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A triphenylamine-based polymer with anthraquinone side chain as cathode material in lithium ion batteries
Electrochimica Acta 283 (2018) 1284e1290
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A triphenylamine-based polymer with anthraquinone side chain as cathode material in lithium ion batteries Wanrong Huang, Tao Jia, Guangying Zhou, Sha Chen, Qiong Hou*, Yuhai Wang**, Suilian Luo, Guang Shi, Bingjia Xu School of Chemistry and Environment, South China Normal University, Guangzhou, 510631, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2018 Received in revised form 2 July 2018 Accepted 11 July 2018
A triphenylamine-based polymer poly[N-(anthraquinone-2-yl)-N,N-diphenylamine] (PDPA-AQ) with anthraquinone side chain was synthesized by a simple chemical oxidative polymerization. Due to the introduction of anthraquinone units with a high specific capacity in the side chain, the polymer PDPA-AQ has a high theoretical specific capacity, about twice of polytriphenylamine (PTPA). The lithium ion batteries based on PDPA-AQ as cathode active material and lithium foil as anode were assembled and their electrochemical properties were investigated. For a comparison, the electrochemical performance of the monomer DPA-AQ was also studied. Both monomer DPA-AQ and polymer PDPA-AQ have high initial discharge specific capacity, about 129 mAh g1 and 159 mAh g1 respectively, which are higher than the theoretical specific capacity of PTPA-based cells. The polymer PDPA-AQ has better discharge specific capacity, cycle performance and rate performance than monomer DPA-AQ, which shows that the design idea of introducing an electrochemically active carbonyl compound into the polytriphenylamine side chain can improve the specific capacity of the PTPA. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Triphenylamine polymer Anthraquinone Lithium ion batteries Cathode active material
1. Introduction As a new generation of green high-energy battery, lithium-ion batteries have been widely applied to mobile phones, notebook computers, camcorders, digital cameras and other fields because of their features such as high-capacity, high-cycle stability and pollution-free. In the configuration of lithium ion battery, the cathode material accounts for a large proportion, so the performance of the lithium ion battery depends to a large extent on the characteristics of the cathode material. At present, inorganic cathode materials widely used are mainly lithium transition metal oxides such as LiCoO2, LiMn2O4 and LiFePO4, etc [1e6]. Compared with inorganic cathode materials, organic cathode materials have the advantages of high theoretical specific capacity, rich raw materials, environmental friendliness, strong structural design, and system safety, which are potential energy storage materials with wide application prospects [7e16].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Hou), [email protected] (Y. Wang). https://doi.org/10.1016/j.electacta.2018.07.062 0013-4686/© 2018 Elsevier Ltd. All rights reserved.
Polytriphenylamine (PTPA) has a highly conductive polyparaphenylene (PPP) back-bone and an electroactive polyaniline unit with high energy density, which exhibited a reversible, rapid, and stable radical redox reaction during charge-discharge processes. So the TPA-based polymers have been explored recently as the electrode active material applied in supercapacitors [17e19] and lithium-ion batteries [20e25]. The first PTPA-based Lithiumion battery reported by J.K. Feng et al. [20] gives a high average discharge voltage of 3.8 V and a capacity of 103 mAh g1, and exhibits excellent rate performance and cycle stability. Subsequently, PTPA cathode materials with nitroxyl radicals or electronwithdrawing nitro or cyano groups [21] in the side chains, TPAbased organic-metal framework structures (MOFs) [22] and microporous polymers [23,24] have been reported. In general, there are a few researches on lithium-ion batteries based on PTPA and its derivatives. Therefore, there is still much room for research on PTPA and its derivatives. According to the calculation formula of theoretical specific ca1 pacity, Ct ¼ nF (where Ct, n, F, and Mw are the theoMw mAh g retical specific capacity, the transferred electron number in each structural unit, the Faraday constant, and the molecular weight of the structural unit respectively), the theoretical specific capacity Ct
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depends on the ratio of n to Mw. The larger the ratio, the higher the theoretical specific capacity. In PTPA chains, each structural unit has only one nitrogen free radical active site, while it has a relatively large molecular weight. So the theoretical specific capacity of PTPA is relatively low, about 109 mAh g1, which limits its commercial application. The theoretical specific capacity of the TPA-based polymer can be improved by introducing a compound with a high n/Mw ratio into the polymer chain. Due to containing two redox-active carbonyl groups, anthraquinone has a high n/Mw ratio and thus have a high theoretical specific capacity of 259 mAh g1. There have been many literature on anthraquinone-based compounds [26e28] and polymers [29e31] as electrode materials for lithium ion batteries, however, there have been no reports on the application of TPA-like polymers with anthraquinone side chains in lithium-ion batteries. Herein, a TPA-based polymer poly[N-(anthraquinone-2-yl)-N,N-diphenylamine] (PDPA-AQ) with a anthraquinone side chain is synthesized, in which anthraquinone unit was linked to the side chain of polymer via nitrogen atom. Each structural unit of this polymer contains three redox active sites including a nitrogen radical active center in diphenylamine and two carbonyl groups in anthraquinone unit, so the polymer PDPA-AQ has a high theoretical specific capacity of 214 mAh g1, which is about twice of PTPA. The CR2025 half-cells were assembled with PDPA-AQ as cathode active material and lithium foil as anode and their electrochemical properties were studied. As for comparison, the electrochemical performances of monomer N-(anthraquinone-2-yl)-N,N-diphenylamine (DPA-AQ) were discussed. 