Abstract: Charge-transfer complexes have been extensively used as cathode materials in fabrication of solid-state organic batteries. However, the low mechanical strengths of the charge-transfer complexes have restricted their proper application in device formation. The polymer composite of these materials have been prepared and used in fabrication of solid-state organic batteries to overcome this problem. The pressed pellets of the polymer composites of phenothiazine-iodine in poly (vinyl chloride) (PVC) and polystyrene (PS) have been used as cathodes in contact with zinc as anode metal. The electrochemical characterization of these cells such as open-circuit voltages, short-circuit currents, their time and temperature dependence have been studied. The impedance analyses have been done to understand the nature of the electrode reaction.
INTRODUCTION
Charge-transfer materials based on iodine have been extensively used as cathode materials in the fabrication of electrochemical cells[1-4]. Gutmann et al.[1] have studied the effects of different metallic contacts such as barium, calcium, magnesium and aluminum etc., on the electrochemical behavior of these cells[1]. Solid-state lithium cells based on Charge-transfer Complexes (CTC) of benzidine-iodine and perylene-iodine have been fabricated[5]. These cells have high open-circuit voltage at room temperature. Electrochemical cell based on CTC of graphite-iodine has been fabricated by Chandra et al.[6] This cell had an open-circuit voltage of 0.68 volts at room temperature.
Gutmann et al.[7] have proposed a reaction mechanism for cells based on both iodine free and iodine complexes. They reported that the formation of metal halide was the energy producing reaction in such type of cells. Scrosati et al.[8] have reported the formation of insulating layer between cathode and anode as a result of the metal halide formation. Formation of insulating layer in these cells has also been confirmed by impedance analysis[9]. A solid-state battery based on coal-iodine complex has been fabricated by Friedel[10]. The open-circuit voltage of 1.9 Volts has been found for this cell.
The low mechanical strengths of the CTC have restricted their potential application in device fabrication[11]. The mechanical strengths of these materials have been improved by preparing composites of these complexes with insulating polymers[10]. These composites have high mechanical strengths while retaining other properties such as electrical conductivity, electrochemical behavior etc. In this communication, we present a detailed study of electrochemical behavior of the cells based on composites of phenothiazine-iodine (1:2) in poly (vinyl chloride) and polystyrene. Various electrochemical parameters such as open-circuit voltage, short-circuit current and their dependence on time and temperature have been studied. The impedance and dielectric behaviour of these batteries have been also studied in order to understand the nature of electrode reactions.
MATERIALS AND METHODS
Phenothiazine (Aldrich) was used after recrystallization in ethanol and iodine (SD Fine-Chem. Ltd.) was used after purification by sublimation from KI+I2 mixture (1:1.25). The poly (vinyl chloride) and polystyrene (GSC) were used as received. All other chemical used were of AR grade. The phenothiazine-iodine (Ptz-I2) complex was prepared by mixing the hot solutions of phenothiazine and iodine (in 1:2 molar ratio) in diethyl ether[12]. The composites of CTC with poly (vinyl chloride) (PVC) have been prepared by dissolving the required amount of CTC in excess of benzene and were introduced in PVC matrix by diffusion, whereas the composites of CTC with polystyrene (PS) have been prepared by solvent evaporation method. Here the required amounts of CTC and PS were dissolved in excess of benzene separately and then mixed together to get the composites. Then the solvent was evaporated at room temperature with constant stirring. The composite so obtained was grounded with the help of pastel mortal. The fine powders of the composites were pressed in pellets form under 10 tones load using a hydraulic press.
The cells were fabricated in a pellet holder, by introducing the pellet of the materials in between the anode metal (zinc) and inert electrode (platinum). The fabricated cell was placed in a glass chamber containing fused calcium chloride in order to protect it from the atmospheric moisture. The electrical measurements were done on a Source-Measure-Unit (Keithley, Model-236), Picoammeter (Keithley, Model-485), LCZ meter (Keithley, Model-3330) and temperature controller (Century, CT-180). The compressive strengths were measured on Amsler Universal Testing Machine.
RESULTS AND DISCUSSION
Mechanical properties: The compressive strengths of these composites were measured on a pellet of uniform dimensions by taking fixed weight of the materials. The compressive strength of these composites was found to be increasing as the amount of insulating polymer increases (Table 1). Pure charge-transfer complex has compressive strength 192.3 kg cm-2, whereas the composite having 80 wt.% CTC in PS has compressive strengths of 323.5 kg cm-2, which increases to 521.7 kg cm-2 for 20 wt.% CTC, which was almost comparable to that of polystyrene. Similar trends have been observed for composites of CTC with PVC.
