Freestanding Cathode Electrode Design for High-Performance

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A Freestanding Cathode Electrode Design for High-Performance Sodium Dual-Ion Battery Hsiang-Ju Liao, Yu-Mei Chen, Yu-Ting Kao, Ji-Yao An, Ying-Huang Lai, and Di-Yan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08429 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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The Journal of Physical Chemistry

A Freestanding Cathode Electrode Design for HighPerformance Sodium Dual-Ion Battery Hsiang-Ju Liao, Yu-Mei Chen, Yu-Ting Kao, Ji-Yao An, Ying-Huang Lai, Di-Yan Wang* Department of Chemistry, Tunghai University, Taichung 40704, Taiwan

ABSTRACT

In this work, both advantages of sodium-ion batteries and dual-ion batteries have been combined in an innovated sodium-ion-based dual-ion battery (SDI) system using a Na metal film as anode and a freestanding meso-carbon micro bead film (FS-MCMB) as cathode. FS-MCMB in SDI battery exhibited a superior working performance with the specific capacity of 83.6 mAh/g and a remarkable long-term stability over 300 cycles. The SDI battery with FS-MCMB exhibited an advantage of high mass density loading in range of 2~7.5 mg-cm-2, which were equal to a comparable capacity of 78~83 mAh/g. The electrochemical impedance analysis indicated that FS-MCMB provided a superior permeability, resulting in facilitating electrolyte infiltration into MCMB structure. In-situ XRD and ex-situ Raman Spectroscopy were utilized to characterize the intercalating/deintercalating

process

of

PF6-

anions

into/out

of

MCMB

during

charging/discharging processes. Finally, theoretical calculations further confirmed the structural arrangement of PF6- anions in the graphite layers.

ACS Paragon Plus Environment

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Introduction Efficient electrical energy storage technology has attracted intense attention due to high energy demand in the next generation of electric vehicles and stationary systems. Electrochemical rechargeable batteries have been considered as a good approach for energy storage. The cost effective earth-abundant metal ion batteries, such as sodium1, magnesium2 and aluminum3 ion batteries are especially attractive because of their high specific energy in comparison with traditional lithium ion batteries4. Recently, the development of dual-ion battery has become an important study for alternative energy storage system, in which charge/discharge mechanism involves both cation and anion’s intercalation reactions toward anode and cathode electrodes, respectively. In most studies, graphite materials usually were employed as anode and cathode as well. Those dual ion batteries exhibited a high working potential in comparison to conventional metal ion batteries. Some reports studied the behavior of Li+ and TFSI- ions intercalation/deintercalation into/out of graphite anode and cathode with the electrolyte system of Pyr14TFSI-LiTFSI in dual-ion battery.5-10 Most of the studies demonstrated dual-ion battery with graphite as both cathode and anode in LiPF6 based system.11-16 Some literatures reported alloying process in anode and intercalation process in cathode when charging the dual-ion battery with LiPF6 and NaPF6 electrolyte.17-19 Many reports have investigated the intercalation mechanism of anion intercalating into graphite in the Li-ion-based system.5-17 However, few of studies explored the phenomena in the Na-ion-based system20-22. The sodium element is abundant on Earth and more economical than lithium element.1, 23 The current problem of sodium dual ion battery is its low discharge potential range.20-22, 24 Therefore, developing a new cathode electrode for sodium dual-ion (SDI) battery with improving working potential and storage capacity is still an important issue to be resolved.