2. Experimental section 2.1. Syntheses and characterization of materials 2.1.1. Synthesis of monomer DPA-AQ [32] Under a nitrogen atmosphere, diphenylamine (1.694 g, 10 mmol), 2-bromoanthraquinone (4.305 g, 15 mmol), potassium carbonate (2.072 g, 15 mmol), and dimethyl sulfoxide (45 mL) were successively added to a dry three-neck flask. Then Pd(OAc)2 (115 mg, 0.5 mmol) was added and the reaction was carried out at 120 C for 24 h. After cooling to room temperature, the reaction solution was added dropwise to a saturated ammonium chloride solution, separated by suction filtration, and the solid was washed with water until neutral. Drying at 80 C for 24 h gives a red crude product which was purified by column chromatography on silica gel/petroleum ether and dichloromethane (v/v ¼ 3:1) to give the final product as an orange-red solid powder (2.562 g, 68%). 1H NMR (500 MHz, CDCl3, d): 7.19 (s, 2H), 7.21e7.22 (m, 4H), 7.25e7.27 (dd, 1H), 7.35e7.38 (m, 4H), 7.71e7.78 (m, 3H), 8.11e8.12 (d, 1H), 8.21e8.23 (dd, 1H), 8.28e8.30 (dd, 1H). 13C NMR (125 MHz, CDCl3, d): 116.95, 124.31, 125.70, 126.26, 126.45, 127.08, 128.28, 129.28, 130.03, 133.63, 134.10, 135.02, 137.12, 145.12, 145.74, 152.93, 181.73, 183.52. FT-IR (KBr, v): 3057, 1672, 1568, 1487, 1323, 715. 2.1.2. Synthesis of polymer PDPA-AQ Anhydrous ferric chloride (486 mg, 3 mmol) and 50 mL chloroform were added to a three-necked flask under a nitrogen atmosphere and stirred at 40 C for 0.5 h. Then a solution of DPA-AQ (375 mg, 1 mmol) in chloroform (30 mL) was added dropwise and reacted 24 h. After the reaction was completed, 500 mL methanol was added and the mixture was filtered. The solid was washed with methanol and deionized water, Soxhlet extracted with dichloromethane and dried at 80 C for 24 h to obtain an orange-red polymer PTPA-AQ (357 mg, 95%). FT-IR (KBr, n): 3067, 3032, 1670, 1571, 1490, 1325, 824. Soluble polymer Soxhlet extracted was used for the 1 H NMR and 13C NMR tests. 1H NMR (500 MHz, CDCl3, d): 7.24e7.26
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(m, 2H), 7.27e7.28 (d, 1H), 7.33e7.42 (m, 4H), 7.58e7.64 (m, 2H), 7.73e7.79 (m, 2H), 7.85 (s, 1H), 8.15e8.20 (dd, 1H), 8.23e8.25 (d, 1H), 8.30e8.31 (d, 1H). 13C NMR (125 MHz, CDCl3, d): 116.96, 124.32, 125.69, 126.26, 126.45, 127.08, 128.27, 129.28, 130.03, 133.62, 134.10, 135.01, 137.11, 145.11, 145.73, 152.92, 181.72, 183.52. 2.1.3. Characterization of materials 1 H NMR and 13C NMR spectra of the monomer and polymer were measured by AV III, Ascend 500 HD (Bruker Tm, Switzerland). The Fourier transform infrared (FT-IR) absorption spectra were carried out on Nicolet 6700 (Thermo Fisher Nicolet, America). The thermogravimetric analysis (TGA) was taken using Netzschsta 409P €tebau GmbH, Germany) instrument at the heating (NETZSCHGera rate of 10 C min1 under an air atmosphere. The microscopic morphology features were examined on a ZEISS Ultra 55 FESEM (Carl Zeiss, Germany) at an accelerating voltage of 20 kV or 30 kV. 2.2. Electrochemical characterization CR2025 coin-type cells were assembled in an argon-filled glove box for electrochemical experiments. The cathode includes active material DPA-AQ or PDPA-AQ, acetylene black as conductive agent, and polyvinylidene fluoride (PVDF) as binder (4:5:1 wt%) on Al foils, then the pole pieces were vacuum-dried at 120 C for 12 h to completely remove the solvent. The solvent is N-methylpyrrolidone (NMP). Lithium foil was used as the anode, and the electrolyte was a solution of 1 mol L1 LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl ethyl carbonate (EMC) (EC/DMC/ EMC ¼ 1:1:1 v/v/v). The cyclic voltammetry characteristic curves were tested at a scan rate of 0.1 mV s1 and a scan voltage range of 1.5 Ve4.5 V or 1.8 V-4.5 V (vs. Li/Liþ) on a CHI760E electrochemical workstation (Chenhua Co., China). The charge-discharge tests were measured on a CT2001A Neware battery test system (Wuhan Land Co., China) with a voltage range of 1.5 Ve4 V (vs. Li/Liþ). The AC impedance tests were conducted on the CHI760E electrochemical workstation (Chenhua Co., China) at a frequency of 0.1e10 kHz. The electrochemical characterization of the cells was measured at a constant current density at room temperature. 3. Results and discussion 3.1. Structural characterization and thermal stability of PDPA-AQ The polymer PDPA-AQ was synthesized by a simple chemical oxidative polymerization method and its synthetic route is shown in Scheme 1. Polymer PDPA-AQ has a low solubility in common organic solvents and its structure was characterized using FT-IR and 1 H NMR and 13C NMR spectra. Fig. 1a shows the FT-IR spectra of monomer DPA-AQ and polymer PDPA-AQ. As can be seen from Fig. 1a, polymer PDPA-AQ and monomer DPA-AQ exhibit similar material structures. The absorption peaks at 3032 cm1, 1670 cm1, 1571 cm1 and 1490 cm1, 1325 cm1 are attributed to the CeH stretching vibration of the benzene ring, the C]O stretching vibration, the aromatic ring C]C skeleton vibration, and the CeN single bond stretching vibration respectively. The absorption peak of PDPA-AQ at 824 cm1 is a CeH out-of-plane bending vibration from 1,4-disubstituted benzene rings. Soluble polymer from Soxhlet extraction was used to test the 1H NMR and 13C NMR spectra of the polymer PDPA-AQ. Compared with the 1H NMR spectrum of the monomer DPA-AQ, the corresponding chemical shift of the polymer PDPA-AQ shifted toward the lower field, and the absorption of H at the j position in the polymer was significantly weakened (Fig. S1a). The 13C NMR spectrum of the polymer is
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Scheme 1. Synthetic route of polymer PDPA-AQ.
