Article pubs.acs.org/Macromolecules
Cyclopentadithiophene−Terephthalic Acid Copolymers: Synthesis via Direct Arylation and Saponification and Applications in Si-Based Lithium-Ion Batteries Chun-Feng Yao,† Kuo-Lung Wang,† Hsin-Kai Huang,‡ Yen-Jen Lin,† Yun-Yang Lee,† Chun-Wei Yu,‡ Cho-Jen Tsai,‡ and Masaki Horie*,† †
Department of Chemical Engineering and ‡Department of Material Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *
ABSTRACT: This study investigates conjugated polymers comprising of cyclopentadithiophene and dimethyl terephthalate or terephthalic acid units for use in Si-based lithium-ion batteries. Cyclopentadithiophene tethered with 2-ethylhexyl or triethylene glycol side chains and 2,5-dibromoterephthalate are copolymerized via palladium-complex-catalyzed direct arylation. The dimethyl ester groups in the dimethyl terephthalate unit are converted to the carboxyl groups via saponification. The polymers are mixed with Si nanoparticles to fabricate an anode electrode for use in lithium-ion batteries. Compared with polymers before saponification, the batteries with the electrodes incorporating the saponified polymers have greater specific capacities of up to 2500 mA h g−1 (for total anode weight) at a second cycle and greater stability. Electrolyte uptake test and scanning electron microscopy are used to verify the battery performance.
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INTRODUCTION
drastic volume change of Si, have also been used as binders.23−26 However, in these traditional binder systems, carbonaceous materials must be added to construct an electronic and ionic pathway for the redox reaction, which complicates the optimization of the material composition during battery fabrication. In addition, the formation of complicated interfaces between Si, polymer binders, and carbonaceous materials in the ternary system affects the degradation processes of LIBs. Thus, it is crucial to simplify the material system by using different functional polymer binders. Functionalized conjugated-polymer binders, which have electronic and lithium-ion conductivities and bond efficiently to Si materials, are desirable because these polymers allow such carbonaceous materials to be omitted from the anode electrode. Liu and co-workers reported the use in LIBs of conjugated-polymer binders consisting of random copolymers of fluorene, fluorenone, and methylbenzoic ester units with Si (or Sn) anode materials and achieved high performance (e.g., greater than 3000 mAh g−1 for 50 cycles).27−30 To date, numerous alternating conjugated copolymers have been applied to organic electronic devices such as organic photovoltaic (OPV) cells and organic field-effect transistors (OFETs).31−36 These conjugated polymers and the abovementioned conjugated-polymer binders were synthesized by polycondensations such as palladium-complex-catalyzed Suzuki
Lithium-ion batteries (LIBs) are promising for energy storage in portable electronic devices such as smartphones, tablets, and PCs and for electric vehicles.1,2 Some important advantages of LIBs are a large storage capacity, low self-discharge, and long charge−discharge cycling lifetime without significant memory effect.3−5 High-performance LIBs have been achieved by using carbonaceous materials, such as graphite, as anode electrode materials.6 To increase the capacity of LIBs, silicon (Si)-based materials have recently received great interest because Si has a gravimetric capacity (4200 mAh g−1) that is an order of magnitude greater than current graphite anodes (372 mAh g−1).7−11 However, Si-based materials undergo ca. 400% volume expansion during lithiation and delithiation, leading to degradation of the anode electrode and a resulting rapid fade of capacity.12,13 To reduce the effect of the volume expansion of Si, various nano- and microstructural Si anode materials (e.g., Si nanowires,14 Si thin film,15 hollow Si nanospheres,16 Si nanotubes,17 and porous Si18) have been used for LIBs. In addition, functional polymers, which do not participate in the electrochemical reactions, have been used as polymer binders to suppress degradation of Si-based anode electrodes. For example, linear polymers of carboxymethyl cellulose (CMC),19 poly(acrylic acid) (PAA),20 and poly(vinyl alcohol)21 have been used as polymer binders for Si-based materials, leading to improvements in cycling life due to interactions between Si and carboxyl or hydroxyl groups in polymer binders.22 Cross-linked polymers, which have mechanical durability to endure the © XXXX American Chemical Society
Received: June 28, 2017 Revised: August 8, 2017
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DOI: 10.1021/acs.macromol.7b01355 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
obtain P[CPDT(EH)-TPA] and P[CPDT(EG)-TPA], respectively. These polymer binders were synthesized via directarylation polymerization followed by saponification. The polymers were then used as conducting-polymer binders for Si nanoparticles for use as anode electrodes in LIBs. As illustrated in Figure 1, the polymers with two carboxyl groups in TPA are expected to bind more strongly to Si nanoparticles than previously reported P[CPDT(EH)-B)] with a monocarboxyl group. Furthermore, due to interchain hopping of lithium ions, [CPDT(EG)-TPA] with triethylene glycol monomethyl ether (EG) groups is expected to have higher ionic conductivity than polymers with nonpolar 2-ethylhexyl (EH) groups.