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The investigation of intercalating mechanism of anions and related structural transformation of electrode materials is key to improving the performance of dual-ion battery. PF6-, one of anions, has been used as a good candidate as intercalant in cathodic reaction of graphite materials. Several reports11,

16, 22, 25

indicated that the stage number of PF6- anions

intercalating into graphite cathode for Li dual-ion battery and the cointercalation of organic solvent ions have been revealed by in-situ X-ray diffraction (XRD) and theoretical calculation during charging process. It was observed that the diffusion coefficient of PF6- decreased due to the increasing concentration of PF6- in graphite layers upon charging the battery with galvanostatic intermittent titration technique.15 Moreover, the diffusivity of anions PF6- in graphitic carbon was found to be higher than that of Li+ in LiFePO4. Therefore, PF6- anion system could be a great potential and applied as intercalant of graphite cathode reaction for SDI battery. In this work, we utilized meso-carbon micro-bead (MCMB) as the cathode material without using metal foil as the current collector, which was so-called freestanding (FS) method for high-performance sodium dual-ion (SDI) battery. A high loading amount in the SDI battery achieved 2-7.5 mg-cm-2, and the batteries exhibited superior discharge capacity of 78-83 mAh/g at the current density of 100 mA-g−1 in the potential range of 5.0-3.0 V. Notably, the SDI battery also demonstrated good cycle stability which possessed high capacity of 83.6 mAh/g even after 300 cycles at a discharging current density of 100 mA-g−1 with the capacity retention rate of 91.7 %. To clarify the mechanism of PF6- ions intercalation into graphite, ex-situ Raman spectroscopy and in-situ XRD measurement were performed to investigate the structural variation of MCMB freestanding film during charging and discharging process. DFT calculation was also employed to understand the changes of PF6-molecular structural intercalating in graphite layers.


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Experimental Section MCMB Freestanding Film/ MCMB-Al foil Preparation. First, we prepared the slurry by mixing meso carbon micro beads (MCMB) (90%) with polyvinylidene fluoride (PVDF, from Union Chemical Ind. Co., LTD) (10%) in N-methyl-2pyrrolidone (NMP, from Alfa Aesar). Secondly, after rigorously stirred for 2 h, the slurry was casted on a sheet of Cu foil (Ubiq Tech Co., LTD.) to form a uniform film and was dried at 150 °C for 2 h under vacuum. Third, we prepared 2 M FeCl3(aq) (Alfa Aesar, 98%, anhydrous) and placed the MCMB film on the surface of the FeCl3 solution for 10 min to etch the Cu foil. Last, we washed the freestanding MCMB film with DDI water and dried the film at 80 °C for 30 min under vacuum. The preparation method of MCMB cast on Al foil was the same as MCMB freestanding film with the substrate of Al foil (Ubiq Tech Co., LTD.) instead of Cu foil and without the etching process. Fabrication of MCMB/SDI battery Coin cell The MCMB/SDI battery was constructed with CR 2032 coin cell (Ubiq Tech Co., LTD.) by using 1.69 cm2 of Na metal film anode, 1 cm2 of MCMB freestanding film cathode, 3.61 cm2 of glassy fiber separator (Whatman Grade GF/D Glass Fiber Filter Paper without Binder, Dia: 4.7cm, 2.7um) and 120 µL of 1 M NaPF6 (ACROS, 98.5+ %, pure) which was dissolved in an organic mixture of ethylene carbonate (EC): dimethyl carbonate (DMC) = 1:1 (ACROS, 99+ %, extra dry). The coin cell was fabricated in glove box where the concentration of H2O and O2 was lower than 0.1 ppm. After the fabrication of MCMB/SDI battery, the battery rested for 6 h before charging-discharging process.


