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Electrochemical performance of all-solid lithium ion batteries with a polyaniline film cathode
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Electrochemical performance of all-solid lithium ion batteries with a polyaniline film cathode Ji-Woo Oh a, Rye-Gyeong Oh a, Yongku Kang b, Kwang-Sun Ryu a,∗ a b
Department of Chemistry, University of Ulsan, Ulsan 680-749, Republic of Korea Advanced Materials Divisions, Korea Research Institute of Chemical Technology, Daejeon 305-600, Republic of Korea
a r t i c l e
i n f o
Article history: Received 17 March 2015 Revised 2 June 2015 Accepted 3 June 2015 Available online xxx Keywords: Lithium ion batteries Cathode Conducting polymer Polyaniline Flexible battery
a b s t r a c t We have prepared a high-density polyaniline (PANI) paste (50 mg/mL), with similar physical properties to those of paints or pigments. The synthesis of PANI is confirmed by Fourier transform infrared (FT-IR) spectroscopy. The morphologies of PANI, doped PANI, and doped PANI paste are confirmed by scanning electron microscopy (SEM). Particles of doped PANI paste are approximately 40–50 nm in diameter, with a uniform and cubic shape. The electrochemical performances of doped PANI paste using both liquid and solid polymer electrolytes have been measured by galvanostatic charge and discharge process. The cell fabricated with doped PANI paste and the solid polymer electrolyte exhibits a discharge capacity of ∼87 μAh/cm2 (64.0 mAh/g) at the second cycle and ∼67 μAh/cm2 (50.1 mAh/g) at the 100th cycle. © 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
1. Introduction Thin, flexible, lightweight, and environmentally friendly batteries for high-performance sportswear, wearable displays, and new classes of portable power have become focused. For such electronic applications, a liquid electrolyte should be avoided and the electrode materials should have adequate mechanical flexibility for solid-stated flexible. Especially, printed electrodes offer more flexibility than common electrodes. As cathode materials, conducting polymers offer many advantages, such as high conductivity, mechanical flexibility, light weight, and cost-efficiency, compared to the metal-containing materials. Among the conducting polymers, polyaniline (PANI) is particularly useful because of its low cost, simple polymerization, and high yield. Additionally, PANI is highly stable in its conductive form and satisfies most of the basic requirements of electrode materials for light-weight batteries. Several techniques have been used in the fabrication of thin films such as thermal evaporation, electro-polymerization [1–5], spin-coating [6], dipping [7], electrophoretic patterning [8], and printing [9]. However, it is difficult to apply these techniques to produce PANI films due to its insolubility in common solvents. Recent breakthroughs in the synthesis and fabrication of dispersed PANI in solvents have focused on overcoming these processing difficulties. However, the fabrication of these electrodes requires a re-print process and a large amount of binder to obtain suitable ∗
Corresponding author. Tel.: +82 52 259 2763; fax: +82 52 259 2383. E-mail address: [email protected] (K.-S. Ryu).
density and thickness. In addition, the dispersants in the PANI solution degrade the electrochemical performance and lower the solution viscosity as impurities. Recently published papers for flexible devices based on the conducting polymers have focused on the development of different aspects of the various applications such as flexible sensors, organic solar cells, display applications, and thin film manufacturing processes [10–14]. Current studies are investigating lithium secondary batteries with the PANI cathode and a gel polymer electrolyte. Nevertheless, these devices still contain non-solid-state components [3,15]. In this study, therefore, we aimed to fabricate a high-density (50 mg/mL) PANI paste with suitable physical properties for printing. The PANI-pasted substrates printed via screen printing had a smooth surface without rough particles. PANI paste can be used as a conducting material in various applications, including plastic, clothes, and flexible substrates. We measured the electrochemical performance of both doped PANI and doped PANI paste with a liquid electrolyte and a solid polymer electrolyte. Furthermore, we measured the charge and discharge capacities of doped PANI paste as a function of electrode thickness to investigate the marginal efficiency of electrodes. 2. Experimental 2.1. Materials Aniline (>99.5%, Aldrich) monomers were purified by filtration ˚ and distillation before through aluminum oxide (∼150 mesh, 58 A) being stored in a refrigerator. Ammonium persulfate ((NH4 )2 S2 O8 ,
http://dx.doi.org/10.1016/j.jechem.2015.08.008 2095-4956/© 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
Please cite this article as: J.-W. Oh et al., Electrochemical performance of all-solid lithium ion batteries with a polyaniline film cathode, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.08.008
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Fig. 3. UV–vis spectra of (1) PANI-EB, (2) doped PANI, and (3) doped PANI paste.
Fig. 1. Fabrication processes of doped PANI and doped PANI paste.
PANI paste from the colloidal solution. A screen printing system (Simple Silky, Jaesung Engineering) was used to print the PANI paste. Ultraviolet–visible spectroscopy (UV–vis) was performed using an Optizen analyzer (Mecasys Co.). A Varian FT-IR 1000 instrument was used for the Fourier transform infrared (FT-IR) spectroscopy. The morphology and particle size information were obtained from field emission scanning electron microscopy (FE-SEM) (Supra 40, Carl Zeiss Co., Ltd.) analysis. The electrochemical performance was estimated by galvanostatic charge and discharge process and cycling tests (WBCS3000, WonAtech). The electrochemical stability window of the solid polymer electrolyte was measured by cyclic voltammetry (CV, Ivium Tech., IVIUMnSTAT). A stainless steel plate was used for the working electrode and lithium foil (FMC Co.) was used for the counter and reference electrodes. These test cells were assembled by sandwiching the solid polymer electrolyte between two electrodes. The ionic conductivity was measured by an impedance analyzer (Zahner Elektrik, model IM6) with an amplitude of 10 mV in a frequency range from 1 Hz to 1 MHz. The electrochemical impedance spectroscopy (EIS) measurements were conducted using a SP-300 (Biologic) at a fully discharged state over the frequency range from 100.0 kHz to 0.01 Hz at an amplitude of 10 mV at room temperature.
