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Ultra-Long Life Organic Sodium Ion Batteries Using Polyimide/MultiWalled Carbon Nanotubes Nanocomposite and Gel Polymer Electrolyte James Manuel, Xiaohui Zhao, Kwon-Koo Cho, Jae-Kwang Kim, and Jou-Hyeon Ahn ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04561 • Publication Date (Web): 19 May 2018 Downloaded from http://pubs.acs.org on May 19, 2018

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ACS Sustainable Chemistry & Engineering

Ultra-Long Life Organic Sodium Ion Batteries Using Polyimide/Multi-Walled Carbon Nanotubes Nanocomposite and Gel Polymer Electrolyte James Manuel,†,‡ Xiaohui Zhao,†,ξ,,‡ Kwon-Koo Cho, § Jae-Kwang Kim, †

⊥,*

and Jou-Hyeon Ahn†,*

Department of Chemical and Biological Engineering and Research Institute for Green Energy Convergence Technology, Gyeongsang National University, 900, Gajwa-dong, Jinju 52828, Republic of Korea

ξ

Soochow Institute for Energy and Materials InnovationS, College of Physics, Optoelectronics and Energy, Soochow University, Suzhou 215006, China

§

Department of Materials Engineering and Convergence Technology and RIGET, Gyeongsang National University, 501 Jinju-daero, Jinju 52828, Republic of Korea ⊥

Department of Solar & Energy Engineering, Cheongju University, Cheongju, Chungbuk, 28503, Republic of Korea

Corresponding Authors *E-mail (J.K. Kim) [email protected] *E-mail (J.H. Ahn) [email protected] ACS Paragon Plus Environment

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Keywords: polyimides, multi-walled carbon nanotubes, composite, gel polymer electrolyte, organic sodium ion battery

ABSTRACT

Organic cathode materials are of great interest for application in batteries due to their abundant availability and environmental compatibility. An approch to make long chain molecules of these organic material in order to overcome the problem of dissolution in liquid electrolyte (LE) and incroporating a highly conducting material to enhance the poor electric conductivity of these materials would be of great research interest. In this work, a novel polyimide (PI)/multi-walled carbon nanotube (MWCNT) nanocomposite is prepared as the cathode material for organic Naion batteries (NIBs), via a two-step imidization reaction using perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and diaminopropane (DAP) to form an insoluble PI. The MWCNT in the composite serves as the conductive channel to maximize the utilization of the active material in the electrode. Furthermore, a three-dimensional fiber network is prepared from an electrospun polyacrylonitrile nanofibrous membrane and used as a gel polymer electrolyte (GPE) with efficient electrolyte uptake and high ionic conductivity. The combination of PI/MWCNT nanocomposite cathode and GPE results in a highly efficient organic NIB with ultra-long life span of 3000 cycles and stable cycle performance at high C-rates.

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Introduction The extensive use of fossil fuels by humankind over the last 250 years has caused serious environmental consequences, mainly through the large quantity of carbon dioxide released.1 There is an increasing demand for alternative energy sources with a smaller carbon footprint, such as wind- and solar-generated electricity. These sources often require large-scale energy storage devices in practice. Currently, lithium ion batteries (LIBs) are widely used in most portable devices and are considered candidates for large-scale energy storage, due to their high energy density and long cycle life.2 In spite of continued improvements in LIBs, however, challenges such as high cost, safety and environmental issues, and reduced performance at low temperature still hinder their application in grid scale storage. Sodium (Na) ion batteries (NIBs) could become an alternative to the expensive LIBs, because sodium is the 4th most abundant metal on earth and has worldwide availability.3,4 Although many cathode materials have been developed for NIBs, most of them have low reversible capacity.5 Therefore, it is imperative to develop high-performance cathode materials for future NIBs. Since the discovery of polyacetylene and subsequently its doping to achieve high conductivity 30 years ago, researchers have investigated electrochemically active polymers for potential applications in battery cathodes.6-8 Attempts were made to overcome their inherent drawbacks, such as low electric conductivity, undesired dissolution in electrolytes, and poor thermal stability.9-12 Among the electrochemically active polymers, aromatic polyimides (PIs) have received significant attention, due to their exceptional mechanical properties, high chemical resistance, and thermo-oxidative stability.13 The electrochemistry of aromatic PIs was first studied by Mazur et al. in 1987.14 In 2010, Song et al. reported the first use of PIs as active

