A High-Performance Ionic Liquid Based Gel Polymer Electrolyte

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Energy, Environmental, and Catalysis Applications

A High-Performance Ionic Liquid Based Gel Polymer Electrolyte Incorporating Anion-Trapping Boron Sites for All-Solid-State Supercapacitors Application Mengyuan Jin, Yifan Zhang, Chaojing Yan, Yanbao Fu, Yanhui Guo, and Xiaohua Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00083 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

A

High-Performance

Ionic

Liquid

Based

Gel

Polymer

Electrolyte Incorporating Anion-Trapping Boron Sites for All-Solid-State Supercapacitors Application Mengyuan Jin2, Yifan Zhang2, Chaojing Yan2, Yanbao Fu3, Yanhui Guo2*, Xiaohua Ma1*

1

Institute of Special Materials and Technology, Fudan University, Shanghai, 200433,

China 2

3

Department of Materials Science, Fudan University, Shanghai, 200433, China Environmental and Energy Technologies Division, Lawrence Berkeley National

Laboratory, University of California, Berkeley, CA 94720, USA

ABSTRACT: A high-performance boron-containing gel polymer electrolyte (GPE) with semi-interpenetrating polymer network (IPN) structure was successfully prepared by a rapid and easy one-step polymerization process assisted with UV light, exploiting PEO as a polymer host, the novel borate ester monomer (BEM) as the cross-linker, and LiClO4 and EMIMBF4 both as the plasticizer and electrolytic salt. Owing to the incorporation of anion-trapping boron sites, the ionic conductivity of the as-prepared GPE at room temperature can be up to 5.13 mS cm-1. And the boron-containing gel polymer electrolyte (B-GPE) exhibits favorable mechanical strength, excellent thermal stability and extremely low flammability. Moreover, the all-solid-state symmetric supercapacitor using B-GPE as electrolyte and reduced graphene oxide as electrode was fabricated and exhibited a broad potential window

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(3.2 V). The all-solid-state symmetric supercapacitor based on B-GPE can still reach a high energy density of 27.62 Wh kg-1 with a power density of 6.91 kW kg-1 at high current density of 5 A g-1. After 5000 cycles at current density of 1 A g-1, the all-solid-state supercapacitor with B-GPE displays decent capacitance retention of 91.2%.

KEYWORDS: gel polymer electrolyte, photopolymerization, anion-trapping boron sites, ionic liquid, semi-IPN, all-solid-state supercapacitors


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1. INTRODUCTION Supercapacitors (SCs) or electrochemical capacitors, as the energy storage devices which can bridge the power/energy gap between traditional capacitors and batteries, are considered to be pretty potential candidates for the application in portable electronics, hybrid electric vehicles and other large-scale electrical equipment, owing to their high power densities and outstanding cycle performance.1-3 Compared with the conventional liquid electrolytes, polymer electrolytes have attracted considerable research attentions with the increasing demand for flexible, lightweight and wearable energy devices. Polymer electrolytes used in all-solid-state SCs not only avoid the safety risk such as leakage and volatility, but also greatly reduce the cost of encapsulation and provide more design opportunities for novel configuration of devices.4-7 Polymer electrolytes are usually divided into two types: gel polymer electrolytes (GPEs) (consisting of polymer matrix, plasticizer and supporting salt) and solid polymer electrolytes (SPEs) (without any solvent).8 However, given that the ultralow room-temperature ionic conductivity of SPEs limits the further utilization in SCs, GPEs are preferable and have been extensively prepared and investigated for all-solid-state SCs applications.9-12 Among the various polymer framework materials involving poly(ethylene oxide) (PEO), poly(vinylidene fluoride) (PVDF), poly(methylmethacrylate) (PMMA), poly(acrylonitrile) (PAN), PEO and its derivatives have been widely used in GPEs as matrix attributed to the strong interaction between the ethylene oxide (EO) groups and



metal ions.8,13,14 Nevertheless, the PEO-based GPEs swollen with organic solvents usually have poor mechanical strength and may result in the short circuit of devices. In order to solve the problems of weak mechanical properties, the interpenetrating chemically cross-linked polymer network can be introduced to improve dimensional stability and simultaneously lower the crystallinity of PEO.15-18 Since many investigations have indicated that the ionic conductivity of GPEs significantly influences the electrochemical and capacitance performance of devices,19,20 it is urgently compulsory to prepare original GPEs equipped with superior ion transport properties. With an empty p-orbital, boron atom can serve as an acidic center that interacts with anion of electrolyte salt to improve the dissociation of cation-anion pairs and consequently enhance ionic conductivity.21 In recent years, a number of previous papers have been published where boron-containing segments were incorporated into the polymer matrix,22,23 the inorganic filler24,25 and the electrolyte salts26,27 of GPEs via different synthetic methods. The boronic acids react easily with diols to form borate esters and the esterification or transesterification reaction can proceed under mild conditions28, which provides more opportunities to design novel boron-containing polymers or monomers utilized in GPEs to improve the conductivity. Moreover, the electrochemical stability window (ESW) of supporting electrolytes largely restricts the operating voltage window of SCs that considerably impacts on the energy density. In comparison with the aqueous electrolyte exhibiting narrow ESW (~1.2V) and organic electrolyte with relatively higher ESW (~2.7V),