batteries. 3.2. Electrochemical performance Fig. 2 shows the CV curves of DPA-AQ and PDPA-AQ containing the first four scans with a voltage range of 1.5 Ve4.5 V or 1.8 V4.5 V at a scan rate of 0.1 mV s1. DPA-AQ has two pairs of distinct reversible redox peaks at 2.4 V/2.2 V (vs. Li/Liþ) and 4.2 V/3.7 V (vs. Li/Liþ), where the redox peak at 2.4 V/2.2 V (vs. Li/Liþ) corresponds to the redox reaction of the carbonyl group on the anthraquinone
Fig. 1. (a) FT-IR spectra of monomer DPA-AQ and polymer PDPA-AQ; (b) TG curves of monomer DPA-AQ and polymer PDPA-AQ under air atmosphere.
similar to that of the monomer (Fig. S1b). The above analyses show that the polymer has been successfully synthesized. Fig. 1b shows the TG curves of DPA-AQ and PDPA-AQ under air atmosphere. The temperature at 5% loss of mass is 274 C for DPAAQ and 415 C for PDPA-AQ respectively, which are much higher than the working temperature of the lithium-ion battery (generally between 20 C and 80 C). This shows that the thermal stability of DPA-AQ and PDPA-AQ satisfies the requirements of the lithium-ion
Fig. 2. CV curves of DPA-AQ (a) and PDPA-AQ (b) with a voltage range of 1.5Ve4.5 V or 1.8V-4.5 V (vs. Li/Liþ) and a scan rate of 0.1 mV s1.
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unit [33] and the redox peak at a higher potential of 4.2 V/3.7 V (vs. Li/Liþ) is attributed to the redox reaction of the radical cations during the doping and dedoping of diphenylamine [18]. Similar to DPA-AQ, polymer PDPA-AQ also has two pairs of corresponding redox peaks and its CV curve has no significant changes in terms of potentials and currents in the first four cycles, confirming that PDPA-AQ is redox active and operable as an active material in lithium ion batteries. There is only one pair of reversible redox peaks for AQ unit in PDPA-AQ, similar to the literature [34] in which AQ shows one pair of redox peaks. For the polymer PDPA-AQ, the redox peak of the anthraquinone unit is much weaker than that of diphenylamine. This phenomenon is similar to the triphenylaminebased compound Cu-TCA (H3TCA ¼ tricarboxytriphenylamine) [22] and hollow microporous triphenylamine networks H-MTPN-TCNE (TCNE ¼ tetracyanobutadiene) [35]. PDPA-AQ has same structural unit with DPA-AQ, so their electrochemical reaction mechanism is similar. Therefore, only the possible electrochemical reaction mechanism of the polymer PDPA-AQ is listed in Scheme 2. It includes the intercalation/deintercalation of lithium ions on the carbonyl group of the anthraquinone unit at low potential and the doping/dedoping of the nitrogen radical of the diphenylamine main chain at high potential. Fig. 3a and S2 are initial charge/discharge curves of the cells based on DPA-AQ or PDPA-AQ as cathode active material with a current density of 0.1C. Both of the discharge curves have two sloping discharge plateaus at 3.6 V-4.0 V and 1.8 V-2.3 V, corresponding to the insertion of lithium ions of the diphenylamine unit and the anthraquinone unit, respectively, which is consistent with the redox potential of the CV curve. The structural unit of the polymer PDPA-AQ is the same as the monomer DPA-AQ, so the polymer PDPA-AQ has equal theoretical specific capacity of 214 mAh g1 with DPA-AQ. Under the same conditions, the initial discharge specific capacity is 129 mAh g1 for DPA-AQ and 159 mAh g1 for PDPA-AQ. From the CV curves of the polymer PDPA-AQ, the discharge specific capacity seems to be mainly contributed by the
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diphenylamine unit. However, it can be seen from Fig. 3a and S2 that the specific discharge capacity of the anthraquinone units is about 62% of the initial discharge specific capacity, which shows that each anthraquinone unit has two electrons participating in the electrochemical reaction during charge and discharge. This result is consistent with the fact that each anthraquinone unit in the polymer chain can theoretically insert two lithium ions. Fig. 3a also shows that the specific capacities contributed by the dedoping of the nitrogen radical cations and by the lithium intercalation of the carbonyl groups in the polymer PDPA-AQ are higher than the corresponding monomer DPA-AQ. Therefore, although DPA-AQ and PDPA-AQ have the same theoretical specific capacity, the initial specific capacity of PDPA-AQ is higher than that of DPA-AQ. This phenomenon that the specific capacities of monomers and polymers have large differences also exists in the literature [33,36]. It is noteworthy that PTPA cathodes have shown the discharge specific capacities in a range of 62e100 mAh g1 in the literature [23,27]. The theoretical and practical specific capacity of polymer PDPA-AQ is higher than that of PTPA, which can be ascribed to multiple electrochemically active sites of the DPA-AQ unit. The experimental results indicate that the specific capacity of PTPA can be increased by introducing the anthraquinone unit into the PTPA chain. Cyclic stability is an important parameter for evaluating the performance of the cells. Cycling performance of the cells with DPA-AQ or PDPA-AQ as cathode active material at a current density of 0.1C is shown in Fig. 3b. Monomer DPA-AQ exhibits serious capacity degradation in the initial 3 cycles from 129.7 mAh g1 down to 67.3 mAh g1, and then the specific capacity decayed slowly and the capacity is about 47 mAh g1 after 100 cycles. Similar to the monomer DPA-AQ, the specific capacity of the polymer PDPA-AQ also decreased during the initial cycles, but the capacity decay was slower than the monomer DPA-AQ. In order to elucidate the reasons for the drop in cycling performance, we tested the solubility of the monomer DPA-AQ and the polymer PDPA-AQ in the
Scheme 2. Possible electrochemical reaction mechanism of polymer PDPA-AQ.
