Drawbacks of Low Lattice Energy Ammonium Salts for Ion-Conducting Polymer Electrolyte Preparation: Structural, Morphological and Electrical Characteristics of CS:PEO:NH4BF4-Based Polymer Blend Electrolytes

Mohamad A Brza; Shujahadeen B Aziz; Muaffaq M Nofal; Salah R Saeed; Shakhawan Al-Zangana; Wrya O Karim; Sarkawt A Hussen; Rebar T Abdulwahid; Mohd F Z Kadir

doi:10.3390/polym12091885

Drawbacks of Low Lattice Energy Ammonium Salts for Ion-Conducting Polymer Electrolyte Preparation: Structural, Morphological and Electrical Characteristics of CS:PEO:NH₄BF₄-Based Polymer Blend Electrolytes

Polymers (Basel). 2020 Aug 21;12(9):1885. doi: 10.3390/polym12091885.

Authors

Mohamad A Brza¹, Shujahadeen B Aziz^{2

3}, Muaffaq M Nofal⁴, Salah R Saeed⁵, Shakhawan Al-Zangana⁶, Wrya O Karim⁷, Sarkawt A Hussen², Rebar T Abdulwahid^{2

8}, Mohd F Z Kadir⁹

Affiliations

¹ Manufacturing and Materials Engineering Department, Faculty of Engineering, International Islamic University of Malaysia, Kuala Lumpur 50603, Gombak, Malaysia.
² Hameed Majid Advanced Polymeric Materials Research Lab., Department of Physics, College of Science, University of Sulaimani, Sulaimani 46001, Kurdistan Regional Government, Iraq.
³ Department of Civil Engineering, College of Engineering, Komar University of Science and Technology, Sulaimani 46001, Kurdistan Regional Government, Iraq.
⁴ Department of Mathematics and General Sciences, Prince Sultan University, P.O. Box 66833, Riyadh 11586, Saudi Arabia.
⁵ Charmo Research Center, Charmo University, Peshawa Street, Chamchamal, Sulaimani 46001, Kurdistan Regional Government, Iraq.
⁶ Department of Physics, College of Education, University of Garmian, Kalar 46021, Kurdistan Regional Government, Iraq.
⁷ Department of Chemistry, College of Science, University of Sulaimani, Qlyasan Street, Sulaimani 46001, Kurdistan Regional Government, Iraq.
⁸ Department of Physics, College of Education, University of Sulaimani, Old Campus, Sulaimani 46001, Kurdistan Regional Government, Iraq.
⁹ Centre for Foundation Studies in Science, University of Malaya, Kuala Lumpur 50603, Malaysia.

Abstract

In the present work it was shown that low lattice energy ammonium salts are not favorable for polymer electrolyte preparation for electrochemical device applications. Polymer blend electrolytes based on chitosan:poly(ethylene oxide) (CS:PEO) incorporated with various amounts of low lattice energy NH₄BF₄ammonium salt have been prepared using the solution cast technique. Both structural and morphological studies were carried out to understand the phenomenon of ion association. Sharp peaks appeared in X-ray diffraction (XRD) spectra of the samples with high salt concentration. The degree of crystallinity increased from 8.52 to 65.84 as the salt concentration increased up to 40 wt.%. These are correlated to the leakage of the associated anions and cations of the salt to the surface of the polymer. The structural behaviors were further confirmed by morphological study. The morphological results revealed the large-sized protruded salts at high salt concentration. Based on lattice energy of salts, the phenomena of salt leakage were interpreted. Ammonium salts with lattice energy lower than 600 kJ/mol are not preferred for polymer electrolyte preparation due to the significant tendency of ion association among cations and anions. Electrical impedance spectroscopy was used to estimate the conductivity of the samples. It was found that the bulk resistance increased from 1.1 × 10⁴ ohm to 0.7 × 10⁵ ohm when the salt concentration raised from 20 wt.% to 40 wt.%, respectively; due to the association of cations and anions. The low value of direct current (DC) conductivity (7.93 × 10^-7 S/cm) addressed the non-suitability of the electrolytes for electrochemical device applications. The calculated values of the capacitance over the interfaces of electrodes-electrolytes (C₂) were found to drop from 1.32 × 10^-6 F to 3.13 × 10^-7 F with increasing salt concentration. The large values of dielectric constant at low frequencies are correlated to the electrode polarization phenomena while their decrements with rising frequency are attributed to the lag of ion polarization in respect of the fast orientation of the applied alternating current (AC) field. The imaginary part of the electric modulus shows obvious peaks known as conduction relaxation peaks.

Keywords: degree of crystallinity; dielectric properties; impedance spectroscopy; low lattice energy ammonium salts; morphology study; polymer blend electrolyte.