Conjugated Microporous Polymers with Tunable Electronic Structure

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Conjugated Microporous Polymers with Tunable Electronic Structure for High Performance Potassium-Ion Batteries Chong Zhang, Yu Qiao, Peixun Xiong, Wenyan Ma, Panxing Bai, Xue Wang, Qi Li, Jin Zhao, Yunfeng Xu, Yu Chen, Jing Hui Zeng, Feng Wang, Yunhua Xu, and Jia-Xing Jiang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b08046 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 3, 2019

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Conjugated Microporous Polymers with Tunable Electronic Structure for High Performance Potassium-Ion Batteries Chong Zhang†‖, Yu Qiao‡‖, Peixun Xiong§, Wenyan Ma†, Panxing Bai§, Xue Wang†, Qi Li‡, Jin Zhao§, Yunfeng Xu†, Yu Chen†, Jing Hui Zeng†, Feng Wang※, Yunhua Xu§*, Jia-Xing Jiang†* †

Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University),

Ministry of Education, Key Laboratory for Macromolecular Science of Shaanxi Province, Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, Shaanxi, 710062, P. R. China. ‡

Graduate School of System and Information Engineering, University of Tsukuba, 1-1-1,

Tennoudai, Tsukuba 305-8573, Japan. §

School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, P. R.

China. ※

Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of

Technology, Wuhan, 430073, P. R. China.

ABSTRACT: Conjugated microporous polymers (CMPs) with π-conjugated skeletons show great potential as energy storage materials due to their porous structure and tunable redox nature. However, CMPs and the structure-performance relationships have not been well explored for potassium-ion batteries (KIBs). Here, we report the structure-engineered CMP anodes with tunable electronic structures for high performance KIBs. The results demonstrate that electronic structure of the CMPs plays an important role in enhancing potassium storage capability, including the lowest unoccupied molecular orbital (LUMO) distribution, LUMO energy level and band gap, which can be finely tuned by synthetic control. It was revealed 1

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that the poly(pyrene-co-benzothiadiazole) (PyBT) with optimized structure delivers a high reversible capacity of 428 mAh g1, and shows an excellent cycling stability over 500 cycles. Our findings provide a fundamental understanding in the design of CMP anode materials for high performance potassium-organic energy storage devices.

KEYWORDS: conjugated microporous polymer; building block; electronic structure; potassium-ion battery Energy storage is of great importance for sustainable development. Considering the ever-increasing demand for energy storage devices, particularly for the large-scale energy storage systems, it is highly desirable to develop advanced energy storage technologies beyond lithium-ion batteries (LIBs). As a kind of competitive energy storage devices, potassium-ion batteries (KIBs) have received much recent interest due to the resource affordability, natural abundance of K element and the similar redox potential of K+/K (2.93 V vs standard hydrogen electrode) to that of Li+/Li (3.04 V).1,2 Nonetheless, the large volume change caused by the insertion/extraction of large K ions commonly leads to poor cycling stability.3-8 Therefore, it is still a big challenge to achieve viable anode materials for KIBs with high capacity and stable cycling performance.2,9-13 Conjugated microporous polymers (CMPs),14,15 as a powerful platform for challenging ever-worsening environment and energy related issues, have been intensively explored as adsorbents,16,17 heterogeneous catalysts,18,19 light-harvesting materials,20,21 light-emitting materials,22,23 optoelectronic materials,24,25 and photocatalysts.26-29 Owing to the extended -conjugation along the polymer skeletons, excellent physicochemical stability, plentiful porous structure, and high surface area, CMPs have been emerging as promising energy storage materials.30,31 The conjugated polymer skeleton enables CMPs a reversible dopingdedoping capacity as the conventional conjugated polymers.32-34 Meanwhile, the 2


