Preparation and Characterization of PAN–KI Complexed Gel Polymer Electrolytes for Solid-State Battery Applications
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Bull. Mater. Sci., Vol. 39, No. 4, August 2016, pp. 1047–1055. c Indian Academy of Sciences. DOI 10.1007/s12034-016-1241-8 Preparation and characterization of PAN–KI complexed gel polymer electrolytes for solid-state battery applications N KRISHNA JYOTHI1,2, K K VENKATARATNAM3, P NARAYANA MURTY2 and K VIJAYA KUMAR1,∗ 1Solid State Ionics Research Laboratory, Department of Physics, K L University, Vaddeswaram 522502, India 2Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar 522510, India 3Department of Physics, Malaviya National Institute of Technology Jaipur, Jaipur 302017, India MS received 4 July 2015; accepted 16 February 2016 Abstract. The free standing and dimensionally stable gel polymer electrolyte films of polyacrylonitrile (PAN): potassium iodide (KI) of different compositions, using ethylene carbonate as a plasticizer and dimethyl formamide as solvent, are prepared by adopting ‘solution casting technique’ and these films are examined for their conductivi- ties. The structural, miscibility and the chemical rapport between PAN and KI are investigated using X-ray diffrac- tion, Fourier transform infrared spectroscopy and differential scanning calorimetry methods. The conductivity is enhanced with the increase in KI concentration and temperature. The maximum conductivity at 30◦C is found to be 2.089 × 10−5 Scm−1 for PAN:KI (70:30) wt%, which is nine orders greater than that of pure PAN (<10−14 Scm−1). The conductivity-temperature dependence of these polymer electrolyte films obeys Arrhenius behaviour with activa- tion energy ranging from 0.358 to 0.478 eV. The conducting carriers of charge transport in these polymer electrolyte films are identified by Wagner’s polarization technique and it is found that the charge transport is predominantly due to ions. The better conducting sample is used to fabricate the battery with configuration K/PAN + KI/I2+ C + electrolyte and good discharge characteristics of battery are observed. Keywords. Polyacrylonitrile; solution casting technique; differential scanning calorimetry; X-ray diffraction; electrical properties. 1. Introduction The smoothness of this metal makes it easier for the compo- nents of the battery to have good contact to accomplish and Recently a lot of attention has been paid to the research acti- keep contact with other components in the battery. Moreover, vities of developing gel polymer electrolytes for solid-state potassium is more moisture resistant than lithium [13]. battery applications with good conductivity. In view of the Compared with the other polymers, PAN-based elec- above fact that solid-state battery has many advantages, trolytes have been extensively studied because of their good such as higher energy density, solvent-free condition, leak chemical, flame resistance and electrochemical stability proof, easy process ability and light weight [1–5]. Several [14–17]. PAN is one of the most important fibre-forming poly- earlier studies in this field are focused upon polyethylene mers and is widely used because of its high strength, abra- oxide (PEO), polyvinyl alcohol, polyvinyl pyrrolidone (PVP) sion resistance and good insect resistance [18]. It is used to and polyacrylonitrile (PAN) complexed with NaI, NaClO3, produce a large variety of products, including ultrafiltration NaYF4, KYF4 and LiCF3SO3 [6–12]. Although very few membranes, hollow fibres for reverse osmosis, fibres for tex- investigations are conducted on potassium complexed poly- tiles, oxidized flame retardant fibres like PANOX and carbon mer electrolytes, still no attention is paid on potassium salt fibre. Although the conductivity of pristine PAN is less than complexed PAN-based polymer electrolytes. 10−14 Scm−1, this static problem confine its further applications. Lithium batteries have advantages such as high energy PAN is usually synthesized using free radical polymer- density and rechargeability, which make them commercially ization. Generally it forms copolymers with acrylonitrile, active. Despite their advantages, they experience the ill effects methyl acrylate, acrylonitrile and methyl methacrylate. PAN of operational limitations, for example, high cost, trouble in has a glass transition point of about 107◦C and melting point taking care of atmospheric conditions due to high reactiv- of about 319◦C, and it decomposes at this temperature [19]. ity. So, the research has been focused on potassium salts due To enhance the conductivity of polymer electrolytes, sev- to their several advantages over lithium counterparts. Potas- eral techniques are suggested in the literature, including sium is less expensive and much more abundant than lithium. the use of blend polymers, addition of a ceramic filler, plasticizers and radiation. Two plasticizing solvents ethylene carbonate (EC) and dimethyl formamide (DMF) are chosen ∗Author for correspondence ([email protected]) as solvents in this study. EC has high dielectric constant and 1047 1048 N Krishna Jyothi et al low viscosity, which weakens the columbic force between DSC, TA Instruments, at a heating rate of 10◦Cmin−1 under cation and anion of the salt. Consequently, salt separation nitrogen atmosphere in the temperature range of 40–350◦C. takes place. Further, plasticizers decrease the glass transi- tion temperature of the polymer electrolyte and accordingly enhance the segmental motion of the polymer backbone and 2.4 Electrical studies create free volume. Therefore, the ions can migrate easily through the void, as a result ionic conductivity increases [20]. 