Poly(fluorene-alt-naphthalene diimide) as n-type polymer electrodes

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Poly(fluorene-alt-naphthalene diimide) as ntype polymer electrodes for energy storage Kasturi Sarang, Andrea Miranda, Hyosung An, Eun-Suok Oh, Rafael Verduzco, and Jodie L. Lutkenhaus ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00164 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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ACS Applied Polymer Materials

Poly(fluorene-alt-naphthalene diimide) as n-type polymer electrodes for energy storage Kasturi T. Sarang†, Andrea Miranda‡, Hyosung An†#, Eun-Suok Oh§, Rafael Verduzco*∥⊥, Jodie L. Lutkenhaus*†¥ †Artie

McFerrin Department of Chemical Engineering, Texas A&M University, College Station,

Texas 77843, United States ‡Department

§School

of Chemistry, Rice University, Houston, Texas 77005, United States

of Chemical Engineering, University of Ulsan, Ulsan 44611, South Korea

∥Department

of Materials Science and NanoEngineering, Rice University, Houston, Texas

77005, United States ⊥Department

of Chemical and Biomolecular Engineering, Rice University, Houston, Texas

77005, United States ¥Department

of Materials Science and Engineering, Texas A&M University, College Station,

Texas 77843, United States

1 ACS Paragon Plus Environment

ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

Redox-active polymers, especially conjugated polymers, are of interest for use in battery electrodes due to their conductivity, synthetic versatility, and processability. Conjugated polymers store energy via a doping mechanism, in which counter ions compensate the charge generated on the conjugated backbone as a result of oxidation or reduction. Here, n-type redox-active poly(fluorene-alt-napthalene diimide) is investigated as an organic battery electrode. The naphthalene diimide (NDI) unit is redox-active and the polyfluorene (PF) unit provides πconjugation and redox activity. However, the nature and mechanism of energy storage for PFNDI is not well-described. Further, n-type polymers in general have high impedance in the doped state and stability issues. It is hypothesized here that the synergy between the PF and NDI units may address these issues. Cyclic voltammetry, galvanostatic cycling, and impedance spectroscopy are utilized to demonstrate that PFNDI stores charge by cathodic doping with Li+ ions. PFNDI is reversibly doped with 86% capacity retention up to 500 cycles while maintaining 99.8% coulombic efficiency. The relative stability is attributed to the resonance at the NDI unit, along with piconjugation along the backbone. With added carbon, a PFNDI composite electrode showed an improved rate performance and capacity over pure PFNDI. The highest capacity recorded for the composite electrode was 39.8 mAh/g at 0.5 C, corresponding to an n-doping level of 1.6 Li+ ions/repeat unit. This work demonstrates that PFNDI is a candidate n-type polymer for reversible energy storage.

Keywords. Lithium-ion battery; conjugation; redox active polymers; n-type polymers, naphthalene diimide unit; polyfluorene unit.


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Introduction Electrochemical energy storage is gaining importance in daily life due to the ever-increasing demand for energy storage devices, specifically batteries, for applications in portable electronics, electric vehicles, and grid-level energy storage.1-2 The most common batteries use critically important and geographically scarce metal oxides, such as lithium and cobalt.2-4 Polymer-based batteries5-9 offer an alternative to metal oxide batteries. An important component of the polymerbased battery is the electrode, which relies upon redox-active polymers (RAPs) that store energy by a reversible chemical conversion mechanism called doping.6, 10 RAPs offer several advantages such as excellent electrode processability, simple redox reaction, and fine-tuning of electrochemical properties by modifying the chemical structure.7 The cathode consists of a high potential p-type RAP that becomes positively charged upon oxidation and doping with anions, and the anode consists of a low potential n-type RAP that becomes negatively charged upon reduction and doping with cations. P-type RAPs such as polypyrrole,2, 11-13 polythiophene,14-15 polyaniline,1619

and poly(3,4-ethylenedioxythiophene) (PEDOT)13, 20 are well-established as fairly reversible

cathodes. Several organic molecules have been used as n-type RAPs for electrodes, but they suffer from electrochemical instability, dissolution, high impedance, and other issues.21-24 At present, identifying and expanding the portfolio of viable n-type RAP anodes is a major challenge for the area of polymer-based batteries.24-26 Several studies on n-type RAPs have focused on the issues of capacity, solubility, and impedance.23, 27-29 Zhang et al. studied RAPs, poly(benzobisimidazobenzophenanthroline) (BBL) and its derivative SBBL, which stored 22.5 and 12.5 Li+ ions and had capacities of 1787 and 1603 mAh/g, respectively, at a C-rate of 3 C.27 However, both showed rapid capacity decline after 50



