Al foil was tested by cyclic voltammetry in 1.0 M LiClO4 , EC/DMC at a scan rate of 0.1 mV/s, the results was illustrated in Fig. 3(b). As can be seen, the sharp cathodic peak at 2.0 V of the monomer disappeared, instead of two new cathodic peaks at ca. 2.3 and 2.4 V and one broad anodic peak at ca. 3.0 V, respectively. The corresponding peak potential differences are 0.7 and 0.6 V for 3.0/2.3 and 3.0/2.4 V redox pair, respectively. So, the peak potential separation of PDTTA (polymer) between correspond oxidation and reduction peaks (ca. 0.6 V) is much smaller than that of the DTTA (monomer, ca. 1.1 V), which can be attributed to the electrocatalytic effect of the main chain of polyphenyl. Overall, as the existence of the shrink of the potential peak gap of the redox of the S S bond (reduced from 1.1 to 0.6 V), we can make some conclusion that as a result of monomer DTTA oxidation to PDTTA, some kind of changes occur that may be attributed to the intramolecular interaction of polyphenyl main chain and S S side bonds, which is similar to the effect that has been observed in polyaniline/DMcT  and PDTAn . That is to say, not only the polyaniline but also the polyphenyl can act as the electrocatalyst to enhance the electrochemical process of the cleavage and recombination process of S S bond.
Fig. 4. The second cycle of discharging curve for: DTTA (a) and PDTTA (b) in 1.0 M LiClO4 , EC/DMC (1:1, v/v). Current density: 30 mA/g; cutoff voltages: 1.4–4.4 V.
records the second cycle of the discharging curves of the DTTA and PDTTA electrode, respectively. It can be seen from Fig. 4(a) that the Li/LiClO4 /DTTA battery shows the capacity of 257 mAh/g and has only one obvious discharging platform near 2.6 V with the mean-voltage of about 2.6 V. However, the Li/LiClO4 /PDTTA battery (Fig. 4(b)) shows the capacity of 422 mAh/g and has a series of discharging platform appeared at ca. 3.1, 2.6 and 1.9 V with the mean-voltage of 2.6 V, which might be due to the discharge of the polyphenyl and the S S bond, respectively. From the above discharging data, we can calculate that the above two materials have energy density of 668 Wh/kg (DTTA) and 1097 Wh/kg (PDTTA) at the second cycle, respectively. Fig. 5 shows the cycling curves of the two materials. The results of the Li/LiClO4 /DTTA battery indicate that the specific capacity reached 257 mAh/g at second circle but decreased dramatically (Fig. 5, curve A), due to a rapid escape of the monomer from the cathode during the charge–discharge process. The initial discharge capacity of
3.3. The charge–discharge experimental results The charge–discharge test for a lithium battery with DTTA and PDTTA as cathode material was performed at a current density of 30 mA/g and both of the two materials showed their highest capacity at the second cycle after the first charging–discharging cycle for activation. Fig. 4(a and b)
Fig. 5. The specific capacity of: DTTA (A) and PDTTA (B) in 1.0 M LiClO4 , EC/DMC (1:1, v/v). Current density: 30 mA/g; cutoff voltages: 1.4–4.4 V.
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the PDTTA cathode is 422 mAh/g, which should include both contributions of polyphenyl and S S bonds of PDTTA. The capacity of the PDTTA could maintain 170 mAh/g at the 44th (Fig. 5, curve B), which was much better than that of the monomer (only 29 mAh/g at the 44th). That is to say, the cycle stability was dramatically enhanced when the PDTTA was used as the cathode material, which should profit from the polymer’s special structure. During charge–discharge process, the main chains of PDTTA were maintained, and the reversible ring-opening reduction of the disulfide bond was expected to perform in each monomer unit, leading to the improvement of the cyclic efficiency. When compared the charging–discharging property with the PABTH and PDTAn (reported as the polyorganodisulfide for secondary lithium anode materials by our previous studies [3,10]), the PDTTA electrode has many obvious advantages. The capacity and energy density of PDTTA reached 422 mAh/g and 1097 Wh/kg, which is much higher than that of PABTH (300 mAh/g and 630 Wh/kg) and PDTAn (225 mAh/g and 585 Wh/kg). On the other hand, the cycle ability of the PDTTA is much better than the two previous materials, the PDTTA maintained capacity of 170 mAh/g at the 44th cycle, while the previous tow materials left not more than 100 mAh/g after 15th cycle. 4. Conclusion A new redox system, PDTTA, containing of a polyphenyl main chain and two six-membered cycles with S S bonds on both side chain of benzene has been successfully designed and synthesized, then presented as a high-energy storage material for secondary lithium batteries. The enhanced redox properties of PDTTA are confirmed by the cyclic voltammetry, which is due to the intramolecular electrocatalytic effect of benzene moiety and thiolate anions. The charge–discharge test shows an initial discharge capacity of 422 and 170 mAh/g at the 44th cycle with a mean-voltage of 2.6 V, which is
much higher than that of the monomer (DTTA), indicating an improved reversibility. Further studies on higher capacity and better cyclic ability of PDTTA cathode are under research in our group.
Acknowledgement We gratefully acknowledge financial support by the National Natural Science Foundation of China (No. 29833090).
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