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ionic liquids (ILs) possess wider ESW (~4.5V), which is propitious to improve the energy and power density of SCs.3,29 In addition, ILs have other unique properties such as high ionic conductivity, outstanding thermal/electrochemical stability, negligible volatility, and low flammability.30 Therefore, ILs are the suitable candidate to substitute traditional liquid electrolytes of SCs and have attracted considerable investigation attention.20,29,31,32 In our previous study, the interaction between the heterocyclic borane groups and ClO4- has been confirmed by DFT calculation and the prepared boron-containing solid polymer electrolyte exhibited high ionic conductivity.23 Based on that, a novel boron-containing functional cross-linker was designed and synthesized through a simple transesterification reaction, which can effectively improve the ionic conductivity and mechanical strength of GPE at the same time. In this work, we report on the preparation, physicochemical and electrochemical characterization, and the application on the supercapacitors of boron-containing gel polymer electrolyte (B-GPE). The semi-IPN boron-containing GPE were prepared through one-step polymerization of a mixture containing a novel borate ester monomer (BEM), PEO and 1 M LiClO4/EMIMBF4 assisted with UV light. The prepared B-GPE exhibits high ionic conductivity (5.13 mS cm-1), favorable mechanical strength and excellent thermal stability. Furthermore, the all-solid-state symmetric supercapacitor using B-GPE as electrolyte and reduced grapheme oxide (r-GO) as electrode was fabricated and measured, exhibiting excellent energy and power delivery, outstanding cycle stability and good tolerance to the high temperature.



2. EXPERIMENTAL SECTION Materials. PEO (average Mv = 3×105), poly(ethylene glycol) methacrylate (PEGMA, average Mn = 500), poly(ethylene glycol) dimethacrylate (PEGDA, average Mn = 750), the photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA, 99%), trimethyl borate (TMB, ≥99.0%) were purchased from Sigma-Aldrich. Glycerol monomethacrylate (GMMA, 96%) was obtained from J&K Scientific. Acetonitrile (Sinopharm, 99.0%) was refluxed with CaH2 (Sinopharm) and distilled under reduced pressure, then stored with molecular sieves (4 Å, Sinopharm) before use. Lithium perchlorate (LiClO4, battery grade, Sigma-Aldrich) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4, 99.0%, Sigma-Aldrich) were stored in an argon filled glove box (MBRAUN, LABstar, O2 and H2O content < 0.1 ppm). All the other reagents were used without any further purification. Synthesis of Novel Borate Ester Monomer (BEM). The novel borate ester monomer (BEM) was synthesized through a simple transesterification reaction33,34, as presented in Scheme 1. TMB (2.08 g, 0.02 mol) and GMMA (3.20 g, 0.02 mol) were dissolved in 50 mL anhydrous acetonitrile and the solution was stirred at 60 °C for 1 h. PEGMA (10 g, 0.02 mol) was subsequently added to the solution and continuously stirred for another 2 h at 60 °C. After the reaction, the residual solvent was removed by evaporation under reduced pressure and the remaining viscous liquid was dried under vacuum at 60 °C for 24 h. The final product was obtained with almost 100% yield and stored in an argon filled glove box. Scheme 1. Synthesis of the novel borate ester monomer (BEM)


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Preparation of Semi-IPN Boron-Containing Gel Polymer Electrolytes. The semi-interpenetrating polymer network (IPN) gel polymer electrolytes incorporating anion-trapping boron sites were prepared via one-step solution-casting method assisted with UV light. PEO, BEM and photoinitiator DMPA (3 wt% of BEM content) were added to 1 M LiClO4/EMIMBF4 electrolyte solution with different mass ratios and stirred to obtain homogeneous viscous mixer at 60 °C. The precursor solution was poured into a 0.5 mm thick polytetrafluoroethene (PTFE) groove and exposed to the UV light (MXGainLAB40-40A, 1750 mW cm−2) for 5 minutes to form the semi-IPN structure. The whole procedure of GPE preparation is exhibited in Scheme 2 in detail. As shown in Table S1 (Supporting Information), in comprehensive consideration of ionic conductivity at room temperature and mechanical properties, the best mass ratio of GPE consist of PEO, BEM and 1 M LiClO4/EMIMBF4 is 1:15:45. The prepared GPE with this mass ratio containing borate ester functional groups is abbreviated as B-GPE. In order to understand the role that borate ester functional groups play in GPE system, PEGDA was used as the substitute of BEM to prepare the GPE with the same mass ratio for comparison, named as P-GPE. (Scheme S1, Supporting Information)



Scheme 2. Schematic illustration for the preparation process of semi-IPN gel polymer electrolytes incorporating anion-trapping boron sites.

Fabrication of All-Solid-State Supercapacitors. R-GO was obtained simply by a reduction reaction between hydrazine hydrate and graphene oxide (GO) synthesized via a modified Hummer method35,36. First, 80 wt% rGO, 10 wt% acetylene black and 10 wt% PVDF were homogeneously mixed with N-methyl-2-pyrrolidone (NMP) to form the slurry. Then the obtained slurry was coated on the rounded nickel foam substrate (14 mm diameter and 1 mm thickness) and dried at 110 °C under vacuum for 12 h. The r-GO electrode (0.12 mm thickness) was finally prepared under the pressure of 3 MPa. An all-solid-state symmetric supercapacitor consists of two r-GO electrodes and the GPE film sandwiched between them. All the components were assembled into 2016 coin cell in glove box. While, the conventional SCs consist of


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two working electrodes and a separator (Celgard, 2730) using 1 M LiClO4/EMIMBF4 as liquid electrolyte. Characterization of BEM and the boron-containing GPE. The 1H-NMR, 13