or Stille coupling polymerizations between aryl dihalide and aryldiboronic acid or aryldistannyl monomers.28,32−36 However, these reactions have some inherent drawbacks. First, the synthesis process connecting the monomer precursors with diboronic acid or distannyl is time-consuming. Second, the purification process presents difficulties owing to the instability of these functional groups during column chromatography. Finally, some of the reagents, and in particular the organostannyls used in Stille coupling, are highly toxic. Therefore, environment-friendly and potentially scalable reactions that require only simple processing is a key issue for the synthesis of conjugated-polymer binders and mass production of LIBs that incorporate them. To address these challenges, C−H direct arylation37−47 was used to synthesize conjugated copolymers, some of which were used as active materials in OPVs and OFETs.35,36,48 In addition, we recently communicated the synthesis of the conjugatedpolymer P[CPDT(EH)-B)] (Figure 1), which consists of 4,4-
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EXPERIMENTAL SECTION
Materials and General Method. All reactions were conducted under a dry nitrogen atmosphere in a Schlenk tube. CPDT(EH) was obtained by modification of a published method.50 Pd(OAc)2 and pivalic acid were purchased from Sigma-Aldrich and used without further purification. Before polymerization, monomers were purified by size exclusion chromatography by using a JAI LC-9204 with a column JAIGEL-1H-40 eluted with HPLC-grade chloroform. 1H NMR spectra were recorded at room temperature by using a VarianUnity INOVA-500 spectrometer. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of polymer powders were recorded by using a Bruker VERTEX 80v spectrometer. Matrixassisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectra were obtained by using a Bruker Autoflex III TOF/TOF equipped with a nitrogen laser (337 nm) in positive-ion mode. The molecular weights of the polymers were determined by gel permeation chromatography (GPC) using a JASCO 870 UV detector, a 880 pump, and American Polymer Standards Corporation Ultrastyragel columns (Serial 2-15-89 A, B, and C) eluted with tetrahydrofuran (THF) as the solvent. Polystyrenes were used as standards. The molecular weight of P[CPDT(EG)-TPA] was determined by GPC using a JAI LC-9204 with a column JAIGEL-3H-40 eluted with chloroform. GPC elution curves are shown in Figure S1 of the Supporting Information. UV−vis absorption spectra of polymers in THF solutions and of films spincoated on glass substrates were recorded at room temperature by using a JASCO V-630 UV−vis spectrophotometer. The thin films were spincoated by using 30 μL of a polymer solution (2 mg of polymer in 2 mL of THF) on 1.5 cm × 4.0 cm glass substrates. The electric resistance of polymer films was measured using DY2300 electrochemical analyzer with a scan rate of 2 V s−1. Synthesis. Dimethyl-2,5-dibromoterephthalate. The monomer was synthesized by using a method modified from the literature.51 2,5Dibromoterephthalic acid (5.00 g, 15.4 mmol) was dissolved in methanol, and the solution was cooled to 0 °C. Next, thionyl chloride (9.90 g, 0.416 mol) was added dropwise. The mixture was refluxed for 8 h under a nitrogen atmosphere. The solution was transferred into a separatory funnel, and 100 mL of water was added. The organic layer was extracted by using ethyl acetate, dried over MgSO4, and filtered. After the solvent was removed in a vacuum, the residual solid was recrystallized from methanol to give white crystals (3.50 g, 65%). 1H NMR (CDCl3, 500 MHz, RT): δ 8.03 (s, 2H, aromatic), 3.94 (s, 6 H, ester). 4,4-Di[2-[2-(2-methoxyethoxy)ethoxy]ethyl]-4H-cyclopenta[2,1b:3,4-b′]dithiophene [CPDT(EG)]. The monomer was synthesized by using a method modified from the literature.52 Cyclopenta[2,1-b:3,4b′]dithiophene (1.00 g, 5.60 mmol) was dissolved in DMSO (30 mL), and the solution was purged with nitrogen for 30 min. NaI (60 mg, 0.40 mmol) and KOH (1.26 g, 22.4 mmol) were added, and the solution was stirred for another 10 min. 1-Bromo-2-(2-(2-methoxyethoxy)ethoxy)ethane (3.43 g, 11.8 mmol) was added, and the solution was stirred at room temperature for 24 h in dark. The mixture was extracted with diethyl ether (100 mL × 4). The organic layers were combined and washed with brine (100 mL), dried over MgSO4, filtered, and concentrated in a vacuum. The resulting crude brown oil
Figure 1. Chemical structure of polymer binders and schematic illustration of polymer binder−Si nanoparticle system used as anode in lithium-ion batteries (LIBs).
bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene [CPDT(EH)] and benzoic acid (B) units, via direct-arylation polymerization followed by saponification reaction.49 The polymer was used as a conductive polymer binder for Si nanoparticles for use as an anode electrode in LIBs and exhibited efficient binding ability to the Si nanoparticles in LIBs. However, no detailed synthesis or characterization of the polymer was reported. Modifying the diverse chemical structures of these polymer binders, and particularly accounting for the effects of side chains, is crucial for obtaining highperformance LIBs. Here, we report the synthesis of a series of conjugatedpolymer binders consisting of CPDT(EH) or 4,4-di[2-[2-(2methoxyethoxy)ethoxy]ethyl]-4H-cyclopenta[2,1-b:3,4-b′]dithiophene [CPDT(EG)] and terephthalic acid (TPA) to B
DOI: 10.1021/acs.macromol.7b01355 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules was purified by flash column chromatography using diethyl ether and acetone (2:1 vol %) as eluent to give CPDT(EG) as a brown oil (1.80 g, 68%). 1H NMR (CDCl3, 500 MHz, RT): δ 7.08 (d, J = 5 Hz, 2H, 2CPDT), 6.91 (d, J = 5 Hz, 2H, 3-CPDT), 3.51−3.49 (m, 4H), 3.44− 3.40 (m, 8H), 3.31−3.26 (m, 10H), 2.97 (t, J = 5 Hz, 4H), 2.19 (t, J = 5 Hz, 4H). P[CPDT(EH)-DMT] (DMT = Dimethyl Terephthalate). CPDT(EH) (500 mg, 1.24 mmol), dimethyl-2,5-dibromoterephthalate (436 mg, 1.24 mmol), and K2CO3 (429 mg, 3.10 mmol) were dissolved in anhydrous N-methyl-2-pyrrolidone (NMP) (6.2 mL) in a Schlenk tube. Pd(OAc)2 (27.9 mg, 0.12 mmol) and anhydrous pivalic acid (38 mg, 0.37 mmol) were added, and the reaction mixture was stirred at 70 °C for 20 h under a nitrogen atmosphere. After cooling to room temperature, the reaction mixture was poured into methanol, and the red precipitate was collected by filtration. The red powder was washed by Soxhlet extraction with ethanol and hexane and extracted with methyl ethyl ketone (MEK). The MEK fraction was concentrated in vacuum to provide a red solid of P[CPDT(EH)-DMT] (120 mg, 25%). 1H NMR (CDCl3, 500 MHz, RT): δ 7.78 (s, 2H, aromatic), 7.03 (s, 2H, 3-CPDT), 3.81 (s, 6H, −OCH3), 1.91 (broad, 4H, CH2), 1.23−0.64 (m, 30H, 2-EH). GPC (polystyrene standards in THF): Mn = 46 400, Mw/Mn = 3.35. UV−vis: λmax = 462 nm in THF, λmax = 462 nm in film. P[CPDT(EH)-TPA]-17k. P[CPDT(EH)-DMT] (Mn = 46 400, 150 mg) was dissolved in a mixture of THF and MeOH (volume ratio = 2:1, 168 mL). 20 M KOH(aq) (15 mL) was added, and the reaction mixture was stirred at 50 °C for 24 h under a nitrogen atmosphere. After cooling, the solution was concentrated in vacuum, and the solution was acidified by adding 5 N HCl(aq) (ca. 90 mL). The powder was collected by filtration, washed with water (50 mL × 10 times), and dried in a vacuum to obtain a red powder of P[CPDT(EH)-TPA]-17k (140 mg, 98%). 