Core–shell heterostructured composites of carbon nanotubes and imine-linked hyperbranched polymers as metal-free Li-ion anodes

Yu Dou; Jianye Zhang; Xiaoyan Han; Qiming He; Yingkui Yang

doi:10.1515/ntrev-2022-0046

Article Open Access

Core–shell heterostructured composites of carbon nanotubes and imine-linked hyperbranched polymers as metal-free Li-ion anodes

Yu Dou , Jianye Zhang , Xiaoyan Han , Qiming He and Yingkui Yang

Published/Copyright: February 9, 2022

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From the journal Nanotechnology Reviews Volume 11 Issue 1

Abstract

An in situ Schiff-base condensation between p-phthalaldehyde (PPD) and 1,3,5-tris(4-aminophenyl)benzene (TAPB) or 1,3,5-tris(4-aminophenyl)triazine (TAPT) was actualized in the presence of carbon nanotubes (CNTs), producing imine-linked hyperbranched poly(PPD-TAPB) and poly(PPD-TAPT)-coated CNTs (abbreviated as CNT@HBP-1 and CNT@HBP-2, respectively). Such quasi-1D core–shell heterostructures are interleaved to build robust 3D networks with porous internal channels, which are favorable for efficient electron transport and ion diffusion, exposing active sites, fast redox kinetics, and high electrochemical utilization. When used as Li-ion anodes, both CNT@HBP-1 and CNT@HBP-2 exhibit larger specific capacity, better rate performance, and higher cycling stability compared to their pure polymers. Furthermore, CNT@HBP-2 delivers higher reversible capacities of 351 mA h g⁻¹ at 0.05 A g⁻¹, and 81 mA h g⁻¹ at 1.0 A g⁻¹, respectively, compared to CNT@HBP-1 (335 and 56 mA h g⁻¹). Besides, CNT@HBP-2 retains 268 mA h g⁻¹ over 100 cycles at 0.1 A g⁻¹, and 617 mA h g⁻¹ in the 500th cycles at 0.5 A g⁻¹, respectively, outperforming CNT@HBP-1 (155 and 256 mA h g⁻¹). Further improvements in the electrochemical performance for CNT@HBP-2 relative to CNT@HBP-1 are attributable to the incorporation of additional redox-active triazine units into HBP-2. This work would unlock insights into the rational development of metal-free polymer-based electrodes for rechargeable batteries.

Keywords: imine-based hyperbranched polymers; carbon nanotubes; core–shell structures; metal-free electrodes

1 Introduction

Lithium-ion batteries (LIBs) have long demonstrated as the main power sources for consumer electronics and electric vehicles [1,2,3]. It is generally known that LIBs are mainly based on transition-metal compounds as cathodes (usually LiFePO₄ and LiCoO₂) and graphite or Si/C as anodes [4,5,6,7]. However, such electrode materials are suffering from their intrinsically limited Li-storage performance, gradually exhausted mineral resources, and environmental challenges of greenhouse effect and heavy metal pollution [8,9,10]. Therefore, the development of eco-sustainable and cost-efficient materials has become an urgent requirement to seek the future substitutions for the commercially used inorganic electrode materials [11,12,13].

In this context, organic redox polymers as potential electrode alternatives for green LIBs have recently attracted extensive attention due to their merits of high specific capacities, fast redox kinetics, structure designability, resource abundance, low cost, and sustainability and recyclability [14,15,16,17]. In the last years, polymers containing redox-active units of free radicals, organosulfur, imine, carbonyl, and azo groups have been frequently used in organic LIBs [18,19,20]. However, most polymers show high redox potentials (over 2.0 V vs Li/Li⁺) with their reversible lithiation/delithiation processes in the voltage range of 1.5–3.5 V (vs Li/Li⁺) [21]. Therefore, such polymers are preferred as cathode materials for LIBs [22,23], and there are few reports on polymer-based anodes so far [24].