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charge-discharge cycles with only 66 and 58% capacity retention for BBL and SBBL respectively. According to Zhang et al., this decline might be attributed to the low degree of polymerization.27 Elsewhere, oligoazaacenes, which are electron-deficient ladder-type macromolecules, have shown capacities of 500 mAh/g at 2.5 C for 1000 cycles.29 Along similar lines, Sun et al. used polymers enriched with aromatic rings bearing oxygen and/or nitrogen heteroatoms to reduce its dissolution in electrolyte,30 which led to an increase in the abundance of electrochemical active sites for Li+ ions doping.31 Yao et al. synthesized poly{[N,N’-bis(2-octyldodecyl)-1,4,5,8napthalenedicarboximide-2,6-diyl]-alt-5,5’(2,2’bithiophene)} - (P(NDI2OD-T2)), for which naphthalene diimide (NDI) has aromatic groups with nitrogen heteroatoms. P(NDI2OD-T2) exhibited stable capacity of 54 mAh/g for 3000 cycles at 10 C.32 The low capacity of P(NDI2ODT2)) relative to others was because of the side chains installed to enhance solubility and processing. Liu

et

al.

synthesized

poly(9,9-dioctylfluorene-co-fluorenone-co-methylbenzoic

ester)

(PFFOMB) by adding carbonyl (C=O) and methylbenzoic ester (MB) functional groups to a conjugated polyfluorene (PF) unit14 for use as an n-type conductive binder in silicon electrodes. The carbonyl group lowered the lowest unoccupied molecular orbital (LUMO) of the PFFOMB polymer, which improved the ability of PFFOMB to be reduced. These results highlight the growing interest in redox-active conjugated polymers in which a redox-active group reversibly stores energy and a conjugated unit assist in electron conduction.33-34 One such polymer with both redox-active and conjugated units is poly(fluorene-alt-napthalene diimide) (PFNDI) (Figure 1a). Although not yet studied in the context of energy storage, PFNDI (theoretical capacity of 48.6 mAh/g) is of interest because of the dual nature of the fluorene and naphthalenediimide units. The NDI unit accepts 2 Li+ ions per repeat unit at a potential of ~2.5 V vs. Li/Li+ and exhibits battery-like behavior– Figure 1b.32, 35-37 However polyNDIs are generally 4 ACS Paragon Plus Environment

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insulating, so large amounts of carbon are required to facilitate the energy storage. On the other hand, PFs have conductivities around 10-4-10-2 S/cm (when doped with a base such as potassium or tert-butoxide),38 and modest redox activity above 4 V vs. Li/Li+.39-41 As an alternating copolymer, PFNDI has a LUMO level of -3.61 eV and a highest occupied molecular orbital (HUMO) level of -5.93 eV, and this high HOMO energy level is indicative of its possible stability in a reducing environment.10, 36 The possible interplay between these two units within a copolymer is not yet understood or characterized as an n-type battery electrode for energy storage. Here, we investigate the electrochemistry of n-type PFNDI and its suitability as an anode for electrochemical energy storage. PFNDI has been explored previously for solar cells,42 in which the fluorene unit acted as an electron-donating block. We hypothesized that the PF unit would, accordingly affect the redox activity of the n-type PFNDI, allowing for reversible energy storage of in a reducing environment without dissolution. Our approach uses cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). We first investigate the electrochemical reversibility under various voltage windows, centering upon the window in which the NDI unit is reversibly active by Li+ doping. Using electroanalytical chemistry, we calculated the diffusion coefficients of Li+ ions in the polymer matrix, and analyzed the impedance of polymer in doped and de-doped states to understand the mechanism of doping and energy storage. Finally, we compared the performance of pure PFNDI against composite electrodes of PFNDI with carbon black (CB) and with silicon.



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Figure 1. (a) Structure of poly(fluorene-alt-napthalene diimide) (PFNDI) showing the redox active NDI unit (blue) and the conjugated flourene unit (purple). (b) Doping of a naphthalene diimide (NDI) unit with Li+ ions. Upon the first reduction, one carbonyl oxygen is doped with a Li+ ion, and upon the second reduction, a second carbonyl oxygen is doped. The reverse process takes place during oxidation. Voltages are vs. Li/Li+.

Experimental section Materials For the PFNDI synthesis, 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester and (4,9-dibromo-2,7-bis(2-octyldodecyl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone were acquired from Sigma Aldrich and SunaTech Inc. Tetraethylammonium hydroxide, 20 wt% solution in water was acquired form ACROS organics. Indium tin oxide (ITO) coated glass substrates were purchased from Delta Technologies (7 mm x 50 mm x 0.7 mm, resistance 5-10 Ω, 6 ACS Paragon Plus Environment

