Article pubs.acs.org/Macromolecules
Charge−Discharge with Rocking-Chair-Type Li+ Migration Characteristics in a Zwitterionic Radical Copolymer Composed of TEMPO and Trifluoromethanesulfonylimide with Carbonate Electrolytes for a High-Rate Li-Ion Battery Hiroshi Tokue, Tomoaki Murata, Haruka Agatsuma, Hiroyuki Nishide,* and Kenichi Oyaizu* Department of Applied Chemistry, Waseda University, Tokyo 169-8555, Japan S Supporting Information *
ABSTRACT: Redox-active copolymer containing organic robust radical, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), and charge neutralizing anion, trifluoromethanesulfonylimide (TFSI−) was synthesized as a cathode material of a Li-ion battery. The copolymer, poly(2,2,6,6-tetramethylpiperidin-1-oxy-4-yl methacrylate-co-styrenesulfonyl(trifluoromethanesulfonyl)imide) (P(TMA-co-TFSI)), was designed to give rise to the Li+ migration during its charge−discharge process, based on the selfcharge compensation of TEMPO with TFSI− bound to the polymer chain in a widely used electrolyte system for Li-ion battery, organic carbonate mixtures. Copolymerization was performed to achieve efficient self-charge compensation with uniformly distributed TFSI− units. The P(TMA-co-TFSI) layer electrode exhibited reversible redox reaction at 0.73 V vs Ag/AgCl. Electrochemical measurements combined with quartz crystal microbalance analysis evidenced that the redox reaction involved the Li+ migration in binary system of ethylene carbonate and diethyl carbonate. A test cell fabricated with the P(TMA-co-TFSI) cathode exhibited high discharging voltage of 3.7 V and high-rate charge−discharge capability at 30 C (i.e., full charging in 2 min). groups allowed high specific capacity of 466 mAh g−1 supported by the surface Faradaic reaction on reduced graphene oxides17 and swift charge−discharge within 1 min (>60 C) based on the fast electron self-exchange reaction between nearby TEMPO groups.18,19 Meanwhile, the reversible one-electron oxidation to produce cations from neutral groups (p-type redox reaction) in TEMPO-substituted polymers changes salt concentration during charge−discharge due to the anion migration for charge neutralization of the N-oxoammonium cation. The change in salt concentration makes excess amount of electrolyte salt necessary for ionic conduction and might bring about safety problem caused by the lithium deposition in contrast to the “rocking-chair-type” Li+ migration20−22 observed in inorganic transition-metal oxides such as LiCoO2,23 LiMn2O4,24 and LiFePO4.25 in Li-ion batteries. The rocking-chair-type Li+ migration promises the constant concentration of the electrolyte during the charge−discharge process, which enables a reduction of the electrolyte amount avoiding precipitation and
1. INTRODUCTION The demand for enhanced charge-storage system has been increased because of a rapid development of electrical vehicles and a widespread use of portable devices.1,2 Since a series of polymers containing redox-active groups such as ferrocene,3,4 thianthrene,5 triphenylamine,6 carbazole,7,8 and anthraquinone9 were discovered, their charge−discharge properties have been investigated to incorporate the unique characteristics as an organic electrode-active material: flexible, nonexplosive and nontoxic properties into a secondary battery.10−12 However, the performance of the organic-based battery has often been limited by low output voltage9 and slow charge transport through the polymer layer5 preventing swift charge−discharge. Recently, we have developed a new class of polymer electroactive materials containing organic robust radicals, such as 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)13 and 2,2,5,5tetramethyl-1-pyrrolidinoxy (PROXYL),14 per repeating unit, which are so-called radical polymers. Organic-based battery composed of TEMPO-substituted polymer cathode and Li anode demonstrated relatively high output voltage of ∼3.6 V resulting from the reversible redox reaction between a nitroxide radical and an N-oxoammonium cation at a positive potential.15,16 Moreover, the densely populated TEMPO © XXXX American Chemical Society
Received: November 5, 2016 Revised: February 15, 2017
A
DOI: 10.1021/acs.macromol.6b02404 Macromolecules XXXX, XXX, XXX−XXX
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the carbonate mixture with high voltage, giving rise to an organic Li-ion battery featured by the high power density.
