High-Energy and High-Power Nonaqueous Lithium-Ion Capacitors

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High-energy and High-power Non-aqueous Lithium Ion Capacitors Based on Polypyrrole/Carbon Nanotube Composites as Pseudocapacitive Cathodes Cuiping Han, Ruiying Shi, Dong Zhou, Hongfei Li, Lei Xu, Tengfei Zhang, Junqin Li, Feiyu Kang, Guoxiu Wang, and Baohua Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02781 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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ACS Applied Materials & Interfaces

High-energy and High-power Non-aqueous Lithium Ion Capacitors Based on Polypyrrole/Carbon Nanotube Composites as Pseudocapacitive Cathodes Cuiping Hana‡, Ruiying Shib,c‡, Dong Zhoud‡, Hongfei Lib, Lei Xub,c, Tengfei Zhanga, Junqin Lia*, Feiyu Kangb,c, Guoxiu Wangd*, Baohua Lib* a

College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060, China

bShenzhen

Geim Graphene Center, Graduate School at Shenzhen, Tsinghua University, Shenzhen

518055, China cSchool

of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China

dAustralia

Centre for Clean Energy Technology, Faculty of Science, University of Technology

Sydney, Sydney, New South Wales 2007, Australia ‡

These authors contributed equally to this work.

* Corresponding authors: [email protected] (J. Li), [email protected] (G. Wang), [email protected] (B. Li)

Abstract The energy density of present lithium ion capacitors (LICs) are greatly hindered by the limited specific capacities of porous carbon electrodes. Herein, we report the development of a nonaqueous LIC system by integrating two reversible electrode processes, i.e. anion doping/undoping in a core-shell structured polypyrrole/carbon nanotube (Ppy@CNT) composite cathode and Li+ intercalation/deintercalation in a Fe3O4@carbon (C) anode. The hybrid Ppy@CNT is utilized as a promising pseoducapacitive cathode for non-aqueous LIC applications. The Ppy provides high pseudocapacitance via the doping/undoping reaction with PF6- anions. Meanwhile the CNT backbone significantly enhances the electrical conductivity. The as-developed composite delivers noteworthy capacities with exceptional stability (98.7 mAh g-1 at 0.1 A g-1 and retain 89.7% after cycling at 3 A g-1 for 1000 times in Li-half cell), which outperforms state-of-art porous carbon cathodes in present LICs. Furthermore, when paired with Fe3O4@C anodes, the as-developed LICs demonstrate a superior energy density of 101.0 Wh kg-1 at 2,709 Wh kg-1, and still maintains 70 Wh kg-1 at increased power density of 17,186 W kg-1. The findings of this work provides new 1

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knowledge on the cathode materials for LICs. Keywords:

Lithium

Ion

Capacitors;

Non-aqueous;

Polypyrrole;

Carbon

Nanotube;

Pseudocapacitance

1. Introduction The soaring growth of portable devices and electric transportations greatly relies on efficient energy storage technologies1-4. The past decades have witnessed significant improvement in current energy storage systems, including electrochemical capacitors and batteries5-9. In particular, supercapacitors (SCs) are attractive for fast charging/discharging and long cycle life applications, while lithium ion batteries (LIBs) are dominant in high energy applications due to their suitable energy storage features. Recently, lithium ion hybrid capacitors (LICs) are constructed to merge the advantages of the above two devices. They are assembled using a capacitive cathode in conjunction with a battery anode within Li-ion-conducting organic electrolyte6,

10-11.

In this

architecture, the capacitive cathode provides high rate and long lifespan via the electric double layer capacitive (EDLC) effect with PF6- anions, while the battery anode with high Li storage capacity ensures high energy of the LICs10, 12-13. Moreover, compared with aqueous electrolyte, organic electrolyte has higher decomposition potential, thus the operating voltage range of this devices are much wider than aqueous ones. To date, a series of electrode materials have been explored for LICs. The LIB-type anodes, such as Li4Ti5O12 (175 mAh g-1)14, graphite (372 mAh g-1)15, Li3VO4/N-doped carbon (529 of mAh g1)16,

antimony (Sb, 660 mAh g-1), titanium carbide (TiC, 851 mAh g-1)10, silicon (Si, 4200 mAh

g-1)17 etc., generally deliver high specific capacities to guarantee high energy density. In the case of cathodes, the most widely used AC materials exhibit relatively low specific capacities (usually less than 50 mAh g-1) owing to their limited charge accumulation via surface adsorption/desorption mechanisms (i.e. electric double layer capacitance (EDLC))18, which will undoubtedly sacrifice the energy density of LICs. As a result, great efforts have been made focusing on replacing AC with specially-designed porous carbons, such as activated carbon fibers (ACFs)6, graphene19, biomass or polymer derived AC18, 20-23. Although these could lead to an increase in capacity, they are still not sufficient to satisfy the dramatically increased requirements for high energy storage. Moreover, during the synthesis of porous carbon, chemical activation 2


