Article pubs.acs.org/cm
Cite This: Chem. Mater. 2018, 30, 5726−5732
Indirect Synthesis Route toward Cross-Coupled Polymers for High Voltage Organic Positive Electrodes Klemen Pirnat,* Jan Bitenc, Alen Vizintin, Andraz Krajnc, and Elena Tchernychova National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia
Chem. Mater. 2018.30:5726-5732. Downloaded from pubs.acs.org by CHALMERS UNIV OF TECHNOLOGY on 10/21/18. For personal use only.
S Supporting Information *
ABSTRACT: Cross-coupling polymerization using Ni(COD)2 is a simple method to obtain redox active polymers. It requires readily available dihalogenated quinones as starting compounds. In this article, we propose a new synthesis route to obtain a high voltage redox active polymer, poly(9,10-phenanthrenequinone) (PFQ), as a positive electrode material, because direct polymerization with Ni(COD)2 cannot be used for synthesis of high voltage quinone polymers. Thus, we protected the quinone groups with acetyl groups before polymerization reaction, developing an indirect synthesis route. As a positive electrode material in a lithium battery, PFQ shows 400 mV higher voltage (2.54 V vs Li/Li+) compared to the more known para counterparts, due to the ortho quinone structure. PFQ was tested in several electrolytes, and the best performance was obtained in 1 M trifluoromethanesulfonimide lithium salt dissolved in a mixture of 1,3-dioxolane and dimethoxyethane (1 M LiTFSI/DOL +DME). Furthermore, PFQ redox utilization was improved by the addition of reduced graphene oxide (rGO) during polymerization (PFQ/rGO) to obtain 153 mAh/g of specific capacity. The PFQ/rGO composite showed very good capacity retention; 91% of starting capacity was retained after 500 cycles at C/5 coupled with good rate capability.
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with respect to lithium and also other monovalent (Na,1 K2) and bivalent (Mg,3,4 Zn5) cations. Organic positive electrodes could be produced from bioderived feedstock and at lower temperatures than those based on inorganic materials, which would lower the overall carbon footprint of the battery production.6−9 However, they often suffer from capacity degradation due to dissolution of the active material into the electrolyte. There are several strategies to mitigate the dissolution: grafting of organic molecules onto a solid support,10,11 use of selective separators,12 and preparation of insoluble organic polymers.6−9,13,14 However, implementation of these strategies often comes at a cost of a decreased electrochemical performance of the positive electrode. For example, grafting introduces additional electrochemically nonactive mass, whereas selective separators are often bulky and have limited performance at higher current densities. Similarly, preparation of polymers includes an additional linker group into the structure (vinyl, methyl group, S, etc.), which increases the molecular weight of the monomer unit. By contrast, direct cross-coupling of electrochemically active groups is an elegant way to retain the capacity of the simple organic molecule, while synthesizing insoluble polymers with stable capacity performance. This approach was successfully demonstrated by Song et al., who cross-coupled an
INTRODUCTION Climate change and decreasing reserves of the readily available and low-cost fossil fuels are inciting our society for a shift toward renewable energy resources, i.e., wind, solar, hydropower, tidal, etc. While the latter can provide sustainable energy to meet our demands, energy production from renewable sources is often time dependent. This raises concerns about the stability of the electrical grid and energy supply. Thus, energy storage devices will be a necessary intermediate piece of the energy supply chain to balance the mismatches between energy production and demand. Currently, major energy storage solutions rely mainly on pumped hydro storage, but this type of energy storage has a relatively low energy density and requires a suitable geographical configuration of terrain to operate efficiently. Batteries offer an alternative, significantly more energy dense systems and are expected to play an increasingly important role in the future electrical energy storage. When considering batteries for the energy storage, price and sustainability are among the most important factors. While Liion batteries are the best battery technology on the market, concerns about the toxicity of certain metals currently used (Co, Ni) together with the high price and limited availability of Li are encouraging research of alternative Li systems such as Li-S, Li-O2 and beyond-Li systems like Na-ion, Mg, Al batteries and different metal−organic batteries. The latter are especially interesting because organic positive electrodes have already demonstrated reversible electrochemical performance © 2018 American Chemical Society
Received: June 4, 2018 Revised: August 2, 2018 Published: August 3, 2018 5726
DOI: 10.1021/acs.chemmater.8b02329 Chem. Mater. 2018, 30, 5726−5732
Chemistry of Materials