lack of the electrolyte salts. The reduction of the electrolyte consequently improves the energy density in an overall cell. To accomplish the rocking-chair-type charge−discharge characteristics maintaining the excellent features in radical polymers, we have proposed zwitterionic pendant design inspired by self-doping concept in conducting polymers.18,26 Self-charge compensation with anions bound to the polymer chain during the redox process in TEMPO-substituted polymer containing sulfonate anion gave rise to the rocking-chair-type Li+ migration. Alternating tendency in poly(TEMPO-methacrylate-stat-vinylsulfonic acid) helped the self-charge compensation with the sulfonate anion in an aqueous electrolyte.26 In this context, replacement of vinyl sulfonic acid with more hydrophobic styrenesulfonate was attempted to allow the selfcharge compensation in a nonaqueous electrolyte toward the Li-ion battery,18 taking advantage of the enhanced swellability. However, the self-charge compensation was still being limited in organic carbonate mixtures composed of ethylene, dimethyl and diethyl carbonates (EC, DMC, and DEC, respectively) which are currently employed as the electrolyte solvents in the Li-ion battery owing to their high ionic conductivity, high anodic stability and low toxicity.27 We anticipated that the Li+ migration during the redox process should be achieved even in the organic carbonate mixture by incorporating trifluoromethanesulfonylimide (TFSI−) as a charge neutralizing anionic group into the TEMPO-substituted polymers. Lithium bis(trifluoromethanesulfonyl)imide has been regarded as one of the promising electrolytes in the Li-ion battery28−30 because of the strong delocalization of the negative charge,31,32 resulting in high degree of ionic dissociation and consequent high ionic conductivity.33 Polymers bearing TFSI− group have recently been investigated as a polymer electrolyte for safe battery.34−36 They showed excellent single-ion conductivity,34 thermal stability35 and mechanical property36 compared to the conventional sulfonate-bearing polymers. The hydrophobic nature in the TFSI− group with excellent dissociation properties in organic solvents is expected to facilitate the selfcharge compensation (Scheme 1). In this paper, redox
2. EXPERIMENTAL SECTION 2.1. Materials. Sodium 4-vinylbenzenesulfonate was purchased from Sigma-Aldrich. Vapor grown carbon fiber (VGCF) was purchased from Showa Denko Co. Polyvinylidene difluoride (PVDF) was purchased from Kureha Chemical Co. All other reagents were purchased from Tokyo Chemical Industry Co. and Kanto Chemical Co., Inc. They were used without further purification. All solvents were distilled for purification prior to use. The glassy carbon (GC) electrode was purchased from ALS Co., which was cleaned by sonication with isopropyl alcohol using AIWA AU-16C ultrasonic cleaner before use. The indium tin oxide (ITO)/glass electrode was obtained from Asahi Glass, which was cleaned by plasma etching prior to use. 2.2. Apparatus. 1H and 13C NMR spectra were recorded on a JEOL JNM-LA 500 and Bruker AVANCE 600 spectrometers with chemical shifts downfield from tetramethylsilane (TMS) as an internal standard. Infrared spectra were obtained by means of a JASCO FT-IR 6100 spectrometer. ESI-MS spectroscopy was done for the negative ion mode using the ion-trap Thermo Quest FINNIGAN LCQ DECA. Molecular weight measurements were performed by gel permeation chromatography using TOSOH HLC 8220 instrument with DMF containing 0.1 M LiCl as an eluent. The film thicknesses were estimated using a KLA Tencor P6̅ contact stylus profiler. ESR spectra were recorded using a JEOL JES-TE200 spectrometer with a 100 kHz field modulation frequency and a 0.1 mT width. The magnetization and magnetic susceptibility were measured using Quantum Design MPMS-7 SQUID (superconducting quantum interference device) from 10.0 to 300 K in a 0.5 T field. 2.3. Electrochemical Measurements. Electrochemical analyses were carried out using an ALS electrochemical analyzer 760 EW. A GC plate (2.5 cm × 2.5 cm) was employed as a counter electrode, and a commercial Ag/AgCl was employed as a reference electrode. The formal potential of the ferrocene/ferrocenium couple was +0.45 V vs the Ag/AgCl electrode. P(TMA-co-TFSI) solution in acetonitrile/ DMF = 1/1 (v/v) was spin coated to form a layer with a thickness of ca. 150 nm on a GC electrode using MIKASA 1H-D3 spin coater for estimation of diffusion coefficients Dapp in polymer layers and for ac impedance analysis. Dapp were determined from the slope of Cottrell plots obtained from potential-step chronoamperometry according to i = nFACT(Dapp/πt)1/2, where n, F, A, CT, and t are numbers of electrons, the Faraday constant, the working electrode area, the concentration of the pendant redox site and time, respectively. AC impedance analyses were performed using a Zahner IM6/ZENNIUM system. The frequency range was set from 1.0 × 106 to 1 × 102 Hz with an ac amplitude of 10 mV. The bc bias voltage was set at the halfwave potential. Thereby, 50% of the nitroxide radical units were supported to be oxidized to the oxoammonium cation at the bc vias voltage. A Hokuto Denko charge−discharge analytical instrument (HJ1001SM8) was used to measure the electrical charge−discharge characteristics of a test cell. The cyclability performance of the test cell was examined by repeated charge−discharge galvanostatic cycles at different current densities. The cutoff potentials were 2.5−4.2 V vs Li/ Li+. The polymers on the substrates were swollen and potassium cation was exchanged to lithium cation through repeated scan of electrode potential by cyclic voltammetry using lithium salts as an electrolyte prior to electrochemical measurements. 