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Materials 2018, 11, x FOR PEER REVIEW 4 of 20 Materials 2018, 11, 2567 4 of 18 Figure 1. The structure of typical polyimides suitable for use in NIBs. Aromatic polyimides are redox-active polymers (Figure 1), and hence are very promising energy Aromatic polyimides are redox‐active polymers (Figure 1), and hence are very promising energy storage electrode materials [36]. The carbonyls of the polyimide provide the active sites for the redox storage electrode materials [36]. The carbonyls of the polyimide provide the active sites for the redox reaction. The carbonyls can interact with sodium ions via enolization, involving two one-electron reaction. The carbonyls can interact with sodium ions via enolization, involving two one‐electron reduction steps to yield sequentially the anion radical and the dianion (Scheme 1). reduction steps to yield sequentially the anion radical and the dianion (Scheme 1). Scheme 1. Schematic representation of the redox reaction of aromatic polyimides. Scheme 1. Schematic representation of the redox reaction of aromatic polyimides. The polyimides currently used in NIBs are usually derived from 3,4,9,10- perylenetetracarboxylic The polyimides currently used in NIBs are usually derived from 3,4,9,10‐ perylenetetracarboxylic dianhydride (PTCDA) (1, 2) [37,38], 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) dianhydride (PTCDA) (1, 2) [37,38], 1,4,5,8‐naphthalenetetracarboxylic dianhydride (NTCDA) (3, 4) (3, 4) [39,40] and pyromellitic dianhydride (PMDA) (5) [41]. The average discharge voltage of [39,40] and pyromellitic dianhydride (PMDA) (5) [41]. The average discharge voltage of polyimides polyimides becomes progressively lower when the aromatic core changes from PTCDA (1.94 V) becomes progressively lower when the aromatic core changes from PTCDA (1.94 V) to NTCDA (1.89 to NTCDA (1.89 V) and to PMDA (1.73 V) [37]. This is due to the increased energy of the lowest V) and to PMDA (1.73 V) [37]. This is due to the increased energy of the lowest unoccupied molecular unoccupied molecular orbital (LUMO) and thus the decreased average discharge voltage. Therefore, orbital (LUMO) and thus the decreased average discharge voltage. Therefore, PTCDA‐derived PTCDA-derived polyimides are most commonly investigated as cathode materials for NIBs [37,38], polyimides are most commonly investigated as cathode materials for NIBs [37,38], while the NTCDA‐ while the NTCDA- and PMDA-derived polyimides are studied as anode materials [39–41]. Improved and PMDA‐derived polyimides are studied as anode materials [39–41]. Improved cycling stability and cycling stability and increased gravimetric capacity can be achieved by employing larger conjugated increased gravimetric capacity can be achieved by employing larger conjugated aromatic cores and aromatic cores and reducing the molecule weight of inactive chains, respectively. reducing the molecule weight of inactive chains, respectively. The working voltage of the polyimides can be manipulated by introducing an electron The working voltage of the polyimides can be manipulated by introducing an electron withdrawing group. For example, compared with the polyimide containing an ethylene connecting withdrawing group. For example, compared with the polyimide containing an ethylene connecting unit, polyimide (6) with a connecting sulfonyl group has a 0.14 V higher charge-discharge voltage, unit, polyimide (6) with a connecting sulfonyl group has a 0.14 V higher charge‐discharge voltage, resulting from the decreased electron density of the redox-active carbonyl by inductive effects [42]. resulting from the decreased electron density of the redox‐active carbonyl by inductive effects [42]. A very attractive advantage of polyimides as electrode materials for organic NIBs is their A very attractive advantage of polyimides as electrode materials for organic NIBs is their insolubility in electrolytes, which results in good cycling stability. However, only half of the insolubility in electrolytes, which results in good cycling stability. However, only half of the carbonyls carbonyls can be involved in the redox reactions and so the capacity of polyimides is usually less than can be involved in the redox reactions and so the capacity of polyimides is usually less than 140 mAh 1‐140 mAh·g−1 (Table 2). Further development of polyimide electrode materials requires this bottleneck g (Table 2). Further development of polyimide electrode materials requires this bottleneck to be to be overcome. Copolymerization of the imide with a high-capacity quinone is a promising strategy overcome. Copolymerization of the imide with a high‐capacity quinone is a promising strategy to to obtain polyimides with enhanced capacity. Xu et al. prepared two imide-quinone copolymers obtain polyimides with enhanced capacity. Xu et al. prepared two imide‐quinone copolymers with high with high reversible capacities of−192 mAh·g−1 (7) and 16−15 (8) mAh·g−1 [43]. A number of other reversible capacities of 192 mAhꞏg (7) and 165 (8) mAhꞏg [43]. A number of other imide‐quinone imide-quinone copolymers (9, 10, 11, 12) have also displayed improved capacities [44]. In addition, copolymers (9, 10, 11, 12) have also displayed improved capacities [44]. In addition, introduction of introduction of electron withdrawing groups into the polyimide chain can increase the capacity of the electron withdrawing groups into the polyimide chain can increase the capacity of the polyimides. Li polyimides. Li et al. reported that the introduction of the triazine ring can increase the sodium-ion et al. reported that the introduction of the triazine ring can increase the sodium‐ion storage capability storage capability (13, 14), through the synergistic effects of electron withdrawing amide groups and (13, 14), through the synergistic effects of electron withdrawing amide groups and triazine rings [45]. triazine rings [45]. 2.1.2. Polyquinones 2.1.2. Polyquinones The polyquinones employed as electrode materials for NIBs are usually sulfur‐containing The polyquinones employed as electrode materials for NIBs are usually sulfur-containing polyquinones, obtained by the polycondensation reaction of quinone monomers and sulfides [46–49]. polyquinones, obtained by the polycondensation reaction of quinone monomers and sulfides [46–49]. Due to the reductive capacity of the quinonyl groups, a side redox reaction between the quinonyl Due to the reductive capacity of the quinonyl groups, a side redox reaction between the quinonyl groups and sulfide anions may occur during the formation of the polyquinone main chains. Therefore, groups and sulfide anions may occur during the formation of the polyquinone main chains. Therefore, the synthesis is usually followed by subsequent purification or oxidation steps [47,49]. the synthesis is usually followed by subsequent purification or oxidation steps [47,49]. As is shown in Figure 2, the structure of the polyquinone chains is determined by the molecular As is shown in Figure 2, the structure of the polyquinone chains is determined by the molecular structure of quinone monomers. The polyquinones usually contain a chain of quinone rings separated structure of quinone monomers. The polyquinones usually contain a chain of quinone rings separated by heteroatoms with an unshared electron pair. Though the heteroatoms are not the active sites and do by heteroatoms with an unshared electron pair. Though the heteroatoms are not the active sites and do not contribute to the sodium storage, the heteroatoms can enhance the discharge voltage [48]. not contribute to the sodium storage, the heteroatoms can enhance the discharge voltage [48]. Similar to polyimides, polyquinones usually undergo two-electron redox reactions through reversible electrochemical association of sodium ions with its redox-active quinonyl groups (Scheme 2).PDF Image | Polymer Electrode Materials for Sodium-ion Batteries
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