C-NMR and

11

B-NMR spectra of BEM and reactants dissolved in d-DMSO were

obtained using a Bruker spectrometer (500 MHZ) with tetramethylsilane (TMS) as the internal reference. Fourier transform infrared (FT-IR) spectra was recorded on Nicolet Nexus 470 spectrometer in the frequency range from 400 to 4000 cm-1. The morphology of the boron-containing GPEs was analyzed by a field-emission scanning electron microscopy (FE-SEM, FEI Nova NanoSEM 450). The X-ray diffraction (XRD) measurement was performed on an X-ray diffractometer (Bruker AXS D8 Advance) using Cu Kα radiation. The thermal stability was evaluated via thermogravimetric Analysis (TGA, TA Instruments Q500) under N2 flow at a heating rate of 20 °C min-1 from 40 °C to 600 °C. Stress-strain test of GPE membranes was performed using a CMT4104 universal tensile testing machine at a tensile rate of 2mm min-1 at room temperature. The ionic conductivity of the GPEs between 0 °C and 90 °C was measured by electrochemical impedance spectroscopy (EIS) using CHI 660E electrochemical workstation in the frequency range from 1 Hz to 1 MHz with potential amplitude of 5 mV. Each sample was kept at each testing temperature for at least 0.5 h before measurement. The GPE films were sandwiched between two stainless-steal electrodes for measurement. The ionic conductivity of GPE was calculated according to the equation (1):



(1)

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𝜎𝜎 = 𝑑𝑑/(𝑅𝑅 × 𝑆𝑆) where σ is the ionic conductivity of GPE (mS cm-1), d is the thickness of the

GPE membrane (cm), the resistance R is the intercept at the real axis in the impedance Nyquist plot (Ω), S is the contact area between GPE and stainless steel electrode (cm2). Electrochemical Characterization of All-Solid-State Supercapacitors. All the electrochemical tests were carried out at ambient temperature using a symmetric two-electrode capacitors configuration on a CHI 660E electrochemical workstation. Cyclic voltammetry (CV) measurements were performed in the potential range from 0 V to 3.2 V at various scan rates (10 mV s-1, 20 mV s-1, 50 mV s-1, 100 mV s-1 and 200 mV s-1). Galvanostatic charge and discharge (GCD) was conducted in the same potential range at various current densities (0.5 A g-1, 1 A g-1, 2 A g-1, 3 A g-1, 5 A g-1 and 10 A g-1). EIS test was carried out in the frequency range from 0.1 Hz to 1 MHz with potential amplitude of 5 mV. The mass specific capacitance (Cs, F/g) of the symmetric SCs device was calculated based on the GCD curves according to the following equation: 𝐶𝐶s = (𝐼𝐼 × ∆𝑡𝑡)/(𝑚𝑚 × ∆𝑉𝑉)

(2)

where I is the discharge current (A), △t is the discharge time (s), m is the total

mass (g) of active materials on two electrode, △V is the actual voltage excluding IR

drop during the discharge process (V).


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The energy density (E, Wh kg-1) and power density (P, kW kg-1) of the symmetric SCs was calculated by using equations (3) and (4): 1

(3)

𝐸𝐸 = 2 𝐶𝐶s ×△ 𝑉𝑉 2

(4)

𝑃𝑃 = 𝐸𝐸/△ 𝑡𝑡

where Cs (F/g) is the mass specific capacitance of the symmetric SCs based on the total mass, △V (V) is the actual voltage excluding IR drop, △t (s) is the discharge time.

3. RESULTS AND DISCUSSION Physicochemical Characterization of Borate Ester Monomer (BEM). The successful synthesis of borate ester monomer (BEM) was confirmed by 1H-NMR and FT-IR spectroscopy. Figure 1a shows the 1H-NMR spectra of the reactants (GMMA and PEGMA) and the product BEM. Compared with 1H-NMR spectra of the reactants (GMMA and PEGMA) where the proton peaks perfectly correspond to their structure, the hydroxyl responding proton peaks at 4.55 ppm (PEGMA, signal e), 4.66 ppm (GMMA, signal f) and 4.92 ppm (GMMA, signal g) disappear, verifying the formation of borate ester structure. Overlapping signals observed at 6.03 ppm (a+l), 5.69 ppm (b+m) and 1.88 ppm (c+k) are assigned to methacrylate groups of BEM. Signals in the range of 3.51−4.20 ppm (d+h+i+j) consist of peaks attributed to



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ethylene oxide units of PEGMA and GMMA. The borate ester structure of BEM was further analyzed by 11B-NMR spectra as presented in Figure 1b, where single peak at 23.046 is ascribed to the tricoordinated boron atom.

13

C-NMR analysis was also

performed to confirm successful synthesis of BEM. (Figure S1, Supporting Information) The FTIR spectra of the reactants (GMMA, PEGMA and TMB) and the product (BEM) are revealed in Figure 1c. Obviously, the hydroxyl peaks in the range of 3200−3600 cm-1 are almost invisible in the spectrum of BEM, in comparison with the peaks at 3417 cm-1 (−OH, stretching vibration, GMMA) and 3412 cm-1 (−OH, stretching, PEGMA), indicating the formation of borate ester structure. In addition, the peak at 1353 cm-1 is assigned to the stretching vibration of B−O bond on BEM, deviating slightly from the peak at 1363 cm-1(B−O, stretching, TMB). The peaks appearing at 2876 cm-1 (−CH3, stretching), 1719 cm-1 (C=O, stretching), 1638 cm-1 (C=C, stretching), 1172 cm-1 and 1110 cm-1 (C−O, stretching, EO units) are well in accordance with the structure of BEM.