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process (such as KOH etching) is conducted to introduce abundant micropores, but is environmentally unfriendly. Besides this issue, the complex and high cost preparation process also restricts their further applications. Therefore, the key towards high performance LIC is to explore promising cathode candidates with improved capacities but no demand for chemical activation. As alternatives to porous carbons, pseudocapacitive electrodes has gained extensive research interest owing to their high theoretical capacitances through Faradaic redox reactions5, 24. Among them, polypyrrole (Ppy) as an electronic conducting polymer demonstrates relatively high theoretical capacitance, easy polymerization, and good stability in ambient atmosphere25-26, thus making it promising electrode candidates for SCs and lithium/sodium ion batteries27,28 , unfortunately, yet not for LICs. The reversible doping/undoping of electrolyte anions in the backbone of Ppy polymer can provide a specific capacitance far exceeding that of AC. Therefore, the employment of Ppy as a novel cathode may open a new way to significantly enhance the energy density of non-aqueous LICs. In this study, we employed Ppy anchored on carbon nanotubes (Ppy@CNT) as pseudocapacitive cathode to enhance the energy densities of LICs. The in situ coating of a Ppy shell onto a CNT core was achieved through a simple chemical oxidative polymerization approach. The Ppy shell provides high specific capacity via the doping/un-doping reaction with electrolyte anions (PF6-). The CNT core not only dramatically enhances the electrical conductivity, but also provides a robust framework for Ppy polymer to improve its capacitive performance. Impressively, compared with pure CNTs that are based on an EDLC mechanism, the as-prepared Ppy@CNT composites exhibit a significantly improved electrochemical property. The optimized composite (ie. Ppy@CNT with CNT content of 25 wt.%) exhibits a noteworthy specific capacity of 98.7 mAh g-1 at 0.1 A g-1 in LiPF6-based organic carbonate electrolyte together with superiorrate performance and cycle stability. This is obviously superior to state-of-art reported AC cathodes, which deliver limited specific capacities via EDLC processes in present LICs. When matched with Fe3O4@C anode, the as-fabricated LIC shows an excellent energy density of 101.0 Wh kg-1 at 2,709 W kg-1, and successfully retains 70.0 Wh kg-1 even at 17,186 W kg-1. The above findings verified the feasibility of enhancing the energy density of LICs through exploiting pseudocapacitive materials. 3



2. Experimental 2.1 Synthesis of Ppy@CNT cathode The Ppy@CNT cathode was prepared as below: Typically, acidified CNTs (5 wt.%, Guangzhou Hongwu Mater. Tech. Co., Ltd.) were firstly added into 100 mL of purified water, followed by addition of pyrrole monomer (99%, Aladdin) and stirring for 1.5 h. Subsequently, 100 mL 0.115 M (NH4)2S2O8 solution was dropwisly added into the CNT and pyrrole monomer mixture with stirring for 4 h at 0 °C. Then the resulting products was filtered, washed and vacuum dried overnight in an oven at 60 °C (named as Ppy@CNT). In this, pyrrole monomer were oxidized by (NH4)2S2O8 and in situ polymerized to a long-chain polypyrrole (PPy) layer on CNT framework. To obtain optimized performance, the Ppy@CNT with CNT content (based on the mass of pyrrole) of 10%, 25% and 40% were prepared. The as-prepared Ppy@CNT composites were labeled as Ppy@CNT-x, where x represent the content (%) of CNT. For comparison, pure CNT and Ppy served as control samples. 2.2 Synthesis of Fe3O4@C anode For detailed synthesis steps of Fe3O4@C, please refer to our previous works29. 2.2 Materials characterization The scanning electron microscopy (SEM, HITACH S4800, 5kV) and high resolution transmission electron microscope (HRTEM, TECNAIG2 F30) combined with energy dispersive spectrometry (EDS, Oxford) were used to investigate the morphology of Ppy@CNT and Fe3O4@C. The fouriertransform infrared (FTIR) spectra were conducted on Thermo Scientific Nicolet iS 50. Raman spectra were studied on a Raman spectrometer using 532 nm laser (HORIBA LabRAM HR800). The phase composition of Ppy@CNT and Fe3O4@C were investigated using X-Ray Diffraction (XRD, Bruker D8 Advance with Cu J radiation). The elemental composition and valence states was analyzed with X-Ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe II, Al K9 radiation). The nitrogen adsorption/desorption isotherms were characterized by a Micromeritic ASAP2020M+C adsorption/desorption analyzer under 77 K. The specific surface area was calculated on Brunauer-Emmett-Teller (BET) method, while the pore diameter distribution was defined by Barrett-Joyner-Halenda (BJH) method. The electrical conductivity of Ppy@CNT composites were measured using four-point probe method (LORESTA GP MCP-T610). The detail testing procedures were described in the previously reported paper30. 4


