Study of Relaxation Dynamics and Ion Conduction Mechanism of Composite Polymer Electrolyte and Gel Polymer Electrolyte

Abstract

The increasing demand for miniaturized portable electrical energy sources has led towards intensive research on developing efficient electrochemical energy storage/conversion devices. Based on the capability of delivering continuous energy for a longer period of time or quick charge-discharge capabilities, these devices can be divided into energy and current sourcing devices. Among these devices, batteries show intermediate power density along with energy density. At present in most of the commercially available devices, liquid organic carbonate electrolytes having conductivity values close to 10􀀀3 Scm􀀀1 are being used. Although liquid electrolyte shows a high conductivity value, they possess a serious safety concern. Therefore, prior importance is given to developing a polymer electrolyte with comparable ionic conductivity at ambient temperature. Polymer electrolyte has the prospect to improve various key properties of lithium based batteries when used as the electrolyte. These properties include design flexibility, safety, cyclability, energy and power density etc. However, polymer electrolytes are having a serious drawback of low ionic conductivity limiting its potential application. Therefore primary interest is given in the preparation of polymer electrolytes with high ionic conductivity at room temperature. Achievement of the desired level of ambient temperature ionic conductivity (_ 10􀀀3 Scm􀀀1) is still an open problem. Literature suggests that to improve the ionic conductivity of polymer electrolytes several strategies such as plasticization, copolymerization, fabrication of composite/nano-composite etc. have been studied extensively. These techniques mainly concentrate on increasing the amorphous content of polymer electrolytes in order to favour ion mobility to increase the ionic conductivity. In this regard, optimization of ionic conductivity of polymer electrolytes is carried out in the present investigation for composite polymer electrolytes and gel polymer electrolytes. In addition to the process of optimization, prior importance is also given on the understanding the ion conduction mechanism in these two class of polymer electrolytes.

In this study three different series of polymer composite electrolytes are prepared using polyethylene oxide as the host polymer, lithium triflate as salt and nanocrystalline zirconia, titania and organo-modified hydrophobic montmorillonite clay as fillers. In addition to this a series of gel polymer electrolyte is also prepared by blending polymer host and 1 molar lithium triflate electrolyte solution consisting of a mixture of ethylene carbonate and diethyl carbonate as solvent. Phase formation of the filler materials, composite nature of polymer composite electrolytes and blended polymer host matrix prepared for gel polymer electrolytes are studied using X-ray diffraction technique. Surface morphology of all these materials is studied using FE-SEM. Polymer salt interactions are investigated using FTIR. Ionic conductivity is measured over a wide range of temperature for getting proper idea about its temperature dependent behaviour. In all these electrolytes, we have achieved room temperature ionic conductivity up to the order of 10􀀀5 S cm􀀀1. This is nearly two order higher in magnitude than conventional polymer-salt complexes at room temperature. Though we are successful in increasing the ionic conductivity by almost two orders in magnitude at room temperature, there exist a huge scope for further improvement in terms of the magnitude of the ionic conductivity. For this reason, a proper understanding of ion conduction mechanism is necessary. Ionic transport mechanism is probed using broadband dielectric spectroscopy over a wide range of frequency and temperature. Relaxation dynamics at different length and time scale is analyzed using broadband dielectric spectroscopy in order to get a proper idea about the ion conduction processes taking place at the microscopic level. The physical parameters that aids in increasing the ionic conductivity of these materials are also studied with observations made from broadband dielectric spectroscopy.

An in-depth step by step analysis of the data obtained from electrical characterizations are carried out. The temperature-dependent ionic conductivity for polymer composite electrolytes are found to follow VTF behaviour, indicating there exist coupling between ionic conductivity and polymer segmental motion. Segmental relaxation time also follow similar behaviour. To explain and investigate the coupled nature of ion conduction mechanism, ion diffusivity analysis is carried out by employing Trukhan model. The outcome of these analysis also supports the coupled nature of ion conduction process. Empirical laws like Jonscher power law, double power law and different models like RFEBM, Ngai coupling model, MIGRATION model are used to describe the frequency and temperature dependent ionic conductivity of polymer electrolytes. Havriliak -Negami expression is used to analyze the relaxation phenomenon present in polymer electrolytes. Study of ion conduction mechanism in polymer nanocomposite electrolyte suggest ionic conduction and segmental relaxation are coupled physical process. In the case of polymer gel electrolytes, polymer host does not play any significant role in ionic conduction but only provide the mechanical stability to the absorbed liquid electrolytes.

Proper understanding of ion conduction mechanism will help us for preparing good quality polymer electrolytes with high room temperature ionic conductivity, excellent mechanical, thermal and electrochemical stability. By achieving the aforementioned desired properties, the solid polymer electrolytes can replace the organic carbonate liquid based electrolytes commonly used in most of the portable energy storage/conversion devices.