2.4. Electrochemical Quartz Crystal Microbalance (EQCM) Analysis. P(TMA-co-TFSI) solution in acetonitrile/DMF = 1/1 (v/v) was spin coated to form a layer with a thickness of ca. 50 nm on a GC QCM electrode with the diameter φ = 0.5 cm. Change in the resonant frequency Δf was measured simultaneously with the cyclic voltammetry performed at a scan rate of 10 mV s−1. A SEIKO EG&G QCA 922 was employed to obtain the resonant frequency. The obtained Δf was converted to the mass Δm changing during the redox reaction of the polymer layer by employing Sauerbrey’s equation:37
Scheme 1. Mechanism of Li+ Migration and Self-Charge Compensation with Trifluoromethanesulfonylimide (TFSI−) Group in the Copolymer during the Redox Process
copolymer containing both TEMPO and TFSI− groups, poly(2,2,6,6-tetramethylpiperidin-1-oxy-4-yl methacrylate-costyrenesulfonyl(trifluoromethanesulfonyl)imide) (P(TMA-coTFSI)), was synthesized with controlled compositions to combine the Li+ migration characteristics in the organic carbonate mixture and fast charge−discharge capability at a positive potential. The unprecedented redox properties led to the rapid rocking-chair-type charge−discharge behavior18,26 in B
DOI: 10.1021/acs.macromol.6b02404 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Table 1. Radical Polymerization of TMA Precursor and Styrenesulfonyl(trifluoromethanesulfonyl)imide unit ratiob (mol %)
feed ratio (mol %)
a
Fineman−Rossc
entry
1
2
yield (%)
Mn (×10 )
Mw/Mn
m
n
F /f
F( f − 1)/f
1 2 3 4 5 6 7
83 75 67 50 33 25 17
17 25 33 50 67 75 83
12 34 32 24 19 27 25
1.8 1.1 1.3 1.0 1.0 0.73 0.98
1.8 1.7 1.9 1.8 1.6 1.7 1.5
83 74 65 48 32 23 17
17 26 35 52 67 77 83
5.2 3.1 2.1 1.1 0.51 0.36 0.20
4.0 2.0 0.93 −0.063 −0.53 −0.76 −0.80
a
5
2
Calculation on the weight base. bDetermined by 1H NMR. cF = [1]/[2], f = m/n.
Scheme 2. Synthesis of Radical Copolymer P(TMA-co-TFSI)
Δf = −
2f0 2 1/2
A(ρ0 μ0 )
tion (10 000 rpm for 10 min) was performed to yield PTFSI as a yellow solid (0.94 g, 2.66 mmol). Yield: 94%. Mn = 2.9 × 104, PDI = 1.2. 1H NMR (500 MHz, D2O, 25 °C, TMS): δ = 7.59 (br, 2H; ph CH2), 6.61 (br, 2H; ph CH2), 1.44 (br, 3H; vinyl CH). IR (KBr, cm−1): 1334 (vSO), 1261−933 (vC−F). 2.7. Copolymerization of 4-Methacryloyloxy-2,2,6,6-tetramethylpiperidin-1-oxyl and Potassium 4-Styrenesulfonyl(trifluoromethanesulfonyl)imide. Copolymerization of both monomers was performed at feed molar ratios of 5:1, 3:1, 2:1, 1:1, 1:2, 1:3, and 1:5. The copolymerizations were performed in DMF for 2 h at 70 °C. The total concentration of the monomers was 1.0 mol L−1 with AIBN (1.0 mmol L−1). The resulting polymer was precipitated from excess amount of diethyl ether twice and then purified by dialysis and ultracentrifugation (10 000 rpm for 20 min, three times) with DMF to remove any excess unreacted monomer. Yields and molecular weights of all copolymers are listed in Table 1. Characterization of the polymer synthesized via copolymerization at a monomer feed ratio of 1:1 was performed as follows. 1H NMR (500 MHz, C3D7NO, 25 °C, TMS): δ = 7.56 (br, 2H; ph CH2), 7.03 (br, 2H; ph CH2), 4.95 (br, 1H; piperidine methine), 1.73 (br, 2H; CH), 1.05 (br, 4H; CH2). IR (KBr, cm−1): 2973 (vC−H), 1716 (vCO), 1331 (vSO), 1253−933 (vC−F). 2.8. Oxidation of the Precursor Polymers. The precursor polymer 3 (100 mg) was dissolved in DMF (5 mL) and mCPBA (5 equiv for 1 moiety) was added, which was then kept stirring for 4 h at room temperature. The resulting polymer was precipitated from excess amount of DMF twice and collected by ultracentrifugation (10 000 rpm for 20 min, three times). The product was then thoroughly dried in a vacuum oven. Characterization of the resulting free radical was described in the Results and Discussion. IR (KBr, cm−1): 2981 (vC−H), 1698 (vCO), 1307 (vSO), 1237−1021 (vC−F). 2.9. Preparation of Carbon Composite Electrodes and Cell Fabrication. PVDF (10 mg) was dissolved in N-methyl-2-pyrrolidone (NMP) (5 mL). PTFSI, PTMA/PTFSI composite (molar ratio of 1/3 based on monomer unit) or P(TMA-co-TFSI) (m/n = 1/1 or 1/3 from entry 4 or 6 in Table 1, respectively) (10 mg) and VGCF (80 mg) were added into the PVDF solution. After the addition of NMP (5 mL) into the mixture, they were milled for 20 min at 30 s−1 using FRITSCH PULVERISETTE 23 ball mill grinder. The resulting composite was pasted on an ITO electrode or an aluminum foil and then dried under vacuum for 4 h at 70 °C. A coin cell was fabricated by stacking the composite cathode with a separator film and lithium metal as the anode.