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inherent porous structure and high surface area can provide large void space to accommodate charge carriers and large contact surface areas to shorten the ion diffusion distance, ensuring a high electrochemical activity and fast kinetics.30,35-38 In addition, the highly crosslinked polymer structures can effectively suppress the dissolution of active materials in organic electrolytes, enhancing the cycling stability.35,39 To date, a range of CMPs electrode materials has been developed for energy storage devices,30,39-41 particularly for LIBs. While the potential of CMPs as electrodes has been demonstrated, the associated design rules and the structure–property relationships have not yet been well explored. In this work, we present an approach to tune systematically the electronic structure of CMPs, and demonstrate the influence of electronic structure on the electrochemical performance of the resulting CMP anodes for KIBs. The results demonstrated that the electrochemical performance of the resultant CMPs is strongly dependent on the lowest unoccupied molecular orbital (LUMO) distribution, which can be finely tuned by synthetic control. The delocalized LUMO distribution benefits the high degree of charge delocalization along the polymer skeleton during the charge-discharge processes, enabling a high redox activity. The delocalized charge also decreases the charge density of redox active sites, restraining the side reaction between polymer and electrolyte to form solid-electrolyte interphase (SEI).42 The low LUMO energy level and narrow band gap endow the CMPs with a high electron affinity and conductivity. As a result, the polymer PyBT with a high degree of charge delocalization, a low LUMO energy level and a narrow band gap delivers a high reversible capacity of 428 mAh g1 at 30 mA g1, and shows an excellent cycling stability with the capacity retention of 272 mAh g1 after 500 cycles at 50 mA g1. The K ion storage behavior in PyBT was also investigated by the ex situ Fourier transform infrared (FT-IR) spectroscopy.

3


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RESULTS AND DISCUSSION Preparation and characterization of the CMPs. Two series of CMPs were synthesized via Pd(0)-catalyzed

Suzuki-Miyaura

polycondensation

of

bromated

benzene

(Bz)

or

benzothiadiazole (BT) with different tetra-functionalized building blocks of biphenyl (Ph), spirobifluorene (SF) and pyrene (Py) derivatives, respectively. No Pd residue was detected by the energy dispersive X-ray spectroscopy (EDS) measurement for all CMPs after exhaustive purification (Figure S1), implying that the Pd catalyst was completely removed. Figure 1a & b show the notional polymer structures of the resulting CMPs. Unlike organic small molecules and most linear conducting polymers, these CMPs are insoluble in common organic solvents because of their highly crosslinked structures and rigid polymer skeletons (Figure S2), thus being capable of delivering high cycling stability. FT-IR spectra confirmed the polymer structures with the characteristic peaks of the aromatic ring at 1456 cm1 and the C=N stretching vibrations at 1546 and 1625 cm1 for the CMPs (Figure S3).28,43-45 The polymer structures were also characterized by solid state 13C NMR spectroscopy (Figure S4). The peaks ranging from 120 to 152 ppm are assigned to the C atoms in aromatic rings. The peak at 159 ppm corresponds to the C atom of the C=N bond in benzothiadiazole unit.28 All the CMPs display impressively thermal stability, with a decomposition temperature up to 400 o

C in nitrogen atmosphere (Figure S5a). Powder X-ray diffraction profiles revealed an

amorphous structure of the CMPs (Figure S5b), which is in accordance with the reported CMPs.27,30,46 Scanning electron microscope (SEM) images showed that all the CMPs have nano-particle morphologies with particle sizes of 30200 nm, except for PhBT, which exhibits a nano-fiber morphology with the diameter of about 15 nm (Figure S6).

4


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Benzene-containing CMPs

(a)

Benzothiadiazole-containing CMPs N S N

S N N

S N

N N N S

S

N S N

N

N

N

N

800

PhBz PyBz SFBT

700 600

SFBz PhBT PyBT

500 400 300 200 100 0 0.0

0.2

0.4

0.6

0.8

1.0

N S N

S N N

N N S

N S N

N

N S

PyBT

7

(c)

S

SFBT

PhBT

Differential volume (cm3 g-1)

(b)

PyBz

SFBz

PhBz

Quantity adsorbed (cm3 g-1)

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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6

(d) PyBT

5

SFBT

4

PhBT

3

PyBz

2

SFBz

1

PhBz

0 1

10

Pore diameter (nm)

Relative pressure (P/Po)

Figure 1. The notional polymer structures of (a) the Bz-containing CMPs and (b) the BT-containing CMPs. (c) Nitrogen adsorption (filled symbols)desorption (open symbols) isotherms. (d) Pore size distribution curves calculated by NL-DFT. The nitrogen adsorption–desorption isotherms indicated the existence of abundant micropores in these CMPs showing the high nitrogen adsorption at the low relative pressure (P/P0 < 0.001, Figure 1c).47 Among these polymers, SFBT exhibits the typical type I adsorption–desorption isotherms, featuring microporous structure, while other CMPs show the mixture adsorption isotherms of type I and II.47 The hysteresis loop upon the N2 5