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Fig. 3. (a) First charge and discharge curves of the cells based on DPA-AQ or PDPA-AQ as cathode active material; (b) Cycling performance of the cells with DPA-AQ or PDPA-AQ as cathode active material at a current density of 0.1 C; (c) Rate performance of cells based on DPA-AQ as cathode active material; (d) Rate performance of cells with PDPA-AQ as cathode active material.
electrolyte by immersing the cathode in the electrolyte. Fig. S3 is the photographs of the monomer DPA-AQ and polymer PDPA-AQ electrodes soaked in the electrolyte for different time before and after 100 cycles at 0.1C. In Fig. S3, when the DPA-AQ electrode before cycles was immersed in the electrolyte for 0.5 h, the color of the electrolyte changed from colorless to orange, then the color does not change. Even if the polymer electrode was immersed for 24 h, the color of the electrolyte changed only a little, indicating that the solubility of the polymer in the electrolyte was much lower than that of the monomer. The color of the electrolyte is invariable when the polymer cathode was soaked in electrolyte for 24 h after 100 cycles at 0.1 C, which means that the polymer PDPA-AQ tends to stabilize once the easily soluble molecules are dissolved. The above results indicate that the polymer PDPA-AQ has superior cycle stability than the monomer DPA-AQ and the decrease of cycle performance for the polymer PDPA-AQ is mainly due to partial dissolution of the polymer in the electrolyte. The rate performance represents the ability of rapidly charging and discharging of the electrode active material. In order to evaluate the rate performance of DPA-AQ and PDPA-AQ, the chargedischarge tests were carried out at current densities from 0.1C to 2C and then back to 0.1C, and the results are shown in Fig. 3c and d. From 0.1C to 2C, the discharge capacity of DPA-AQ decreases from 129 mAh g1 to 38 mAh g1, and when the current density gets back to 0.1C, the capacity can recover to 71 mAh g1. For the polymer PDPA-AQ, the capacity decreases from 151 mAh g1 to 50 mAh g1, and when discharged again at 0.1C, the capacity can
recover to 106 mAh g1. Consistent with the results of the cycling performance, the polymer PDPA-AQ has better rate performance than the monomer DPA-AQ. The above analysis demonstrates that the polymer PDPA-AQ has a superior overall electrochemical performance than the monomer DPA-AQ. In order to further elucidate the electrochemical properties of the polymer PDPA-AQ, the surface morphologies of the cathodes before and after 100 cycles were tested. Fig. 4 shows the surface morphologies of the cathodes observed by SEM. For the DPA-AQ electrodes, DPA-AQ and conductive agent were uniformly distributed in fine particles, indicating that DPA-AQ and conductive agent have good compatibility. After 100 cycles of charge and discharge, the morphology was almost unchanged compared to that before the cycle, and no structural change or destruction was observed. Similar to the DPA-AQ electrode, the morphology of the PDPA-AQ electrodes before and after 100 cycles was also hardly changed. This shows that PDPA-AQ is very stable, has a low solubility in the electrolyte, and therefore has good cycle stability. Except observing the surface morphologies, we also tested the electrochemical impedance spectroscopies (EIS) of the polymer electrodes before and after 100 cycles (Fig. 5). The semicircle represents the charge transfer resistance (Rct) in the electrochemical reaction process. The inclined line is associated with the Warburg impedance (Zw), representing the Liþ ions diffusion inside the bulk phase of the PDPA-AQ materials. It can be seen from Fig. 5 that both the charge transfer resistance and the lithium ion diffusion resistance before cycles are lower than that after 100 cycles, which
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Fig. 4. SEM images of cathodes (a) DPA-AQ before cycle; (b) DPA-AQ after 100 cycles; (c) PDPA-AQ before cycle; (d) PDPA-AQ after 100 cycles.
twice of PTPA. CR2025 coin-type cells were assembled with PDPAAQ or DPA-AQ as cathode active materials and lithium metal as anodes. The initial discharge specific capacity of polymer PDPA-AQ is 159 mAh g1, which is higher than that of monomer DPA-AQ. Moreover, the cycle stability and rate performance of the polymer PDPA-AQ are better than the monomer DPA-AQ because of the low solubility of the polymer PDPA-AQ in the electrolyte. The specific capacity of the polymer PDPA-AQ is higher than the theoretical capacity of PTPA, indicating that the design idea of introducing carbonyl compounds into the side chain of PTPA is feasible, and this work provides an effective method for the design of TPA-based cathode materials. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2018.07.062. Fig. 5. Electrochemical impedance spectroscopies (EIS) of the polymer electrodes before and after 100 cycles at 0.1C.
shows that the electrode before cycles has better conductivity. This can also explain the decrease of electrochemical performance after cycles.