Δm (1)
where f 0, A, ρ0, and μ0 are the fundamental frequency of the QCM (9.1 MHz), the electrode area (0.20 cm2), the density of quartz (2.7 g cm−3) and the shear modulus of quartz (3.0 × 10−11 dyn cm−2), respectively. 2.5. Synthesis of Potassium 4-Styrenesulfonyl(trifluoromethanesulfonyl)imide. Potassium 4-styrenesulfonyl(trifluoromethanesulfonyl)imide was synthesized according to the method described in a previous report,34 with slight modifications as follows. The monomer precursor, 4-styrene sulfonyl chloride, was synthesized employing sodium 4-vinylbenzenesulfonate and oxalyl chloride as starting materials. The condensation reaction of the monomer precursor with trifluoromethanesulfonamide was then carried out, and the product was neutralized with corresponding carbonate salt. See Supporting Information for details. Yield: 34%. 1H NMR (500 MHz, D2O, 25 °C, TMS): δ = 7.84 (d, 3J(H,H) = 8.5 Hz, 2H; ph CH2), 7.51 (d, 3J(H,H) = 8.5 Hz, 2H; ph CH2), 6.77 (dd, 3 J(H,H) = 11.0, 18.0 Hz 3H; vinyl CH), 5.91 (d, 3J(H,H) = 17.5 Hz, 1H; vinyl CH2); 5.34 (d, 3J(H,H) = 11.5 Hz, 1H; vinyl CH2); IR (KBr, cm−1): 1400 (vSO), 1329−989 (vC−F); ESI-MS (m/z): calcd 314.3; (M−) found 314.0. 2.6. Homopolymerizations of 4-Methacryloyloxy-2,2,6,6tetramethylpiperidin-1-oxyl and Potassium 4-Styrenesulfonyl(trifluoromethanesulfonyl)imide. Poly(4-methacryloyloxyTEMPO) (PTMA), was synthesized by anionic polymerization according to a procedure described in the literature.38 Yield: 54%. Mn = 7.1 × 104, polydispersity index (PDI) = 1.0. The radical density was estimated to be 2.37 × 1021 unpaired electrons g−1 (0.95 radicals per monomer unit) by SQUID measurement using the Curie plots and the values for saturated magnetization. IR (KBr, cm−1): 2976 (vC−H), 1726 (v CO), 1147 (vC−O). See Supporting Information for experimental details. Poly(4-styrenesulfonyl(trifluoromethanesulfonyl)imide) (PTFSI) was synthesized by freeradical polymerization according to the previous literature,34 with slight modifications as follows. Potassium 4-styrenesulfonyl(trifluoromethanesulfonyl)imide (1.0 g, 2.83 mmol) and ammonium peroxodisulfate (10 mg, 4.38 mmol) were added to H2O (20 mL) and stirred for 24 h at 80 °C under nitrogen atmosphere. After evaporation, the resulting polymer was dissolved in acetonitrile and precipitated from excess amount of THF twice. The ultracentrifugaC
DOI: 10.1021/acs.macromol.6b02404 Macromolecules XXXX, XXX, XXX−XXX
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3. RESULTS AND DISCUSSION 3.1. Synthesis of P(TMA-co-TFSI). P(TMA-co-TFSI) was synthesized as shown in Scheme 2. Since swelling in an organic carbonate mixture is necessary to perform fast redox reaction for charge−discharge including the charge neutralization process with the fixed anions in the copolymer, hydrophobic styrene and methyl methacrylate chain structures were employed. Copolymerization of 2,2,6,6-tetramethyl-4-piperidyl methacrylate and potassium 4-styrenesulfonyl(trifluoromethanesulfonyl)imide and subsequent oxidation of piperidine moiety to produce the nitroxide radical were performed under the conditions mentioned in the Experimental Section. Diffusion-ordered spectroscopy (DOSY) NMR of P(TMA-co-TFSI) synthesized via copolymerization at a monomer feed ratio of 1:1 (m/n = 1/1 from entry 4 in Table 1) is shown in Figure 1. All proton peaks derived from each monomer unit were recorded with the same diffusion constant, which evidenced the formation of the copolymer.