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one side coated). Dichloromethane, isopropyl alcohol, acetone, and anhydrous chloroform used for dissolving PFNDI were purchased from VWR. Super P carbon black (CB) (0.04 µm particle size, 62 m2/g surface area) and stainless-steel substrates (15.5 mm diameter x 0.2 mm thick) were purchased from MTI corporation. Anhydrous lithium perchlorate (LiClO4), anhydrous acetonitrile (MeCN), 1-methyl-2-pyrrolidinone (NMP) and the electrolyte composed of 1 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate:diethylene carbonate (EC:DEC) (1:1 v/v) were purchased from Sigma Aldrich for silicon anode studies. The reference electrode used was a saturated silver/silver chloride (Ag/AgCl) electrode purchased from Pine Research Instrumentation. Lithium metal foil (0.75 mm thick x 19 mm wide) and Aliquat 336 were purchased from Alfa Aesar. Polypropylene separator (Celgard 3501) (19 mm diameter x 0.025 mm thick) was purchased from Celgard. Silicon nanoparticles (98+% purity, 20-30 nm size, 80-120 m2/g surface area) were purchased from US Research Nanomaterials. Electrode preparation and assembly PFNDI polymer electrode ITO-coated glass and stainless-steel substrates were cleaned by sonicating sequentially in dichloromethane, isopropyl alcohol, distilled water, and acetone. They were then dried using nitrogen gas. Furthermore, the ITO-coated glass substrates were cleaned using ozone plasma treatment (Harrick PDC-32G) for 20 minutes. PFNDI was dissolved in chloroform (2 mg/ml) and drop-cast on the conducting side of cleaned ITO or the stainless-steel substrate to yield an active material loading of approximately 0.3–0.4 mg/cm2. The PFNDI polymer electrode was dried in a vacuum oven at 75 °C for 6 h to remove residual chloroform.



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PFNDI+CB composite electrode To obtain a composite electrode of PFNDI and CB, 60 wt% PFNDI and 40 wt% CB were mixed in chloroform and bath sonicated for 30 minutes to form a homogeneous dispersion. This mixture was then drop-cast onto clean ITO-coated glass. The electrodes were dried following the same procedure as described in the previous section. Silicon electrode with PFNDI polymeric binder To make a silicon electrode with PFNDI as the conductive binder, 65 wt% silicon nanoparticles, 15 wt% PFNDI, and 20 wt% CB were dispersed in NMP by sonicating the mixture for 30 minutes to form a slurry. This slurry was then drop-cast onto a stainless steel substrate, dried in air, and then dried in a vacuum oven at 120 °C for 3-4 h to remove the residual NMP. The active material loading of Si/PFNDI/CB electrode was around 0.3-0.4 mg/cm2 with total areal loading of 0.5 mg/cm2. PFNDI polymer characterization Polymer molecular weights and polydispersities were determined using an Agilent 1200 module equipped with three PSS SDV columns in series (100, 1000, and 10,000 Å pore sizes), an Agilent variable wavelength UV/vis detector, a Wyatt technology HELEOS II multiangle laser light scattering (MALLS) detector (λ = 658 nm), and a Wyatt Technology Optilab reX RI detector. This system enables GPC with simultaneous refractive index (GPC-RI), UV/vis (GPC-UV/vis), and MALLS detection. THF was used as the mobile phase at a flow rate of 1 ml/min at 40 °C. ProtonNMR (1H NMR) spectra were recorded using tetramethylsilane as the internal standard in (d3chloroform) CDCl3 on a 400 MHz Bruker multinuclear spectrometer. Samples were placed in 5 mm outer diameter tubes and the concentration was ~10 mg/ml.


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Electrochemical characterization A three-electrode electrochemical cell was used to perform electrochemical measurements on PFNDI and PFNDI+CB electrodes. PFNDI or PFNDI+CB drop-cast onto ITO-coated glass was the working electrode, Ag/AgCl was the reference electrode, platinum wire was the counter electrode, and 0.5 M LiClO4 in MeCN was used as the electrolyte. The three-electrode setup was connected to a potentiostat/galvanostat (Solartron Electrochemical Interface 1287), and cyclic voltammetry (CV) at varying scan rates (1 mV/s, 5 mV/s, 10 mV/s, 20 mV/s, and 50 mV/s), galvanostatic charge-discharge at different C-rates (0.5 C, 1 C, 2 C, 5 C, 10 C, 50 C, 100 C, 200 C, and 500 C), and cycling at a constant rate of 10 C (500 cycles) were performed. The C-rate was calculated using the theoretical capacity of PFNDI based on two Li+ ions per repeat unit (48.6 mAh/g). Electrochemical impedance spectroscopy (EIS) was performed using Gamry Interface 1000, Gamry Instruments with 50 mV amplitude and 100 kHz to 5 mHz frequency range. For the PFNDI/silicon polymer anode, the voltage window was 0.01 V to 1 V vs. Li/Li+, and a two-electrode half-cell was used. PFNDI coated stainless-steel was used as the working electrode and lithium metal was used as the counter and reference electrode. Celgard separators were used between these two electrodes and 0.5 M LiClO4 in MeCN was used as the electrolyte. The Solartron instrument was used to conduct cyclic voltammetry at 1 mV/s and galvanostatic cycling at different C-rates (0.1 C, 0.2 C, 0.5 C, and 1 C). A Gamry instrument was used to perform EIS as stated before. A two-electrode half-cell was also used to study Si/PFNDI/CB=65/15/20 working electrodes with lithium metal as the counter and reference electrodes, Celgard separators, and 1 M LiPF6 in EC:DEC (1:1 v/v) as the electrolyte. The Solartron was used to perform cyclic voltammetry at 1



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mV/s and galvanostatic cycling at 0.1 C. For these experiments, the C-rate was calculated using the theoretical capacity of silicon (3579 mAh/g).