The past years has witnessed considerable progress on organic LIBs with polymer electrodes [25]. Nevertheless, there are two challenges that need to be addressed. First, organic polymers are usually electrical insulators which often result in sluggish redox kinetics, and inferior power capability [26]. Generally improved strategies are to combine with electrically conductive additives such as graphene and carbon nanotubes (CNTs) by ex situ physical blending and in situ chemical polymerization methods [27,28]. Another issue involves a low electrochemical utilization (usually less than 50%) of polymers and hence, an inferior practical capacity [29,30]. To alleviate current concerns, it is highly desired but challenging to develop metal-free polymer-based anode materials for sustainable LIBs.

Herein, we report on the fabrication of imine-based hyperbranched polymer-coated CNTs by the in situ Schiff-base condensation reaction of p-phthalaldehyde (PPD) with 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 1,3,5-tris(4-aminophenyl)triazine (TAPT), respectively. This process leads to forming core–shell heterostructures of CNTs encapsulated in hyperbranched poly(PPD-TAPB) and hyperbranched poly(PPD-TAPT), which are accordingly abbreviated as CNT@HBP-1 and CNT@HBP-2, respectively. The thicknesses of the HBP-1 and HBP-2 outer shell are about 18.9 and 19.2 nm, respectively. Both CNT@HBP-1 and CNT@HBP-2 anodes display larger capacities, higher rate, and cycling capabilities in comparison with pure HBP-1 and HBP-2 anodes. Remarkably, the CNT@HBP-2 anode delivers a higher reversible capacity of 351 mA h g⁻¹ at 0.05 A g⁻¹ and retains 617 mA h g⁻¹ over 500 cycles at 0.5 A g⁻¹, compared to CNT@HBP-1 (335 and 256 mA h g⁻¹) due to the presence of additional redox-active triazine sites in HBP-2. This work may provide new insights into the rational crafting of metal-free polymer-based electrodes for rechargeable batteries.

2 Experimental

2.1 Materials

TAPT was synthesized as described elsewhere [31]. CNTs were purchased from Chengdu Organic Chemicals Co. Ltd, CAS, China. Trifluoroacetic acid and PPD were purchased from Shanghai Aladdin Industrial Co. LTD., China. TAPB and 4-aminobenzonitrile were provided by Shanghai Hongyan Bio-medicine Co. Ltd and Shanghai Macklin Biochemical Co., Ltd, China. Dimethyl sulfoxide (DMSO), N-methyl-2-pyrrolidone (NMP), and other chemicals were supplied by Sinopharm Group Chemical Reagent Co. Ltd, China. All chemicals and solvents were used as received.

2.2 Synthesis of hyperbranched polymer-encapsulated CNTs

CNTs (0.066 g) were ultrasonically dispersed into DMSO (30 mL) for 30 min at room temperature. To this stable suspension were then added PPD (0.121 g, 0.9 mmol) and TAPB (0.211 g, 0.6 mmol) or TAPT (0.213 g, 0.6 mmol) under ultrasonic condition for another 5 min. The resultant dispersion was held at 150°C for 12 h under magnetic stirring in an Ar atmosphere. Finally, the final product was collected by filtering and washing the black powder with ethanol to remove free polymers. After vacuum-drying at 80°C for 12 h, CNT/hyperbranched poly(PPD-TAPB) and CNT/hyperbranched poly(PPD-TAPT) composites were obtained and accordingly abbreviated as CNT@HBP-1 and CNT@HBP-2, respectively. As control experiments, pure hyperbranched poly(PPD-TAPB) (HBP-1) and hyperbranched poly(PPD-TAPT) (HBP-2) were also synthesized in the absence of CNTs under identical experimental procedures.