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4. Conclusion
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The triphenylamine-based polymer PDPA-AQ with anthraquinone unit in the side chain was synthesized by chemical oxidative polymerization, which has low solubility in common organic solvents, and the temperature of mass loss of 5% is 415 C, with high thermal stability. Due to each structural unit of PDPA-AQ containing multiple electrochemically active sites, the polymer PDPA-AQ has a high theoretical specific capacity of 214 mAh g1, which is about
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A triphenylamine-based polymer with anthraquinone side chain as cathode material in lithium ion batteries Wanrong Huang, Tao Jia, Guangying Zhou, Sha Chen, Qiong Hou*, Yuhai Wang**, Suilian Luo, Guang Shi, Bingjia Xu School of Chemistry and Environment, South China Normal University, Guangzhou, 510631, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2018 Received in revised form 2 July 2018 Accepted 11 July 2018
A triphenylamine-based polymer poly[N-(anthraquinone-2-yl)-N,N-diphenylamine] (PDPA-AQ) with anthraquinone side chain was synthesized by a simple chemical oxidative polymerization. Due to the introduction of anthraquinone units with a high specific capacity in the side chain, the polymer PDPA-AQ has a high theoretical specific capacity, about twice of polytriphenylamine (PTPA). The lithium ion batteries based on PDPA-AQ as cathode active material and lithium foil as anode were assembled and their electrochemical properties were investigated. For a comparison, the electrochemical performance of the monomer DPA-AQ was also studied. Both monomer DPA-AQ and polymer PDPA-AQ have high initial discharge specific capacity, about 129 mAh g1 and 159 mAh g1 respectively, which are higher than the theoretical specific capacity of PTPA-based cells. The polymer PDPA-AQ has better discharge specific capacity, cycle performance and rate performance than monomer DPA-AQ, which shows that the design idea of introducing an electrochemically active carbonyl compound into the polytriphenylamine side chain can improve the specific capacity of the PTPA. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Triphenylamine polymer Anthraquinone Lithium ion batteries Cathode active material
1. Introduction As a new generation of green high-energy battery, lithium-ion batteries have been widely applied to mobile phones, notebook computers, camcorders, digital cameras and other fields because of their features such as high-capacity, high-cycle stability and pollution-free. In the configuration of lithium ion battery, the cathode material accounts for a large proportion, so the performance of the lithium ion battery depends to a large extent on the characteristics of the cathode material. At present, inorganic cathode materials widely used are mainly lithium transition metal oxides such as LiCoO2, LiMn2O4 and LiFePO4, etc [1e6]. Compared with inorganic cathode materials, organic cathode materials have the advantages of high theoretical specific capacity, rich raw materials, environmental friendliness, strong structural design, and system safety, which are potential energy storage materials with wide application prospects [7e16].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Hou), [email protected] (Y. Wang). https://doi.org/10.1016/j.electacta.2018.07.062 0013-4686/© 2018 Elsevier Ltd. All rights reserved.
Polytriphenylamine (PTPA) has a highly conductive polyparaphenylene (PPP) back-bone and an electroactive polyaniline unit with high energy density, which exhibited a reversible, rapid, and stable radical redox reaction during charge-discharge processes. So the TPA-based polymers have been explored recently as the electrode active material applied in supercapacitors [17e19] and lithium-ion batteries [20e25]. The first PTPA-based Lithiumion battery reported by J.K. Feng et al. [20] gives a high average discharge voltage of 3.8 V and a capacity of 103 mAh g1, and exhibits excellent rate performance and cycle stability. Subsequently, PTPA cathode materials with nitroxyl radicals or electronwithdrawing nitro or cyano groups [21] in the side chains, TPAbased organic-metal framework structures (MOFs) [22] and microporous polymers [23,24] have been reported. In general, there are a few researches on lithium-ion batteries based on PTPA and its derivatives. Therefore, there is still much room for research on PTPA and its derivatives. According to the calculation formula of theoretical specific ca1 pacity, Ct ¼ nF (where Ct, n, F, and Mw are the theoMw mAh g retical specific capacity, the transferred electron number in each structural unit, the Faraday constant, and the molecular weight of the structural unit respectively), the theoretical specific capacity Ct
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depends on the ratio of n to Mw. The larger the ratio, the higher the theoretical specific capacity. In PTPA chains, each structural unit has only one nitrogen free radical active site, while it has a relatively large molecular weight. So the theoretical specific capacity of PTPA is relatively low, about 109 mAh g1, which limits its commercial application. The theoretical specific capacity of the TPA-based polymer can be improved by introducing a compound with a high n/Mw ratio into the polymer chain. Due to containing two redox-active carbonyl groups, anthraquinone has a high n/Mw ratio and thus have a high theoretical specific capacity of 259 mAh g1. There have been many literature on anthraquinone-based compounds [26e28] and polymers [29e31] as electrode materials for lithium ion batteries, however, there have been no reports on the application of TPA-like polymers with anthraquinone side chains in lithium-ion batteries. Herein, a TPA-based polymer poly[N-(anthraquinone-2-yl)-N,N-diphenylamine] (PDPA-AQ) with a anthraquinone side chain is synthesized, in which anthraquinone unit was linked to the side chain of polymer via nitrogen atom. Each structural unit of this polymer contains three redox active sites including a nitrogen radical active center in diphenylamine and two carbonyl groups in anthraquinone unit, so the polymer PDPA-AQ has a high theoretical specific capacity of 214 mAh g1, which is about twice of PTPA. The CR2025 half-cells were assembled with PDPA-AQ as cathode active material and lithium foil as anode and their electrochemical properties were studied. As for comparison, the electrochemical performances of monomer N-(anthraquinone-2-yl)-N,N-diphenylamine (DPA-AQ) were discussed. 