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anthraquinone moiety to prepare a polyanthraquinone (PAQ) polymer.15 PAQ exhibited extremely stable cycling performance (98.1% of initial capacity was retained after 1000 cycles at 1 C) and good rate capability (69% of initial capacity retained at 20 C). Apparently the only downside of this material was its relatively low discharge voltage of 2.14 V vs Li/Li+. A low voltage becomes especially deterring when considering polymers for metal−organic batteries beyond Li, due to a higher redox potential of any metal with respect Li. Thus, there is an evident need to increase the voltage of cross-coupled polymers, while maintaining the electrochemical stability and rate capability of organic polymers. It is well-known that ortho quinones have a higher redox potential compared to their para counterparts, due to a better coordination of the Li+ ion, which increases the binding energy of Li.16−19 Thus, in order to increase the operation voltage of quinone-based polymers, we here propose the preparation of a cross-coupled polymer with carbonyl groups in the ortho position. It is worth noting that the synthesis of such high voltage organic polymers using Ni(COD)2 can be rather complex as already demonstrated on the example of poly(benzoquinone) where additional protection and deprotection reaction steps had to be included.20 Here we report a successful synthesis of a new high voltage polymer, poly(9,10-phenanthrenequinone) (PFQ), with carbonyl groups in the ortho position as well as its electrochemical behavior in several Li electrolyte systems. On the basis of the behavior of different systems, the electrochemical performance of PFQ is optimized with the addition of rGO and compared with PAQ.
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Article
RESULTS AND DISCUSION
Phenanthrenequinone (FQ) was chosen as a starting compound of the synthesis, due to an analogous molecular structure to anthraquinone (AQ). Additionaly, the carbonyl groups in FQ are in the ortho position. FQ was brominated to obtain a brominated derivative of FQ-monomer 1, which is a precursor for different types of polymerizations, including cross-coupling. Unfortunately, direct cross-coupling of monomer 1 into poly(9,10-phenanthrenequinone) (PFQ) was not successful. FQ is a stronger oxidant than AQ with around 400 mV higher electrochemical potential. It is very likely that FQ monomer (1) oxidizes the Ni0(COD)2 complex. As a result, a reduced monomer 2 is obtained instead of a PFQ polymer, as evident from the corresponding IR spectra (Figure S1). Our results and various literature reports show consistently that low voltage polymers, e.g., PAQ (2.14 V vs Li/Li+), P15AQ (2.09 V vs Li/Li+),21 can be directly polymerized using halogenated quinone monomers, while higher voltage materials, such as PFQ (2.54 V vs Li/Li+) and PPBQ (2.7 V vs Li/Li+),22 require an indirect polymerization route using protected (nonoxidizing) monomers. We estimate that the upper limit of stability window of the Ni0(COD)2 complex lies in the range between 2.14 and 2.54 V vs Li/Li+ (Figure 1).
EXPERIMENTAL SECTION
Synthesis. Synthesis was started by bromination of phenanthraquinone with N-bromosuccinimide (NBS)/H2SO4 to obtain a brominated derivative of FQ denoted as monomer 1. Monomer 1 was used for direct synthesis of PFQ, which however was not successful. Thus, an indirect route consisting of five synthesis steps was devised. The first monomer was reduced into monomer 2 with Sn/HCl, which was further acetylated to monomer 3. The latter was polymerized using Ni(COD)2, in the presence of 1,5-cyclooctadiene and 2,2′-bipyridyl to obtain polymer 4. Polymer 4 was deprotected with LiAlH4, resulting in PFQ_L. In the last step, PFQ_L was oxidized using 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) to obtain the PFQ polymer. The overall yield was 82%, which may be considered excellent for a five-step reaction. To obtain a PFQ/rGO composite, rGO was added to the reaction mixture during the polymerization reaction of monomer 3. All further details about the synthesis can be found in the Supporting Information (SI). Analysis. The prepared compounds were analyzed using IR spectroscopy, liquid NMR, magic angle spinning (MAS), solid-state NMR, elemental analysis, X-ray diffraction (XRD), thermogravimetric analysis coupled with mass spectrometer (TGA-MS), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). Information about the specific conditions of measurements can be found in the Supporting Information. Electrochemical Analysis. All the electrochemical tests were carried out in Swagelok electrochemical cells. Electrodes were prepared by mixing 60% of active material, 30% of Printex XE2 carbon black, and 10% of polytetrafluoroethylene binder (PTFE) in isopropanol using a planetary ball mill. The electrodes were prepared on Al mesh. All the electrochemical tests were performed on VMP3 and MPG2 galvanostat/potentiostats from Bio-Logic using the potential window of 1.5−3.5 V vs Li/Li+.