2.3. Synthesis of PANI and the processing method for PANI paste
Fig. 2. FT-IR spectrum of PANI-EB.
98%, used as an initiator), hydrochloric acid (HCl, 37%), and N-methyl2-pyrrolidinone (NMP, 99.5%) were purchased from Aldrich. 2.2. Instrumentation A ball-mill (pulverisette 6, FRITSCH) was used for mixing. The electric conductivities were measured by 4-point probe (CMTSR1000N, AIT Co.) after coating the polymer electrolytes onto a prepatterned ITO cell. The crystallinity of the doped PANI and doped PANI paste was characterized with X-ray powder diffraction (XRD, Rigaku UltraX, Rigaku Co.) diffractometer equipped with Cu Kα ra˚ in the 2θ range of 10° –60° with a step size diation (λ = 1.5418 A) ° of 0.02 /s. A centrifuge (HA12, Hanil Co.) was used to separate the
PANI was synthesized via the chemical oxidative polymerization method: after 10 mL of aniline monomer (10.22 g, 0.1 mol) was dissolved in 350 mL of 1.0 mol/L HCl, 5.75 g of ammonium persulfate in 20 mL of 1.0 mol/L HCl solution was slowly added dropwise into the suspension at the reaction temperature of 0–5°C until the dark suspension turned green, which indicated that polymerization of the aniline monomer had commenced. After 24 h polymerization, the solution was filtered and washed several times with water and acetonitrile until the filtrate became colorless. PANI (emeraldine base, EB) was obtained by stirring the polymer in 1.0 mol/L NH4 OH for 10 h and the solution was filtered and vacuum dried at 60°C overnight. Finally, doped PANI powder was prepared by stirring in 1.0 mol/L HCl solution. At the same time, PANI-EB was dissolved in NMP to form a PANI solution (2%), which was doped with 1.0 mol/L HCl and mixed for 2 h. The doped PANI solution was concentrated into PANI paste with a density of 50 mg/mL using the centrifuge. The density of PANI paste was measured from the dried weight of 1 mL of PANI paste. Fig. 1 shows an outline of the processing method for doped PANI and doped PANI paste.
Please cite this article as: J.-W. Oh et al., Electrochemical performance of all-solid lithium ion batteries with a polyaniline film cathode, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.08.008
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Fig. 4. SEM images of (a, b) PANI-EB, (c, d) doped PANI, and (e, f) doped PANI paste.
2.4. Fabrication of electrodes for 2032 coin-type cells and conditions for electrochemical analysis The electrodes were prepared by casting and pressing a mixture of 70 wt% active materials, 10 wt% polyvinylidene difluoride (PVDF) binder, and 20 wt% carbon black (Super P) in NMP on aluminum foil, followed by drying for 32 h at 50 °C. Electrodes of various thicknesses (35 μm, 55 μm, 70 μm, and 85 μm) were prepared using the same process. Coin-type cells (CR 2032) were fabricated in an argon-filled glove box. The electrolyte was 1.15 mol/L LiPF6 in an ethylene carbonate-diethyl carbonate-dimethyl carbonate (EC-DEC-DMC, 3:2:5 volume ratio) solution. Celgard 2400 film was used as a separator with the counter and reference electrodes of lithium. The all-
solid-state lithium ion batteries were made with the solid polymer electrolyte without the separator. To compare the electrochemical properties of doped PANI paste with those of doped PANI, CV was conducted in the voltage range of 2.0–4.1 V at a scan rate of 0.05 mV/s. Galvanostatic charge and discharge test was carried out in the voltage range of 2.0–4.1 V at a current density of 5 μA/cm2 for 100 cycles at room temperature (25 °C). The thickness of the electrodes was 35 μm for both doped PANI and doped PANI paste. 2.5. Preparation of the solid polymer electrolyte The solid polymer electrolyte was prepared by in situ crosslinking of a homogeneous precursor solution. The solution was composed of
Please cite this article as: J.-W. Oh et al., Electrochemical performance of all-solid lithium ion batteries with a polyaniline film cathode, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.08.008
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Fig. 5. XRD profiles of (a) doped PANI and (b) doped PANI paste.
the cross-linker (polyethylene glycol diacrylate (PEGDA)), the plasticizers (tetraethylene glycol dimethylether (TEGDME) and polyethylene glycol dimethyl ether (PEGDME), a lithium salt (LiCF3 SO3 ), and an initiator (t-amyl peroxybenzoate (APO)). The precursor solution was injected into the coin-cells in a wet form with the nonwoven separator placed between the cathode electrode (doped PANI or doped PANI paste) and the anode electrode. All samples were prepared in an argon-filled glove box, and were thermally crosslinked in an oven at 80 °C for 30 min. More information about the synthesis of the solid polymer electrolyte can be found in the literature [16–18]. 2.6. Production of electrodes by screen printing PANI paste was mixed with 10 wt% PVDF using a mixing machine to increase the viscosity and adhesive properties. Aluminum foil was placed under the screen printer to serve as a conductive substrate, and then the PANI paste was loaded and printed onto the substrate with a squeegee. 3. Results and discussion The synthesis of PANI-EB was confirmed by FT-IR spectroscopy, as shown in Fig. 2. The peaks of PANI evident at 1,588, 1,492, 1,302, and 1,142 cm−1 correspond to C=N and C=C stretching modes of the quinoid and benzenoid rings, which are specific to PANI, and to C—N stretching (—N—benzenoid—N—) and C=N stretching (—N=quinoid=N—), respectively. The two other peaks at 832 cm−1
Fig. 6. (a) Cyclic voltammogram at a scan rate 0.05 mV/s, (b) charge-discharge profiles between 2.5 and 4.1 V at a constant current density of 5 μA/cm2 , and (c) cyclic performance at a constant current density of 5 μA/cm2 of doped PANI and doped PANI paste.
and 677 cm−1 correspond to C—H out-of-plane bending and the C=C bending vibration, respectively. The presence of PANI-EB, doped PANI, and doped PANI paste was confirmed by UV–vis as shown in Fig. 3. All samples have a peak around 327 nm caused by the π -π ∗ transition of the benzenoid structure. PANI-EB has a broad peak at 620 nm, corresponding to the molecular exciton by localized three-ring charge transfer band [19]. On the other hand, the peak at 620 nm is absent in the spectra of doped PANI and doped PANI paste, whereas an absorption peak at
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Fig. 8. (a) Cyclic voltammogram and (b) ionic conductivity of the solid polymer electrolyte.