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2.3 Electrochemical characterization The electrochemical performance of Ppy@CNT composites and Fe3O4@C were measured via coin cells, in which Li metal was used as counter electrode. The electrode was prepared using a slurring coating method as described in the previously reported paper 29. The areal mass loading of Fe3O4@C anode and Ppy@CNT composites cathodes were about 1.3 mg cm-2 and 3.9 mg cm-2, respectively. The compressed density of Ppy@CNT-25% electrode was characterized after compressing by a roll squeezer. Li-half cell with Ppy@CNT or Fe3O4@C electrode was assembled in an Ar-filled glove box. The electrolyte is 1 M LiPF6 in a mixture of ethylene carbonate, diethyl carbonate and dimethyl carbonate (EC/DEC/DMC, 1:1:1 in volume) with 2 wt.% fluoroethylene carbonate (FEC) (Dongguan Shanshan Tech., China). The electrolyte amount used to assemble half coin cell and LIC was 80 uL. The separator is a Celgard 2400 film. Galvanostatic charging/discharging (GCD) and cyclic voltammetry (CV) tests of Ppy@CNT were conducted on an electrochemical workstation (Bio-logic, VMP3). The Fe3O4@C were tested on a Land battery testing system. Considering the relatively low Coulumbic efficiency for metal oxides, the Fe3O4@C anodes were pre-cycled for 5 times within 0.005-3 V at 0.1 A g-1 and ended with lithiation to 1 V in a half cell before fabricating the LICs. Then LICs based on Ppy@CNT cathode and pre-lithiated Fe3O4@C anode (3:1 in mass) were fabricated. The electrochemical characterizations of LIC were performed within 1-4 V on VMP3 (Bio-logic). The calculation method of specific capacity and energy density were detailed illustrated in reported literatures6, 29.

3. Results and discussion 3.1 Structural and physical characterization Figure 1a shows the synthesis process of Ppy@CNT composites, which was obtained through an in situ chemical oxidation method. Acidified CNTs contains rich surface functionalities, including -OH, -COOH, and other oxygen containing groups, which not only enable the uniform dispersion of CNT in purified water, but also function as the nucleation sites for Ppy. Therefore, when (NH4)2S2O8 were added into the CNT and pyrrole monomer mixture solution to trigger the polymerization reaction, Ppy was coated in situ on the CNT surfaces, resulting in a Ppy@CNT core-shell composite structure. Specifically, during the initiation step, the oxidation of a pyrrole 5




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SEM and TEM were used to study the morphology of Ppy@CNT composites and pure CNT. It was observed that pure CNTs are smooth fibers with diverse diameters range from 5 nm to 10 nm and wall thickness of 3-4 nm (Figure 1b, 1c and Figure S1). After the composition with Ppy, the Ppy@CNT composites show a rough surface with obviously enlarged diameter (Figure 1c and S2). TEM characterization of Ppy@CNT-25% display a well-defined core-shell structure, and the Ppy shell (15-20 nm) is uniformly coated on the CNTs surface (Figure 1d). The inset of Figure 1d demonstrates nearly completely graphitized lattice fringe of CNT, which is consistent with pure CNT (Figure S1). Furthermore, the elemental colour mapping images demonstrate the C, N and O elements in Ppy@CNT-25%, which indicate a uniform coating of Ppy along the CNT (Figure 1e). As shown in Figure 2a, the phase compositions of as-prepared Ppy@CNT composites were further confirmed by XRD. Pure CNT displays two diffraction peaks at 2;=25.8° and 43.2° (the black line), which are the characteristic (002) and (100) planes of the hexagonal graphite structure, respectively33. Upon in situ polymerization of Ppy, the sharp peak of CNT gradually disappears while a broad and weak hump centered at 2;=21.6o is observed, indicating the amorphous structure of coated Ppy34. (a)