hyperfine split trimodal spectrum for TEMPO due to the oxygen-centered nitroxide unpaired electron.39 The unimodal signal indicated densely populated nitroxide radicals bound to the polymer chain. The g-value measured in the ESR spectrum (g = 2.0054) was smaller than those obtained for TEMPO (g = 2.0062) under the same conditions. It can be ascribed to the difference of environmental polarity40 of the unpaired electrons, induced by the nearby TFSI− group in the random copolymer. The radical density of the copolymer was estimated to be 7.57 × 1020 unpaired electrons g−1 by SQUID measurement (0.78 radicals per monomer unit based on the composition determined by 1H NMR). Excess amount of the charge neutralizing anion moiety decreases the theoretical redox capacity calculated from the molecular weight per radical site and the sparsely populated TEMPO moiety impedes the electron self-exchange reaction, which deteriorate the power characteristics. Meanwhile, excess amount of TEMPO moiety results in the redox reaction involving the charge neutralizing process by electrolyte anions, which is ascribed to the stoichiometric imbalance. Therefore, we focused on P(TMA-co-TFSI) (m/n = 1/1, entry 4) in the investigation of the electrochemical properties. Although the theoretical capacity of the copolymer (48 mAh g−1) is lower than that of corresponding homopolymer, PTMA (111 mAh g−1), due to the presence of the electro-inactive TFSI− moiety, the charge storage capacity in an overall cell can be increased because the copolymer contains not only charge storing TEMPO but also charge neutralizing TFSI− before charging and it contributes to minimize the electrolyte amount. 3.2. Electrochemical Properties of P(TMA-co-TFSI). Prior to the electrochemical measurements, the countercation of the fixed TFSI−, the potassium cation, in the P(TMA-coTFSI) layer on the substrate was exchanged to the lithium cation (see Experimental Section for details), so that the influences of the potassium cations as the counterion on the electrochemical properties were removed. A negligible influence of a trace amount of the potassium cation on the charge− discharge performance allowed us to replace the ion exchange process in the synthesis of the monomer35 with the simple electrochemical pretreatment. Ion migration during the redox process of P(TMA-co-TFSI) was investigated by electrochemical quartz crystal microbalance (EQCM). Cyclic voltammogram obtained from the layer of P(TMA-co-TFSI) on a GC QCM electrode exhibited a reversible redox wave at E1/2 = 0.73 V vs Ag/AgCl due to the p-type redox reaction at the TEMPO radical (Figure 2a). The separation of the anodic and cathodic peak potentials was 108 mV at a sweep rate of 10 mV s−1. The narrow peak-to-peak separation was suggestive of the rapid charge transport through the layer. The Li + compensating for TFSI− anion in the P(TMA-co-TFSI) layer was excluded for electroneutrality during the oxidation of the polymer as shown in Figure 2b, leading to the decrease of the polymer layer weight and hence the increase in the resonance frequency in the EQCM response (Figure 2a). On the other hand, PTMA which does not contain the charge neutralizing TFSI− anion showed the weight change opposite to the copolymer-modified electrode because of the electrolyte anion insertion for charge neutralization of the N-oxoammonium cation during the oxidation.26 The change in the polymer layer weight provided a decisive diagnosis for the ion migration characteristics of the polymer. The weight change is thus able to be detected as the resonant frequency changes of the QCM electrodes (see Experimental Section for details).
Figure 1. DOSY spectrum of P(TMA-co-TFSI) precursor (m/n = 1/ 1) in C3D7NO.