Results and discussion Synthesis of PFNDI was adapted from a prior report,42 and the details are provided in the Supporting Information. PFNDI as-synthesized had a molecular weight of approximately 41.0 kDa and a dispersity index of 1.7. PFNDI was drop-cast from chloroform onto ITO-coated glass to yield a 1-5 µm thick polymer film (typical loading of 0.3-0.4 mg/cm2). Cyclic voltammetry and galvanostatic cycling was performed in a three-electrode cell over three different voltage windows to identify the region contributing most to energy storage. Zhou et al. studied PFNDI polymer having a different side chain in the voltage range of 2.2 V to 5.2 V vs. Li/Li+, for which the onset of reduction was 3.68 V.42 In other reports, polymers with NDI units were typically studied in voltage windows ranging from 1.7 V to 3.2 V vs. Li/Li+.23, 28, 32, 35-36, 43-45 The ranges explored here include a low voltage region (LVR, 1.7 V to 3.7 V), a high voltage region (HVR, 3.7 V to 5.2 V), and the full voltage region (FVR, 1.7 V to 5.2 V). The reference electrode was Ag/AgCl, and the electrolyte was 0.5 M LiClO4 in acetonitrile. Krtil and Novak showed that LiClO4 in acetonitrile was stable up to 5.5 V vs. Li/Li+.46 The potential values are reported with respect to the Li/Li+ redox couple by correcting the potential according to the relative reference potentials. In Figure 2, electrochemical activity appears in the LVR and FVR scans, with the LVR showing higher specific current than the FVR. There is not remarkable electrochemical activity in the HVR scan (Figure 2b).


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Figure 2. (a) Cyclic voltammograms of PFNDI at a scan rate of 1 mV/s in various voltage windows: low voltage region (LVR, 1.7 V to 3.7 V), high voltage region (HVR, 3.7 V to 5.2 V), and the full voltage region (FVR, 1.7 V to 5.2 V). Tests were conducted in a three-electrode cell, with PFNDI as the working electrode, Ag/AgCl as the reference electrode, platinum wire as the counter electrode, and 0.5 M LiClO4 in acetonitrile as the electrolyte. (b) Cyclic voltammogram of PFNDI at a scan rate of 1 mV/s for three cycles in HVR. Figure 3a shows cyclic voltammograms for the LVR scan. Two pairs of peaks occurring at different potentials were observed, indicating the presence of two redox reactions (Figure 1b). Upon the first reduction (2.53 V), a carbonyl group on the NDI unit gains an electron; for charge neutrality a Li+ cation dopes the negatively charged NDI unit. As the voltage decreases further, a 11 ACS Paragon Plus Environment


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second reduction occurs at the NDI unit (2.32 V); resulting in another carbonyl group’s reduction and doping with Li+. The peak current of the second reduction peak is notably less than that of the first reduction peak. When the potential is reversed, oxidation and first (de)lithiation of the NDI unit occurs (2.54 V). Upon increasing the potential further, a second oxidation event occurs at the NDI unit (2.75 V). The peak separation (ΔEp) of 220 mV for both reactions and overlapping cyclic voltammogram curves at fixed scan rate indicate that the redox reactions are quasi-reversible. The PFNDI polymer electrode was cycled galvanostatically in the LVR at different C-rates ranging from 1 C to 50 C, where 1C indicates the current required to discharge the theoretical capacity in one hour. Figure 3b shows that the specific capacity decreases from 24.6 mAh/g at 1 C to 6.5 mAh/g at 50 C, maintaining approximately 99.8 % coulombic efficiency (CE) at all Crates. Also, two plateau regions were observed during both discharge (2.58 V, 2.32 V) and charge (2.45 V, 2.67 V), as shown in Figure S2 which is in accordance with the redox peak positions in Figure 3a. This confirms that two redox reactions occur for the NDI unit, corresponding to a (de)lithiation process.23, 32, 35-36, 45


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Figure 3. Cyclic voltammogram of PFNDI at 1 mV/s for three cycles in (a) LVR and (c) FVR. Galvanostatic charge-discharge at different C-rates in (b) LVR and (d) FVR. Tests were conducted in a three-electrode cell, with PFNDI as the working electrode, Ag/AgCl as the reference electrode, platinum wire as the counter electrode, and 0.5 M LiClO4 in acetonitrile as the electrolyte. Figure 3c shows cyclic voltammetry of PFNDI for the FVR at 1 mV/s for 3 cycles. Redox peaks were observed only for the NDI units and no notable redox peaks were beyond 3.0 V. To further confirm the lack of activity, a cyclic voltammogram in the HVR alone exhibited no peaks and little electrochemical activity, Figure 2b. The feature observed at ~4.9 V vs. Li/Li+ was due to ITO degradation.47 Although polyfluorenes are redox active generally in the region of 3.2 – 5.2 V vs. Li/Li+,39-40 no such activity was observed here. By these observations, we conclude that the electrochemical activity of PFNDI is dominated by the NDI unit. The rate performance of PFNDI with varying C-rate in the FVR (Figure 3d) was evaluated and compared to that in the LVR. For the FVR, the specific capacity of PFNDI at 1 C was around 15 13 ACS Paragon Plus Environment