(b) CNT

990

1473 1104 1199 1321

1545

CNT

1417 1056

1044 966 915

Ppy@CNT-40%

930

Ppy@CNT-40%

(c) CNT

G band

D band

Ppy@CNT-40% Ppy@CNT-25%

15

30

45

Ppy@CNT-25%

Ppy@CNT-25%

Ppy@CNT-10%

Ppy@CNT-10%

2 Theta (°)

(d)

60

75

1000

2000

-1

Ppy@CNT-10%

3000

1800

Raman shift (cm ) C

O

1500

1200

900 -1

600

Wavenumber (cm )

(e) Ppy@CNT-25%

C 1s

(f)

N 1s

Ppy@CNT-25%

N

Ppy@CNT-40%

+

C-N

-NH -

3

sp C

-NH-

Ppy@CNT-25% C=O

C-O +

=NH -

O-C=O 2

Ppy@CNT-10%

1200

900

=N-

sp C

600

300

Binding energy (eV)

0

290

288

286

284

Binding energy (eV)

282 406

404

402

400

398

396

Binding energy (eV)

Figure 2. Physical characterization of Ppy@CNT and pure CNT. (a) XRD characterization, (b) Raman and (c) FTIR spectra of pure CNT and Ppy@CNT composite. (d) XPS survey results of Ppy@CNT-10%, 7



Ppy@CNT-25% and Ppy@CNT-40%. Detailed (e) C1s and (f) N1s spectra of Ppy@CNT-25%.

Figure 2b demonstrates the Raman spectra of as-prepared samples. In case of Raman spectrum of CNT, two primary peaks located at around 1348 and 1581 cm-1are clearly observed, which are typical D band and G band of carbonaceous materials6-7. In comparison, the Raman spectra of all Ppy@CNT composites reveal four extra Ppy characteristic peaks at 930, 990, 1056, and 1417 cm-1. In detail, the peak at 930 cm-1 is caused by the ring deformation vibration of bipolarons (i.e., protonated benzenoid amine (-NH+-) while the peak located at 990 cm-1 is the ring deformation of polarons (i.e., protonated quinonoid imine (=NH+-)).35 The characteristic peaks at 1056 and 1417 cm-1 are assigned to C-H in-plane deformation and C-N stretching vibration of the aromatic ring35. It should be emphasized that the intensity ratio of ID/IG in CNT clearly change after Ppy coating, demonstrating the strong interaction between the CNT and the Ppy. It is verified that highly conductive Ppy@CNT composites could be obtained owing to the enhanced interaction between Ppy and the CNTs, which will be favorable for the anchoring of Ppy conjugated backbones onto the surface of CNT. FTIR further displays the detailed chemical bonding information of Ppy@CNT (Figure 2c). In detail, characteristic FTIR peaks of Ppy appear at 1560 and 1473 cm-1, which originates from the stretching vibration of C-C and C-N bond in pyrrole ring, and suggest the presence of Ppy in the Ppy@CNT composite36. The bands of C-N stretching vibration and C-H in-plane deformation in Ppy are centered at 1321 and 1044 cm-134. Meanwhile, the bands at 915 and 1199 cm-1 suggest the doped state of Ppy, while the peaks located at 790 and 966 cm-1 indicate the polymerized pyrrole stucutre34,

37-38.

It is seen that no extra dopant was added during the polymerization reaction.

Therefore, the doping state of Ppy is most likely induced by the strong interaction between Ppy and CNT. This can be further proved by the C=O stretching vibration (1652 cm-1) in -COOH groups of the acidified CNTs. This peak gradually shifts to 1702 cm-1 for Ppy@CNT-10%, which may suggest the doping of Ppy molecules with a carboxylic group of acidified CNT25, 36. Generally, pristine Ppy is in an insulating state without doping. Hence, the doping of Ppy from the carboxylic groups in acidified CNT may increase the protonation level of Ppy, which can greatly improve the electrical conductivity of Ppy. This can be further confirmed by the following XPS analysis. As seen from Figure 2d, three distinct peaks at 284.8, 400.4and 530.8 eV were detected in the 8