2.3 Material characterization

Fourier transform infrared (FT-IR) spectra were recorded on a Thermo Nicolet NEXUS 470 spectrometer (USA). X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance (Germany) X-ray diffractometer with a Cu Kα source. Thermogravimetric analysis (TGA) was measured on a Netzsch TG 209 F3 Tarsus analyzer (Germany) at a heating rate of 10°C min⁻¹ under a N₂ atmosphere. Raman spectra were collected on a Thermo Scientific DXR spectrometer (USA) with an excitation laser λ = 532 nm. Elemental analysis of C, N, and H was detected by Elementar Vario MICRO Cube (Germany). Transmission electron microscope (TEM, Tecnai G220 S-TWIN, FEI, USA) and scanning electron microscope (SEM, SU8010, Hitachi, Japan) were used to determine material morphologies. Energy dispersive X-ray (EDX) elemental mappings were collected on a scanning transmission electron microscope (STEM, Talos F200X).

2.4 Electrochemical measurements

The working electrode was fabricated by casting a slurry mixture of active materials, acetylene black, and poly(vinylidene fluoride) (ratio: 6:3:1 in weight) in NMP onto the copper foil, followed by drying at 80°C under vacuum for 12 h. The mass loading of active materials was about 1.0 mg cm⁻² on each circular electrode slice with a diameter of 10 mm. The as-made anode was assembled into a 2032-type half-cell using the lithium foil as the counter electrode. 1.0 M LiPF₆ dissolved in a mixture of ethylene carbonate, dimethyl carbonate, and ethyl–methyl carbonate (ratio: 1:1:1 in volume) was used as the electrolyte. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) data were collected on a Chenhua CHI 660 electrochemical workstation (Shanghai, China) in the range of 0.01 to 3 V. The rate performance and cycling stability were carried out on a Land CT2001A battery testing system (Wuhan, China). All electrochemical measurements were actualized at room temperature.

3 Results and discussion

3.1 Synthesis and structural characteristics

Figure 1 demonstrates chemical structures and synthesis routes of hyperbranched polymers and the composites with CNTs. CNTs are well known to be easily dispersed in polar DMSO due to their well-matched solubility parameters and surface tensions [32,33]. DMSO has also been reported as an excellent reaction medium to synthesize Schiff-base polymers with high yields [34]. Furthermore, phenyl-enriched monomers (PPD, TAPB, and TAPT) and the synchronously-formed conjugated hyperbranched polymers (HBP-1, and HBP-2) enable strong π–π non-covalent interactions with CNTs during the in situ polymerization process [35]. Accordingly, this allows for an intimate contact between CNTs and polymers to build robust core–shell nanostructures.

Figure 1

Chemical structures and synthesis routes of hyperbranched polymers (HBP-1 and HBP-2) and the composites with CNTs (CNT@HBP) by Schiff-base condensation.

Figure 2 shows SEM images of HBP-1, HBP-2, CNTs, and their composites. Both HBP-1 and HBP-2 exhibit near-spherical particles with average diameters of 915 and 196 nm, respectively. Bare CNTs have an average diameter of 41 nm. In contrast, CNT@HBP-1 (Figure 2c) and CNT@HBP-2 (Figure 2e and f) display large average diameters of about 77 nm, due to the homogeneous coating of hyperbranched polymers onto the surface of CNTs during the in situ polycondensation process. Apparently, such quasi-1D CNT@HBP-1 and CNT@HBP-2 are intertwined with each other to form 3D networks with porous internal channels, which are favorable for fast charge transport and high electrochemical utilization [36].

Figure 2

Typical SEM images of (a) HBP-1, (b) CNTs, (c) CNT@HBP-1, (d) HBP-2, and (e and f) CNT@HBP-2.

TEM tests also reveal that HBP-1 (Figure 3a) and HBP-2 (Figure 3d) exhibit near-spherical particles with average diameters of 908 and 186 nm, respectively. In contrast to pure CNTs (Figure 3b), CNT@HBP-1 (Figure 3c) and CNT@HBP-2 (Figure 3e and f) are uniformly covered by polymer shells with average thicknesses of 18.9 and 19.2 nm, respectively. These observations are in good accordance with SEM images, approving the formation of core–shell heterostructures through in situ Schiff-base condensation of organic monomers in the presence of CNTs.