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mAh/g, which was lower than that in the LVR window (24.6 mAh/g). Also, the capacity decrease with increasing C-rate for the FVR was more pronounced than in the LVR. This may be attributed to electrochemical degradation of the PFNDI or of the ITO current collector.47 Thus, PFNDI should be utilized in the LVR rather than the FVR under these conditions. For this reason, we chose to investigate PFNDI in further detail in the LVR. Cyclic voltammetry at different scan rates (Figure 4a) was performed on PFNDI in the LVR from 1 mV/s to 50 mV/s to evaluate the diffusion of Li+ ions into the polymer matrix, which could explain the reduced capacity at higher C-rates, as evident in Figure 3b. As scan rate increased, the peak separation also increased. At 50 mV/s, the two peaks became indistinguishable due to diffusion limitations. To investigate this, the peak current was plotted with respect to the square root of the scan rate, and a linear relationship was observed, Figure 4b. The reduction peak at 2.53 V became indistinguishable at high scan rates. Thus, to plot current at higher scan rates, the peak potential at the lowest scan rate of 1 mV/s was used for the plot. At this potential, the current at all other scan rates was determined and plotted, Figure 4b. From the linear fit, the diffusion coefficient was calculated using the Randles-Sevcik equation (Eqn. 1) for a quasi-reversible process.16, 48-50 For a transfer coefficient α=0.5, 𝑖𝑝 = 0.3508 𝑛𝐹𝐴𝐶0

∗

(

𝑛𝐹𝑣𝐷𝐿𝑖 + 𝑅𝑇

0.5

)

(1)

where, ip is the peak current (A), n is the number of electrons involved in redox reaction, F is Faraday’s constant (96485 C/mol), A is contact area between PFNDI and the electrolyte (for simplicity, the geometric area of the electrode in cm2 is used), C0* is the concentration of redox active species (8.6 x 10-3 mol/cm3), ν is the scan rate (V/s), DLi+ is the diffusion coefficient (cm2/s), R is the universal gas constant (8.314 J/mol K) and T is the temperature (298 K). The calculated 14 ACS Paragon Plus Environment

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average diffusion coefficient for the Li+ ion in the PFNDI was 5 x 10-11 cm2/s, which is similar in magnitude to the n-type polymer P(NDI2OD-T2).32 Thus, the capacity fade at higher C-rates may be attributed to the slow diffusion of Li+ ions.

Figure 4. (a) Cyclic voltammogram of PFNDI at different scan rates in the LVR (1.7 V to 3.7 V) to show the shift of the redox peaks. (b) Plot of peak current vs. scan rate0.5 to calculate the diffusion coefficient using the Randles-Sevcik equation, Eqn. 1. The solid lines represent fits to the data. The Faradaic and non-Faradaic contributions to charge storage for the PFNDI electrode were evaluated using cyclic voltammetry at different scan rates (Figure 5, see SI for a full description of the calculation). The non-Faradaic (capacitive) contribution was relatively small as compared to the Faradaic (diffusion-controlled) contribution. This is in agreement with the linear 15 ACS Paragon Plus Environment


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relationship for peak current with the square root of the scan rate in Figure 4b, which confirms diffusion-controlled behavior. The b-value was also calculated as a function of potential, which further provides an indication of the relative Faradaic (b=0.5) and non-Faradaic (b=1) contributions. In Figure S4, the b-value was around 0.7 at 1.7 V, decreased to 0.2 at 2.5 V, and then gradually increased to a b-value of 1 at 3.7 V. This trend corresponds with the range of potentials for which the diffusion-controlled redox reaction was most prominent.

Figure 5. Plot of i(V)/ν0.5 vs. ν0.5 at selected potentials for (a) anodic (lithiation) and (b) cathodic (delithiation) scans in cyclic voltammetry. The plots were used to calculate a1 (x-intercept) and a2 (slope) from 𝑖(𝑉)/𝜈0.5 = 𝑎1 + 𝑎2𝜈0.5. (c) Cyclic voltammogram at 5 mV/s for PFNDI. The Faradaic, non-Faradaic, and total charge storage contributions are shown in pink, purple, and green, respectively. (d) Total charge stored with Faradaic and non-Faradaic contributions with respect to scan rate. The calculation details are available in the Supporting Information.