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Electrochemical Characterization In galvanostatic test, the upper and lower cut-off potentials were set at 5.0 and 3.0 V vs. Na respectively with CT-3008-5V5mA-164 battery testing machine from Neware Techonology ltd. In cyclic voltammetry test, the scanning potential range was 3.0 – 5.0 V vs. Na with a scan rate of 0.1 mV/s. In electrochemical impedance spectroscopy test, the frequency range was 0.1105 Hz. Both of the cyclic voltammetry test and the electrochemical impedance spectroscopy test were performed on AUTOLAB PGSTAT302N (Metrohm). Structural Characterization Scanning Electron Microscope (SEM) and In-situ X-ray diffraction measurement The SEM images were acquired with JEOL JSM-6510 Scanning Electron Microscope. The in-situ XRD measurement was taken with FS-MCMB/Na coin cell charging and discharging at a constant current density of 100 mA-g-1 with the cut-off voltage of 3.0 and 5.0 V vs. Na. The coin cell was drilled a hole with a diameter of 0.3 mm in the center of upper/lower lids and spacer, and the holes of upper/lower lids were covered with Kapton tape to prevent electrolyte from contacting with air. In-situ XRD analysis was performed at the beamline of BL23A small/wide angle X-ray scattering (SWAXS) at Taiwan Light Source (TLS) and the beamline of 09A at the Taiwan Photon Source (TPS) in National Synchrotron Radiation Research Center. Ex-situ Raman Measurement Ex-situ Raman spectra were collected on UniRAM micro-Raman spectrometer with a laser wavelength of 532 nm. The data acquisition time was 10s and accumulated for 1 time with laser power of 5 mW. The wavelength of laser excitation source was calibrated with a silicon


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wafer at 520 cm-1 before sample examination. The laser source was from Cobalt DPSS LASER RL532C. The laser line was focused onto the sample using a Leica DM 2500M optical microscope with 50 objective. FS-MCMB samples with different charging and discharging states were removed from the coin cell in the glove box. To avoid a reaction between the cathode and air/moisture in the ambient atmosphere, the FS-MCMB cathode was positioned in a pouch with a glassy window, and the pouch was sealed with heat-sealer. The sealed samples were immediately removed from the glove box for ex-situ Raman measurement. Theoretical Calculations The first principles calculation was performed based on the density functional theory in the CASTEP code. The ultrasoft pseudopotential and the Perdew-Burke-Ernzerhof generalized gradient approximation were also used. The energy cutoff was set as 330 eV and used for plane waves. Electronic structure was optimized until the force on each atom was smaller than 0.03 eV Å-1. A 3 X 3 X 2 k-point mesh was used for Brillouin-zone integrations. Due to the Van der Waals interactions, the DFT-D approach was used for the correction in the GICs. The simulation of XRD pattern was obtained by using the Powder Diffraction function in Reflex module included in software Materials studio. Results and Discussion Figure 1a shows a schematic illustration of FS-MCMB film preparation for rechargeable SDI battery. First, MCMB slurry is prepared by mixing MCMB powder and PVDF in N-methyl pyrrolidone solvent (Step A). The resulting slurry is casted onto a sheet of Cu foil to obtain a uniform MCMB film. After dried at 150 °C for 2 h (Step B), the MCMB on Cu film is formed. The Cu foil is then etched by immersing the film into the iron chloride (FeCl3) solution (Step C)


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to form a freestanding (FS) MCMB film. Afterwards, the FS-MCMB film is rinsed with deionized water to remove the residual FeCl3 (Step D), and then dried at 80 °C for 0.5 h under vacuum condition (Step E). The resulting FS-MCMB film is obtained (see photograph in Step E of Figure 1). The loading amount of FS-MCMB film can be adjusted from 2 to 7.5 mg-cm-2 in the film by adding different amount of MCMB in the slurry. A schematic fabrication process of FS-MCMB/SDI battery coin cell is depicted in the supporting information (Figure S1). After FSMCMB film is put on the upper lid of coin cell, 120 µL electrolyte of 1 M NaPF6 (EC: DMC=1:1) is added into coin cell, followed by glassy fiber separator, Na metal, stainless steel spacer, spring and lower lid. It is noticeable that no current collector was used in the fabrication of FS-MCMB/SDI battery. To explore the electrochemical performance of FS-MCMB/SDI battery, the cyclic voltammetry (CV) test is obtained to understand reversible oxidation and reduction potential of FS-MCMB in the potential range of 3.0 to 5.0 V vs. Na, as shown in Figure 2a. It is clear that there are seven distinct peaks at 4.16, 4.56, 4.66, 4.71, 4.81, 4.90 and 4.94 V during the charging process and four reduction peaks appear at 4.00, 4.59, 4.72 and 4.86 V in the discharging process. These peaks could be attributed to the different structure arrangements of PF6- anions intercalated in the graphite layer. The galvanostatic results of the SDI battery performance in Figure 2b show the charging and discharging curves of the battery with FS-MCMB film and that with MCMB film with Al current collector (MCMB-Al) under similar loading amount of electrodes (~5.5 mg-cm-2) in both batteries. It is found that the battery with FS-MCMB demonstrates a higher specific capacity (80.5 mAh/g) than that of MCMB-Al (62.7 mAh/g) at 300th cycle. Moreover, the charging plateau of the battery with MCMB-Al is higher than that of FS-MCMB, while the discharging plateau of MCMB-Al is lower than that of FS-MCMB. It