2.4a Conductivity measurements: For conductivity mea- In the present work, we have developed and study the elec- surements, circular discs of diameter 2 cm were cut from the trical properties of PAN-based KI complexed gel polymer films. A disc sample was sandwiched between a pair of silver electrolytes and verify their potential in solid-state battery electrodes and spring loaded into a sample holder of lab- applications. made conductivity setup [21]. Room temperature conductivi- ties of films were measured using HIOKI 3532-50 LCR Hitester in 50 Hz to 5 MHz frequency range. 2. Experimental 2.1 Materials 2.4b Transport properties: Wagner’s polarization method was carried out at room temperature on disc-shaped samples, Polyacrylonitrile (PAN; MW = 1,50,000 g mol−1), ethy- which were sandwiched between silver electrodes. Polariza- lene carbonate (EC) were from Sigma Aldrich and potas- tion current was measured with respect to time in response sium iodide (KI), dimethyl formamide (DMF) were from to an applied DC bias of 1.5 V. Ionic transport (tion) and Merck. All reagents were used, as received, without further electronic transport (tele) numbers were calculated using the purification. graph of polarization current vs. time. 2.2 Preparation of the gel polymer electrolyte films 2.4c Fabrication of polymer battery: Potassium polymer battery was fabricated by sandwiching the salt complexed Solution casting technique was used to prepare gel polymer film as electrolyte between anode and cathode pellets electrolyte films. In this method, KI was first dissolved in the with the arrangement [K (anode)/(PAN + KI) (polymer mixture of EC and DMF with continuous stirring at 60◦Cfor electrolyte)/(I2 + C + electrolyte) (cathode)]. This entire 12 h. Then the polymer PAN was mixed with the prepared assembly was finally covered in the sample holder. In this solution and the mixture was stirred up to 36 h to obtain a battery, potassium metal was used as the anode (negative homogeneous solution. Subsequently, the prepared solution electrode). For the preparation of cathode, an accurate mix- was poured into polypropylene Petri dishes and evaporated ture of iodine (I2), graphite (C) and the electrolyte material slowly at room temperature under vacuum until the gel film in the wt% of 5:5:1, respectively, was obtained by mechani- was formed. The polymer electrolyte samples were trans- cal grinding. It was pressed in the form of pellet at a pressure ferred into desiccators for further drying before the test. The of 200 MPa after proper mixing of constituents. The battery films formed were mechanically stable, transparent and free parameters, short circuit current (SCC), open circuit voltage standing with thickness range of 150 μm. (OCV), current density, energy density, power density and discharge time were evaluated. 2.3 Structural characterizations 2.3a X-ray diffraction: To investigate the structure of the 3. Results and discussion gel polymer electrolyte films, X-ray diffraction (XRD) spec- tra were carried out using an X’pert diffractometer (Philips 3.1 XRD analysis PANalytical) with CuKα radiation (λ = 1.5403 A),˙ and the diffraction patterns were recorded for 2θ values between 10◦ XRD measurements are performed on PAN-based KI com- and 90◦. plexed films to examine the crystallinity and the amorphous nature of the films. Figure 1 is a comparative study of the XRD patterns of pure PAN and PAN doped with different 2.3b Fourier transform infrared spectroscopy: Fourier KI concentration regions. The comparative study shows that transform infrared (FTIR) absorption spectra were recorded the intensity of the crystalline peak of PAN decreases grad- with the help of the Perkin Elmer FTIR spectrometer (Spec- ually with the addition of KI salt (up to 30 wt%) to the trum II USA) in the wavenumber range of 450–4000 cm−1 polymer, significant decrease in the degree of crystallinity with spectral resolution of 2 cm−1. of the electrolyte films. It may be due to the disturbance in the semicrystalline structure of the film by an addition of KI 2.3c Differential scanning calorimetry: The glass transition salt. The XRD pattern of the pure PAN film shows a crys- ◦ temperature (Tg) and melting temperature (Tm) of the gel poly- talline peak at 2θ = 17 , corresponding to orthorhombic mer electrolyte films were determined using an Auto Q20 PAN (110) reflection [22]. The XRD patterns show that the PAN–KI complexed gel polymer electrolytes 1049 Figure 1. X-ray diffraction patterns of pure PAN, pure KI salt and PAN:KI complexed films. semicrystalline structure is with the presence of mixed crys- (60:40) film the crystallinity increased to 24%. From these talline and amorphous phases.