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cathode materials for LIBs, achieving a discharge capacity of around 180 mAh g-1 for 100 cycles.10 Nevertheless, these researchers have pointed out that the synthesis of PIs should be improved to optimize the molecular structure. Stable prolonged cycle performance of PI cathode materials over 5000 cycles was reported by Wang et al15. Recently reports about PI by Banda et al and Dai et al showed that PI can be an attractive cathode active material for prolonged stable cycle performance16,17. Meanwhile, gel polymer electrolytes (GPEs), which have good electrolyte penetration and high ionic conductivity, have been widely reported for fabricating LIBs that are both flexible and safe.18 In a GPE, an organic electrolyte is added to the a porous membrane and thus entrapped molecule can form a wide network which not only help to improve the ionic conduction through the membrane, but also help to improve structural stability of GPE.19 GPE based on the polyacrylonitrile (PAN) host polymer has many advantages including high ionic conductivity, high electrolyte uptake, and thermal stability, making it a promising candidate to be used in LIBs.20,21 Vondrák et al. have reported that smaller ions such as Li+ which are easily embedded or captured by the polymer matrix have lower mobility in GPEs than larger ions such as Na+. Hence, GPEs may provide even better electrochemical performance in NIBs than in LIBs.22 In this work, we have prepared a new PI/multi-walled carbon nanotube (MWCNT) nanocomposite by a two-step imidization process. First, polyamic acid (PAA) was synthesized with perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and diaminopropane (DAP). The PTCDA-DAP based PIs are structurally stable, insoluble in electrolytes, and afford high theoretical capacities with their two anhydride groups attached to the stable aromatic ring.23-25 In the second step, the imide formation was carried out at specific temperatures. During the polymerization of PAA, MWCNTs were added to form a PI/MWCNT nanocomposite, which

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was then used as cathode material for an organic NIB. Meanwhile, a GPE with high ionic conductivity based on electrospun nanofibrous PAN membrane was prepared with 1 M NaClO4 solution in ethylene carbonate (EC)/propylene carbonate (PC)/dimethoxyethane (DME) (1:1:1 volume ratio) for use as polymer electrolyte. The combination of the PI/MWCNT nanocomposite cathode and PAN-based GPE was expected to promote the electrochemical performance of the resulting organic NIB.

RESULTS AND DISCUSSION

Figure 1. Schematic representation of the synthesis of PI/MWCNT nanocomposite.

Synthesis and characterization of PI and PI/MWCNT nanocomposite. The PI/MWCNT nanocomposite was prepared by a two-step polycondensation reaction according to the synthetic route shown in Figure 1. In the first step, polycondensation of PTCDA and DAP in NMP in the presence of MWCNT leads to the formation of a PAA/MWCNT nanocomposite. In the second step, PI/MWCNT nanocomposite is formed by the imidization of PAA during the stepwise thermal treatment up to 400 °C. A uniform nanocomposite consisting of interconnected

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MWCNT interwoven in the aggregated PI particles was observed in FE-SEM and TEM images (Figure 2). With its excellent mechanical stability, MWCNT serve as a conductive network in the nanocomposite and facilitate fast electron transfer in the cathode, thus maximizing the utilization of the active material.

Figure 2. (a) FE-SEM and (b) TEM images of the PI/MWCNT nanocomposite.