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indicates that the voltage hysteresis in MCMB-Al/Na battery is larger than that in FS-MCMB/Na battery. This phenomenon could be related to the contact resistance of each component in the battery26. Thus, the electrochemical impedance spectroscopy (EIS) in Figure 3 is utilized to understand the different contact resistance in the battery with the cathode of FS-MCMB and MCMB-Al. It is found that the impedance of charge transfer of the battery with FS-MCMB is similar with that of MCMB-Al (Figure 3a). To investigate the difference of diffusion process between FS-MCMB and MCMB-Al, the infinite Warburg plot is shown in Figure 3b. By plotting –Z’’ vs. ω-1/2, the slope of the plot refers to Warburg coefficient, which is inversely proportional to the square root of diffusion coefficient. The slope of MCMB-Al is found to be higher than that of FS-MCMB, indicating that the diffusion coefficient of FS-MCMB is higher than that of MCMB-Al. Therefore, the results represent that the diffusion process of PF6- in FS-MCMB is faster than that of PF6- in MCMB-Al. It is believed that fast diffusion behavior of FS-MCMB cathode is attributed to the higher permeability for electrolyte to infiltrate the structure of the FSMCMB cathode electrode than that of MCMB-Al electrode. According to the analysis of impedance spectroscopy, FS-MCMB was found to be a better choice as the cathode electrode for SDI battery in comparison to MCMB-Al. To test working performance of FS-MCMB at different mass-loading amount in SDI battery, FS-MCMB films with different loading weight density in the range of 2~7.5 mg-cm-2 were well-prepared. Table 1 demonstrates a similar discharging capacity value of 78~83 mAh/g for various FSMCMB films with different loading amounts. The related galvanostatic curves of FS-MCMB and MCMB-Al with different loading amount at 300th cycle are shown in supporting information (Figure S3). Overall results indicate that the SDI battery with FS-MCMB film exhibits superior


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performance with higher mass loading amount, offering higher storage capacity in favor of further practical applications. The rate performance of the FS-MCMB/SDI battery system is shown in Figure 4a. By increasing charging and discharging current density from 1 C to 5 C (1C = 100 mA-g-1), the battery exhibits similar discharging capacity under different current densities. At high chargingdischarging rate of 5 C, the battery still delivers a capacity of 56.8 mAh/g and a Coulombic efficiency of 96.5 %. When the current density is returned to 1C, the capacity recovers to 74.9 mAh/g as same as tested in the previous one. The corresponding galvanostatic curves of the SDI battery for each current density are displayed in the supporting information (Figure S4). To test FS-MCMB/SDI battery’s stability performance, the long-term charging-discharging test is shown in Figure 4b. It is found that after 300 cycles, the discharge capacity of FS-MCMB/SDI battery remains at 82.0 mAh/g under the current density of 100 mA-g-1 with a steady Coulombic efficiency of 91.1 %; while significantly, the galvanostatic curves of the FS-MCMB/SDI battery recorded at the 200th to 300th cycle are almost identical (inset of Figure 4b). To see if the battery with higher FS-MCMB loading amount can also perform well, long-term charging-discharging test of battery with each MCMB weight is shown in Figure S5. The result shows that the batteries with higher weight value can still deliver a stable discharging capacity of ~81.4 mAh/g with a Coulombic efficiency of ~91.9 % over 300 cycles. Our results strongly support that the performance of FS-MCMB/SDI battery is highly improved in comparison with previous reports18, 20, 22, 24. The estimated energy density of FS-MCMB/SDI battery can achieve 230.1 Wh/kg with MCMB density range of 2~7.5 mg-cm-2 (the calculation method of energy density is described in Supporting Information).