2. Experimental section 2.1. Syntheses and characterization of materials 2.1.1. Synthesis of monomer DPA-AQ [32] Under a nitrogen atmosphere, diphenylamine (1.694 g, 10 mmol), 2-bromoanthraquinone (4.305 g, 15 mmol), potassium carbonate (2.072 g, 15 mmol), and dimethyl sulfoxide (45 mL) were successively added to a dry three-neck flask. Then Pd(OAc)2 (115 mg, 0.5 mmol) was added and the reaction was carried out at 120 C for 24 h. After cooling to room temperature, the reaction solution was added dropwise to a saturated ammonium chloride solution, separated by suction filtration, and the solid was washed with water until neutral. Drying at 80 C for 24 h gives a red crude product which was purified by column chromatography on silica gel/petroleum ether and dichloromethane (v/v ¼ 3:1) to give the final product as an orange-red solid powder (2.562 g, 68%). 1H NMR (500 MHz, CDCl3, d): 7.19 (s, 2H), 7.21e7.22 (m, 4H), 7.25e7.27 (dd, 1H), 7.35e7.38 (m, 4H), 7.71e7.78 (m, 3H), 8.11e8.12 (d, 1H), 8.21e8.23 (dd, 1H), 8.28e8.30 (dd, 1H). 13C NMR (125 MHz, CDCl3, d): 116.95, 124.31, 125.70, 126.26, 126.45, 127.08, 128.28, 129.28, 130.03, 133.63, 134.10, 135.02, 137.12, 145.12, 145.74, 152.93, 181.73, 183.52. FT-IR (KBr, v): 3057, 1672, 1568, 1487, 1323, 715. 2.1.2. Synthesis of polymer PDPA-AQ Anhydrous ferric chloride (486 mg, 3 mmol) and 50 mL chloroform were added to a three-necked flask under a nitrogen atmosphere and stirred at 40 C for 0.5 h. Then a solution of DPA-AQ (375 mg, 1 mmol) in chloroform (30 mL) was added dropwise and reacted 24 h. After the reaction was completed, 500 mL methanol was added and the mixture was filtered. The solid was washed with methanol and deionized water, Soxhlet extracted with dichloromethane and dried at 80 C for 24 h to obtain an orange-red polymer PTPA-AQ (357 mg, 95%). FT-IR (KBr, n): 3067, 3032, 1670, 1571, 1490, 1325, 824. Soluble polymer Soxhlet extracted was used for the 1 H NMR and 13C NMR tests. 1H NMR (500 MHz, CDCl3, d): 7.24e7.26
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(m, 2H), 7.27e7.28 (d, 1H), 7.33e7.42 (m, 4H), 7.58e7.64 (m, 2H), 7.73e7.79 (m, 2H), 7.85 (s, 1H), 8.15e8.20 (dd, 1H), 8.23e8.25 (d, 1H), 8.30e8.31 (d, 1H). 13C NMR (125 MHz, CDCl3, d): 116.96, 124.32, 125.69, 126.26, 126.45, 127.08, 128.27, 129.28, 130.03, 133.62, 134.10, 135.01, 137.11, 145.11, 145.73, 152.92, 181.72, 183.52. 2.1.3. Characterization of materials 1 H NMR and 13C NMR spectra of the monomer and polymer were measured by AV III, Ascend 500 HD (Bruker Tm, Switzerland). The Fourier transform infrared (FT-IR) absorption spectra were carried out on Nicolet 6700 (Thermo Fisher Nicolet, America). The thermogravimetric analysis (TGA) was taken using Netzschsta 409P €tebau GmbH, Germany) instrument at the heating (NETZSCHGera rate of 10 C min1 under an air atmosphere. The microscopic morphology features were examined on a ZEISS Ultra 55 FESEM (Carl Zeiss, Germany) at an accelerating voltage of 20 kV or 30 kV. 2.2. Electrochemical characterization CR2025 coin-type cells were assembled in an argon-filled glove box for electrochemical experiments. The cathode includes active material DPA-AQ or PDPA-AQ, acetylene black as conductive agent, and polyvinylidene fluoride (PVDF) as binder (4:5:1 wt%) on Al foils, then the pole pieces were vacuum-dried at 120 C for 12 h to completely remove the solvent. The solvent is N-methylpyrrolidone (NMP). Lithium foil was used as the anode, and the electrolyte was a solution of 1 mol L1 LiPF6 in ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl ethyl carbonate (EMC) (EC/DMC/ EMC ¼ 1:1:1 v/v/v). The cyclic voltammetry characteristic curves were tested at a scan rate of 0.1 mV s1 and a scan voltage range of 1.5 Ve4.5 V or 1.8 V-4.5 V (vs. Li/Liþ) on a CHI760E electrochemical workstation (Chenhua Co., China). The charge-discharge tests were measured on a CT2001A Neware battery test system (Wuhan Land Co., China) with a voltage range of 1.5 Ve4 V (vs. Li/Liþ). The AC impedance tests were conducted on the CHI760E electrochemical workstation (Chenhua Co., China) at a frequency of 0.1e10 kHz. The electrochemical characterization of the cells was measured at a constant current density at room temperature. 3. Results and discussion 3.1. Structural characterization and thermal stability of PDPA-AQ The polymer PDPA-AQ was synthesized by a simple chemical oxidative polymerization method and its synthetic route is shown in Scheme 1. Polymer PDPA-AQ has a low solubility in common organic solvents and its structure was characterized using FT-IR and 1 H NMR and 13C NMR spectra. Fig. 1a shows the FT-IR spectra of monomer DPA-AQ and polymer PDPA-AQ. As can be seen from Fig. 1a, polymer PDPA-AQ and monomer DPA-AQ exhibit similar material structures. The absorption peaks at 3032 cm1, 1670 cm1, 1571 cm1 and 1490 cm1, 1325 cm1 are attributed to the CeH stretching vibration of the benzene ring, the C]O stretching vibration, the aromatic ring C]C skeleton vibration, and the CeN single bond stretching vibration respectively. The absorption peak of PDPA-AQ at 824 cm1 is a CeH out-of-plane bending vibration from 1,4-disubstituted benzene rings. Soluble polymer from Soxhlet extraction was used to test the 1H NMR and 13C NMR spectra of the polymer PDPA-AQ. Compared with the 1H NMR spectrum of the monomer DPA-AQ, the corresponding chemical shift of the polymer PDPA-AQ shifted toward the lower field, and the absorption of H at the j position in the polymer was significantly weakened (Fig. S1a). The 13C NMR spectrum of the polymer is
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Scheme 1. Synthetic route of polymer PDPA-AQ.