The unit ratios of P(TMA-co-TFSI) were calculated from the integrated area ratios of the peaks derived from proton bound to 4-carbon position of piperidine ring and those derived from aromatic region’s protons in NMR spectra (Table 1, Figure S1). The NMR spectra were obtained from the precursor polymers before oxidation to produce nitroxide radical. Reactivity ratios of each monomer (r1 = 0.97 and r2 = 1.07) were determined by Fineman−Ross method (Figure S2), which suggested the progress of the random copolymerization. Since TEMPO and TFSI− groups need to be positioned close to each other for the self-charge compensation, P(TMA-co-TFSI) composed of the random monomer sequence is advantageous to demonstrate the Li+ migration compared with the composite of each homopolymer (vide inf ra). After the oxidation of the precursor copolymer, the resulting free radical was characterized by ESR spectroscopy. A DMF solution of the copolymer (m/n = 1/1, entry 4) showed a broad signal even in a dilute solution (Figure S3), in contrast to the D
DOI: 10.1021/acs.macromol.6b02404 Macromolecules XXXX, XXX, XXX−XXX
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the electrochemical system depended on the salt concentration. On the other hand, the insertion process of the electrolyte anion for the charge neutralization during the oxidation of PTMA was thermodynamically disfavored. Therefore, the effect of the increase in the salt concentration upon the half-wave potential (E1/2) is opposite between P(TMA-co-TFSI) and PTMA. The correlation between the salt concentration and E1/2 is expressed by the Nernst equation.18 eqs 2 and 3 are for P(TMA-co-TFSI) and PTMA, respectively (see Supporting Information for derivation of the equations), ° ′ − co − TFSI) + 2.3 E1/2 = E P(TMA °′ E1/2 = E PTMA − 2.3 Figure 2. Cyclic voltammogram obtained for the layer of P(TMA-coTFSI) on a GC QCM electrode in EC/DEC (3/7 = v/v) containing 0.1 M LiClO4 at a scan rate of 10 mV s−1 with a layer thickness of 50 nm.
RT log[Li+]s F
RT log[ClO4 −]s F
(2)
(3)
where E°,́ R, T, and F are the formal potential, gas constant, the absolute temperature and the Faraday constant, respectively, and s represents the solution. E1/2 for the redox reaction of P(TMA-co-TFSI) increased almost linearly with the increase in logarithm of the salt concentration, in contrast to the negative correlation observed for PTMA (Figure S4), corresponding to the difference in the ion migration characteristics between P(TMA-co-TFSI) and PTMA. The slope of the E1/2 increase for P(TMA-co-TFSI) was 23 mV, which was smaller than the theoretical slope calculated from eq 2 (2.3RT/F = 59 mV). It can be explained by the changes in the swollen state of the copolymer layer during the redox process. The negative slope for PTMA was −64 mV, which almost agreed with the theoretical slope calculated from eq 3 (−2.3RT/F = −59 mV). Although we employed carbonate-based electrolytes whose compositions were different between in figures, the same composition ratios between EC and DEC in each comparison promised the rational discussion on the difference of the electrochemical properties of (co)polymers. Note that the rocking-chair-type redox behavior, confirmed by both EQCM measurements and the redox potential shift, was achieved in one of the most appropriate electrolytes for the practical Li-ion battery, binary system of EC/DEC, despite their moderate donor properties (donor numbers for EC and DEC are 16.0 and 16.4, respectively43) to coordinate Li+ for ionic dissociation of the fixed anions in the copolymer. The ionic dissociation in the fixed anionic group was significant for the Li+ migration because the fixed anion should compensate for the Noxoammonium cation prior to an electrolyte anion in the charge balancing process. Therefore, it seems that the high degree of dissociation in TFSI− group even in an organic carbonates44 allowed the Li+ migration in EC/DEC. The comparison of the cyclic voltammograms obtained in an electrolyte with low concentration revealed the contributions of the charge neutralizing anionic group, emphasizing the advantage of the rocking-chair-type behavior, the constant electrolyte concentration during redox reactions.18 The narrower peak-to-peak separation obtained from poly(TEMPO-substituted methacrylate-co-styrenesulfonate) (130 and 360 mV for 0.1 and 0.01 M LiTFSI in DMF, respectively) than those obtained for PTMA (160 and 450 mV for the same electrolytes, respectively) indicated that the fixed anionic group removed the increase of the solution resistance caused by the accumulation of the electrolyte anions in the polymer layer during oxidation, as a result of the rocking-chair-type Li+ migration.