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To clarify the structural differences between before and after charging-discharging process for 300 cycles, the SEM images of FS-MCMB were obtained. Figure 5 displays that a few small particles on the surface of each MCMB beads after 300 cycles (Figure 5c-d) is found in comparison with that of original FS-MCMB (Figure 5 a-b). It may be attributed to the expansion of graphite layer in MCMB, which is caused by PF6- anions intercalation during charging process. The whole structure of MCMB large beads remain intact after chargingdischarging for 300 cycles. It indicates that the periodic intercalation and deintercalation of PF6ions do not damage the structure of MCMB, explaining the highly reversible chargingdischarging performance of the FS-MCMB/SDI battery. Figure 6a shows the investigation of the structural transformation of MCMB during charging and discharging process via ex-situ Raman Spectroscopy. The selected different charging and discharging states for Raman measurement are pointed out in Figure 6b. When the battery is charged from the potential of 3.00 to 4.12 V, the E2g band of MCMB separates into two peaks of 1583 and 1605 cm-1. After further charging the battery to 4.50 V, which is on the first charging potential plateau, a single peak of 1614 cm-1 can be found. Along the climbing potential slope from 4.78 to 4.86 V, a conspicuous blue shift of E2g shows up and achieves the highest Raman shift of 1630 cm-1 (E2g2(b)) at 4.86V. During discharging process, the Raman spectra exhibits the opposite trend. Along with the descending slope from 4.84 to 4.35 V, the signal E2g2(b) disappears, and the red-shifting of E2g2(i) appears when discharging to lower potential. The trend of Raman shift during charging and discharging process indicates there is intercalation/deintercalation process occurred in MCMB materials.27-28 In addition, when discharging to 3.00 V, the signal of E2g2(i) emerges, and the Raman signal returns to original


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MCMB status, pointing out that the structure of MCMB is preserved after anions intercalation/deintercalation processes. Figure 7a exhibits the determination of the highest intercalating stage that the highest extent of PF6- ions can be stored in FS-MCMB/SDI battery through in-situ XRD analysis. Each XRD pattern of FS-MCMB/SDI battery for different charging and discharging states (left) is directly consistent with the galvanostatic curve (right). Initially, the characteristic (002) signal of FS-MCMB is found at 2θ~26.50°. When gradually charging the SDI battery to 4.86 V, the intensity of the peak at 26.50° decreases along with the appearance of several peaks at 8.16°, 24.10°, 33.05°. When charging the SDI battery to 4.93 V, the above signals become weaker while the peaks at 11.41°, 23.04° and 35.24° emerge. Figure 7b demonstrates the XRD pattern of fully charged status (the green line in Figure 7a). The peaks at11.41°, 23.04° and 35.24° become dominant in the XRD pattern, which are assigned as (001), (002) and (003), respectively. By using Bragg’s law, the observed distance of graphite layers of MCMB at the Miller lattice of (002) and (003) are 3.88Å (d(002)) and 2.56 (d(003)) Å, respectively. In order to determine the stage number of fully charged status, the value of d(002)/d(003) is calculated9, which gives the value of ~1.51. From the stage calculated in graphite intercalated compound7, 29, our FS-MCMB cathode reaches stage 1 when the SDI battery is charged to 5 V. Furthermore, a relationship between unit cell repeat distance (Ic), intercalatant gallery height (di), stage number (n) and observed distance (d00l) can be calculated as Equation (1)30. ‫ܫ‬௖ = ݀௜ + ሺ݊ − 1ሻ × 3.3996 = ݈ × ݀଴଴௟