Figure 3

Typical TEM images of (a) HBP-1, (b) CNTs, (c) CNT@HBP-1, (d) HBP-2, and (e and f) CNT@HBP-2.

Further STEM observations disclose that single conductive hollow CNT as an inside axis is fully encapsulated in HBP-1 (Figure 4a) and HBP-2 (Figure 4b) with an evident contrast. The EDX elemental mapping analysis also verifies the presence of high-level elemental N signals in both CNT@HBP-1 and CNT@HBP-2, implying the formation of imine bonds (‒CH═N‒) in the HBP-1 and HBP-2 shells.

Figure 4

High-angle annular dark-field STEM images and the corresponding EDX elemental mappings of (a) CNT@HBP-1 and (b) CNT@HBP-2.

Chemical structures of the as-synthesized products were further examined by spectroscopy techniques. As shown in the FT-IR spectra (Figure 5a), the characteristic bands in the wavenumber range of 1,700‒1,200 cm⁻¹ suggest the disappearance of aldehyde stretching at 1,690 cm⁻¹ form PPD, and the emergence of C═N stretching at 1,630 cm⁻¹ [37], due to the formation of imine-linked chains in HBP-1, HBP-2, CNT@HBP-1, and CNT@HBP-2. The absorption bands at about 1,510 cm⁻¹ correspond to the characteristic vibration of triazine rings in HBP-2 and CNT@HBP-2 [38]. Raman spectra (Figure 5b) show that two characteristic D and 2D bands for CNTs also appear at ca. 1,330 and 2,660 cm⁻¹ in CNT@HBP-1 and CNT@HBP-2 [39], respectively. The characteristic bands of aromatic C‒H at 1,170 cm⁻¹ for HBP-1 and HBP-2 can be also detected in CNT@HBP-1 and CNT@HBP-2 [37]. Moreover, the coupled characteristic peaks (C═C/C═N) centered at 1,600 cm⁻¹ for polymers and G-band of CNTs are almost completely overlapped. XRD patterns (Figure 5c) demonstrate weak peaks at 2θ = 26° in CNT@HBP-1 and CNT@HBP-2, corresponding to the (002) lattice plane of CNTs. However, polymers and their composites made in our experimental conditions are amorphous states, which are different from crystalline covalent organic frameworks [40]. The coupling of CNTs with polymers obviously increases the thermal stability of the latter (Figure 5d). CNTs exhibit a very tiny loss at 800°C while the major weight loss originates from polymers. The accurate composition was further estimated by the elemental analysis. The theoretical N and C contents are 8.4 and 86.8% for HBP-1, and 16.8 and 79.0% for HBP-2, based on their repeating units. The actual N fractions are 6.1% within CNT@HBP-1 and 12.4% within CNT@HBP-2, respectively. The N signal is not detectable in bare CNTs. Accordingly, it can be calculated that the loading amounts of CNTs are about 27.4% in CNT@HBP-1 and 26.2% in CNT@HBP-2, respectively, by assuming that all N moieties come from imine-linked polymers.

Figure 5

(a) FT-IR spectra, (b) Raman shifts, (c) XRD patterns, and (d) TGA curves of HBP-1, HBP-2, CNTs, CNT@HBP-1, and CNT@HBP-2.