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Figure 1. (a) 1H-NMR spectrum of the reactants (GMMA and PEGMA) and the product BEM. Character a-m represents the H atom with different chemical shifts. (b) 11

B-NMR spectrum of the product BEM. (c) FT-IR spectra of the reactants (GMMA,

PEGMA and TMB) and the product BEM. Characterization of Boron-Containing Gel Polymer Electrolytes (B-GPE). The optical image of obtained B-GPE membrane with a thickness of 300-400 µm is presented in Figure 2a, indicating that the B-GPE film synthesized by one step solution-casting method assisted with UV light is free-standing, thin and uniform. From the SEM image of the surface and cross-section of B-GPE membrane shown in



Figure 2b and Figure S2a (Supporting Information), the morphologies of the surface and cross-section are very even and smooth, which reveals that the semi-IPN polymer host can absorb a large amount of liquid component (EMIMBF4) to form a homogeneous ion gel and solid salt LiClO4 is completely dissociated in the gel system. To investigate the structure of the polymer host of B-GPE, the polymer matrix was separated from B-GPE as described in Supporting Information, named B-GPE matrix. In contrast with the smooth appearance of the B-GPE film surface, the surface of B-GPE matrix exhibited in Figure 2c is rugged and raised polymeric backbones can be obviously observed. The magnified SEM images presented in Figure 2d and 2e demonstrate that there are lots of grooves of various shapes and sizes on the surface of B-GPE matrix. And the similar morphology of the cross-section of the B-GPE matrix film in Figure S2b (Supporting Information) further clarified that plenty of empty microdomains exist on the surface and in the inside of B-GPE matrix membrane, which can hold quantities of liquid electrolyte and effectively enhance the ionic conductivity of B-GPE. As can be seen in the SEM image of the interface between r-GO electrode and B-GPE (Figure 2f), the B-GPE membrane compactly sticks to the surface of r-GO electrode, which can significantly lower the interface resistance and improve the electrochemical performance of the all-solid-state SCs. Furthermore, the element (C, O, B and N) mapping of B-GPE film verifies that the prepared B-GPE is highly homogeneous.


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Figure 2. (a) The optical image of B-GPE membrane with a thickness of 300-400 µm. SEM image of (b) the surface of B-GPE membrane, (c) the rugged surface of B-GPE matrix, (d, e) the magnified image with many grooves and (f) the interface between r-GO electrode and B-GPE. (g) Element mapping image of the surface of B-GPE membrane. The UV irradiation polymerization reaction of PEO, BEM and 1 M LiClO4/EMIMBF4 mixture was elucidated by FT-IR spectroscopy. As can be observed from the FT-IR spectrum of EMIMBF4 in Figure 3a, the appearance of peaks at 3165 and 3123 cm-1 (C−H, aromatic stretching , imidazole ring), 2991 cm-1 (C−H, aliphatic stretching), 1635 cm-1 (C=N, ring stretching), 1574 and 1463 cm-1 (C=C, ring stretching), 1172 (aromatic C-H, deformation vibration) and 1063 cm-1 (C+−BF4−, stretching) are almost accordant with the previous reported result.37 In comparison with the FT-IR spectrum of EMIMBF4, other appearing peaks at 3420 cm-1 (−OH,



stretching), 2876 cm-1 (−CH3, stretching) and 1719 cm-1 (C=O, stretching) in the spectrum of B-GPE indicate that the prepared B-GPE system includes all the components (PEO, BEM and 1 M LiClO4/EMIMBF4). However, the peak at 1635 cm-1 of B-GPE spectrum (C=N bond of EMIMBF4) that is similar to the peak value of C=C bond may disturb the judgment of polymerization reaction. Therefore, liquid electrolyte EMIMBF4 was removed to obtain the polymer host of B-GPE as mentioned above. The FT-IR spectrum of B-GPE matrix shows that the C=C bond peak at 1638 cm-1 completely disappeared compared with the spectrum of BEM, verifying the complete polymerization reaction of BEM. The X-ray diffraction (XRD) patterns of pure PEO, B-GPE matrix and B-GPE are demonstrated in Figure 3b. The sharp characteristic peaks at the 2θ value of 19.16° and 23.33° exhibited in the XRD pattern of pure PEO reveals the semi-crystalline structure of PEO, which is detrimental to ion transport and thus greatly lower the ionic conductivity.38 As can be seen from the XRD pattern of B-GPE matrix, the cross-linked framework of polymerized BEM incorporating into PEO polymer host to form semi-IPN structure (see Scheme 2) resulted in distinct reduction in the intensity and crystalline of XRD characteristic peaks of PEO. The addition of liquid electrolyte 1 M LiClO4/EMIMBF4 further decreased the degree of crystallinity and no obvious characteristic XRD peaks of PEO can be observed from the XRD pattern of B-GPE, suggesting that the whole gel polymer electrolyte system is nearly amorphous. The reduction of the degree of crystallinity can be also confirmed by the DSC thermograms shown in Figure S3 (Supporting Information).