The formation of PI was confirmed with FTIR spectra, and the vibrational bands are in agreement with the reported data.23 Figure 3a compares the FTIR spectra of PTCDA, PAA, and PI. The characteristic absorption bands of PI groups were present: the absorption bands at 1691, 1655, 1342, and 744 cm-1 correspond to the imide C=O asymmetric stretching, imide C=O symmetric stretching, imide C–N stretching, and imide C=O symmetric in-plane bending of PI, respectively. Thus, the spectrum clearly validates the formation of PI from PAA in the PI/MWCNT nanocomposite. The XRD patterns of PI, MWCNT, and PI/MWCNT are presented in Figure 3b, in which the combined patterns of PI and MWCNT phase purity and semicrystalline nature for the nanocomposite could be clearly detected in PI/MWCNT as reported earlier24. Reduced crystallite size of active electrode material can reduce the solid-state diffusion distance for Na+ ions, thereby improve the ionic conductance. The crystalline structure of [Type here]


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PTCDA reduced to semi-crystalline nature by the polymerization of PTCDA and the addition MWCNT which contribute to the enhanced electrochemistry25. The TGA curves of MWCNT, PI, and PI/MWCNT nanocomposite are presented in Figure 3c. Weight loss by degradation was observed for both PI and PI/MWCNT nanocomposite in the temperature range of 450–650 °C, and both curves followed a similar pattern. Since there was no weight loss for MWCNT up to 650 °C, the TGA curve of the nanocomposite only represents the thermal behavior of PI. Amount of MWCNT added to the reaction mixture to get 65 wt.% of active material in the final product. Pristine PI showed a weight loss of 60 wt.% and by analyzing the ash content, the weight loss of PI/MWCNT corresponds to the presence of ~35 wt% of MWCNT in the final product, which is in accordance with the high yield of the reaction.

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Figure 3. (a) FTIR spectra of PTCDA, PAA/MWCNT, and PI/MWCNT nanocomposite. (b) XRD patterns and (c) TGA curves of PI, MWCNT, and PI/MWCNT nanocomposite.

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(a)

(b)

Figure 4. (a) FE-SEM image and (b) electrolyte uptake of electrospun PAN nanofibrous membrane. LE: 1 M NaClO4 in EC/PC/DME in 1:1:1 volume ratio.

Morphology and electrochemical properties of GPE. FE-SEM image of the nanofibrous PAN membrane suggests that it consists of a three-dimensional network of fibers with a large number of cavities (Figure 4a). Based on five different FE-SEM images of the membrane, the fiber diameter was in the range from 800 nm to 2.4 µm, with an average fiber diameter (AFD) of 1.8 µm. The high porosity of the membrane resulted in high electrolyte uptake with fast penetration of LE to form the GPE. The porosity of the PAN nanofibrous membrane was determined from its density and the mass of its n-butanol uptake during 1 h, compared to the density and mass of the polymer membrane. The nanofibrous PAN membrane showed a high porosity of 82%, which is much higher than the value of the commonly used Celgard separator.27 Figure 4b shows the percentage of LE uptake, with the maximum value of 440 wt% attained within 5 min. The high electrolyte uptake of the nanofibrous membrane is expected to contribute to the high ionic conductivity of GPE and improved electrochemical properties of NIBs.

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Figure 5. (a) LSV spectra, (b) interfacial resistance, (c) electrolyte resistance, and (d) temperature-dependent ionic conductivity of GPE based on electrospun nanofibrous PAN membrane impregnated with 1 M NaClO4 in EC/PC/DME (1:1:1 volume ratio). The electrochemical stability window is an important parameter for evaluating the stability of GPE in a NIB within its operating voltage, and it was determined by LSV. The LSV curve obtained through a Na/GPE/SS cell is shown in Figure 5a. The GPE exhibited a high anodic stability at 5.1 V, which is sufficient for a NIB system. The effective interaction between PAN nanofibrous membrane and LE is probably responsible for the enhanced electrochemical stability of NIBs. The interfacial property of the symmetric sodium cell constructed with GPE was