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survey XPS spectra of Ppy@CNT composites, which are attributed to C, N, and O elements, respectively. Detailed elemental content is listed in Table 1. The detailed C1s spectrum is deconvoluted into six peaks, which represent sp2 hybridized carbon (284.1 eV), sp3 hybridized carbon (284.9 eV), C-N (285.5 eV), C-O (286.6 eV), C=O (287.3 eV), and O-C=O (288.7 eV), respectively (Figure 2e)36, 39. The detected C-N group originates from the pyrrole molecules, which suggests the successful integration of Ppy with CNTs. Moreover, detailed N1s spectrum of Ppy@CNT-25% could be fitted into four peaks at 402.2, 400.9, 399.9 and 398.2 eV, which corresponds to the protonated quinonoid imine (=NH+-), protonated benzenoid amine (-NH+-), benzenoid amine (-NH-) and quinonoid imine (=N-) groups, respectively (Figure 2f and Figure S3)40. The protonated imine and protonated amine cations are usually regarded as polarons and bipolarons of the PPy polymer, which can serve as the active site for electrochemical reaction. The ratio of (=NH+- and -NH+-)/(=NH+-, -NH+-, -NH- and =N-) (noted as N+/Ntotal) can indicate the protonation level of Ppy40. As shown in Table 1, the Ppy in the Ppy@CNT-25% possessed the highest protonation level over other Ppy@CNT composites, indicating more available reactive sites for electrochemical reaction40. Note that the protonation is partially caused by the doping of Ppy layer by the carboxylic group of acidified CNT, which is well consistent with the above FTIR analysis. Furthermore, the Ppy@CNT-25% composite demonstrated the highest electrical conductivity of 1.3×10-1 S cm-1, which is much higher than that of Ppy@CNT-10% (1.8×10-3 S cm-1) and Ppy@CNT-40% (3.2×10-4 S cm-1) composites (Table 1). The porous structure of Ppy@CNT composites with various mass ratios are shown in Figure S4. The N2-adsorption isotherms of all samples show typical features of IV type isotherms with well-defined hysteresis loop at the P/P0>0.8, indicating the mesopore-dominated porous structure. The SSA of Ppy@CNT-10%, Ppy@CNT-25% and Ppy@CNT-40% are measured to be 120.9 m2 g-1, 139.7 m2 g-1 and 158.2 m2 g-1, respectively. The difference in SSA may be caused by different CNT contents. The pore diameter distribution of the all Ppy@CNT composites exhibit an obviously peak centered at the pore diameter values of 20 nm. Such mesopores can improve the electrochemical properties via promoting the formation of electrical double layer and decreasing the ion transfer impedance. 9



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Table 1 Physical properties of Ppy@CNT with different ratios.

Composition

Electrical N+/Ntotal

Sample

conductivity (S cm-1)

C%

O%

N%

Ppy@CNT-10%

70.3

15.9

13.8

46.2%

1.8×10-3

Ppy@CNT-25%

72.5

15.3

12.2

48.7%

1.3×10-1

Ppy@CNT-40%

75.1

14.7

10.2

40.5%

3.2×10-4

3.2 Electrochemical performance of Ppy@CNT Ppy is one of the most frequently used electronic conducting polymers for aqueous Faradaic pseudocapacitive applications. Here, Ppy@CNT is used as the positive electrode to provide pseudocapacitance via the doping/un-doping of electrolyte PF6- anions in organic Li-ion conducting electrolyte, i.e. Ppy+PF6-

[Ppy+]PF6- + e-. During the charging process, the

protonated -NH+- and =NH+- in the monomer units of polymer chain can serve as active site for electrochemical reaction, which are compensated/doped by the PF6- anions from the electrolyte. The discharge process is completely opposite, in which the Ppy is reduced to the neutral state. According to the reaction mechanism, the theoretical specific capacity of Ppy polymer is around 400 mAh g-1. Unfortunately, conductive polymers including Ppy are suffered from low available doping levels (0.3~0.5), leading to limited specific capacity41. The charge storage characteristics of the pure Ppy, pure CNT and Ppy@CNT composites electrodes were characterized by Li half cell. The CV profiles Pure Ppy exhibits a spindle shape with small current density due to its low electronic conductivity (Figure 3a and S5a). While pure CNT present a nearly rectangular CV shape, indicating the dominant EDLC behavior of CNT (Figure 3a and S5b). In comparison, the CV curve of Ppy@CNT-25% demonstrates largely expanded current density and area than that of pure CNTs and pure Ppy (Figure 3a), indicating an excellent electrochemical performance. Moreover, there is a broad hump at around 2.5V, which corresponds to the undoping of PF6- anion from the Ppy backbone during discharge42. To reveal the capacitive storage characteristic of the Ppy@CNT cathode, CV curves of Ppy@CNT-25% were measured at a series of scan rates (Figure 3b). The cathodic peak current and the scan rate follows the equation: i=avb 10


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In which, a and b are adjustable parameters; i represent the peak current (A) and v is the scan rate (V s-1), respectively. In general, b=0.5 represents a diffusion-controlled process, while b=1.0 suggests a capacitive-dominated process. The value of b parameter is calculated from the linear relationship between log (i) and log(

High-Energy and High-Power Nonaqueous Lithium-Ion Capacitors

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