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From the DSC curve of PEO, the endothermic peak at 66.45 °C corresponds to the melting temperature (Tm) of PEO. With the incorporation of cross-linked network and the addition of ionic liquid and lithium salt, the melting temperature dropped to 59.75 °C and the melting enthalpy value (∆Hm) significantly shifted from 152.6 J/g to 9.694 J/g, proving the almost amorphous state of B-GPE.

Figure 3. (a) FT-IR spectra of the mixture (PEO, BEM and 1 M LiClO4/EMIMBF4), the obtained B-GPE film formed by UV irradiation and B-GPE matrix. (b) XRD patterns of pure PEO, B-GPE matrix and B-GPE membrane. As discussed above, the structure of B-GPE matrix shown in SEM image can absorb quantities of liquid electrolyte and the dramatic decrease of crystallinity of PEO revealed in XRD analysis can accelerate the ion transport. Both of them are beneficial to effectively improve the ionic conductivity of B-GPE, indicating the high ionic conductivity of B-GPE. The room temperature ionic conductivity of boron-containing gel polymer electrolyte with different mass ratios of components (PEO:BEM:1 M LiClO4/EMIMBF4) is listed in Table S1 (Supporting Information). Boron-containing GPE in the mass ratio of 1:12:45 (named B-GPE) exhibits highest



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ionic conductivity of 5.13 mS cm-1 at 25 °C. The conductivity performance is outstanding compared with other previous reported papers (Table S2, Supporting Information), in coordination to the investigation results of SEM and XRD. Temperature impendence of ionic conductivity (log σ versus 1000/T) of B-GPE, P-GPE (adding PEGDA as the substitute of BEM) and 1 M LiClO4/EMIMBF4 with commercial separator from 0 to 90 °C can be observed in Figure 4. The thermal dependence of conductivity (log σ versus 1000/T ) obeys the Vogel–Tamman–Fulcher (VTF) equation, which is commonly employed to clarify the ion transport in amorphous polymer electrolyte11,39:

σ = A𝑇𝑇 −1/2 exp(−𝐸𝐸a /R(𝑇𝑇 − 𝑇𝑇0 ))

(5)

Where A is the pre-exponential factor, T is the absolute temperature, Ea is a constant proportional to the activation energy for conduction, R is the ideal gas constant (8.314 J mol-1 K-1) and T0 is a parameter correlated to the glass transition temperature. These VTF fitting parameters (A, Ea and T0) for the B-GPE, P-GPE and 1 M LiClO4/EMIMBF4 with commercial separator are listed in Table 1. The glass transition temperature (Tg) of B-GPE and P-GPE were measured by DSC (Figure S4, Supporting Information) and are also summarized in Table 1. The pre-exponential factor A reflects the number of charge carriers and the highest value of A (0.894 S cm-1 K-1/2) and the highest conductivity are observed for B-GPE, indicating that the value of A is proportional to the ion conductivity. A close value of the Tg to T0 for B-GPE and P-GPE is normal and well rationalized. And the VTF fitting parameters Ea of B-GPE (2.09 kJ mol-1) is lower than that of P-GPE (3.28 kJ mol-1), demonstrating


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that the movement of ions in B-GPE is easier than that in P-GPE, which is probably ascribed to the incorporation of anion-trapping boron sites.40 The anion-trapping properties of Lewis acidic boron sites can be evaluated by measuring the lithium ion transference number (tLi+). The calculated tLi+ values of B-GPE, P-GPE and 1 M LiClO4 /EMIMBF4 with commercial separator are 0.626, 0.360 and 0.193 shown in Figure S5 (Supporting Information), illustrating the role of boron sites in the immobilization of the anion. As a result, the incorporated anion-trapping boron sites can improve the dissociation of cation-anion pairs and consequently enhance ionic conductivity of B-GPE, in coordination with the higher ionic conductivity of B-GPE (5.13 mS cm-1 at 25 °C) than that of P-GPE (2.14 mS cm-1) listed in Table 1. Table 1. The values of ionic conductivity (σ) at 25 °C and VTF fitting parameters in Figure 4.

Sample

σ at 25 °C (mS cm-1)

A (S-1 cm-1 Ea (kJ mol-1)

T0 (K)

Tg (K)

-1/2)

K

B-GPE

5.13

0.894

2.09

197.16

205.09

P-GPE

2.14

0.848

3.28

178.09

187.94

1 M LiClO4/EMIMBF4 (with commercial separator)

2.88

0.119

0.99

207.30

/

Apart from ionic conductivity, mechanical strength of GPE membrane is another essential property for the application in the all-solid-state SCs. The stress-strain curves of B-GPE is shown in Figure S6. The tensile strength of B-GPE film is 1.30 MPa with a maximum strain of 62.8%, which is enough for the use in all-solid-state SCs as polymer electrolyte.