Fig. 7. (a) Casting test of doped PANI paste electrode using screen printing and (b) dried electrode.
450 nm caused by the semiquinoid radical cation does appear [20]. Moreover, a near infrared absorption peak from 600 nm to 1000 nm also appears as a broad cationic radical polaron band. This peak is free carrier tailing and is caused by delocalization of electric charge carriers such as metals with high conductivity [21,22]. Among these samples, doped PANI paste exhibited the highest absorption band at 327 nm, which is closely related to the doping state of PANI. These results implied that doped PANI paste has a higher electric conductivity than doped PANI has. The electrical conductivity of doped PANI and doped PANI paste was 1.09 × 10−1 and 1.70 × 10−1 S/cm, respectively. The measurements were taken on sheets of the material (15 mm × 15 mm × 50 μm) made by kneading the mixture. The mixture consisted of doped PANI (90 wt%) or doped PANI paste (90 wt%) and polytetrafluoroethylene (PTFE, 10 wt%). The morphologies of PANI-EB, doped PANI, and doped PANI paste are shown in Fig. 4. The PANI-EB shows a granular structure, which is common in PANI powder. This powder was prepared through the precipitation polymerization using strong oxidants and a high concentration of aniline under strongly acidic conditions (pH < 2.5) [23,24]. Granules were made from randomly aggregated nucleates due to the high concentration of nucleates that were produced during the short induction period. Aggregated nucleates initiated the growth of granules through the starburst growth of PANI chains. The hydrophobic
nucleates were adsorbed onto the completed PANI granules as droplets and then initiated the growth of new granules on their surface. The particles had a primary diameter of 20–50 nm and a secondary diameter of 10–100 μm. The long fiber-like shape of the PANIEB is related to the π -π ∗ interaction between molecules. Doped PANI shows a similar morphology to that of PANI-EB after doping with HCl, as shown in Fig. 4(b) and 4(d). Fig. 4(e) and 4(f) shows the images of doped PANI paste after machine processing. Overall, each particle has a uniform shape and diameter of approximately 40–50 nm with a highly crystalline structure. We considered that this shape have been induced by the dopant, which was attributed to the minimum size of nucleates. Fig. 5 exhibits the XRD profiles of doped PANI and doped PANI paste. A comparison of XRD patterns for these materials is useful because the crystallinity of PANI is related to the electric conductivity and doping level of the material. The XRD patterns of doped PANI indicate reflections at 16.7° , 22.4° , and 26.5° , which are ascribed to the periodicity parallel and perpendicular to the polymer chains. The peaks at 22.4° and 26.5° indicate the local crystalline property of PANI [25,26]. The similar intensity of the peaks at 22.4° and 26.5° indicates that the doping level of doped PANI is fairly low under 0.5 [Cl]/[N] [25]. On the other hand, the peaks of doped PANI paste exhibit reflections at 16.7° , 22.4° , 26.5° , 27.5° , and 30.2° with sharper peaks than that of doped PANI. The new peaks at 27.5° and 30.2° are related to an increase in the crystallinity and the doping level of the material. Furthermore, the intensity of the peak at 22.4° decreased compared with that of doped PANI, indicating that the amorphous regions
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Fig. 9. (a,b) Discharge profiles of the cell using doped PANI paste electrode and the liquid electrolyte at first and the 100th cycle, respectively, and (c,d) discharge profiles of the cell using the solid polymer electrolyte without a separator at first and the 100th cycle, respectively.
decreased and the doping level increased by approximately 0.5 [Cl]/[N] in doped PANI paste [26–28]. This result corresponds to the UV–vis spectra, conductivity measurements, and FE-SEM results. Fig. 6 shows the electrochemical performances of doped PANI and doped PANI paste. There are two similar pairs of redox peaks in each curve of both doped PANI and doped PANI paste, which are caused by the oxidation of PANI to its radical cation (polaron) [29]. Redox peaks of doped PANI are broad, whereas those of doped PANI paste are distinctly divided. The oxidation and reduction peaks are located at 3.5 and 3.25 V, respectively, which correspond to the redox peaks of the electrochemical doping/de-doping reactions of H+ or Cl− [30]. The charge-discharge profiles of 35 μm-thick, doped PANI and doped PANI paste were measured between 2.5 and 4.1 V at a constant current density of 5 μA/cm2 (0.1 C) at room temperature. In the electrochemical process, the charge separation mechanism in the electrolyte is followed by the intercalation mechanism. In the charge process, a site having an unpaired electron is generated in PANI. The oxidized PANI generates stable positive charge due to its π conjugated system. In the discharge process, however, a site, which is unstable and has a paired electron, is generated by accepting one electron [2]. The voltage drop in passing from charge to discharge is relatively small, indicating that the cell has a low resistance. Doped PANI and doped PANI paste show discharge capacities of 74.4 and 110.9 mAh/g, respectively. The discharge capacities of the cells are reduced from 81.2 and 112.4 mAh/g at the 10th cycle to 56.8 and 86.8 mAh/g at the 100th cycle, respectively. Fig. 7 shows the images taken with a digital camera of representative samples printed on the aluminum foil. The measured thickness of the printed PANI paste was 4 μm and the tab-density was calculated to be 2.5 mg/cm3 . After oven drying at 50 ° C, electrodes printed with the doped PANI paste exhibited excellent adhesion. Fig. 8(a) shows a cyclic voltammogram of the solid polymer electrolyte with a plasticizer. Significant oxidative degradation of the