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desorption branch could be ascribed to the elastic deformations or swelling behavior of the porous polymers caused by the high gas adsorption quantity.48 The pore size distribution of the CMPs was calculated from nonlocal density functional theory (NL-DFT). All the CMPs, except for SFBT, show a similar micropore diameter of around 0.94 nm with spot mesopores at around 2.1 and 5.0 nm (Figure 1d). SFBT displays mainly micropores centered at around 1.0 and 1.3 nm, which agrees with the nitrogen adsorption–desorption isotherms. The Brunauer-Emmet-Teller (BET) surface areas were measured to be 377, 673, 1214, 463, 305 and 493 m2 g1 for PhBz, SFBz, PyBz, PhBT, SFBT and PyBT, respectively. PyBz shows the highest surface area among these resulting CMPs, which could be attributed to the high rigidity of the polymer skeleton due to the planarity molecular structure of pyrene, and the lower steric hindrance of the building block of benzene.49-51 Structure-performance relationships of the CMPs anodes for KIBs. The fundamental electrochemical performance of the CMPs for KIBs as anodes was investigated using coin-type cells with K as the counter electrode. The solution of 0.8 M KPF6 in ethylene carbonate/diethyl carbonate was used as the electrolyte. Figure 2a & b show the galvanostatic chargedischarge (GCD) curves of the second cycle for the KIBs fabricated from the Bz-containing and BT-containing CMPs at 50 mA g1. The sloping potential plateaus at 1.32.5 V was observed in the charge profiles of the BT-containing CMPs (Figure 2b), which can be ascribed to the dedoping of BT units.42 The BT-containing CMPs show higher polarization than the Bz-containing CMPs (Figure 2a & b), which could be attributed to the introduction of electronwithdrawing BT unit increases the electron affinity of the BT-containing CMPs. Among the BT-containing CMPs, PyBT shows significantly increased discharge potential because of its low LUMO level (Figure 3a) and the small polarization (Figure 2b).52-54 The reversible capacity (charge capacity at the second cycle) are 215, 183, 170, 139, 242 and 338 mAh g1 for PhBz, SFBz, PyBz, PhBT, SFBT and PyBT, respectively 6


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(Figure 2a, b). In the case of the Bz-containing CMPs, the discharge capacities decrease when varying the building blocks from Ph to SF and to Py. For the BT-containing CMPs, however, it is interesting that the reversible capacities display a contrary trend to that of the Bz-containing CMPs. Recently, we found that the surface area shows a significant influence on the electrochemical performance of thiophene-containing polymers.30 However, the BET surface area and the reversible capacity in these CMPs do not share the same trend (Figure 1c, 2a & b). The possible reason could be ascribed to that all of these CMPs have high BET surface area; thus the influence of specific surface area on the redox activity is not distinct in these resulting CMPs. Therefore, the difference in the capacity changing trend for the two series of CMPs implies that there exists a strong relationship between the n-doping capability of the polymers and their chemical and electronic structures. In order to uncover the structure-performance relationships of the CMPs, we adopted the BT-containing CMPs as the examples to investigate the influence of the structural evolution on their redox activity. The cyclic voltammetry (CV) measurements at different scan rates from 0.1 to 1.5 mV s1 were conducted to investigate the redox behavior of the BT-containing CMPs (Figure 2c & S7). The peak current intensity shows a linear relationship as the square roots of the scanning rates (Figure 2d), indicating that the capacities of the CMPs are mainly contributed from the diffusion controlled process,5 although they have high surface area (>300 m2 g1), which might be attributed to the relatively low electron conductivity of these organic semiconducting electrode materials. The diffusion coefficients of K+ ions in the CMPs were calculated from galvanostatic intermittent titration technique (GITT, Figure 2e & S8). SFBT shows a slightly higher diffusion coefficient than PhBT and PyBT, indicating that the mass transformation is not the limitation at such low current density (30 mA g1).55 It should be noted that the three BT-containing CMPs show the same level of ion diffusion rate, although they have different porosities (Please see Supporting Information and Figure S8 for 7


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the explanation in detail). The kinetics of the CMPs was further analyzed using electrochemical impedance spectroscopy (EIS) measurement (Figure 2f). The EIS data were ﬁtted with equivalent circuits (Figure S9 & Table S1). PyBT and SFBT show the similar charge transfer resistance, lower than that of PhBT, indicating faster redox reaction kinetics in PyBT and SFBT electrodes. 3.0

(a)

2.5

Voltage ( V vs. K+/K)

Voltage ( V vs. K+/K)

3.0

2.0 1.5

-1

50 mA g PhBz SFBz PyBz

1.0 0.5 0.0

0

100

200

300

400

2.0 1.5

-1

50 mA g PhBT SFBT PyBT

1.0 0.5 0.0

500

(b)

2.5

0

Specific capacity (mAh g-1)