Figure 4. Temperature impendence of ionic conductivity (log σ versus 1000/T) of B-GPE, P-GPE and 1 M LiClO4/EMIMBF4 with commercial separator. The solid lines represent VTF fitting results. It is well known that traditional organic liquid electrolyte such as propylene carbonate (PC) and DMF are flammable and tend to trigger explosion when the devices are exposed to the conditions such as high temperature, short circuits and overcharging.41 The combustion tests of pure liquid electrolyte (1 M LiClO4/DMF), GPE based on PEO and BEM polymerized with 1 M LiClO4/DMF as liquid electrolyte and B-GPE membrane are exhibited in Figure 5a-c. When the pure liquid electrolyte (1 M LiClO4/DMF) and GPE with 1 M LiClO4/DMF as liquid electrolyte were set on fire, they were highly combustible and burst into flame quickly. On the contrary, when the B-GPE was set on fire, the film cannot be ignited and demonstrated an extremely low flammability, which can provide a higher safety performance when applied in all-solid-state SCs. Moreover, the thermal stability of B-GPE was evaluated by TGA and the TGA thermograms of B-GPE, pure PEO, 1 M LiClO4/EMIMBF4 and EMIMBF4 are shown in Figure 5d. EMIMBF4 and pure PEO are thermally stable up to about 400 °C and show well-defined one-step degradation


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processes. While the decomposition temperature of 1 M LiClO4/EMIMBF4 is slightly lower (about 360 °C) compared with EMIMBF4. The prepared semi-IPN B-GPE exhibits a two-step weight loss process: the first step occurs at around 278 °C corresponding to the decomposition of the B-GPE matrix (see Figure S7, Supporting Information). The second one corresponds to the degradation of 1 M LiClO4/EMIMBF4 occurring at 360 °C. Overall, the prepared B-GPE is thermally stable up to about 278 °C under N2 atmosphere at 5% weight loss. In comparison with GPE containing 1 M LiClO4/DMF that decomposed at only 90 °C with 5% weight loss (Figure S7, Supporting Information), B-GPE demonstrates better thermal stability for application in all-solid-state SCs. The extremely low flammability and remarkable thermal stability of B-GPE are pretty significant to improve the safety of SCs.

Figure 5. Flammability comparison: the combustion test of (a) pure liquid electrolyte



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(1 M LiClO4/DMF), (b) gel polymer electrolyte based on PEO and BEM polymerized with 1 M LiClO4/DMF as liquid electrolyte and (c) B-GPE membrane. (d) TGA thermograms of B-GPE, pure PEO, 1 M LiClO4/EMIMBF4 and EMIMBF4. Electrochemical Performance of All-Solid-State Supercapacitors. The electrochemical performance of the B-GPE based all-solid-state supercapacitor with two r-GO electrodes was evaluated by cyclic voltammetry (CV) and galvanostatic charge and discharge (GCD) tests. For comparison, the all-solid-state supercapacitor with P-GPE and conventional supercapacitor with 1 M LiClO4/EMIMBF4 were also fabricated and tested in the same way. And all the electrochemical characterizations were performed at room temperature. As shown in Figure S8 (Supporting Information), the electrochemical stability window of all-solid-state supercapacitor with B-GPE is 0-3.2 V. Figure 6a shows the CV curves for a symmetric supercapacitor using B-GPE as electrolyte at different scan rates from 10 to 200 mV s-1. The B-GPE based all-solid-state supercapacitor exhibits a wide potential window of 3.2 V and similar rectangular shape of the CV loops. The charge-discharge curves of all-solid-state supercapacitor with B-GPE at different current densities from 0.5 to 10 A g-1 in the voltage range of 0−3.2 V are depicted in Figure 6b and symmetrical charge-discharge plots indicate a reversible ion adsorption/desorption process at the surface of r-GO electrode.42 The CV and GCD testing results of all-solid-state supercapacitor

with

P-GPE

and

conventional

supercapacitor

with

1

M

LiClO4/EMIMBF4 can be seen from Figure S9 (Supporting Information). To investigate the electrochemical performance of B-GPE based all-solid-state


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supercapacitor more intuitively, the CV and GCD measurements of supercapacitor with B-GPE were compared with those of supercapacitors with P-GPE and 1 M LiClO4/EMIMBF4. Figure 6c and Figure 6d reveal the combined CV profiles of all-solid-state SCs with B-GPE and P-GPE, and conventional supercapacitor with 1 M LiClO4/EMIMBF4 at scan rates of 25 mV s-1 and 200 mV s-1. As shown in Figure 6c, all the CV curves at low potential scan rate of 25 mV s-1 show nearly rectangular shape, indicating the ideal double layer capacitor behavior.43 The all-solid-state supercapacitor with B-GPE illustrates almost the same capacitive performance as the conventional one with 1 M LiClO4/EMIMBF4, but higher than that of all-solid-state supercapacitor with P-GPE. When the potential scan rate reaches 200 mV s-1, all the rectangular CV shapes become distorted mainly owing to the charge transport resistance.9 However, the high-rate voltammogram of B-GPE based all-solid-state supercapacitor shows higher capacitive performance than the other two devices, suggesting the effective charge storage process using B-GPE as electrolyte.



Figure 6. Electrochemical characterization of the all-solid-state supercapacitor with B-GPE: (a) CV curves at different scan rates from 10 mV s-1 to 200 mV s-1; (b) GCD curves at different current densities from 0.5 A g-1 to 10 A g-1 in the voltage range of 0−3.2 V. Comparison of CV measurements of all-solid-state SCs with B-GPE and P-GPE, and conventional supercapacitor with 1 M LiClO4/EMIMBF4 at scan rates of (c) 25 mV s-1 and (d) 200 mV s-1. The charge-discharge curve of B-GPE device at a current density of 1 A g-1 was compared to that of the counterpart with P-GPE and 1 M LiClO4/EMIMBF4. All of them demonstrate a nearly triangular linear behavior as shown in Figure 7a, which is in agreement with the capacitive charge-discharge mechanism of the ideal double layer capacitor43, just as the CV curves reveal. At the current density of 1 A g-1, the B-GPE based all-solid-state supercapacitor exhibits a comparable mass specific