(1)


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By putting d002 value into Equation (1), the intercalant gallery height (di) and unit cell repeat distance (Ic) both equal to 7.76Å, which are consistent with the result from in-situ XRD measurement, showing the fitted size of PF6- anion perfectly. To explore the molecular structure of PF6- anion when intercalating into graphite layers, the density functional theory (DFT) was applied to simulate the orientation of intercalated anions. Based on calculating from the charging specific capacity in our experimental result and intercalant gallery height, a periodic unit cell for the model was constructed by two layers of 24 graphitic carbon atoms with one PF6- molecule inserted in the middle of carbon layer and another PF6- molecule situated in the next space layer. Each PF6- ion was surrounded with 24 C atoms averagely when PF6- anions were under fully charged status. In simulated geometry result (Inset of Figure 7c), the PF6- molecule shows tilted octahedral shape. Additionally, the simulated XRD pattern (Figure 7c) fits our experimental XRD pattern at fully charged status. Therefore, at fully charged status (stage-1), it is believed that the PF6- anions display an arrangement of tilted structure when intercalated in the graphite layers. Several studies have reported that different kind of carbon materials were utilized as cathode materials in SDI battery with NaPF6 system (Table S1, in the supporting information). Some problems, such as lower working potential or less storage capacity still have been occurred in the previous reports. For example, Aladini’s work reported a capacity of 55 mAh/g and a Coulombic efficiency of 90 % with KS6 graphite as a cathode and sodium metal as an anode.22 Sheng et al. used expanded graphite as a cathode and tin metal as an anode and the battery showed a capacity of 70 mAh/g with a Coulombic efficiency of 94 %.18 Although Fan et al. demonstrated a sodium dual-ion battery with a capacity of 103 mAh/g and a cyclic retention of 81.8 % after 800 cycles, it only performed a low discharging potential of 3.5 V.24 Furthermore,


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the above-mentioned reports revealed that the weight density of the graphite cathode is not high (2-5 mg-cm-2). It is notable that our FS-MCMB/SDI battery demonstrates high infiltration ability of electrolyte, resulting in an excellent capacity in NaPF6 system under high mass loading. Our design for SDI battery overcomes the disadvantages of previous reports and opens a good route for real practical applications. Conclusions A SDI battery with high performance under high-mass loading MCMB has been successfully developed by exploiting a freestanding fabrication process. It is noteworthy that the battery’s reversible capacity could reach ~81.4 mAh/g over a high potential window of 3.0-5.0 V at a current density of 100 mA-g-1 with FS-MCMB as a cathode, which maximum weight density could achieve 7.5 mg-cm-2. Furthermore, the battery also demonstrated a good rate performance and great stability over 300 recharge cycles with 91.1 % of Coulombic efficiency without obvious structural deterioration at the FS-MCMB film. The current SDI battery with using FSMCMB as cathode electrode could produce an energy density of 230.1 Wh/kg (based on the cathode capacity of 83.6 mAh/g and the masses of active materials in electrodes and electrolyte). The significant insight into sodium dual-ion battery reactions has been investigated by spectroscopic method and theoretical modelling.

ACKNOWLEDGMENTS This work has been financially supported by the Ministry of Science and Technology of Taiwan (MOST 104-2113-M-003-006-MY2 and MOST 106-2113-M-029 -006 -MY2) and Tunghai University. The study for XRD simulation was supported by Prof. Chun-Wei Chen and Mr.