Unit: mV s-1 0.1 0.3 0.5 1 1.5

-0.2 -0.4 -0.6 -0.8 0.0

0.5

1.0

1.5

2.0

2.5

-1

500

0.15 0.10

PhBT SFBT PyBT

0.05 0.00

3.0

0.01

0.02

0.03

Scan rate

1E-10

1/2

((V

-1 1/2

s )

0.04 )

8000

(e)

(f) 6000

-Z'' (ohm)

1E-12 1E-13 1E-14 1E-15

+

400

0.20

Potential (V vs. K+/K)

PhBT SFBT PyBT

1E-16 1E-17

300

(d)

0.0

1E-11

200

0.25

(c) Peak current (A g-1)

Current density (A g-1)

0.2

100


0.4

K diffusivity ( cm2 s )

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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0

20

4000

PhBT SFBT PyBT

2000

40

60

80

0

100

0

Depth of discharge (%)

2000

4000

6000

8000

Z' (ohm)

Figure 2. The GCD curves of the second cycle for (a) the Bz-containing CMPs and (b) the BT-containing CMPs at 50 mA g1. (c) CV curves of PyBT at different scan rates. (d) The 8


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fitted lines between peak current and the square roots of scan rates. (e) Diffusion coefficients calculated from GITT. (f) Electrochemical impedance spectra of PhBT, SFBT and PyBT. The electronic structure is an important factor affecting redox activity of the CMP electrodes, since the redox behavior originates from the n-doping/dedoping process. For example, Yu et al. reported that the band gap shows critical leverage on the redox activity of quinone electrodes.55 The energy band structures of the CMPs were investigated through UV-Vis reflectance spectroscopy (Figure S10) and CV (Figure S11). Both the LUMO energy level and band gap exhibit a decreased tendency from PhBT to SFBT to PyBT, as observed for the Bz-containing CMPs (Figure 3a). Based on the molecular orbital theory, a narrow band gap indicates high electron conductivity, and a low LUMO energy level means a high electron affinity.52 Hence, the decreased LUMO energy level and band gap manifest the increased n-doping ability of the CMPs with the building blocks from Ph to SF and to Py, which is in line with the EIS result. However, it is noteworthy that the reversible capacity for the Bz-containing CMPs shows a decreased tendency from PhBz to SFBz to PyBz with the decreased band gap and LUMO energy level. This could be explained by the difference in LUMO distribution of the CMPs, since electrons are first filled into and extracted from the LUMO orbits of the CMPs during the potassiation and depotassiation process based on the frontier molecular orbital theory, the LUMO distribution also affects the activity of the CMPs during the n-doping process.34,56 In principle, the high delocalization degree of the LUMO orbits leads to the high n-doping capability of conjugated polymer.34 The LUMO distribution of the two series of CMPs was calculated by density functional theory (DFT) using the Gaussian 09 program at B3LYP/6-31G (d) level.57 As for the Bz-containing CMPs, PhBz shows a relative uniform LUMO distribution on the Bz and Ph units (Figure 3b), while the LUMO orbits are mainly concentrated on the SF and Py units for SFBz and PyBz, respectively, which could be 9


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attributed to the increased planarity and conjugation of the building blocks of SF and Py compared to Ph. Owing to the strong electro-withdrawing ability of BT unit, the LUMO orbits mainly spread on the BT unit in the BT-containing CMPs, and expanded LUMO distribution was obtained from PhBT to SFBT and to PyBT with increasing the conjugation degree of the building blocks from Ph to SF and to Py (Figure 3c). This indicates that the electron delocalization is enhanced and the charge density decreases from PhBT to SFBT and to PyBT, leading to the increased n-doping ability. The high charge density of active sites can easily trap alkaline metal ions and trigger the decomposition reaction of the electrolytes, and thus resulting in a high irreversible capacity and a low initial Coulombic efficiency (Figure S12).42,58 This result was further evidenced by the matching degree of the LUMO energy levels between the two building blocks in all of the alternative copolymers. The LUMO energy levels of the building blocks employed in this work were also calculated by DFT and displayed in Figure 3d. For the Bz-containing CMPs, the differences in LUMO energy level between Bz and Ph, SF, Py building blocks are 0.75, 0.91 and 1.56 eV, respectively (Figure 3e). As for the BT-containing CMPs, these values are 1.66, 1.5 and 0.85 eV between BT and Ph, SF, Py building blocks, respectively (Figure 3e). It is well known that the closer LUMO energy level of the building blocks leads to the higher degree of the LUMO delocalization,59 and thus the high n-doping capability. As a result, the polymer PhBz, combined with expanded LUMO distribution and the smallest LUMO energy level difference, shows the highest discharge capacity among the Bz-containing CMPs (Figure 2a), as observed for PyBT (Figure 2b). These prove that, apart from the band gap and LUMO energy level, the LUMO distribution of CMPs and the difference in LUMO energy level between the two building blocks also play crucial roles in the improvement of capacity for CMPs-based KIBs. Therefore, the selection of suitable building blocks is an important basis in the design of CMPs electrodes for high-performance energy storage devices. 10