solid polymer electrolyte started at about 4.67 V (vs. Li+ /Li), and then the current, which is related to the decomposition of the polymer electrolyte, increased gradually when the electrode potential was higher than 4.67 V. Reversible Li plating/stripping cycles were observed between −0.3 and 2.0 V (vs. Li+ /Li). Fig. 8(b) shows the temperature dependence of the ionic conductivity of the solid polymer electrolyte: the ionic conductivity of the solid polymer electrolyte decreases with increasing temperature. The conductivity was measured to be 3.543 × 10−4 S/cm at 30 ° C. Fig. 9 shows the electrochemical performances of the cell comprised of different thicknesses of PANI paste with a liquid electrolyte (a, b) and a solid electrolyte (c, d) in the voltage range of 2.5–4.1 V at a constant current density of 5 μA/cm2 (0.1 C). In the case of a liquid electrolyte, the voltage drop is relatively small, and it reveals the relatively low resistance of the cell. The solid polymer electrolyte, however, has a large voltage drop. The discharge capacity decreased due to the increased resistance related to the low ionic conductivity and wettability of the solid polymer electrolyte. When using the liquid electrolyte, the discharge capacity of electrodes with thicknesses of 35, 55, and 70 μm were 75.62, 107.96, and 146.76 μAh/cm2 , respectively, in which the capacity increased with increasing electrode thickness. Similar results were observed for the solid polymer electrolyte up to electrode thicknesses of 70 μm. However, the capacity of the 85 μm-thick electrode was similar or decreased compared to that of the 70 μm-thick electrode. Moreover, the cells with 85 μm-thick electrodes exhibited the greatest capacity fade after 100 cycles due to the considerable strain between the substrate and the active materials. Overall, the solid polymer electrolyte exhibited an increased IR-drop and a decreased discharge capacity. The electrochemical results are listed in Table 1. Fig. 10 shows the EIS results for the cell comprised of different thicknesses of PANI paste with a liquid electrolyte at room temperature as Nyquist plot. All the spectra are comprised of one semicircle
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Table 1 Electrochemical results as a function of electrode thickness using doped PANI paste. Separator
Polypropylene
Electrolyte
Discharge Electrode thickness (μm) capacity (mAh/g)
1.15 mol/L LiPF6 in 35 EC/DEC/DMC (3/2/5) 55 70 85 Solid polymer electrolyte 35 55 70 85
Discharge capacity (μAh/cm2 )
110.94
75.62
100.46 110.90 91.55 65.17 56.40 63.86 42.46
107.96 146.76 144.77 43.53 59.20 87.60 60.19
Fig. 10. Nyquist plots for doped PANI paste electrode with a liquid electrolyte and the corresponding equivalent circuit.
at high-to-medium frequency with sloping line at low frequency region. The high frequency intercept at the real axis corresponding to the solution resistance (Rs ) is mainly attributed to the electrolyte. The semicircle in the medium frequency is caused by the charge-transfer resistance (Rct ) in parallel with the non-ideal double-layer capacitance (CPE). The slope at low frequency is related to the Warburg impedance (Zw ) assigned to the diffusion of lithium ion in the cathode. The fitted values of the parameter Rct are 725.3, 903.2, 994.5, and 1186.1 , respectively. The Rct increases depending on the thickness of the electrodes. The drastically increased Rct for 85 μm-thick electrode is corresponding to its decreased discharge capacity, indicating that a proper thickness of electrode is necessary to achieve better electrochemical performance. The charge-discharge curves for the liquid electrolyte (Fig. 11a) and the solid polymer electrolyte (Fig. 11b), and cycle performance for 100 cycles (Fig. 11c) of the 70 μm-thick electrodes at a current density of 5 μA/cm2 (0.1 C) are presented. The capacities of the PANI paste with the liquid electrolyte at the 2nd, 50th, and 100th cycle were 146.8, 113.4, and 105.5 μAh/cm2 , respectively, compared to 87.1, 75.2, and 67.2 μAh/cm2 for PANI paste with the solid polymer electrolyte, respectively. The discharge capacity was decreased by 37.2% for the solid polymer electrolyte compared to the liquid electrolyte. Both the liquid electrolyte and the solid polymer electrolyte demonstrate a gradual diminution in discharge capacity over increased cycles: a 35% decrease from the initial discharge capacity of 146.8–105.5 μAh/cm2 (110.9–79.7 mAh/g) for the liquid electrolyte, and a 43.5% decrease from the initial discharge capacity of 87.1–67.2 μAh/cm2 (64.0–50.1 mAh/g) for the solid polymer electrolyte.
Fig. 11. (a,b) Charge-discharge profiles and (c) cyclic performances of doped PANI paste electrode (70 μm-thick) using the liquid electrolyte and the solid polymer electrolyte.