3.2 Lithium-storage performance

Electrochemical performance of anode materials was carefully evaluated in a half-cell using the Li foil as the counter electrode. Figure 6 shows typical CV curves of HBP-1, HBP-2, CNT@HBP-1, and CNT@HBP-2. During the discharge process, the broad cathodic peaks centered at 0.82 V for HBP-1 and 0.77 V for CNT@HBP-1 (Figure 6a), and 0.85 V for HBP-2 and 0.78 V for CNT@HBP-2 (Figure 6b) can be attributed to the insertion of Li⁺ into the C═N bonds from imine bonds and triazine rings while the strong sharp peaks nearly 0.04 V correspond to the addition of lithium to the C═C bonds from aromatic rings [37,41,42]. During the reverse charge sweep, the anodic peaks centered at about 0.31 V are assigned to the extraction of Li⁺ from the C═C subunits, while the shoulder peaks appear at around 1.06 V for HBP-1, 1.02 V for CNT@HBP-1, 1.12 V for HBP-2, and 1.03 V for CNT@HBP-2 arising from the delithiation processes [43]. However, it is difficult to distinguish redox peaks of C═N between imine bonds and triazine rings due to their structural similarity [40]. Of note, both HBP-2 and CNT@HBP-2 exhibit the slightly larger polarization compared to HBP-1 and CNT@HBP-1, owing to the incorporation of electron-withdrawing triazine rings with an increased electron affinity [44]. Furthermore, both CNT@HBP-1 and CNT@HBP-2 present the stronger current response than HBP-1 and HBP-2, implying the higher redox activity enabled by conductive inside cores of CNTs [37].

Figure 6

Stable CV curves of (a) HBP-1 and CNT@HBP-1 and (b) HBP-2 and CNT@HBP-2 at a scan rate of 0.5 mV s⁻¹ in the third cycles.

Figure 7 shows discharge/charge curves of HBP-1, HBP-2, CNT@HBP-1, and CNT@HBP-2 in the initial three cycles. The initial discharge (lithiation)/charge (delithiation) capacities are detected to be 309/130 mA h g⁻¹ for HBP-1 (Figure 7a), 1,287/466 mA h g⁻¹ for CNT@HBP-1 (Figure 7b), 1,311/376 mA h g⁻¹ for HBP-2 (Figure 7c), and 1,520/534 mA h g⁻¹ for CNT@HBP-2 (Figure 7d), respectively. The low Coulombic efficiency (29‒42%) and drastic capacity decay in the first cycle are mainly ascribed to the decomposition of electrolyte and the formation of solid electrolyte interface film [45]. The discharge/charge profiles are then overlapped modestly in the subsequent cycles. Notably, the HBP-2-based anodes show much higher capacities compared to the HBP-1 ones, mainly due to the presence of highly redox-active triazine rings [38,46] as well as smaller sizes of HBP-2 relative to HBP-1. Remarkably, the encapsulation of conductive CNTs further improves the redox activity of HBP-1 and HBP-2 associated with the effective exposure of nano-sized outer shells, affording high electrochemical utilization and large specific capacities [28,37].

Figure 7

Charge/discharge curves of (a) HBP-1, (b) CNT@HBP-1, (c) HBP-2, and (d) CNT@HBP-2 at a current density of 0.1 A g⁻¹ in the first three cycles.

Rate capabilities of electrode materials were further evaluated by galvanostatic charge–discharge, as shown in Figure 8. The CNT@HBP-1 electrode delivers higher reversible capacities at each rate compared to HBP-1 (Figure 8a). Similarly, the CNT@HBP-2 electrode also has higher specific capacities than HBP-2 with increasing the current density (Figure 8b). In particular, CNT@HBP-1 exhibits an average capacity of 335 mA h g⁻¹ at 0.05 A g⁻¹ and 56 mA h g⁻¹ at 1 A g⁻¹, and maintains 269 mA h g⁻¹ after returning to 0.05 A g⁻¹. Meanwhile, CNT@HBP-2 delivers an average capacity of 351 and 81 mA h g⁻¹ at 0.05 and 1 A g⁻¹, respectively, and retains 288 mA h g⁻¹ after resuming to 0.05 A g⁻¹. The lower capacities of HBP-1 and HBP-2 with respect to CNT@HBP-1 and CNT@HBP-2 mainly originate from the bulk congregating morphologies of pure polymers, leading to the insufficient electrolyte penetration, poor accessibility, and slow ion diffusion [23,47]. Furthermore, the slightly higher rate capability and larger specific capacity of CNT@HBP-2 relative to CNT@HBP-1 and polymers are attributed to the presence of triazine rings and CNTs capable of improving redox activity and electronic conduction during the discharge/charge processes [40,48].