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(a) -2.0 Energy levels (eV)

-2.5

LUMO

-2.66 -2.99

-3.0

-3.18

-3.5 -4.0

3.05

2.75

-4.5

2.37

-3.37

-3.64

-3.77

2.14

1.95

-5.78 SFBT

-5.72 PyBT

2.34

-5.0 -5.5 HOMO -5.71 PhBz

-6.0

-5.55

-5.74 SFBz

PyBz

-5.71 PhBT

(b)

PhBz

SFBz

PyBz

(c)

PhBT

SFBT

PyBT

0 -1

0.1 LUMO

-0.65

-0.81 -1.46

-2

-2.31

-3 -4 -5 -6 -7

HOMO

-6.04

-5.6

-5.35 -6.67

-6.72

LUMO difference (eV)

(e)

(d)

Energy level (eV)

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1.8 1.6 1.4 1.2 1.0 0.8 Difference from Bz Difference from BT

0.6 0.4

Ph Bz

Ph

SF

Py

SF

Py

BT

Figure 3. (a) The energy levels for the CMPs. (b) The LUMO diagrams for the Bz-containing Bz Ph SF Py BT CMPs and (c) the BT-containing CMPs. (d) The energy levels and their diagrams for the building blocks. (e) The LUMO energy level differences between the building blocks. 11


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0.1

0.1


0.0 -0.1 -0.2

1st 2nd 3rd

-0.3 0.5

1.0

1.5

2.0

2.5

(b)

0.0 -0.1 -0.2 -0.3

1st 2nd 3rd

-0.4 -0.5

3.0

0.0

0.5

(c)

2.5

1st 2nd 3rd 4th 5th

2.0 1.5

Voltage (V vs. K+/K)

+

2.0

2.5

3.0

3.0

3.0

Voltage (V vs. K /K)

1.5



1.0 0.5 0.0

1.0

0

200

400

600

mA g-1 30 50 100

2.0 1.5 1.0

200

300 500

0.5 0.0

800 1000 1200

(d)

2.5

0


1200 1000

Discharge Charge

800 600

30 50

400

30 mA g 100

200

200

-1

300 500

0 0

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-1

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(e)

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-1

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Discharge Charge Coulombic efficiency

600 400 200 0

60 40 20

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Coulombic efficiency (%)


(a)

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Cycle number

Figure 4. CV curves of (a) PyBT and (b) PyBz at 0.1 mV s1. (c) GCD curves of PyBT in the first 5 cycles at a current density from 30 mA g1. (d) GCD curves of PyBT at current density from 30 to 500 mA g1. (e) Rate performance of PyBT at current density from 30 to 500 mA g1. (f) Cycling stability and Coulombic efficiency of PyBT at 50 mA g1. Electrochemical performance of PyBT. The electrochemical performance of the most redox-active PyBT was further investigated by CV and galvanostatic chargedischarge 12


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measurements. In the first cycle, strong cathodic peaks were observed below 1.0 V, which are attributed to the n-doping of conjugated polymer chains as well as the decomposition of the electrolyte to form SEI films.60 Compared with PyBz, PyBT shows a higher initial reduction voltage than PyBz (Figure 4a & b), indicating that PyBT has a higher activity to accept electrons due to the introduction of electron-withdrawing BT unit.55 In addition, PyBT shows a stronger anodic current response than PyBz, again demonstrating the higher redox activity of PyBT. The CV curves in the subsequent cycles exhibit a typical n-doping/dedoping behavior of conjugated polymers for all of the resulting CMPs (Figure 4a, b & S13).30,40 PyBT shows a high reversible charge capacity of 428 mAh g1 and stable charge-discharge behavior after two cycles at a low current density of 30 mA g1 (Figure 4c). After abstracting the capacity contribution of carbon black (70 mAh g1) in the active electrode (Figure S14), PyBT can still deliver a high reversible capacity of 358 mAh g1. The irreversible capacity for the first cycle is due to the formation of SEI, according with the CV curves (Figure 4a). The low initial Coulombic efficiency might be improved by using artificial SEI61,62 or stable electrolytes.63 In addition, it was found that the initial Coulombic efficiency increases with the delocalized degree of the LUMO distribution (Figure S12), since the delocalized LUMO distribution leads to the low charge density of polymer chains, suppressing the chemical attack of active sites from electrolyte solvents.42 As thus, optimizing the CMP structure to obtain highly delocalized LUMO distribution along the polymer chains can be another efficient strategy to increase the initial Coulombic efficiency. To the best of our knowledge, the reversible capacity for PyBT is among the highest values of reported anodes for KIBs.2,5-7,9,64-68 The rate performance of PyBT was investigated at different current densities from 30 to 500 mA g1 (Figure 4d & e). PyBT can maintain a reversible capacity of 104 mAh g1 even at a high current density of 500 mA g1, demonstrating an excellent rate performance. Impressively, PyBT shows an outstanding long-term cycling stability with a 13