4. Conclusions High density PANI paste was synthesized via the chemical oxidative polymerization method. The particles of PANI paste were approximately 40–50 nm in diameter with strong crystallinity and a cubic shape. The PANI-pasted substrates printed via screen printing had a smooth surface without rough particles. The crystallinity and conductivity of the doped PANI paste were increased compared to those of doped PANI. We measured the electrochemical performance of doped PANI paste using a solid polymer electrolyte and a liquid electrolyte. The 70 μm-thick electrodes with the liquid electrolyte delivered the highest specific capacity of 146.8 and
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105.5 μAh/cm2 (110.9 and 79.7 mAh/g) at the first and the 100th cycle, respectively. On the other hand, the cells with the solid polymer electrolyte delivered the discharge capacity of 87.1 and 67.2 μAh/cm2 (64.0 and 50.1 mAh/g), respectively. These are the first experimental results obtained from an investigation of a solid polymer electrolyte and a conducting polymer as an ink for application to all-solid-state lithium ion batteries. These results may support various future applications and novel processes for electric devices in the field of printing electrodes. Acknowledgments This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Korean Ministry of Knowledge Economy and by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20090093818). References [1] [2] [3] [4] [5]
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Please cite this article as: J.-W. Oh et al., Electrochemical performance of all-solid lithium ion batteries with a polyaniline film cathode, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.08.008
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Journal of Energy Chemistry 000 (2015) 1–8
Contents lists available at ScienceDirect
Journal of Energy Chemistry journal homepage: www.elsevier.com/locate/jechem
Electrochemical performance of all-solid lithium ion batteries with a polyaniline film cathode Ji-Woo Oh a, Rye-Gyeong Oh a, Yongku Kang b, Kwang-Sun Ryu a,∗ a b
Department of Chemistry, University of Ulsan, Ulsan 680-749, Republic of Korea Advanced Materials Divisions, Korea Research Institute of Chemical Technology, Daejeon 305-600, Republic of Korea
a r t i c l e
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Article history: Received 17 March 2015 Revised 2 June 2015 Accepted 3 June 2015 Available online xxx Keywords: Lithium ion batteries Cathode Conducting polymer Polyaniline Flexible battery
a b s t r a c t We have prepared a high-density polyaniline (PANI) paste (50 mg/mL), with similar physical properties to those of paints or pigments. The synthesis of PANI is confirmed by Fourier transform infrared (FT-IR) spectroscopy. The morphologies of PANI, doped PANI, and doped PANI paste are confirmed by scanning electron microscopy (SEM). Particles of doped PANI paste are approximately 40–50 nm in diameter, with a uniform and cubic shape. The electrochemical performances of doped PANI paste using both liquid and solid polymer electrolytes have been measured by galvanostatic charge and discharge process. The cell fabricated with doped PANI paste and the solid polymer electrolyte exhibits a discharge capacity of ∼87 μAh/cm2 (64.0 mAh/g) at the second cycle and ∼67 μAh/cm2 (50.1 mAh/g) at the 100th cycle. © 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
1. Introduction Thin, flexible, lightweight, and environmentally friendly batteries for high-performance sportswear, wearable displays, and new classes of portable power have become focused. For such electronic applications, a liquid electrolyte should be avoided and the electrode materials should have adequate mechanical flexibility for solid-stated flexible. Especially, printed electrodes offer more flexibility than common electrodes. As cathode materials, conducting polymers offer many advantages, such as high conductivity, mechanical flexibility, light weight, and cost-efficiency, compared to the metal-containing materials. Among the conducting polymers, polyaniline (PANI) is particularly useful because of its low cost, simple polymerization, and high yield. Additionally, PANI is highly stable in its conductive form and satisfies most of the basic requirements of electrode materials for light-weight batteries. Several techniques have been used in the fabrication of thin films such as thermal evaporation, electro-polymerization [1–5], spin-coating [6], dipping [7], electrophoretic patterning [8], and printing [9]. However, it is difficult to apply these techniques to produce PANI films due to its insolubility in common solvents. Recent breakthroughs in the synthesis and fabrication of dispersed PANI in solvents have focused on overcoming these processing difficulties. However, the fabrication of these electrodes requires a re-print process and a large amount of binder to obtain suitable ∗
Corresponding author. Tel.: +82 52 259 2763; fax: +82 52 259 2383. E-mail address: [email protected] (K.-S. Ryu).
density and thickness. In addition, the dispersants in the PANI solution degrade the electrochemical performance and lower the solution viscosity as impurities. Recently published papers for flexible devices based on the conducting polymers have focused on the development of different aspects of the various applications such as flexible sensors, organic solar cells, display applications, and thin film manufacturing processes [10–14]. Current studies are investigating lithium secondary batteries with the PANI cathode and a gel polymer electrolyte. Nevertheless, these devices still contain non-solid-state components [3,15]. In this study, therefore, we aimed to fabricate a high-density (50 mg/mL) PANI paste with suitable physical properties for printing. The PANI-pasted substrates printed via screen printing had a smooth surface without rough particles. PANI paste can be used as a conducting material in various applications, including plastic, clothes, and flexible substrates. We measured the electrochemical performance of both doped PANI and doped PANI paste with a liquid electrolyte and a solid polymer electrolyte. Furthermore, we measured the charge and discharge capacities of doped PANI paste as a function of electrode thickness to investigate the marginal efficiency of electrodes. 2. Experimental 2.1. Materials Aniline (>99.5%, Aldrich) monomers were purified by filtration ˚ and distillation before through aluminum oxide (∼150 mesh, 58 A) being stored in a refrigerator. Ammonium persulfate ((NH4 )2 S2 O8 ,
http://dx.doi.org/10.1016/j.jechem.2015.08.008 2095-4956/© 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
Please cite this article as: J.-W. Oh et al., Electrochemical performance of all-solid lithium ion batteries with a polyaniline film cathode, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.08.008
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Fig. 3. UV–vis spectra of (1) PANI-EB, (2) doped PANI, and (3) doped PANI paste.
Fig. 1. Fabrication processes of doped PANI and doped PANI paste.
PANI paste from the colloidal solution. A screen printing system (Simple Silky, Jaesung Engineering) was used to print the PANI paste. Ultraviolet–visible spectroscopy (UV–vis) was performed using an Optizen analyzer (Mecasys Co.). A Varian FT-IR 1000 instrument was used for the Fourier transform infrared (FT-IR) spectroscopy. The morphology and particle size information were obtained from field emission scanning electron microscopy (FE-SEM) (Supra 40, Carl Zeiss Co., Ltd.) analysis. The electrochemical performance was estimated by galvanostatic charge and discharge process and cycling tests (WBCS3000, WonAtech). The electrochemical stability window of the solid polymer electrolyte was measured by cyclic voltammetry (CV, Ivium Tech., IVIUMnSTAT). A stainless steel plate was used for the working electrode and lithium foil (FMC Co.) was used for the counter and reference electrodes. These test cells were assembled by sandwiching the solid polymer electrolyte between two electrodes. The ionic conductivity was measured by an impedance analyzer (Zahner Elektrik, model IM6) with an amplitude of 10 mV in a frequency range from 1 Hz to 1 MHz. The electrochemical impedance spectroscopy (EIS) measurements were conducted using a SP-300 (Biologic) at a fully discharged state over the frequency range from 100.0 kHz to 0.01 Hz at an amplitude of 10 mV at room temperature.