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Shao-Ku Huang in Department of Materials Science and Engineering, National Taiwan University. Also, we thank Dr. U-Ser Jeng for assistance at beamline BL23A at Taiwan Light Source (TLS) and Dr. Hwo-Shuenn Sheu, Dr. Yu-Chun Chuang for assistance at beamline 09A at the Taiwan Photon Source (TPS).

Supporting information available. Detailed description of calculation method of the energy density of FS-MCMB/SDI battery. Galvanostatic curves of SDI battery with different concentration of electrolyte and SDI battery with using different graphite materials.

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10. Beltrop, K.; Meister, P.; Klein, S.; Heckmann, A.; Grünebaum, M.; Wiemhöfer, H.-D.; Winter, M.; Placke, T., Does Size Really Matter? New Insights into the Intercalation Behavior of Anions into a Graphite-Based Positive Electrode for Dual-Ion Batteries. Electrochim. Acta 2016, 209, 44-55. 11. Seel, J. A.; Dahn, J. R., Electrochemical Intercalation of Pf 6 into Graphite. J. Electrochem. Soc. 2000, 147, 892-898. 12. Read, J. A.; Cresce, A. V.; Ervin, M. H.; Xu, K., Dual-Graphite Chemistry Enabled by a High Voltage Electrolyte. Energy Environ. Sci. 2014, 7, 617-620. 13. Ishihara, T.; Koga, M.; Matsumoto, H.; Yoshio, M., Electrochemical Intercalation of Hexafluorophosphate Anion into Various Carbons for Cathode of Dual-Carbon Rechargeable Battery. Electrochem. Solid. St. 2007, 10, A74-A76. 14. Miyoshi, S.; Nagano, H.; Fukuda, T.; Kurihara, T.; Watanabe, M.; Ida, S.; Ishihara, T., Dual-Carbon Battery Using High Concentration Lipf6 in Dimethyl Carbonate (Dmc) Electrolyte. J. Electrochem. Soc. 2016, 163, A1206-A1213. 15. Miyoshi, S.; Akbay, T.; Kurihara, T.; Fukuda, T.; Staykov, A. T.; Ida, S.; Ishihara, T., Fast Diffusivity of Pf6– Anions in Graphitic Carbon for a Dual-Carbon Rechargeable Battery with Superior Rate Property. J. Phys. Chem. C 2016, 120, 22887-22894. 16. Fan, H.; Gao, J.; Qi, L.; Wang, H., Hexafluorophosphate Anion Intercalation into Graphite Electrode from Sulfolane/Ethylmethyl Carbonate Solutions. Electrochim. Acta 2016, 189, 9-15. 17. Zhang, X.; Tang, Y.; Zhang, F.; Lee, C. S., A Novel Aluminum–Graphite Dual‐Ion Battery. Adv. Energy Mater. 2016, 6, 1502588. 18. Sheng, M.; Zhang, F.; Ji, B.; Tong, X.; Tang, Y., A Novel Tin‐Graphite Dual‐Ion Battery Based on Sodium‐Ion Electrolyte with High Energy Density. Adv. Energy Mater. 2017, 7, 1601963. 19. Zhang, F.; Ji, B.; Tong, X.; Sheng, M.; Zhang, X.; Lee, C. S.; Tang, Y., A Dual‐Ion Battery Constructed with Aluminum Foil Anode and Mesocarbon Microbead Cathode Via an Alloying/Intercalation Process in an Ionic Liquid Electrolyte. Adv. Mater. Interfaces. 2016, 3, 1600605. 20. Bordet, F.; Ahlbrecht, K.; Tübke, J.; Ufheil, J.; Hoes, T.; Oetken, M.; Holzapfel, M., Anion Intercalation into Graphite from a Sodium-Containing Electrolyte. Electrochim. Acta 2015, 174, 1317-1323. 21. Aladinli, S.; Bordet, F.; Ahlbrecht, K.; Tübke, J.; Holzapfel, M., Anion Intercalation into a Graphite Cathode from Various Sodium-Based Electrolyte Mixtures for Dual-Ion Battery Applications. Electrochim. Acta 2017, 231, 468-478. 22. Aladinli, S.; Bordet, F.; Ahlbrecht, K.; Tübke, J.; Holzapfel, M., Compositional Graphitic Cathode Investigation and Structural Characterization Tests for Na-Based Dual-Ion Battery Applications Using Ethylene Carbonate:Ethyl Methyl Carbonate-Based Electrolyte. Electrochim. Acta 2017, 228, 503-512. 23. Slater, M. D.; Kim, D.; Lee, E.; Johnson, C. S., Sodium‐Ion Batteries. Adv. Funct. Mater. 2013, 23, 947-958. 24. Fan, L.; Liu, Q.; Chen, S.; Xu, Z.; Lu, B., Soft Carbon as Anode for High-Performance Sodium-Based Dual Ion Full Battery. Adv. Energy Mater. 2017, 7, 1602778. 25. Read, J. A., In-Situ Studies on the Electrochemical Intercalation of Hexafluorophosphate Anion in Graphite with Selective Cointercalation of Solvent. J. Phys. Chem. C 2015, 119, 84388446.