Figure 8

Rate performance of (a) HBP-1 and CNT@HBP-1 and (b) HBP-2 and CNT@HBP-2 at different current densities.

The EIS test was further conducted to evaluate the electrochemical kinetics. As shown in Figure 9a, CNT@HBP-1 (124 Ω) and CNT@HBP-2 (113 Ω) possess the lower charge transfer resistances than HBP-1 (166 Ω) and HBP-2 (147 Ω), because of the efficient electron transport and fast ion diffusion contributed by 3D conductive networks of CNTs. The presence of active triazine rings in HBP-2 also enables a lower resistance of CNT@HBP-2 compared to CNT@HBP-1 [49]. In addition, CNT@HBP-2 also maintains a higher capacity of 268 mA h g⁻¹ after running at 0.1 A g⁻¹ over 100 cycles (Figure 9b), compared to CNT@HBP-1 (155 mA h g⁻¹) and its parent polymer (69 mA h g⁻¹). Their Coulombic efficiencies still remain almost 100%, revealing the high reversibility and cycling stability. Interestingly, the reversible capacities at an enhanced rate of 0.5 A g⁻¹ keep a gradual increase from the 100th to 400th cycles (Figure 9c), possibly due to the electrochemical activation throughout the electrode and superlithiation of aromatic C═C bonds [37,43]. In the prolonged 500th cycle, CNT@HBP-1 and CNT@HBP-2 deliver stable capacities of 617 and 256 mA h g⁻¹, respectively. This implies that the rational integration of triazine and aromatic rings with conductive CNT networks can endow the resulted composite electrodes with abundant active sites, robust electrode structures, and powerful charge transport capability [28,50].

Figure 9

(a) Nyquist plots and (b) cycling stability of HBP-1, HBP-2, CNT@HBP-1, and CNT@HBP-2 at 0.1 A g⁻¹ for 100 cycles, and (c) long-term cycling stability of CNT@HBP-1 and CNT@HBP-2 at 0.5 A g⁻¹ for 500 cycles.

4 Conclusion

Core–shell heterostructures of CNTs and imine-based hyperbranched polymers were successfully fabricated by an in situ Schiff-base condensation between dialdehyde and tris-aminophenyl monomers. The resulted composites were further used as Li-ion anodes, which deliver the larger specific capacity, better rate capability, and higher cycling stability compared to their counterparts of pure polymers. The comprehensively improved electrochemical performance is mainly attributed to the synergistic effects of 3D electrically conductive CNT networks, interconnected porous internal channels, and efficiently exposed active sites. The further improvement in the lithium-storage performance can be also achieved by the incorporation of additional redox-active triazine units into the polymer shell around CNTs. This work would provide a new scenario for the rational crafting of metal-free polymer-based electrodes for the next-generation sustainable batteries.

Funding information: This work was financially supported by National Natural Science Foundation of China (52173091), Program for Leading Talents of National Ethnic Affairs Commission of China (MZR21001), and Hubei Provincial Natural Science Foundation of China (2021CFA022).
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
Conflict of interest: The authors state no conflict of interest.

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Received: 2021-12-18

Revised: 2022-01-21

Accepted: 2022-01-21

Published Online: 2022-02-09

This work is licensed under the Creative Commons Attribution 4.0 International License.

Articles in the same Issue

https://doi.org/10.1515/ntrev-2022-0046

Keywords for this article

imine-based hyperbranched polymers; carbon nanotubes; core–shell structures; metal-free electrodes

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