2.3. Synthesis of PANI and the processing method for PANI paste
Fig. 2. FT-IR spectrum of PANI-EB.
98%, used as an initiator), hydrochloric acid (HCl, 37%), and N-methyl2-pyrrolidinone (NMP, 99.5%) were purchased from Aldrich. 2.2. Instrumentation A ball-mill (pulverisette 6, FRITSCH) was used for mixing. The electric conductivities were measured by 4-point probe (CMTSR1000N, AIT Co.) after coating the polymer electrolytes onto a prepatterned ITO cell. The crystallinity of the doped PANI and doped PANI paste was characterized with X-ray powder diffraction (XRD, Rigaku UltraX, Rigaku Co.) diffractometer equipped with Cu Kα ra˚ in the 2θ range of 10° –60° with a step size diation (λ = 1.5418 A) ° of 0.02 /s. A centrifuge (HA12, Hanil Co.) was used to separate the
PANI was synthesized via the chemical oxidative polymerization method: after 10 mL of aniline monomer (10.22 g, 0.1 mol) was dissolved in 350 mL of 1.0 mol/L HCl, 5.75 g of ammonium persulfate in 20 mL of 1.0 mol/L HCl solution was slowly added dropwise into the suspension at the reaction temperature of 0–5°C until the dark suspension turned green, which indicated that polymerization of the aniline monomer had commenced. After 24 h polymerization, the solution was filtered and washed several times with water and acetonitrile until the filtrate became colorless. PANI (emeraldine base, EB) was obtained by stirring the polymer in 1.0 mol/L NH4 OH for 10 h and the solution was filtered and vacuum dried at 60°C overnight. Finally, doped PANI powder was prepared by stirring in 1.0 mol/L HCl solution. At the same time, PANI-EB was dissolved in NMP to form a PANI solution (2%), which was doped with 1.0 mol/L HCl and mixed for 2 h. The doped PANI solution was concentrated into PANI paste with a density of 50 mg/mL using the centrifuge. The density of PANI paste was measured from the dried weight of 1 mL of PANI paste. Fig. 1 shows an outline of the processing method for doped PANI and doped PANI paste.
Please cite this article as: J.-W. Oh et al., Electrochemical performance of all-solid lithium ion batteries with a polyaniline film cathode, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.08.008
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Fig. 4. SEM images of (a, b) PANI-EB, (c, d) doped PANI, and (e, f) doped PANI paste.
2.4. Fabrication of electrodes for 2032 coin-type cells and conditions for electrochemical analysis The electrodes were prepared by casting and pressing a mixture of 70 wt% active materials, 10 wt% polyvinylidene difluoride (PVDF) binder, and 20 wt% carbon black (Super P) in NMP on aluminum foil, followed by drying for 32 h at 50 °C. Electrodes of various thicknesses (35 μm, 55 μm, 70 μm, and 85 μm) were prepared using the same process. Coin-type cells (CR 2032) were fabricated in an argon-filled glove box. The electrolyte was 1.15 mol/L LiPF6 in an ethylene carbonate-diethyl carbonate-dimethyl carbonate (EC-DEC-DMC, 3:2:5 volume ratio) solution. Celgard 2400 film was used as a separator with the counter and reference electrodes of lithium. The all-
solid-state lithium ion batteries were made with the solid polymer electrolyte without the separator. To compare the electrochemical properties of doped PANI paste with those of doped PANI, CV was conducted in the voltage range of 2.0–4.1 V at a scan rate of 0.05 mV/s. Galvanostatic charge and discharge test was carried out in the voltage range of 2.0–4.1 V at a current density of 5 μA/cm2 for 100 cycles at room temperature (25 °C). The thickness of the electrodes was 35 μm for both doped PANI and doped PANI paste. 2.5. Preparation of the solid polymer electrolyte The solid polymer electrolyte was prepared by in situ crosslinking of a homogeneous precursor solution. The solution was composed of
Please cite this article as: J.-W. Oh et al., Electrochemical performance of all-solid lithium ion batteries with a polyaniline film cathode, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.08.008
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Fig. 5. XRD profiles of (a) doped PANI and (b) doped PANI paste.
the cross-linker (polyethylene glycol diacrylate (PEGDA)), the plasticizers (tetraethylene glycol dimethylether (TEGDME) and polyethylene glycol dimethyl ether (PEGDME), a lithium salt (LiCF3 SO3 ), and an initiator (t-amyl peroxybenzoate (APO)). The precursor solution was injected into the coin-cells in a wet form with the nonwoven separator placed between the cathode electrode (doped PANI or doped PANI paste) and the anode electrode. All samples were prepared in an argon-filled glove box, and were thermally crosslinked in an oven at 80 °C for 30 min. More information about the synthesis of the solid polymer electrolyte can be found in the literature [16–18]. 2.6. Production of electrodes by screen printing PANI paste was mixed with 10 wt% PVDF using a mixing machine to increase the viscosity and adhesive properties. Aluminum foil was placed under the screen printer to serve as a conductive substrate, and then the PANI paste was loaded and printed onto the substrate with a squeegee. 3. Results and discussion The synthesis of PANI-EB was confirmed by FT-IR spectroscopy, as shown in Fig. 2. The peaks of PANI evident at 1,588, 1,492, 1,302, and 1,142 cm−1 correspond to C=N and C=C stretching modes of the quinoid and benzenoid rings, which are specific to PANI, and to C—N stretching (—N—benzenoid—N—) and C=N stretching (—N=quinoid=N—), respectively. The two other peaks at 832 cm−1
Fig. 6. (a) Cyclic voltammogram at a scan rate 0.05 mV/s, (b) charge-discharge profiles between 2.5 and 4.1 V at a constant current density of 5 μA/cm2 , and (c) cyclic performance at a constant current density of 5 μA/cm2 of doped PANI and doped PANI paste.
and 677 cm−1 correspond to C—H out-of-plane bending and the C=C bending vibration, respectively. The presence of PANI-EB, doped PANI, and doped PANI paste was confirmed by UV–vis as shown in Fig. 3. All samples have a peak around 327 nm caused by the π -π ∗ transition of the benzenoid structure. PANI-EB has a broad peak at 620 nm, corresponding to the molecular exciton by localized three-ring charge transfer band [19]. On the other hand, the peak at 620 nm is absent in the spectra of doped PANI and doped PANI paste, whereas an absorption peak at
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Fig. 8. (a) Cyclic voltammogram and (b) ionic conductivity of the solid polymer electrolyte.