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We also evaluated the cycling performance of PFNDI at a high C-rate of 10 C to evaluate electrochemical stability over 500 cycles (Figure 6a, b). As seen from Figure 6, there was a slight increase in capacity during the initial cycling period. This behavior can be attributed to electrolyte penetration into PFNDI, which increases the accessibility of Li+ ions. After about 100 cycles, PFNDI reached a stable discharge capacity of around 18 mAh/g. The coulombic efficiency for the first 10 cycles was ~90 %, which then increased to 95 % within 40 cycles and then remained in the range of 98 %-99 % for remaining cycles. There is a slight drop in the capacity after 300 cycles, but the polymer was still intact on the ITO substrate (i.e., it did not apparently degrade or dissolve in the electrolyte). The capacity retention was around 72 % for the 500th cycle with respect to the highest stable capacity recorded for PFNDI (18 mAh/g). The relatively low capacity for PFNDI may be attributed to the addition of the PF unit, which is not redox active in the potential range investigated here (1.7 V- 3.7 V vs. Li/Li+).39-41 However, we observed that although the PF unit lowered the attainable capacity, the PF unit improved the electrochemical stability by enhancing electronic conduction via conjugation.32



Figure 6. (a) Galvanostatic cycling of PFNDI at a high C-rate of 10 C for 500 cycles. This experiment was repeated twice and yielded similar results. (b) Voltage profile of PFNDI at 10 Crate for 1st, 10th, 50th, 100th, 200th, 400th and 500th cycle during charge and discharge. The cycling performance of PFNDI was better than some previously reported n-type RAPs: for example; polyimide derived from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) showed approximately 47 % capacity retention at the 100th cycle at 0.2 C,36 3,4,9,10perylenetetracarboxylicacid-dianhydride showed 50 % capacity retention at the 90th cycle,51 poly(anthraquinonyl sulfide) exhibited less than 50 % capacity retention over 50 cycles,52 and poly(paraphenylene) had 75 % capacity retention at 0.1 C at the 90th cycle.53 Reasons reported for the previously reported poor cycling performance of these polymers were high impedance in the doped state, dissolution of the polymer in the electrolyte, and very low electrochemical stability. 18 ACS Paragon Plus Environment

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However, some n-type polymers showed similar or much better cycling stability than PFNDI. For example, Yao et al. reported that an n-type polymer, P(NDI2OD-T2), demonstrated a capacity retention of 96 % and an average coulombic efficiency of >99.5 % at 10 C for 3000 cycles.32 Yao et al. also reported on cross-conjugated quinone polymers that retained 96 % capacity at the 250th cycle during cycling at 5 C and very little capacity fade over 1000 cycles at 3 C.28 This cycling stability was attributed to careful design of the polymer backbone and the conjugated side chain to improve conductivity and mechanical integrity of the polymer electrode. A table comparing PFNDI with previously reported n-type polymers is provided in the Supporting Information (Table S1).

PFNDI

performs

comparably

to

3,6-poly(phenanthrenequinone)

(PPQ)54

and

poly(anthraquinonysulfide) (PAQS)52, yet has a lower capacity than (PNDI2OD-T2)32 and BBL27. Here, the improved stability of PFNDI can be attributed to the low impedance in the doped state, which was evaluated using electrochemical impedance spectroscopy (EIS). Figure 7a shows representative Nyquist plots for PFNDI at various potentials, 2.3 to 3.0 V vs. Li/Li+. These potentials were selected because they encompass the two doping events at E1/2 = 2.4 and 2.6 V for the NDI unit. The data is first discussed for the case of 3 V, and then discussed for lower potentials. A capacitive element is observed at potential of 3 V, which indicates the absence of charge transfer reactions and the process of charge build-up at the electrode-electrolyte interface. As the potential is decreased further to 2.6-2.4 V, which coincides with the doping reaction, a Warburg tail appeared as well as two semicircles in the high and medium frequency regions, respectively. The small semicircle in the high frequency region is attributed to the contact resistance.55 The depressed semicircle in the medium frequency region is indicative of the charge transfer resistance for electrochemically doping the polymer. The Warburg tail in the low frequency region is



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indicative of solid-state diffusion of Li+ ions. As the potential decreased further to 2.3 V, the impedance increased again, indicative of an increase in the resistive components. To demonstrate the physical significance of the electrochemical process of doping and de-doping in PFNDI, an equivalent circuit was fitted to the EIS data as shown in Figure 7b. Here, RO represents ohmic resistance; that is, resistance to Li+ ion conduction through the bulk solution to the electrode-electrolyte interface and to the electronic conduction through the PFNDI layer to the ITO-PFNDI interface; CPE represents the constant phase element in consideration of the electrical double layer; RCT is the charge transfer resistance; and WO is the Warburg impedance related to the diffusional effects of Li+ ion. Table 1 summarizes the results of fitting the equivalent circuit to the EIS data. 2 was used as a criterion to determine how well the model fit the experimental data, and since the 2 values were less than 10-2, the model represented a good fit.56 The Nyquist plot begins at an x-intercept (Ro) ranging from 36  to 39  for different dc potentials. The different ohmic resistances could be attributed to the difference in electronic conduction of PFNDI. Its electronic conductivity depends on the state of doping, leading to higher RO for the undoped state. RCT is much lower at imposed dc potentials of 2.6 V and 2.5 V, where the film is doped with Li+ ions than at 2.3 V potential. The value of α in the CPE element describes if it behaves like an ideal capacitor or not. Since the value of α is farther away from 1 (Table 1), the CPE element behaves as a non-ideal capacitor. This can be attributed to surface roughness, non-homogeneous dielectric behavior, and varying electrode thickness.