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high reversible capacity of 272 mAh g1 and a Coulombic efficiency of 99.5 % after 500 charge-discharge cycles (Figure 4f). The electrode material can maintain the nanoparticles morphology well after long-term cycling (Figure S15), demonstrating the high structure stability of PyBT. Potassium storage mechanism of PyBT. To further insight into the K storage mechanism of PyBT, FT-IR spectroscopy was conducted at different discharge-charge states (Figure 5a & b). The pristine PyBT electrode clearly displays the peaks of C=N (1627 cm1),43-45 C=C (1637 and 1618 cm1)69 and CH bonds in pyrene (1198 and 850 cm1).69,70 Along with the discharge process, the intensity of these peaks gradually decreased and finally disappeared at full discharge state at 0.1 V. Similar phenomena were observed by other groups previously.71,72 The decreased intensity of the C=N, CH and C=C peaks in the FT-IR spectra during the potassiaton process can be explained by the n-doping process of the C=N bond of BT unit and the CH, C=C bonds in aromatic rings. After depotassiation, the peaks of C=C, C=N and CH bonds were recovered completely (curves of A and H, Figure 5b), indicating a reversible doping/dedoping process. It was noted that a new peak emerged at 1456 cm1 at discharge state and disappeared upon depotassiation, showing a reversible behavior. This should be assigned to the uptake of K ions due to the n-doping process of the C=N bond and aromatic rings. The recovered C=N peak of PyBT (from point F to G in Figure 5a) implies the accomplishment of dedoping for BT unit. Therefore, the platform in the GCD curves can be assigned to the doping/dedoping of the BT units (Figure 2b), which is absent in the GCD curves of the Bz-containing CMPs without BT units (Figure 2a). As thus, the introduction of BT units benefits to improve the redox activity of the CMP electrodes. According to the above discussion, the proposed K storage mechanism was shown in Figure 5c. Taking into account that the electron-withdrawing ability of BT is stronger than that of Bz, Ph, SF, and Py, the BT unit should have the priority to be doped. This is also supported by the LUMO 14


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distribution, which delocalizes more on BT units. Thus, the BT content in the CMPs may play an important role to affect the n-doping ability of the BT-containing CMPs. The BT contents based on mass ratio of the repeat unit are 64.1, 46.1 and 57.5% for PhBT, SFBT and PyBT, respectively. This does not follow the same trend as the specific capacity of the polymers, again highlighting the significance of electronic structure on the electrochemical performance.


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0 200

(a)

A

(b)

B

C-H

C=N 1456 C=C

C-H

400

B C D E F G H

C

600 800

D

Discharge Charge

1000 1200

E

F

1400 H

A

G 750

3.0 2.5 2.0 1.5 1.0 0.5 0.0 Voltage (V vs K/K+)

1000 1250 1500 -1 Wavenumber (cm ) K

(c)

N S N

S N

N

N

S N

K K

S N

1750

K N

K Discharge Charge K

N

N S

N S

N

N K

N S

K K

N

S N K

Figure 5. Electrochemical and structural evolution of CMP anodes during discharge and charge process. (a) The charge/discharge curves of the first cycle at 30 mA g1 for PyBT. (b) The ex situ FT-IR spectra of PyBT-based battery recorded at different states. (c) The proposed K storage mechanism.