The resonant frequency obtained for the P(TMA-co-TFSI) modified electrode increased during the oxidation accompanied by the potential sweep (Figure 2a), corresponding to the Li+ migration. The magnitude of the frequency change during the redox reaction gave insight into the amount of the inserted and excluded Li+ required for the charge neutralization. The mass change relative to the redox capacity (Δm F/Qcalcd) was 437 g mol−1, which was calculated from the change in the resonant frequency and the capacity obtained by integration of the current coulometrically through the potential sweep in the cyclic voltammogram recorded at the same time. The large deviation of Δm F/Qcalcd from the molar mass of Li+ suggested the solvation of the ion. The solvation number of Li+ in the 1 M LiClO4 EC/DEC (v/v = 1/1) solution was estimated to be 3.1 and 1.1 for EC and DEC, respectively, by Ogumi et al.,41 and the molar mass of solvated Li+ calculated on the basis of these solvation numbers was 392 g mol−1. The molar mass were likely to become larger because the above solvent numbers were estimated with a higher salt concentration than that in the present EQCM measurement and the number of the solvent molecule per Li+ should increase with a decrease in the salt concentration. Indeed, such correlation between the solvation number of Li+ and the salt concentration was reported for various carbonate solvents and their binary mixtures.42,43 These results indicated that N-oxoammonium cations in the copolymer layer were fully compensated by the TFSI− group bound to the polymer chain. Figure 2 displayed a small hysteresis in frequencies between the forward and backward scans, in contrast to the EQCM response obtained with an aqueous electrolyte in our previous report,26 which can be attributed to the relatively low ionic conductivity of the organicbased electrolyte. The result was consistent with the hysteresis behavior observed in the EQCM response of the copolymer containing TEMPO and sulfonate anion in the DMF-based electrolyte in our previous report.18 The ion migration during the redox process was also revealed by the examination of the correlation between the salt concentration and the redox potential under equilibrated conditions. The exclusion process of Li+ from the copolymer layer to the electrolyte solution was thermodynamically favored because of the Li+ concentration difference between them, and thus the driving force arising from the increase in the entropy of E
DOI: 10.1021/acs.macromol.6b02404 Macromolecules XXXX, XXX, XXX−XXX
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of the carbon composite electrode consisting of P(TMA-coTFSI) displayed the single wave at relatively negative potential, 0.72 V. These results suggested that almost all of the redoxactive radical sites throughout the P(TMA-co-TFSI) layer underwent redox reaction involving Li+ migration, which agreed with the results obtained from the EQCM measurement. It can be attributed to the random monomer sequence which allowed all of the TFSI− sites to be within a charge neutralizing range. It is well-known that lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is corrosive toward an aluminum current collector.45 To examine if P(TMA-co-TFSI) can be applied to a cathode-active material without corrosion, the electrode consisting of the PTFSI-carbon composite and the aluminum current collector was prepared. The anodic corrosive current, typically found for polarized aluminum electrodes in the presence of the TFSI− anion,45 was favorably much less significant in the case of the PTFSI composite electrode coated on the aluminum current collector, suggesting that the TFSI− group bound to the polymer chain was not corrosive. The charge−discharge properties of the P(TMA-co-TFSI) cathode was revealed by fabricating a test coin cell composed of a Li anode. The coin cell exhibited high output voltage of 3.67 V based on the two-electrode cyclic voltammogram (Figure 4a), which was considerably higher than the voltage reported for conventional redox polymer cathodes demonstrating the rocking-chair-type Li+ migration in previous literatures.9,12,46,47 The asymmetric redox wave may be ascribed to the slow polymer relaxation processes48 of the oxidized P(TMA-coTFSI) layer. The rigid oxidized layer might retard the charge compensation ion transfer in the polymer layer. On the other hand, the symmetric redox wave was displayed in Figure 3, owing to the efficient formation of the conductive carbon network in the composite. The effective conductive path could avoid the sluggish ion transfer through the thick polymer layer. The two redox peaks in the cyclic voltammogram suggested that the carbon composite electrode produced the redox reactions involving charge compensations with both TFSI− bound to the polymer chain and an electrolyte anion, in contrast to the single wave in Figure 3. It could be caused by the aggregation of the copolymer in the composite. Low viscous DEC could preferentially penetrate into the copolymer placed away from the conductive carbon surface and the ionic dissociation in the fixed anionic group is suppressed by the low-dielectric DEC. Therefore, the optimization of the fabrication procedure of the carbon composite electrode would provide the symmetric single wave due to the uniform conductive path without polymer aggregations. The charge− discharge curves obtained from the test cell showed a plateau voltage near 3.7 V (Figure 4b). It may be added that lower charge−discharge capacities than the theoretical capacity would be caused by polymer aggregation in the composite electrode which could be improved by the optimization of the fabrication process of the composite. At 30 C, the discharging capacity obtained at a current density of 5 C was maintained at 83% (i.e., the rate to fully charge or discharge within 2 min). It should be emphasized that the good rate performance reflected the fast charge storage property of the radical polymers without a negative effect of the fixed anionic moiety. The charge− discharge properties revealed the potential of P(TMA-co-TFSI) and the self-charge compensation approach for high-power Liion battery. Lower formula weight per radical site leads to the larger theoretical redox capacity.14 Therefore, our continuous research will focus on search for compact units not only as the
The influence of the anionic moiety on the charge propagation property was examined by AC impedance analysis and potential-step chronoamperometry. The resistance of charge transfer (Rct) estimated from the diameter of the semicircle in the Cole−Cole plots increased with an increase in the unit ratio of TFSI-substituted styrene (Table 2, Figure S5). Table 2. Electrochemical Parameters of PTMA and P(TMAco-TFSI)
a
polymer
Dapp (10−11 cm2 s−1)
kex (106 M−1 s−1)
Rct (ohm)
PTMA P(TMA-co-TFSI)a P(TMA-co-TFSI)b
3.1 1.3 2.9
1.3 0.6 1.3
37 56 81
m/n = 1/1. bm/n = 1/3.