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monitored by impedance spectroscopy. The semicircle in the impedance spectra (Figure 5b) is attributed to the impedance related to the solid electrolyte interface (SEI) layer at the Na/GPE interface. At the open circuit voltage, the interfacial resistance (Ri) between the sodium electrode and GPE increased continuously with time at room temperature, from an initial value of 820 to 1078 Ω after 6 days. The gradual increase in Ri suggests the formation of an SEI layer due to the reaction between the sodium electrode and the GPE components, resulting in the accumulation of reaction products at the interface. In the temperature-dependent impedance measurements (from -20 to 80 °C), the sodium cells showed a low electrolyte resistance Rb of 2.1 Ω at room temperature and 4.7 Ω at a low temperature of -20 °C, suggesting good affinity between LE and PAN nanofibrous membrane (Figure 5c). The ionic conductivity of GPE increased with temperature, owing to the increased ion mobility. The ionic conductivity was observed to follow the Arrhenius behavior (Figure 5d), although there is a slight curvature in the plot. At room temperature, the ionic conductivity was 3.01×10-3 S cm-1, demonstrating the superior performance of GPE in enhancing the ion migration in the cell and thus improving the electrochemical performance of NIBs.

Mechanism and deep discharge results of PI/MWCNT nanocomposite cell. The redox reaction mechanism of PI during the discharge-charge process is schematically depicted in Figure 6a. Theoretically, each aromatic dianhydride unit could undergo enolization reaction at the four carbonyl double bonds by the insertion of Na+ ions, leading to a theoretical capacity of 249.2 mAh g-1 for the PI. The insertion and extraction of Na+ ions occur in two steps. During the first step of discharge, PI is reduced and the oxygen atoms of two carbonyl groups could combine with Na+ ions at a potential near 2.1 V.16 In the second step, PI undergoes further

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reduction as the remaining two carbonyl groups combine with Na+ in the potential range of 0.9– 1.3 V.28 The reverse reactions (i.e., the extraction of Na+ ions) occur during the charge process. In the deep discharge curve of PI/MWCNT cell at 0.1 C-rate (Figure 6b), the cell delivered a discharge capacity of 1077 mAh g1

.This high capacity is a result of the contributions of MWCNT and Super-P in the electrode, as

Na+ ions are inserted into the C6 rings in the potential range of 1.5–0.0 V.29 However, reports have shown that deep discharge of the cell beyond 1.5 V irreversibly damages the PI structure.2224

Therefore, a more suitable lower limit of discharge voltage is 1.5 V, even though it allows only

half of the theoretical capacity (124.6 mAh g-1) in PI-based cells.

Figure 6. (a) Sodium intercalation reaction mechanism in PI-based cells and (b) first deep discharge curve of the sodium cell with PI/MWCNT nanocomposite and GPE based on

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electrospun nanofibrous PAN membrane impregnated with 1 M NaClO4 in EC/PC/DME (1:1:1 volume ratio) at 0.1 C-rate.

Figure 7. Electrochemical properties of NIBs with PI and GPE based on electrospun nanofibrous PAN membrane impregnated with 1 M NaClO4 in EC/PC/DME (1:1:1 volume ratio). (a) Discharge-charge curves and (b) cycle performance at 5 C-rate in the voltage range of 1.5-3.5 V, (c) specific discharge-charge curves, and (d) cycle performance at different C-rates. Evaluation of Na/GPE/PI cell. The electrochemical performance of the PI and PI/MWCNT nanocomposite electrodes was tested with sodium half-cells at room temperature. Small organic molecules are susceptible to dissolution in electrolyte, leading to active material loss. The polymerization of PTCDA to PI could therefore solve the solubility problem, leading to better cycle performance in cells based on PI or PI/MWCNT nanocomposite electrodes. The dischargecharge curves of the cell based on PI and GPE at 5 C-rate are shown in Figure 7a. The single