Fig. 7. (a) Casting test of doped PANI paste electrode using screen printing and (b) dried electrode.
450 nm caused by the semiquinoid radical cation does appear [20]. Moreover, a near infrared absorption peak from 600 nm to 1000 nm also appears as a broad cationic radical polaron band. This peak is free carrier tailing and is caused by delocalization of electric charge carriers such as metals with high conductivity [21,22]. Among these samples, doped PANI paste exhibited the highest absorption band at 327 nm, which is closely related to the doping state of PANI. These results implied that doped PANI paste has a higher electric conductivity than doped PANI has. The electrical conductivity of doped PANI and doped PANI paste was 1.09 × 10−1 and 1.70 × 10−1 S/cm, respectively. The measurements were taken on sheets of the material (15 mm × 15 mm × 50 μm) made by kneading the mixture. The mixture consisted of doped PANI (90 wt%) or doped PANI paste (90 wt%) and polytetrafluoroethylene (PTFE, 10 wt%). The morphologies of PANI-EB, doped PANI, and doped PANI paste are shown in Fig. 4. The PANI-EB shows a granular structure, which is common in PANI powder. This powder was prepared through the precipitation polymerization using strong oxidants and a high concentration of aniline under strongly acidic conditions (pH < 2.5) [23,24]. Granules were made from randomly aggregated nucleates due to the high concentration of nucleates that were produced during the short induction period. Aggregated nucleates initiated the growth of granules through the starburst growth of PANI chains. The hydrophobic
nucleates were adsorbed onto the completed PANI granules as droplets and then initiated the growth of new granules on their surface. The particles had a primary diameter of 20–50 nm and a secondary diameter of 10–100 μm. The long fiber-like shape of the PANIEB is related to the π -π ∗ interaction between molecules. Doped PANI shows a similar morphology to that of PANI-EB after doping with HCl, as shown in Fig. 4(b) and 4(d). Fig. 4(e) and 4(f) shows the images of doped PANI paste after machine processing. Overall, each particle has a uniform shape and diameter of approximately 40–50 nm with a highly crystalline structure. We considered that this shape have been induced by the dopant, which was attributed to the minimum size of nucleates. Fig. 5 exhibits the XRD profiles of doped PANI and doped PANI paste. A comparison of XRD patterns for these materials is useful because the crystallinity of PANI is related to the electric conductivity and doping level of the material. The XRD patterns of doped PANI indicate reflections at 16.7° , 22.4° , and 26.5° , which are ascribed to the periodicity parallel and perpendicular to the polymer chains. The peaks at 22.4° and 26.5° indicate the local crystalline property of PANI [25,26]. The similar intensity of the peaks at 22.4° and 26.5° indicates that the doping level of doped PANI is fairly low under 0.5 [Cl]/[N] [25]. On the other hand, the peaks of doped PANI paste exhibit reflections at 16.7° , 22.4° , 26.5° , 27.5° , and 30.2° with sharper peaks than that of doped PANI. The new peaks at 27.5° and 30.2° are related to an increase in the crystallinity and the doping level of the material. Furthermore, the intensity of the peak at 22.4° decreased compared with that of doped PANI, indicating that the amorphous regions
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Fig. 9. (a,b) Discharge profiles of the cell using doped PANI paste electrode and the liquid electrolyte at first and the 100th cycle, respectively, and (c,d) discharge profiles of the cell using the solid polymer electrolyte without a separator at first and the 100th cycle, respectively.
decreased and the doping level increased by approximately 0.5 [Cl]/[N] in doped PANI paste [26–28]. This result corresponds to the UV–vis spectra, conductivity measurements, and FE-SEM results. Fig. 6 shows the electrochemical performances of doped PANI and doped PANI paste. There are two similar pairs of redox peaks in each curve of both doped PANI and doped PANI paste, which are caused by the oxidation of PANI to its radical cation (polaron) [29]. Redox peaks of doped PANI are broad, whereas those of doped PANI paste are distinctly divided. The oxidation and reduction peaks are located at 3.5 and 3.25 V, respectively, which correspond to the redox peaks of the electrochemical doping/de-doping reactions of H+ or Cl− [30]. The charge-discharge profiles of 35 μm-thick, doped PANI and doped PANI paste were measured between 2.5 and 4.1 V at a constant current density of 5 μA/cm2 (0.1 C) at room temperature. In the electrochemical process, the charge separation mechanism in the electrolyte is followed by the intercalation mechanism. In the charge process, a site having an unpaired electron is generated in PANI. The oxidized PANI generates stable positive charge due to its π conjugated system. In the discharge process, however, a site, which is unstable and has a paired electron, is generated by accepting one electron [2]. The voltage drop in passing from charge to discharge is relatively small, indicating that the cell has a low resistance. Doped PANI and doped PANI paste show discharge capacities of 74.4 and 110.9 mAh/g, respectively. The discharge capacities of the cells are reduced from 81.2 and 112.4 mAh/g at the 10th cycle to 56.8 and 86.8 mAh/g at the 100th cycle, respectively. Fig. 7 shows the images taken with a digital camera of representative samples printed on the aluminum foil. The measured thickness of the printed PANI paste was 4 μm and the tab-density was calculated to be 2.5 mg/cm3 . After oven drying at 50 ° C, electrodes printed with the doped PANI paste exhibited excellent adhesion. Fig. 8(a) shows a cyclic voltammogram of the solid polymer electrolyte with a plasticizer. Significant oxidative degradation of the