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capacitance (34.35 F g-1) calculated using GCD cycling to that of the conventional one with 1 M LiClO4/EMIMBF4 (36.68 F g-1), higher than that of P-GPE device (28.47 F g-1). In addition, it is clearly observed that the IR drop of B-GPE device (89 mv) is less than that of P-GPE device (244 mv), which is associated with the overall resistance of the supercapacitor and has significant impact on the electrochemical performance of device. Figure 7b summarizes the variation of IR drop with different current densities and shows a linear behavior with the increase of charge current density, where the overall resistance of the supercapacitor is determined from the slope of the linear relationship.44 From Figure 7b, the overall resistance of B-GPE based all-solid-state supercapacitor is a little higher than that of the conventional one with 1 M LiClO4/EMIMBF4 and much lower than that of P-GPE device, making a difference in the capacitance performance of devices. The mass specific capacitances obtained for SCs with B-GPE, P-GPE and 1 M LiClO4/EMIMBF4 calculated using GCD cycling at different current densities are given in Figure 7c. The B-GPE based supercapacitor exhibits a good retention (57.4%) of specific capacitance from a low current density of 0.5 A g-1 to a high one of 10 A g-1, while the P-GPE device shows a drastic drop in capacitance at higher current density, resulting in a poor capacitance retention of 20.5%. The higher capacitance retention of B-GPE based supercapacitor can be ascribed to the lower overall resistance. Nyquist plots of all-solid-state SCs with B-GPE and P-GPE, and conventional supercapacitor with 1 M LiClO4/EMIMBF4 with a frequency range of 10 mHz to 100 kHz are superimposed and revealed in Figure 7d. All the systems show a typical



behavior of capacitive and porous interfaces. As presented in Figure 7d, the Nyquist plots exhibit two feature regions: a depressed semicircle formed at high frequency and a line obtained at low frequency. At the high frequency region, the intercept point on the real axis represents bulk resistance of the supercapacitor including the resistance of the electrolyte and the internal resistance of the electrodes. The diameter of the depressed semicircle is charge transport resistance, which is related to the interface nature of electrolyte and electrode. At the low frequency region, the vertical straight line indicates the ion diffusion behavior and the larger the line slope, the smaller the diffusion resistance.42,45 As can be observed from the inset in Figure 7d, the B-GPE device demonstrates comparable bulk and interface resistance with conventional supercapacitor with 1 M LiClO4/EMIMBF4, which is ascribed to the high ionic conductivity of B-GPE and the compactly connected interface between r-GO electrode and B-GPE (see Figure 2f). Compared with the other two devices, the supercapacitor with P-GPE exhibits larger intercept on the real axis and diameter of semicircular at the high frequency region, suggesting the poor connected interface, which is in accordance with the drastic drop in capacitance performance at higher current density as discussed above. In addition, the B-GPE supercapacitor shows a more vertical line at low frequency indicating the faster ion diffusion behavior.45 On the whole, the B-GPE based all-solid-state supercapacitor demonstrates outstanding capacitance performance and high capacitance retention among the three kinds of devices, which can be attributed to the low overall resistance of device due to the high conductivity of the B-GPE (5.13 mS cm-1) along with the compactly connected


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interface electrode-electrolyte interface.

Figure 7. Comparison of electrochemical performance of all-solid-state SCs with B-GPE and P-GPE, and conventional supercapacitor with 1 M LiClO4/EMIMBF4: (a) GCD curves at a current density of 1 A g-1; (b) IR drop at different current densities from 0.5 A g-1 to 10 A g-1; (c) Specific capacitances of SCs at different current densities; (d) Nyquist plots of SCs with a frequency range of 10 mHz to 100 kHz. The inset reveals a magnified view of the impedance spectra in high frequency region. For testing the thermal resistance of the B-GPE based all-solid-state supercapacitor, the electrochemical measurements were also carried out at various temperature ranged from 0 °C to 80 °C (Figure 8a and Figure S10, Supporting Information). The supercapacitor functioned well in the wide temperature range due to the remarkable thermal stability of B-GPE electrolyte. With temperature rising



from 0 °C to 80 °C, the specific capacitance increased from 21 to 74 F g-1 at current density of 2 A g-1 owing to the enhanced ion mobility. Moreover, at high temperature, the device still maintains the high potential window (3.2 V), suggesting negligible decomposition of the B-GPE. Ragone plots (variation of energy density with respect to power density) of all-solid-state supercapacitor with B-GPE and conventional supercapacitor with 1 M LiClO4/EMIMBF4 are depicted in Figure 8b. Despite conventional supercapacitor with 1 M LiClO4/EMIMBF4 demonstrates lower internal resistance than B-GPE device, the B-GPE supercapacitor still exhibits comparable energy and power density attributed to the higher capacitance retention of B-GPE (Figure 7c). Typical charge-discharge characteristics of symmetric SCs with B-GPE, P-GPE and 1 M LiClO4/EMIMBF4 are listed in Table S3 (Supporting Information). To further understand the outstanding performance of all-solid-state supercapacitor with B-GPE, the energy and power densities achieved by the B-GPE device recorded with GCD tests were compared with those previously reported gel-type symmetric supercapacitors11,12,18,32,46 and it delivers superior energy and power density among those works. (Figure 8c) The all-solid-state B-GPE supercapacitor achieved an excellent energy density of 54.20 Wh kg-1 at the power density of 0.79 kW kg-1 and maintained 16.18 Wh kg-1 at a high power density of 11.42 kW kg-1, where the wide potential window of 3.2 V can improve the energy density effectively because the energy density is the square of the operating voltage shown in equations (3). At the high current density of 5 A g-1, the device with B-GPE can still reach a high energy density of 27.62 Wh kg-1 at a power density of 6.91 kW kg-1. And our device can


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lighten red LED light up to 50 s after charging for only 90s (inset in Figure 8c) Moreover, the cycle stability and durability of B-GPE supercapacitor are evaluated using GCD test at a current density of 1 A g-1 for 5000 cycles (Figure 8d). The capacitance of B-GPE device remained 91.2% of its initial value after 5000 cycles, higher than the capacitance retention (71.3%) of conventional supercapacitor with 1 M LiClO4/EMIMBF4.