batteries. 3.2. Electrochemical performance Fig. 2 shows the CV curves of DPA-AQ and PDPA-AQ containing the first four scans with a voltage range of 1.5 Ve4.5 V or 1.8 V4.5 V at a scan rate of 0.1 mV s1. DPA-AQ has two pairs of distinct reversible redox peaks at 2.4 V/2.2 V (vs. Li/Liþ) and 4.2 V/3.7 V (vs. Li/Liþ), where the redox peak at 2.4 V/2.2 V (vs. Li/Liþ) corresponds to the redox reaction of the carbonyl group on the anthraquinone
Fig. 1. (a) FT-IR spectra of monomer DPA-AQ and polymer PDPA-AQ; (b) TG curves of monomer DPA-AQ and polymer PDPA-AQ under air atmosphere.
similar to that of the monomer (Fig. S1b). The above analyses show that the polymer has been successfully synthesized. Fig. 1b shows the TG curves of DPA-AQ and PDPA-AQ under air atmosphere. The temperature at 5% loss of mass is 274 C for DPAAQ and 415 C for PDPA-AQ respectively, which are much higher than the working temperature of the lithium-ion battery (generally between 20 C and 80 C). This shows that the thermal stability of DPA-AQ and PDPA-AQ satisfies the requirements of the lithium-ion
Fig. 2. CV curves of DPA-AQ (a) and PDPA-AQ (b) with a voltage range of 1.5Ve4.5 V or 1.8V-4.5 V (vs. Li/Liþ) and a scan rate of 0.1 mV s1.
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unit [33] and the redox peak at a higher potential of 4.2 V/3.7 V (vs. Li/Liþ) is attributed to the redox reaction of the radical cations during the doping and dedoping of diphenylamine [18]. Similar to DPA-AQ, polymer PDPA-AQ also has two pairs of corresponding redox peaks and its CV curve has no significant changes in terms of potentials and currents in the first four cycles, confirming that PDPA-AQ is redox active and operable as an active material in lithium ion batteries. There is only one pair of reversible redox peaks for AQ unit in PDPA-AQ, similar to the literature [34] in which AQ shows one pair of redox peaks. For the polymer PDPA-AQ, the redox peak of the anthraquinone unit is much weaker than that of diphenylamine. This phenomenon is similar to the triphenylaminebased compound Cu-TCA (H3TCA ¼ tricarboxytriphenylamine) [22] and hollow microporous triphenylamine networks H-MTPN-TCNE (TCNE ¼ tetracyanobutadiene) [35]. PDPA-AQ has same structural unit with DPA-AQ, so their electrochemical reaction mechanism is similar. Therefore, only the possible electrochemical reaction mechanism of the polymer PDPA-AQ is listed in Scheme 2. It includes the intercalation/deintercalation of lithium ions on the carbonyl group of the anthraquinone unit at low potential and the doping/dedoping of the nitrogen radical of the diphenylamine main chain at high potential. Fig. 3a and S2 are initial charge/discharge curves of the cells based on DPA-AQ or PDPA-AQ as cathode active material with a current density of 0.1C. Both of the discharge curves have two sloping discharge plateaus at 3.6 V-4.0 V and 1.8 V-2.3 V, corresponding to the insertion of lithium ions of the diphenylamine unit and the anthraquinone unit, respectively, which is consistent with the redox potential of the CV curve. The structural unit of the polymer PDPA-AQ is the same as the monomer DPA-AQ, so the polymer PDPA-AQ has equal theoretical specific capacity of 214 mAh g1 with DPA-AQ. Under the same conditions, the initial discharge specific capacity is 129 mAh g1 for DPA-AQ and 159 mAh g1 for PDPA-AQ. From the CV curves of the polymer PDPA-AQ, the discharge specific capacity seems to be mainly contributed by the
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diphenylamine unit. However, it can be seen from Fig. 3a and S2 that the specific discharge capacity of the anthraquinone units is about 62% of the initial discharge specific capacity, which shows that each anthraquinone unit has two electrons participating in the electrochemical reaction during charge and discharge. This result is consistent with the fact that each anthraquinone unit in the polymer chain can theoretically insert two lithium ions. Fig. 3a also shows that the specific capacities contributed by the dedoping of the nitrogen radical cations and by the lithium intercalation of the carbonyl groups in the polymer PDPA-AQ are higher than the corresponding monomer DPA-AQ. Therefore, although DPA-AQ and PDPA-AQ have the same theoretical specific capacity, the initial specific capacity of PDPA-AQ is higher than that of DPA-AQ. This phenomenon that the specific capacities of monomers and polymers have large differences also exists in the literature [33,36]. It is noteworthy that PTPA cathodes have shown the discharge specific capacities in a range of 62e100 mAh g1 in the literature [23,27]. The theoretical and practical specific capacity of polymer PDPA-AQ is higher than that of PTPA, which can be ascribed to multiple electrochemically active sites of the DPA-AQ unit. The experimental results indicate that the specific capacity of PTPA can be increased by introducing the anthraquinone unit into the PTPA chain. Cyclic stability is an important parameter for evaluating the performance of the cells. Cycling performance of the cells with DPA-AQ or PDPA-AQ as cathode active material at a current density of 0.1C is shown in Fig. 3b. Monomer DPA-AQ exhibits serious capacity degradation in the initial 3 cycles from 129.7 mAh g1 down to 67.3 mAh g1, and then the specific capacity decayed slowly and the capacity is about 47 mAh g1 after 100 cycles. Similar to the monomer DPA-AQ, the specific capacity of the polymer PDPA-AQ also decreased during the initial cycles, but the capacity decay was slower than the monomer DPA-AQ. In order to elucidate the reasons for the drop in cycling performance, we tested the solubility of the monomer DPA-AQ and the polymer PDPA-AQ in the
Scheme 2. Possible electrochemical reaction mechanism of polymer PDPA-AQ.