solid polymer electrolyte started at about 4.67 V (vs. Li+ /Li), and then the current, which is related to the decomposition of the polymer electrolyte, increased gradually when the electrode potential was higher than 4.67 V. Reversible Li plating/stripping cycles were observed between −0.3 and 2.0 V (vs. Li+ /Li). Fig. 8(b) shows the temperature dependence of the ionic conductivity of the solid polymer electrolyte: the ionic conductivity of the solid polymer electrolyte decreases with increasing temperature. The conductivity was measured to be 3.543 × 10−4 S/cm at 30 ° C. Fig. 9 shows the electrochemical performances of the cell comprised of different thicknesses of PANI paste with a liquid electrolyte (a, b) and a solid electrolyte (c, d) in the voltage range of 2.5–4.1 V at a constant current density of 5 μA/cm2 (0.1 C). In the case of a liquid electrolyte, the voltage drop is relatively small, and it reveals the relatively low resistance of the cell. The solid polymer electrolyte, however, has a large voltage drop. The discharge capacity decreased due to the increased resistance related to the low ionic conductivity and wettability of the solid polymer electrolyte. When using the liquid electrolyte, the discharge capacity of electrodes with thicknesses of 35, 55, and 70 μm were 75.62, 107.96, and 146.76 μAh/cm2 , respectively, in which the capacity increased with increasing electrode thickness. Similar results were observed for the solid polymer electrolyte up to electrode thicknesses of 70 μm. However, the capacity of the 85 μm-thick electrode was similar or decreased compared to that of the 70 μm-thick electrode. Moreover, the cells with 85 μm-thick electrodes exhibited the greatest capacity fade after 100 cycles due to the considerable strain between the substrate and the active materials. Overall, the solid polymer electrolyte exhibited an increased IR-drop and a decreased discharge capacity. The electrochemical results are listed in Table 1. Fig. 10 shows the EIS results for the cell comprised of different thicknesses of PANI paste with a liquid electrolyte at room temperature as Nyquist plot. All the spectra are comprised of one semicircle
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Table 1 Electrochemical results as a function of electrode thickness using doped PANI paste. Separator
Polypropylene
Electrolyte
Discharge Electrode thickness (μm) capacity (mAh/g)
1.15 mol/L LiPF6 in 35 EC/DEC/DMC (3/2/5) 55 70 85 Solid polymer electrolyte 35 55 70 85
Discharge capacity (μAh/cm2 )
110.94
75.62
100.46 110.90 91.55 65.17 56.40 63.86 42.46
107.96 146.76 144.77 43.53 59.20 87.60 60.19
Fig. 10. Nyquist plots for doped PANI paste electrode with a liquid electrolyte and the corresponding equivalent circuit.
at high-to-medium frequency with sloping line at low frequency region. The high frequency intercept at the real axis corresponding to the solution resistance (Rs ) is mainly attributed to the electrolyte. The semicircle in the medium frequency is caused by the charge-transfer resistance (Rct ) in parallel with the non-ideal double-layer capacitance (CPE). The slope at low frequency is related to the Warburg impedance (Zw ) assigned to the diffusion of lithium ion in the cathode. The fitted values of the parameter Rct are 725.3, 903.2, 994.5, and 1186.1 , respectively. The Rct increases depending on the thickness of the electrodes. The drastically increased Rct for 85 μm-thick electrode is corresponding to its decreased discharge capacity, indicating that a proper thickness of electrode is necessary to achieve better electrochemical performance. The charge-discharge curves for the liquid electrolyte (Fig. 11a) and the solid polymer electrolyte (Fig. 11b), and cycle performance for 100 cycles (Fig. 11c) of the 70 μm-thick electrodes at a current density of 5 μA/cm2 (0.1 C) are presented. The capacities of the PANI paste with the liquid electrolyte at the 2nd, 50th, and 100th cycle were 146.8, 113.4, and 105.5 μAh/cm2 , respectively, compared to 87.1, 75.2, and 67.2 μAh/cm2 for PANI paste with the solid polymer electrolyte, respectively. The discharge capacity was decreased by 37.2% for the solid polymer electrolyte compared to the liquid electrolyte. Both the liquid electrolyte and the solid polymer electrolyte demonstrate a gradual diminution in discharge capacity over increased cycles: a 35% decrease from the initial discharge capacity of 146.8–105.5 μAh/cm2 (110.9–79.7 mAh/g) for the liquid electrolyte, and a 43.5% decrease from the initial discharge capacity of 87.1–67.2 μAh/cm2 (64.0–50.1 mAh/g) for the solid polymer electrolyte.
Fig. 11. (a,b) Charge-discharge profiles and (c) cyclic performances of doped PANI paste electrode (70 μm-thick) using the liquid electrolyte and the solid polymer electrolyte.
4. Conclusions High density PANI paste was synthesized via the chemical oxidative polymerization method. The particles of PANI paste were approximately 40–50 nm in diameter with strong crystallinity and a cubic shape. The PANI-pasted substrates printed via screen printing had a smooth surface without rough particles. The crystallinity and conductivity of the doped PANI paste were increased compared to those of doped PANI. We measured the electrochemical performance of doped PANI paste using a solid polymer electrolyte and a liquid electrolyte. The 70 μm-thick electrodes with the liquid electrolyte delivered the highest specific capacity of 146.8 and
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105.5 μAh/cm2 (110.9 and 79.7 mAh/g) at the first and the 100th cycle, respectively. On the other hand, the cells with the solid polymer electrolyte delivered the discharge capacity of 87.1 and 67.2 μAh/cm2 (64.0 and 50.1 mAh/g), respectively. These are the first experimental results obtained from an investigation of a solid polymer electrolyte and a conducting polymer as an ink for application to all-solid-state lithium ion batteries. These results may support various future applications and novel processes for electric devices in the field of printing electrodes. Acknowledgments This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Korean Ministry of Knowledge Economy and by the Priority Research Centers Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20090093818). References [1] [2] [3] [4] [5]
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Please cite this article as: J.-W. Oh et al., Electrochemical performance of all-solid lithium ion batteries with a polyaniline film cathode, Journal of Energy Chemistry (2015), http://dx.doi.org/10.1016/j.jechem.2015.08.008