Figure 1. Ni(COD)2 stability in comparison with selected redox polymers.
The indirect synthesis route for PFQ consists of five reaction steps, as shown in Figure 2.20 To increase the practically attainable specific capacity closer to the theoretical one, a composite PFQ/rGO material containing 21 wt % of rGO was prepared. In this case, a reduced graphene oxide (rGO) was included in the reaction mixture during the polymerization reaction. The prepared organic materials were characterized using a range of different analytical techniques. In Figure 3, IR spectra of all organic compounds synthesized in this work are presented (monomers 1−3, polymer 4, PFQ_L, and PFQ). To follow the changes in each step, three characteristic IR regions were selected (regions I, II, and III, shaded with gray color). In region I, the CO stretching vibration from an ester group is clearly visible for the acetylated monomer 3 (1769 cm−1) and its corresponding polymer 4 (1772 cm−1). Region II is characteristic for the CO stretching vibration of a quinone group.23 It is clearly seen that the peak at 1674 cm−1 (compound 1) disappears after reduction (monomers 2, 3 and polymer 4) as quinone is converted into hydroquinone and later protected. After deprotection of hydroquinone (compound PFQ_L), the CO peak reappears (1678 cm−1). Most likely, PFQ_L is already oxidized into quinone by the exposure 5727
DOI: 10.1021/acs.chemmater.8b02329 Chem. Mater. 2018, 30, 5726−5732
Article
Chemistry of Materials
Figure 2. Reaction scheme of indirect synthesis of PFQ polymer and PFO/rGO composite.
measurements were performed (Figure 4). 1H MAS NMR did not reveal much information due to extremely broad peaks.
Figure 3. ATR-IR spectra of compounds 1, 2, 3, 4, PFQ_L, PFQ, PFQ/rGO, and rGO. The three most significant regions (I, II, and III) are shown as gray stripes. Figure 4. 1H−13C CPMAS NMR measurements on polymers 4, PFQ_L, PFQ, and PFQ/rGO.
to the air atmosphere. After the oxidation with DDQ (compound PFQ), the IR spectra are identical. Region III is characteristic for the aryl C−O stretching vibrations,24 which should be present in hydroquinone and acylated species. This is in good agreement with experimental spectra: monomer 2 (1200 cm−1), monomer 3 (1198 cm−1), and polymer 4 (1203 cm−1), all of which display these characteristic bands. However, other compounds also have several minor peaks in this region and belong to other vibrations. The C−Br stretching vibration (compounds 1, 2, and 3) is below 667 cm−124 and thus outside of the measured range (4000−650 cm−1). After polymerization, peaks become broader (polymer 4, PFQ_L, PFQ, and PFQ/rGO) compared to monomers (compounds 1, 2, and 3), which is a typically observed phenomenon in the case of polymers. IR spectra of PFQ_L, PFQ, and PFQ/rGO are nearly identical, which means PFQ_L has already oxidized into PFQ. Furthermore, the addition of reduced graphene oxide (rGO) does not have a big influence on the IR spectra: sample PFQ/rGO has only slightly narrower peaks. Due to a low amount of rGO (21 wt %), the characteristic peaks of rGO are not visible in the PFQ/rGO sample. Solid-State NMR Measurements. For all insoluble polymers (4, PFQ_L, PFQ, and PFQ/rGO), MAS NMR
The only difference that could be observed was a broad peak at 1.81 ppm from the acetyl group in polymer 4 (Figure S2). To see the difference between carbons with/without attached hydrogens, two techniques were employed: 1H−13C CPMAS NMR to visualize all the carbons and 1H−13C LG CPMAS NMR to see only carbons directly connected to hydrogens. The MAS NMR spectra can be divided into four different regions. In region (I), carbonyl peaks (CO) are found at 179 ppm from quinone and are present in all deprotected samples, which confirms the oxidized structure of PFQ_L. As there is no hydrogen attached on carbonyl, no signal is observed in the LG CPMAS spectrum. Region (II) represents the carbonyl peak (CO) from the acetyl group at 169 ppm in polymer 4. In region (III), there are aromatic carbons and those without hydrogens have no LG CPMAS signal. Region (IV) represents the acetyl group, which is present only in polymer 4 at 20 ppm. It also shows an LG CPMAS signal confirming the attached hydrogens on the acetyl group. Elemental Analysis. Monomer molecule 3 shows a good agreement with calculated values with the difference between found and calculated values being