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plateau in the discharge curve shows that the electron transfer occurred in a single step in the voltage range of 1.5–3.5 V. In this voltage range, only two electrons take part in the redox reaction, and thus the theoretical capacity is calculated to be 124.6 mAh g-1. The cell showed an initial discharge capacity of 86 mAh g-1 at 5 C-rate, about 70% of the theoretical capacity. The specific capacity initially decreased slowly with cycling and stabilized after 250 cycles. After 1000 cycles, the discharge capacity was 57 mAh g-1, which is about 66% of the initial value (Figure 7b). The cell reached a Coulombic efficiency close to 100% after the first few cycles, implying efficient suppression of active material loss during the redox processes. The rate capability of the cell based on PI with GPE was further tested for 10 discharge-charge cycles at the C-rates of 0.1, 0.2, 0.5, 1, 2, 5, and 10 sequentially using the same cell, and then switched back to 0.1 C-rate. The PI-based cell delivered an initial discharge capacity of 93 mAh g-1 which increased to 100% of the theoretical value of 124 mAh g-1 after 4 cycles, showing highly efficient utilization of the active material. The same cell delivered initial discharge capacities of 114, 109, 105, 103, 85, and 53 mAh g-1 at 0.2, 0.5, 1, 2, 5 and 10 C-rates, respectively, which were about 91, 87, 84, 82, 68, and 43% of the theoretical capacity.

Evaluation of Na/GPE/PI/MWCNT nanocomposite cell. The electrochemical properties of the PI/MWCNT nanocomposite electrode with GPE were studied at room temperature. The reversibility of the cell was studied by CV, with the 1st, 5th, and 10th CV curves presented in Figure S1, Supporting Information. The curves show a broad cathodic peak at around 2.1 V, which corresponds to a single-step reduction of PI with only two Na+ ions inserted into the molecule in the voltage range of 1.5-3.5 V. The reversibility of the cell slowly improved with cycling, since the current of reduction peaks became stronger in later cycles. The discharge-

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charge profiles of the sodium cell using PI/MWCNT nanocomposite and GPE showed one voltage plateau, in accordance with the CV curves (Figure 8a). An ultra-long life span of 3000 cycles at 5 C-rate was

Figure 8. Electrochemical properties of the PI/MWCNT nanocomposite with GPE based on electrospun nanofibrous PAN membrane impregnated with 1 M NaClO4 in EC/PC/DME (1:1:1 volume ratio). (a) Discharge-charge curves at 5 C-rate in a voltage range of 1.5-3.5 V, (b) cycle performance at 5 C-rate, (c) specific discharge-charge curves in a voltage range of 1.5-3.5 V at different C-rates, and (d) cycle performance at different C-rates.

achieved, as shown in Figure 8b. The cell delivered an initial discharge capacity of 98 mAh g-1, which slightly increased to 100 mAh g-1 after a few stabilization cycles and was 82.5 mAh g-1 after 3000 cycles (about 84% of the initial discharge capacity). As illustrated in Figure S2

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(Supporting Information), the cell with PI/MWCNT nanocomposite electrode showed initial capacities of 123.5, 105, and 99 mAh g-1 at 0.1, 1, and 10 C-rates, respectively. Even though the discharge capacity varied slightly at both low and high current densities, the cell retained a maximum Coulombic efficiency throughout 500 cycles, as the discharge capacities of 114, 93, and 76.5 mAh g-1 after 500 cycles at 0.1, 1, and 10 C-rates were about 92, 88.5, and 77% of the initial values, respectively. The variations of the voltage relative to the Na+ insertion/extraction processes are smooth during the entire discharge/charge plateaus, indicating good structural stability of the PI/MWCNT nanocomposite. When compared to the PI cell, the capacity fading in the cell with PI/MWCNT nanocomposite was greatly reduced at both low and high C-rates. Electrochemical performance of PI/MWCNT was also studied with LE to compare with GPE and the data is shown in Figure S3 (Supporting Information). Compared to GPE, the cell with LE showed an inferior performance with an initial discharge capacity of 108 mAh g-1 and maintained a stable capacity of ~111 mAh g-1 at 0.1 C-rate up to 225 cycles, which is lower than that given by the cell with GPE. This superiority arise not only from the high porosity of PAN membrane that drives the more efficient ion movement than Celgard, but also the large amount of entrapped electrolyte within the polymer matrix. The rate capability of the cell with PI/MWCNT nanocomposite electrode was also examined at 0.1, 0.2, 0.5, 1, 2, 5, and 10 C-rates consecutively, and high initial discharge capacities of 125, 123, 120.8, 115.6, 112, 108.5, and 98.3 mAh g-1 were obtained, respectively (Figure 8c). After cycling through these C-rates, the current was switched back to 0.1 C, at which the cell delivered 100% of the theoretical capacity after 70 cycles. After 500 cycles at 0.1 C, the Coulombic efficiency remained 100% with only 6% capacity fading. It further delivered stable specific capacity for another 1400 cycles at 1 C-rate (Figure 8d). The addition of MWCNT in PI