However, the apparent diffusion coefficient (Dapp) determined from the slope of the Cottrell plots (Figure S6) and electron self-exchange rate constants (kex) calculated from Dapp were not affected by the unit ratio that much, although the density of the redox-active radical site in the copolymer decreased as the incorporation of the anionic moiety. The charge propagation is accomplished by the redox reaction between the radical sites, which involves the mass-transfer process of the charge neutralizing ion. The result was indicative of the facilitated charge propagation by the fixed TFSI− group close to TEMPO without the need of the anion diffusion in electrolyte. The redox behavior of P(TMA-co-TFSI) was compared with that of the PTMA/PTFSI composite (molar ratio of 1/3 based on monomer unit) by cyclic voltammetry. The PTMA/PTFSI composite electrode exhibited a two-step wave at E1/2 = 0.69 and 0.86 V vs Ag/AgCl (Figure 3). The two-step wave
Figure 3. Cyclic voltammograms obtained for the PTMA/PTFSI(black solid curve) and P(TMA-co-TFSI)- (red solid curve) carbon composites (PTMA/PTFSI or P(TMA-co-TFSI)/VGCF/PVdF = 1/ 8/1 (w/w/w)) on the ITO electrode in EC/DEC (1/1 = v/v) containing 0.01 M LiTFSI at a scan rate of 20 mV s−1.
indicated that two kinds of reactions occurred in the composite electrode, which were most likely the redox reaction involving the charge neutralization process by anions bound to the polymer chain and in the electrolyte. The first wave at more negative potential can be assigned to the former reaction in light of the thermodynamically favored Li+ exclusion (vide supra). The two redox potentials corresponded to those observed for the P(TMA-co-TFSI) and PTMA layers, respectively, at the same salt concentration in Figure S4, which was consistent with the assignment of the two-step redox behavior in the composite. Remarkably, cyclic voltammogram F
DOI: 10.1021/acs.macromol.6b02404 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. (a) Cyclic voltammogram of a coin cell employing P(TMA-co-TFSI) and Li as cathode- and anode-active materials, respectively with 0.1 M LiClO4 in EC/DEC (3/7 = v/v) solution at a scan rate of 10 mV s−1. (b) Cycle performance at the charge−discharge rate of 10 C. Relative capacity was calculated using observed capacity, ca. 20 mAh g−1. Inset shows charge−discharge curves at various discharging rates of 5, 10, 20, and 30 C.
Notes
redox-active site but also as the charge neutralizing anion, while maintaining the rocking-chair-type behavior to maximize the energy density.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by Grants-in-Aids for Scientific Research (Nos.24225003, 16K14010) from MEXT, Japan. This work was also supported by The Iwatani Naoji Foundation’s research grant. H.T. acknowledges the Leading Graduate Program in Science and Engineering, Waseda University from MEXT, Japan.
4. CONCLUSIONS TEMPO- and TFSI- bearing copolymer, P(TMA-co-TFSI), was synthesized with controlled compositions. The monomer reactivity ratios determined by Fineman−Ross method suggested the progress of random copolymerization providing the favored monomer sequence for the self-charge compensation. The change in the electrode weight during the redox process and the correlation between the redox potential and the salt concentration proved that the P(TMA-co-TFSI) layer demonstrated the rocking-chair-type Li+ migration. Almost all of the redox-active sites in the P(TMA-co-TFSI) layer underwent the Li+ migration even in the carbonate-based electrolyte, which was ascribed to the strongly delocalized negative charge in the TFSI− group and the closely positioned TEMPO and TFSI− units originating from the random monomer sequence. The P(TMA-co-TFSI) composite electrode performed swift charge−discharge at a high output voltage (∼3.7 V) due to the fast p-type redox reaction at the TEMPO group.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02404. Synthetic procedures, characterizations, additional electrochemical performances, and results of corrosion tests (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*(H.N.) E-mail:
[email protected]. Telephone: +813-52863120. Fax: +813-3209-5522. *(K.O.) E-mail:
[email protected]. Telephone: +813-52863214. Fax: +813-3205-4740. ORCID
Kenichi Oyaizu: 0000-0002-8425-1063 G
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