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Fig. 3. (a) First charge and discharge curves of the cells based on DPA-AQ or PDPA-AQ as cathode active material; (b) Cycling performance of the cells with DPA-AQ or PDPA-AQ as cathode active material at a current density of 0.1 C; (c) Rate performance of cells based on DPA-AQ as cathode active material; (d) Rate performance of cells with PDPA-AQ as cathode active material.
electrolyte by immersing the cathode in the electrolyte. Fig. S3 is the photographs of the monomer DPA-AQ and polymer PDPA-AQ electrodes soaked in the electrolyte for different time before and after 100 cycles at 0.1C. In Fig. S3, when the DPA-AQ electrode before cycles was immersed in the electrolyte for 0.5 h, the color of the electrolyte changed from colorless to orange, then the color does not change. Even if the polymer electrode was immersed for 24 h, the color of the electrolyte changed only a little, indicating that the solubility of the polymer in the electrolyte was much lower than that of the monomer. The color of the electrolyte is invariable when the polymer cathode was soaked in electrolyte for 24 h after 100 cycles at 0.1 C, which means that the polymer PDPA-AQ tends to stabilize once the easily soluble molecules are dissolved. The above results indicate that the polymer PDPA-AQ has superior cycle stability than the monomer DPA-AQ and the decrease of cycle performance for the polymer PDPA-AQ is mainly due to partial dissolution of the polymer in the electrolyte. The rate performance represents the ability of rapidly charging and discharging of the electrode active material. In order to evaluate the rate performance of DPA-AQ and PDPA-AQ, the chargedischarge tests were carried out at current densities from 0.1C to 2C and then back to 0.1C, and the results are shown in Fig. 3c and d. From 0.1C to 2C, the discharge capacity of DPA-AQ decreases from 129 mAh g1 to 38 mAh g1, and when the current density gets back to 0.1C, the capacity can recover to 71 mAh g1. For the polymer PDPA-AQ, the capacity decreases from 151 mAh g1 to 50 mAh g1, and when discharged again at 0.1C, the capacity can
recover to 106 mAh g1. Consistent with the results of the cycling performance, the polymer PDPA-AQ has better rate performance than the monomer DPA-AQ. The above analysis demonstrates that the polymer PDPA-AQ has a superior overall electrochemical performance than the monomer DPA-AQ. In order to further elucidate the electrochemical properties of the polymer PDPA-AQ, the surface morphologies of the cathodes before and after 100 cycles were tested. Fig. 4 shows the surface morphologies of the cathodes observed by SEM. For the DPA-AQ electrodes, DPA-AQ and conductive agent were uniformly distributed in fine particles, indicating that DPA-AQ and conductive agent have good compatibility. After 100 cycles of charge and discharge, the morphology was almost unchanged compared to that before the cycle, and no structural change or destruction was observed. Similar to the DPA-AQ electrode, the morphology of the PDPA-AQ electrodes before and after 100 cycles was also hardly changed. This shows that PDPA-AQ is very stable, has a low solubility in the electrolyte, and therefore has good cycle stability. Except observing the surface morphologies, we also tested the electrochemical impedance spectroscopies (EIS) of the polymer electrodes before and after 100 cycles (Fig. 5). The semicircle represents the charge transfer resistance (Rct) in the electrochemical reaction process. The inclined line is associated with the Warburg impedance (Zw), representing the Liþ ions diffusion inside the bulk phase of the PDPA-AQ materials. It can be seen from Fig. 5 that both the charge transfer resistance and the lithium ion diffusion resistance before cycles are lower than that after 100 cycles, which
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Fig. 4. SEM images of cathodes (a) DPA-AQ before cycle; (b) DPA-AQ after 100 cycles; (c) PDPA-AQ before cycle; (d) PDPA-AQ after 100 cycles.
twice of PTPA. CR2025 coin-type cells were assembled with PDPAAQ or DPA-AQ as cathode active materials and lithium metal as anodes. The initial discharge specific capacity of polymer PDPA-AQ is 159 mAh g1, which is higher than that of monomer DPA-AQ. Moreover, the cycle stability and rate performance of the polymer PDPA-AQ are better than the monomer DPA-AQ because of the low solubility of the polymer PDPA-AQ in the electrolyte. The specific capacity of the polymer PDPA-AQ is higher than the theoretical capacity of PTPA, indicating that the design idea of introducing carbonyl compounds into the side chain of PTPA is feasible, and this work provides an effective method for the design of TPA-based cathode materials. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2018.07.062. Fig. 5. Electrochemical impedance spectroscopies (EIS) of the polymer electrodes before and after 100 cycles at 0.1C.
shows that the electrode before cycles has better conductivity. This can also explain the decrease of electrochemical performance after cycles.
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4. Conclusion
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The triphenylamine-based polymer PDPA-AQ with anthraquinone unit in the side chain was synthesized by chemical oxidative polymerization, which has low solubility in common organic solvents, and the temperature of mass loss of 5% is 415 C, with high thermal stability. Due to each structural unit of PDPA-AQ containing multiple electrochemically active sites, the polymer PDPA-AQ has a high theoretical specific capacity of 214 mAh g1, which is about
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