1H NMR (THF-d8, 500 MHz, RT): δ 7.91 (s, 2H, aromatic), 7.29 (s, 2H, 3-CPDT), 2.02 (broad, 4H, CH2), 1.29−0.70 (m, 30H, 2-EH). GPC (polystyrene standards in THF): Mn = 17 100, Mw/Mn = 3.27. UV−vis: λmax = 467 nm in THF, λmax = 472 nm in film. P[CPDT(EH)-TPA]-30k. P[CPDT(EH)-TPA]-30k was synthesized by the same method as that used to prepare P[CPDT(EH)-TPA]-17k, using P[CPDT(EH)-DMT] (Mn = 46 400, 150 mg), a mixture of THF and MeOH (volume ratio = 2:1, 168 mL), and 20 M KOH(aq) (15 mL). The reaction was carried out at 50 °C for 12 h to yield red powder of P[CPDT(EH)-TPA]-30k (138 mg, 97%). GPC (polystyrene standards in THF): Mn = 30 500, Mw/Mn = 3.32. UV−vis: λmax = 469 nm in THF, λmax = 475 nm in film. P[CPDT(EH)-TPA]-60k. P[CPDT(EH)-TPA]-60k was synthesized by the same method as that used to prepare P[CPDT(EH)-TPA]-17k, using P[CPDT(EH)-DMT] (Mn = 69 300, 150 mg), a mixture of THF and MeOH (volume ratio = 2:1, 168 mL), and 20 M KOH(aq) (15 mL). The reaction was carried out at 50 °C for 12 h to yield red powder of P[CPDT(EH)-TPA]-30k (139 mg, 97%). GPC (polystyrene standards in THF): Mn = 61 400, Mw/Mn = 3.27. UV−vis: λmax = 474 nm in THF, λmax = 478 nm in film. P[CPDT(EG)-DMT]. P[CPDT(EG)-DMT] was synthesized by the similar method to that used to prepare P[CPDT(EH)-TPA]-17k, using CPDT(EG) (200 mg, 0.424 mmol), dimethyl-2,5-dibromoterephthalate (149 mg, 0.424 mmol), K2CO3 (147 mg, 1.06 mmol), anhydrous NMP (2.1 mL), Pd(OAc)2 (9.5 mg, 0.042 mmol), and anhydrous pivalic acid (13 mg, 0.13 mmol). The reaction was carried out at 70 °C for 20 h. The polymer was washed by Soxhlet extraction with methanol ad acetone and extracted with CHCl3 to yield red powder of P[CPDT(EG)-DMT] (25 mg, 21%). 1H NMR (CDCl3, 500 MHz, RT): δ 7.84 (s, 2H, aromatic), 7.03 (s, 2H, 3-CPDT), 3.83 (s, 6H, −OCH3), 3.57−3.14 (m, 26H, EG), 2.30 (broad, 4H, CH2). GPC (polystyrene standards in THF): Mn = 36 000, Mw/Mn = 3.74. UV−vis: λmax = 463 nm in THF, λmax = 489 nm in film. P[CPDT(EG)-TPA]. P[CPDT(EG)-TPA] was synthesized by the same method as that used to prepare P[CPDT(EH)-TPA]-17k, using P[CPDT(EG)-DMT] (150 mg) a mixture of THF and MeOH (volume ratio = 2:1, 252 mL), and 20 M KOH(aq) (15 mL). The reaction was carried out at 60 °C for 12 h to yield red powder of
P[CPDT(EG)-TPA] (142 mg, 99%). 1H NMR (CDCl3, 500 MHz, RT): δ 7.92 (s, 2H, aromatic), 7.30 (s, 2H, 3-CPDT), 3.50−3.24 (m, 26H, EG), 2.30 (broad, 4H, CH2). GPC (polystyrene standards in CHCl3): Mn = 15 100, Mw/Mn = 1.43. UV−vis: λmax = 466 nm in THF, λmax = 471 nm in film. Battery Fabrication. Lithium metal was used for the counter electrode. Silicon electrodes were prepared from nanosilicon powder (Alfa aesar, particle diameter ≤50 nm, specific surface area 70−80 m2 g−1) and the conjugated-polymer binders. The Si nanoparticles and the polymer binder (weight ratio = 2:1) were mixed in a 4 mL sample bottle with adequate THT or NMP. This solution or dispersion was mixed by using a planetary centrifugal mixer (Thinky Mixer ARM310) to obtain a well-mixed slurry. The resulting slurry was coated onto copper foils by using a 100 μm blade. The electrode films were dried in a vacuum oven at 80 °C for 12 h. These films were cut to form disks with a 13 mm diameter. Loading of the electrode films ranges from 0.6 to 0.7 mg cm−2. CR2032 coin cells (width = 20 mm, height = 32 mm) were assembled by stacking a porous poly(propene) separator and a lithium foil counter electrode in a glovebox (UNILAB-B, MBraun) under an argon atmosphere (H2O value