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CONCLUSIONS In conclusion, two series of CMPs with different building blocks were synthesized and employed as anodes for KIBs. The comparative study on structure-performance relationships revealed that the electronic structures, particularly the LUMO orbits distribution on the polymer skeleton, have strong effect on the K ions storage capability. The results clearly demonstrate that the high delocalization degree of LUMO orbits endows the CMPs with high n-doping activity. The low LUMO energy level and narrow band gap provide the CMPs with high electrochemical performance due to the enhanced electron affinity and conductivity, the highly porous structure and high surface area provide plentiful active sites and large free space for the accommodation of K ions. As a result, the polymer PyBT consisting of pyrene and benzothiadiazole units shows excellent electrochemical performance for KIBs with a high reversible capacity of 428 mAh g1 at 30 mA g1, and outstanding cycling stability with a capacity retention of 272 mAh g1 after 500 cycles at 50 mA g1. This work highlights the crucial role of electronic structure in the rational design of conjugated microporous polymer anodes for KIBs with high capacity and long cycling life. Considering the various synthetic strategies, design flexibility, tunable electronic structures and the variety of building blocks of CMPs, there is a wealth of opportunity for developing other CMPs electrode materials for high performance energy storage devices. EXPERIMENTAL SECTION Polymer synthesis. All the CMPs were synthesized by Suzuki coupling reaction. A representative experimental procedure for PyBT is given as an example: A mixture of 4,7-dibromo-2,1,3-benzothiadiazole

(588.0

mg,

2.0

mmol),

tetrakis(triphenylphosphine)palladium(0) (15.0 mg, 0.01 mmol), K2CO3 (1.1 g , 8.0 mmol), and 1,3,6,8-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)pyrene (706.0 mg, 1.0 16


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mmol) in water (4 mL) and DMF (20 mL) was stirred at 150 oC for 48 h at nitrogen atmosphere. After cooling down to temperature, the resulting precipitate was collected by filtration and thoroughly washed with methanol, H2O, acetone and CH2Cl2, the product was collected and dried in vacuum at 80 oC for 24 h. Characterization. The thermal properties of the conjugated microporous polymers were evaluated by thermogravimetric analysis (TGA) on a differential thermal analysis instrument (Q1000DSC+LNCS+FACS Q600SDT) over the temperature ranging from 30 to 800 oC with a heating rate of 10 oC min1 under a nitrogen atmosphere. FT-IR spectra of the polymers were collected in transmission on a Tensor 27 FT-IR spectrometer (Bruker) and FT/IR-6200 spectrometer (JASCO) using KBr disks. For the ex situ FT-IR analysis of PyBT at the different voltage, the FT-IR measurements were carried out on the FT-IR-6200 spectrometer. Typically, 64 interferograms were accumulated for one spectrum with a resolution of 4.0 cm1. For pretreatment of the cycled electrode plates, the cycled cells were transferred into an Ar glove box once the discharge (or charge) finished, and the electrodes were twice rinsed by dimethoxyethane (DME, Sigma Aldrich, 99%) to wash/clean off the electrolyte salt and the residual solvent, and then evaporated in a vacuum chamber, connected to the glove box, for ~15 min. The dried electrodes were moved back to the glove box. Then, the electrode film was scratched off (nearly 1×2 mm2) and ground together with potassium bromide (KBr, FT-IR grade, purity of >99 %, Sigma Aldrich) in an Ar-filled glove box. The KBr powder was dried in vacuum at 100 °C for 24 h before test. The mixture powder was pressed into hyaline pellets in vacuum under high pressure (4.0 Mpa) for 5 min. The pellets were rapidly transferred into the IR sample loading chamber, in which continuously purged with argon protection gas. Elemental analysis was conducted on EURO EA30000 Elemental Analyzer. Solid state magic angle spinning

13

C CP/MAS NMR measurement was investigated on a

JEOL RESONRNCE ECZ 400R NMR spectrometer at a MAS rate of 15 kHz. Field-emission 17