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introduced an interconnected 3D conductive network to improve the availability of active sites for the redox reaction, and thus maximized the utilization of active material. Additionally, the GPE with high ionic conductivity enhanced the transfer of ions to both electrodes. The outstanding electrochemical performance of the PI/MWCNT nanocomposite electrode with longterm cycle stability and high Coulombic efficiency can prompt the application of organic NIBs for large-scale energy storage devices (Table S1).

CONCLUSIONS A high-performance PI/MWCNT nanocomposite cathode was synthesized by the polymerization of PTCDA and DAP in the presence of MWCNT. The synthesized PI has lower solubility in LE and reduces active material loss. The interconnected 3D network of MWCNT within and between PI particles enhances electrode conductivity and improves utilization of the active material. GPE based on electrospun PAN nanofibrous membrane with high ionic conductivity was also used to enhance the ion mobility in NIBs. With the combination of PI/MWCNT nanocomposite and GPE, the sodium cell delivered a high discharge capacity close to 100% of the theoretical value at 0.1 C-rate, and an ultra-long life span of 3000 cycles at 5 C-rate. This cell also showed enhanced rate capability up to 10 C-rate, and retained good cycle stability with Coulombic efficiency close to 100% in long-term cycle test. The excellent electrochemical performance of the nanocomposite electrode and the novel cell construction with GPE demonstrate the possibility of improving organic NIBs for next generation energy storage.

MATERIALS AND METHODS

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Preparation of PI/MWCNT composite. MWCNT (0.235 g) was dispersed in Nmethylpyrrolidone (NMP) by sonicating for 10 min in an ultrasonic bath, and then 1 mM of PTCDA (Aldrich) was added. The mixture was transferred into a three-necked round bottom flask that was connected to a condenser. Nitrogen gas was bubbled through the mixture for 30 min and 1 mM of DAP was added to the reaction mixture under magnetic stirring. The reaction was carried out at 60 °C for 12 h. The obtained product was filtered, washed with NMP, and dried at 60 °C for 24 h. The conversion of PAA to PI was carried out by a 5-step thermal imidization process (70 °C for 30 min → 120 °C for 30 min → 200 °C for 60 min → 300 °C for 30 min → 400 °C for 1 h) under nitrogen atmosphere in an automatic temperature-controlled tube furnace (JISICO, J-GAF). For comparative study, PTCDA-DAP based PI without MWCNT was also prepared using the same procedure. Preparation of GPE. The nanofibrous membrane of PAN was prepared using the electrospinning technique as described in our previous publication.20 Typically, a 16 wt% homogenous solution of PAN (MW 150,000, Polysciences) in N,N-dimethylformamide (DMF, Aldrich) was prepared by mechanical ball-milling in a Teflon jar with zirconia balls for 1 h at room temperature, and then degassed to remove the trapped air. The solution was fed through a syringe pump (KD Scientific, Model 210) connected to a 0.6 mm-diameter needle at a constant flow rate of 0.1 ml min-1. A high DC voltage of 20 kV and a distance of 20 cm were used between the needle and an aluminum foil-wrapped stainless steel drum rotating at a speed of 120 rpm. The electrospun membrane was dried overnight at 60 °C and cut into small circular discs of 15 mm diameter for further use. Finally, the GPE was prepared by immersing the membrane discs in 1 M sodium perchlorate (NaClO4) solution in a mixture of EC/PC/DME in 1:1:1 volume