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Figure 7. (a) Nyquist plot at different potentials near and away from redox peaks for PFNDI. The AC amplitude was 50 mV. The frequency range was from 100 kHz to 5 mHz. The zoomed in figure of the pink shaded part is shown on the right-hand side. (b) Equivalent circuit used to model the EIS data. Table 1. Comparison of ohmic resistance (RO), charge transfer resistance (RCT) and constant phase element (CPE) at different potentials.

Potentials (V vs. Li/Li+) Elements

2.3 V

2.4 V

2.5 V

2.6 V

3V

RO ()

38

36

37

36

39

RCT ()

193

93

77

67

N/A

CPE (F)

9.6x10-5

7.2x10-5

6.1x10-5

6.8x10-5

1.2x10-5

α

0.77

0.66

0.67

0.65

0.78

2

0.004

0.01

0.0014

0.0014

0.01 21

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The diffusion coefficient of Li+ ions using EIS data was determined from the Warburg tail in the low frequency region.57-58 From this, a graph of ZRe vs. w-0.5 in the low frequency region was plotted, and the diffusion coefficient was calculated using Eqn. 2 and Eqn. 3 𝐷𝐿𝑖 + =

𝑅2𝑇2 2

2𝐴2𝑛4𝐹4𝐶0 ∗ 𝜎2

𝑎𝑛𝑑 𝑍𝑅𝑒 = 𝑅𝑂 + 𝑅𝐶𝑇 + 𝜎𝜔 ―0.5

(2)

(3)

where, ω is the radial frequency, ZRe is the real part of impedance, RO is ohmic resistance, and RCT is charge transfer resistance. A plot of ZRe vs. w-0.5 gives the slope = σ and intercept = (RO+RCT). Fitting led to a diffusion coefficient for Li+ ions in PFNDI of 3.5 x 10-13 cm2/s at 2.5 V vs. Li/Li+. This value is two orders lower than the one calculated from the Randles-Sevcik equation (Eqn. 1), derived from a solution based semi-infinite system. This difference in magnitude is observed because Randles-Sevcik equation is more appropriate for solution-based diffusion systems and less so for a solid-state system.59 To examine the effect of a conductive additive, we next investigated a composite electrode bearing 40 wt% CB and 60 wt% PFNDI. Scanning electron microscopy images of PFNDI and PFNDI+CB solution in chloroform drop-cast onto copper foil are shown in Figure S5. Polynapthalenediimides are regarded as insulating, and even the PFNDI electrodes exhibited an overall resistance (𝑅𝑂 + 𝑅𝐶𝑇) of 36-231 , depending on potential. We hypothesized that the CB may reduce the overall impedance and enhance the performance by building a conductive percolation network that will enhance electrolyte wettability.60-61 To compare, we galvanostatically cycled both pure PFNDI and PFNDI+CB composite electrodes at 10 C for 500 cycles to determine the electrode stability– Figure 8a. The cycling performance of PFNDI+CB 22 ACS Paragon Plus Environment

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composite electrodes demonstrated a higher capacity retention (91 %) as compared to PFNDI (86 %). The capacity retention was calculated with respect to the highest capacity recorded, which was 34 mAh/g for PFNDI+CB composite electrode and 18 mAh/g for pure PFNDI electrode. This implies both electrodes have high stability and reversibility during lithiation/(de)lithiation process. The rate performance of the PFNDI+CB composite at different C-rates ranging from 0.5 C to 500 C was also examined (Figure 8b). PFNDI+CB composite demonstrated a capacity of 33 mAh/g at 10 C, whereas pure PFNDI demonstrated a capacity of only 14 mAh/g. The PFNDI+CB composite retained 87 % of its initial capacity as the C-rate increased from 0.5 C to 10 C, whereas pure PFNDI only retained 60% of its initial capacity under similar conditions. This result is also supported by the fact that the peak separation for the composite (ΔEp=160 mV, Figure S6a) is lower than that of pure PFNDI (ΔEp=220 mV).22, 48 We also performed EIS on the PFDNI+CB composite electrode (Figure S6b) and the overall impedance was much lower than that of pure PFNDI. The diffusion coefficient for the PFNDI+CB composite calculated using Randles-Sevcik equation is around 1.1x10-9 cm2/s, which is two orders higher than that of pure PFNDI (Figure S7a, b). The diffusion coefficient for PFNDI+CB was also determined using EIS data (Figure S6b) and Eqn. 2 and Eqn. 3. The value of diffusion coefficient calculated was around 6.5 x 10-12 cm2/s which is one order higher than that calculated for pure PFNDI electrode (3.5 x 10-13 cm2/s) using EIS data. This leads to better rate capability by the addition of CB as shown in Figure 8b, as compared to Figure 3b and d. Above all, the polymer did not undergo degradation or delamination even after reaching a rate of 500 C, and the capacity was recovered almost completely at 0.5 C.