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scanning electron microscopy (SEM) (SU8020, Hitachi) was employed to characterize the morphology of the polymers. Powder X-ray diffraction (PXRD) measurement was conducted on X-ray Diffractometer (D/Max-3c). ASAP 2420-4 (Micromeritics) volumetric adsorption analyzer was used to measure surface areas and pore size distributions by nitrogen adsorption and desorption at 77.3 K. The nitrogen adsorption data in the relative pressure (P/P o) ranging from 0.05 to 0.20 were used to calculate the surface area. Pore size distributions and pore volumes were derived from the adsorption branches of the isotherms using non-local density functional theory (NL-DFT). Samples were degassed at 120 °C for 12 h under vacuum (105 bar) before analysis. UV-vis diffuse reflection was collected on a scanning UV-vis spectrophotometer (PerkinElmer, US) with an integrating sphere assembly, using BaSO4 as a reflectance sample. For the calculation of band structures of the CMPs, cyclic voltammetry measurements were carried out on a CHI660E (Chenhua, Shanghai, China) electrochemical workstation in a three-electrode-cell system, glassy carbon electrode as the working electrode, Hg/HgCl2 electrode as the reference electrode, platinum wire as the counter electrode. The polymer sample was mixed with 5 wt% Nafion, then dropped cast on the top of a glassy carbon working electrode, which was dried in a vacuum chamber for 60 min before test. The measurement was carried out in a nitrogen saturated 0.1 M solution of tetrabutylammonium hexafluorophosphate (NBu4PF6) and acetonitrile with a scan rate of 0.1 V s–1. All cyclic voltammetry curves were recorded three times to get the average value. All measurements were calibrated against an internal standard of ferrocene (Fc), the ionization potential (IP) value of which is −4.8 eV for the Fc/Fc+ redox system. The DFT calculations were carried out using Gaussian 09 at the B3LYP/6-31g(d) level. Batteries fabrication and electrochemical measurements. The working electrode was prepared by mixing the active material, acetylene black and sodium alginate in a mass ratio of 5:3:2 using water as solvent and ethanol as dispersant. The resulting slurry was cast onto 18


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copper foil current collector. The foil was rolled into 22 μm thin sheet, and then dried at 50 o

C for 6 h, and then at 110 oC for 12 h in a vacuum oven. The total mass loading of the

electrode material is around 1.4 mg cm2, corresponding to a mass loading around of 0.7 mg cm2 for the active material. The electrodes were cut into disks with the diameter of 9 mm. CR2032 coin-type cells were assembled in an argon-filled glove box with potassium foil as the counter electrode and macroporous polypropylene film (Celgard 2400) as the separator. The batteries were aged 5 h before testing. The electrolyte is 0.8 M KPF6 in a mixture of ethylene carbonate (EC), and diethyl carbonate (DEC) (1:1, v/v). The galvanostatic charge and discharge measurements were carried out by the NEWARE-BTS-5 V/1 mA testing instrument (Neware Co., Ltd., Shenzhen, China) in a voltage ranging from 0.1 to 3 V with the current densities of 30, 50, 100, 200, 300, 500 and 1000 mA g1 at room temperature. The galvanostatic intermittent titration technique (GITT) measurement was performed by a current pulse at 30 mA g1 for 0.5 h followed by a 3 h relaxation process. Cyclic voltammetry (CV) measurements were carried out on a CHI660E electrochemical workstation (Chenhua Co., Ltd., Shanghai, China) at scan rates of 0.1, 0.3, 0.5, 1 and 1.5 mV s1 at room temperature. The Electrochemical impedance spectra measurements were carried out on a CHI660D electrochemical workstation in the frequency range from 100 kHz to 10 mHz at an AC oscillation of 5 mV. ASSOCIATED CONTENT Supporting information The supporting information is available free of charge on the ACS Publications website at https://doi.org/XXX Synthesis and characterization (FT-IR, solid state NMR, XRD, TGA, SEM and UV-vis spectra) of the CMPs, CV curves of CMPs, the GITT profiles and equivalent circuits from EIS data for the BT-containing CMPs, the initial charge/discharge profiles for all the CMPs, 19


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the HOMO diagrams for the CMPs, the charge/discharge curves of carbon black and the resistances of the BT-containing CMPs. AUTHOR INFORMATION Corresponding author: *Email: [email protected]. *Email: [email protected]. Author Contributions ‖

C. Zhang and Y. Qiao contributed equally to this work.

ORCID Jia-Xing Jiang: 0000-0002-2833-4753 Yunhua Xu: 0000-0003-1818-3661 ACKNOWLEDGEMENTS J.-X.J. thanks the financial support from National Natural Science Foundation of China (21574077 & 21304055), 111 project (B14041), the Fundamental Research Funds for the Central Universities (2016TS064 & GK201801001). Y.X. acknowledges the financial support from National Natural Science Foundation of China (Grand No. 51672188). REFERENCES (1) Marcus, Y. Thermodynamic Functions of Transfer of Single Ions from Water to Nonaqueous and Mixed Solvents: Part 3-Standard Potentials of Selected Electrodes. Pure Appl. Chem. 1985, 57, 1129–1132. (2) Eftekhari, A.; Jian, Z.; Ji, X. Potassium Secondary Batteries. ACS Appl. Mater. Interfaces 2017, 9, 4404–4419.

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Table of Content

600

-1

Specific capacity (mAh g )

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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500 400 300 200 100 0

SFBT

PhBT

PyBT

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Conjugated Microporous Polymers with Tunable Electronic Structure

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