Figure 8. (a) CV curves of the all-solid-state supercapacitor with B-GPE under various temperatures from 0 °C to 80 °C at a scan rate of 50 mV s-1. (b) Ragone plots of the all-solid-state supercapacitor with B-GPE and conventional supercapacitor with 1 M LiClO4/EMIMBF4. (c) Ragone plots of the all-solid-state supercapacitor with B-GPE and others from previous articles for comparison (inset: photograph of a LED light powered by the all-solid-state supercapacitor with B-GPE). (d) The cycling



performance of the all-solid-state supercapacitor with B-GPE and conventional supercapacitor with 1 M LiClO4/EMIMBF4 at a current density of 1 A g-1 .

3. CONCLUSIONS In summary, semi-IPN boron-containing GPE was successfully prepared by mixing the novel monomer BEM, PEO and 1 M LiClO4/EMIMBF4 followed by one-step polymerization assisted with UV light. The incorporation of anion-trapping boron sites is beneficial to improve the ionic conductivity and lithium ion transference number of GPE, and the formed semi-IPN enhances the mechanical strength of B-GPE membrane. And the added ionic liquid EMIMBF4 endows the GPE excellent thermal stability and a wider operating potential window up to 3.2 V. As a whole, the prepared B-GPE exhibits favorable mechanical strength, excellent thermal stability, extremely low flammability, high ionic conductivity (5.13 mS cm-1) and lithium ion transference number (0.626), where the last two properties are contributed to the outstanding electrochemical performance of supercapacitor. The all-solid-state supercapacitor based on B-GPE shows a broad electrochemical window (3.2 V) and small bulk and interface resistance with a higher specific capacitance of 34.35 F g-1 than 28.47 F g-1 of P-GPE device at the current density of 1 A g-1. In a wide temperature range from 0 °C to 80 °C, the B-GPE device demonstrates decent capacitive behavior. The Ragone plot of all-solid-state B-GPE based supercapacitor indicates that the device can achieve a maximum energy density of 54.20 Wh kg-1 and a maximum power density of 11.42 kW kg-1. At the high current density of 5 A g-1,


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the device with B-GPE can still reach a high energy density of 27.62 Wh kg-1 at a power density of 6.91 kW kg-1. After 5000 cycles, the symmetric supercapacitor with B-GPE displays better capacitance retention of 91.2% than that of conventional supercapacitor with 1 M LiClO4/EMIMBF4 (71.3%). The all-solid-state B-GPE based supercapacitor exhibited excellent energy and power delivery, outstanding cycle stability and good tolerance to the high temperature, which may bring new opportunities for the device configuration to replace the liquid electrolyte based systems and improve the safety of the devices.



ASSOCIATED CONTENT Supporting Information Supporting Information is available from the ACS Publications website. Room temperature ionic conductivity and mechanical properties of GPE membrane with different mass ratios (Table S1), the ionic conductivity of GPEs based on ionic liquid or boron-containing structure of other papers for 13

comparison (Table S2),

C-NMR spectrum of PEGMA, GMMA and BEM

(Figure S1), the detailed procedure about the separation of the polymer matrix from B-GPE, SEM images of cross-section of B-GPE membrane and B-GPE matrix (Figure S2), DSC curves of PEO and B-GPE membrane (Figure S3), DSC curves of B-GPE and P-GPE (Figure S4) , Chronoamperometry profiles for B-GPE, P-GPE, 1 M LiClO4/EMIMBF4 (Figure S5), Stress-strain curves of B-GPE-1:12:45 and B-GPE-1:12:40 (Figure S6), TGA thermograms of B-GPE, GPE with 1 M LiClO4/DMF and B-GPE matrix (figure S7), the measurement of the electrochemical stability window of all-solid-state supercapacitor with B-GPE (Figure S8), CV and GCD measurements of SCs with P-GPE and 1 M LiClO4/EMIMBF4 (Figure S9), typical charge-discharge characteristics of SCs with B-GPE, P-GPE and 1 M LiClO4/EMIMBF4 (Table S3), GCD curves of the B-GPE supercapacitor under various temperatures from 0 °C to 80 °C at a current density of 2 A g-1 (Figure S10).

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected]; [email protected]. Tel.: +86 021 55664024; fax: +86 021 55664024

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS This work was financially supported by the State Key 973 Program of PRC (2011CB605704), the Research Foundation of Shanghai Academy of Spaceflight Technology (SAST2016110), the National Natural Science Foundation of China (U1201241, 51372041, 51202034 and 51201035) and the China Postdoctoral Science Foundation (2015M571837).

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A High-Performance Ionic Liquid Based Gel Polymer Electrolyte

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