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ratio for 5 min. NaClO4, EC, and DME were purchased from Aldrich, and PC were purchased from Merck. Characterizations. Field emission scanning electron microscopy (FE-SEM) images of the PI/MWCNT nanocomposite were recorded on a Hitachi S-4800 system. The X-ray powder diffraction (XRD) patterns (D2 Phaser Bruker AXS) were recorded with Cu Kα radiation in the range of 10–60°. Fourier transform infrared (FTIR) spectra were recorded on a VERTEX 80v Bruker Optics spectrometer using potassium bromide (KBr, Aldrich) pellets. The content of PI in the PI/MWCNT nanocomposite was confirmed with thermogravimetric analysis (TGA, Q50 TA Instruments) by heating the sample up to 900 °C under nitrogen atmosphere at a heating rate of 5 °C min-1. The porosity of the GPE membrane was determined by the n-butanol uptake method. The electrolyte uptake was measured using a method described elsewhere.15 The electrochemical window of GPE was determined by linear sweep voltammetry (LSV) of Na/GPE/SS cells at a scan rate of 1 mV s-1 over the range of 2–6 V at 25 °C. The interfacial resistance between the GPE and sodium metal electrode was measured at room temperature by the impedance response of Na/GPE/Na cells with a Zahner IM6 frequency analyzer, over the frequency range of 10 mHz–2 MHz at an amplitude of 20 mV. The ionic conductivity (σ) of GPE was calculated using the equation σ = L/(RbA), where L and A are the thickness and area of the GPE, respectively. The bulk resistance Rb was measured by the AC impedance method, with GPE sandwiched between two stainless steel (SS) Swagelok® electrodes, and the impedance measurements were carried out at 20 mV amplitude over the frequency range of 100 mHz to 2MHz. The ionic conductivity as a function of temperature was determined in the temperature range of -20–80 °C.

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The cell was kept at each temperature for 3 h prior to measurement to ensure thermal equilibration of the sample. Electrochemical measurements. The cathodes were prepared by mixing the PI or PI/MWCNT nanocomposite active material with carbon black (Super-P, conductive agent) and poly(vinylidene fluoride) (PVdF, binder) in NMP. The binder content was 10 wt.% and the content of Super-P was adjusted for the active material to maintain a constant PI content (52%) in the electrode. The above components were mixed in a high-energy mixer mill at room temperature for 45 min to obtain a homogeneous slurry. The slurry was cast on Al foil and dried at 60 °C for 12 h. The obtained film was cut into circular discs of 10 mm diameter, and further vacuum-dried for 24 h at 60 °C and was used as cathodes. The active material mass loading was around 2 mg cm-2. Anode was prepared by cutting sodium pieces (Aldrich) with ceramic knife and cut into circular discs with a punch of diameter 11 mm. Two-electrode cells were assembled by sandwiching the GPE between a sodium metal anode and cathode in Swagelok® type circular cells (23 mm in diameter). Cyclic voltammetry (CV) of the electrode was performed at room temperature at a scan rate of 0.1 mV s-1 between 1.5–3.5 V. For the preparation cells with LE, Celgard®2200 with 1 M NaClO4 solution in a mixture of EC/PC/DME in 1:1:1 volume ratio was used instead of GPE. The electrochemical tests of the cells were conducted in an automatic galvanostatic charge-discharge unit (WBCS3000 battery cycler, WonA Tech. Co.) between 1.5– 3.5 V at 25 ºC, at current densities of 0.1, 0.2, 0.5, 1, 2, 5, and 10 C-rates (1 C=249 mA g-1). The electrolyte uptake measurements and test cell fabrication were carried out in an argon-filled glove box with a moisture level < 10 ppm.

ASSOCIATED CONTENT

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Supporting Information Cyclic voltammetry curves and cycle performance of PI/MWCNT nanocomposite with GPE. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail (J.K. Kim) [email protected] *E-mail (J.H. Ahn) [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (No. NRF-2017R1A4A1015711)

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The PI/MWCNT nanocomposite cathode with interconnected 3D network with PAN-based gel polymer electrolyte enables long life-span sodium ion batteries.

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Ultra-Long Life Organic Sodium Ion Batteries ... - ACS Publications

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