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Figure 8. (a) Comparison of cycling performance (discharge capacity and coulombic efficiency) of PFNDI (shown in red) and PFNDI+CB composite (shown in blue) at a C-rate of 10 C for 500 cycles. (b) Rate performance of PFNDI+CB composite at different C-rates from 0.5 C to 500 C. Capacity is reported on the basis of PFNDI mass. As seen from the results, the PFNDI+CB composite showed promise for storing energy via doping with Li+ ions. As an extension, we added n-type PFNDI as a polymeric binder to silicon nanoparticles to evaluate its performance as a conductive anode binder. The electrode was prepared following a mass ratio of Si/PFNDI/CB = 65/15/20 wt% in NMP, which was drop-cast onto stainless steel. This electrode was used as the working electrode in a two-electrode half-cell with lithium metal as the counter/reference electrode and 1 M LiPF6 in EC:DEC (1:1 v/v) as the 24 ACS Paragon Plus Environment

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electrolyte. Figure S8a shows the cycling performance of the silicon electrode at 0.1 C for 10 cycles, and it can be observed that the capacity of the silicon electrode was very low (10 mAh/g) as compared to the theoretical capacity of silicon (3579 mAh/g). The low current in the cyclic voltammogram plot at 0.1 mV/s (Figure S8b) further implies the low electrochemical activity. The cell failed within a few galvanostatic cycles. Overall, n-type PFNDI was a poor binder for silicon as compared to other polyfluorene-type binders14 poly(9,9-dioctylfluorene-co-fluorenone) (PFFO) and PFFOMB.14 Here, the poor performance of PFNDI as a binder for silicon may be attributed to the poor adhesion of the PFNDI to the silicon particles, as well as the continuous formation of the SEI layer.62-63 Therefore, PFNDI is more suitable for use on its own or blended with carbon black as an n-type polymeric electrode.

Conclusion N-type poly(fluorene-alt-naphthalene diimide) (PFNDI) was synthesized via Suzuki coupling and the electrochemical performance as an n-type energy storage electrode was investigated. We demonstrated that this class of redox active conjugated polymer acts as a reversible battery anode. PFNDI with NDI as the redox active unit and PF as the conjugated unit can be stably and reversibly n-doped with Li+ ions. The PFNDI electrode demonstrated a decent cycling stability with capacity retention of 86 % with respect to highest recorded capacity of 18 mAh/g for 500 cycles at 10 C (or ~ 0.74 Li+ ion/repeat unit). Even after repeated cycling, PFNDI did not dissolve in the electrolyte as compared to other reported n-type polymers. The PFNDI+CB composite demonstrated an improved electrochemical performance with higher capacities (35 mAh/g), higher capacity retention (91 % with respect to highest capacity recorded for PFNDI+CB composite electrode), and improved rate performance (capacity of 38 mAh/g at 1 C and 20 mAh/g at 50 C) over pure 25 ACS Paragon Plus Environment


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PFNDI. The highest capacity recorded for the PFNDI+CB composite electrode was 39.8 mAh/g at 0.5 C (or 1.6 Li+ ions/repeat unit). As a binder for silicon anodes, PFNDI exhibited poor performance, which was attributed to its poor adhesion to the silicon particles. Future work should address these issues by increasing the molecular weight and/or installing adhesive groups, such as hydroxyl groups. PFNDI’s reversible charge storage points to its possible application as a battery anode either by itself or with carbon black as an additive.

Supporting information. PFNDI synthesis, GPC and 1H-NMR spectra of PFNDI, differential capacity curve for PFNDI in low voltage region (LVR), galvanostatic charge-discharge in high voltage region (HVR), calculations for Faradaic and non-Faradaic storage contributions, plots for b-value, scanning electron microscopy (SEM) images of PFNDI and PFNDI+CB, cyclic voltammogram comparing PFNDI and PFNDI+CB composite electrode, electrochemical impedance spectroscopy for PFNDI+CB composite electrode, diffusion coefficient calculation for PFNDI+CB composite electrode, electrochemical performance data of silicon electrode with PFNDI as a binder. Author information Corresponding author Email: [email protected] #Present

address: Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgement This material is based upon work supported by the National Science Foundation under Grants 1604666 and 1604682. Authors thank the Materials Characterization Facility at Texas A&M University.

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Poly(fluorene-alt-naphthalene diimide) as n-type polymer electrodes

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