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Figure 1. Schematic illustration of preparation process of freestanding MCMB film.


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Figure 2. (a) Cyclic voltammogram of FS-MCMB/SDI battery in the potential range of 3.0-5.0 V with a scan rate of 0.1 mV/s. (b) Galvanostatic Curves of FS-MCMB/SDI battery and MCMBAl/SDI battery at a current density of 100 mA-g-1 under 300th cycle. The MCMB weigh density of both SDI batteries are ~5.5 mg-cm-2.


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Figure 3. (a) Electrochemical Impedance Spectroscopy of FS-MCMB/SDI battery and MCMBAl/SDI battery after charging-discharging after 300 cycles. (b) The relationship between -Z’’ and ω-1/2 of the related EIS plots, both SDI battery with MCMB weigh density of ~5.5 mg-cm-2.


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Figure 4. (a) Capacity retention of an FS-MCMB/SDI battery cycled at various current densities and (b) Long-term stability test of FS-MCMB/SDI battery with FS-MCMB weight density of ~5.5mg-cm-2 at a current density of 100 mA-g-1.


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Figure 5. SEM images of FS-MCMB films (a-b) before and (c-d) after charging-discharging for 300 cycles, the inset of (d) is with higher magnification.


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Figure 6. (a) Ex-situ Raman Spectra of FS-MCMB cathode electrode through a charge– discharge cycle showing PF6- anion intercalation/de-intercalation into MCMB, which related to (b) Galvanostatic curves of FS-MCMB/SDI battery (red and blue circles stand for selected potentials for ex-situ Raman analysis).


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Figure 7. (a) In-situ XRD pattern of FS-MCMB/SDI battery (right) which is related to galvanostatic curve of FS-MCMB/SDI battery (left) at different charging and discharging state and (b) the selected XRD pattern of FS-MCMB/SDI battery at fully-charged status. (c) Simulated XRD pattern of PF6- anion intercalated at stages-1 and related constructed model of PF6- anions in the graphite layer (the inset).


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Weight density(mg/cm2)

2.34

4.86

5.58

6.84

7.29

Discharging Specific Capacity (mAh/g)

83.60

82.74

82.20

78.11

80.48

Table 1. Discharging Specific Capacity of FS-MCMB with different weight density at 300th cycle.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. Phone: 886-4-23590121 ext. 32222. Fax 886-4-23590426. ORCID Di-Yan Wang: 0000-0003-3084-6050 Notes The authors declare no competing financial interest.


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TOC GRAPHICS


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Freestanding Cathode Electrode Design for High-Performance

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