Shelf life: Due to direct contact of anode and cathode, spontaneous reaction goes on in the cell, which reduced the life of the cell. Scrosati et al.[8] have overcame this problem by introducing a non-reactive polymer film in between the cathode materials and anode metals, which has to be removed at the time of use of battery. In this study, we have measured the open-circuit voltage (Voc) and short-circuit current density (Isc) of the cell without putting any load to the cell. It has been observed that the Voc and Isc were decreasing very slowly with time which shows that the spontaneous reactions go on slowly in these cells and the cells have shelf-life of about 200 h even if no insulating films was inserted between the anode and cathode. The self-life of these cells was more than six months when a non-reactive polymer films was introduced in between cathode materials and anode metal.
| Table 1: | Mechanical strength of the composites of Phenothiazine-iodine with poly (vinyl chloride) and polystyrene |
| * Compressive strengths were measured for samples of dimension A = 0.786 cm2, l = 2.00±0.2 cm | |
| Fig. 1: | Variation of open-circuit voltage with time at 10 k Ω load |
| Fig. 2: | Variation of short-circuit current density with time at 10 k Ω load |
Variation of Voc and Isc with time: The variation of Voc and Isc as a function of time against 10 kΩ external load for the composites of phenothiazine-iodine with PVC and PS has been shown in Fig. 1 and 2, respectively.
| Table 2: | Variation of open-circuit voltage (Voc) and short-circuit current density (Isc) with time at 10 kΩ load |
It has been observed for these cells that the initial Voc of pure charge-transfer complex and its composites were almost equal, but the initial Isc was found to be decreasing on decreasing the content of charge-transfer material in the composites. These observations can be explained on the basis of internal resistance of the cell. On decreasing the concentration of charge-transfer complex in the composites the internal resistance of the cell increases, which reduces the current in the cell. Also the number of reactive iodine molecules per unit contact area of the anode metal decreases as the amount of charge-transfer complex in composite decreases leading to lower amount of electrochemical reaction and hence the short-circuit current[13].
The Voc and Isc for the cell having pure charge transfer complex with configuration of Zn/Ptz-I2/Pt were 0.92 V and 30.40 μA cm-2, respectively which reduced to 0.86 V and 20.87 μA cm-2, respectively after 100 h of discharging of the cell. For composite of poly (vinyl chloride) having 60 wt.% of CTC, the initial Voc and Isc were 0.90 V and 14.85 μA cm-2, respectively after 150 h 0.53 V and 1.57 μA cm-2 was obtained. The operational mechanisms of such type of cells have been reported earlier[1]. During operation, iodine is released from the complex and it enters the layer in ionic form. The released iodide then reacts with zinc and produces energy. The reaction mechanism is as follows:
The detailed electrochemical data for different composition of charge-transfer complex showed that these cells give stable voltage output for about 100 h (Table 2).
Power output: It has been observed that the power output decreases as the amount of charge-transfer complex in the composites decreases (Fig. 3). The decrease in power output with decreasing wt.% of charge-transfer complex is due to the power loss against the internal resistance of the cell. Also the extent of electrochemical reaction will be less, as the less amount of charge-transfer complex will be available at the surface of the pellet in composites having lower weight percent of charge transfer complex.
| Fig. 3: | Variation of power output with time at 10 k Ω load |
| Fig. 4: | Variation of Voc and Isc with temperature for phenothiazine-iodine-PS (60% CTC) |
Variation of Voc and Isc with temperature: The variation of Voc and Isc with temperature of these cells have been studied in the range of 30-110°C (Fig. 4). It has been observed that increase in temperature increases both the open-circuit voltage and short-circuit current density.
| Table 3: | Variation of capacitance of cell with time at 10 kΩ load for composite of Phenothiazine-Iodine with poly (vinyl chloride) and polystyrene (80% CTC) |
| Fig. 5: | Expansion of electrode resistance in complex impedance plot
for phenothiazine-iodine-PVC (60% CTC) system. (a) At the time of cell assembly
(b) After 48 h of discharge (c) After 96 h of discharge |
It was found that beyond 100°C, there was fall in the Isc of these cells. The increase in Isc may be due to thermal activation of electrochemical reaction and the decrease in Isc may be due to liberation of iodine from cathode materials at high temperature.
Various thermodynamic parameters have been calculated for the cell having configuration of Pt/Ptz-I2-PS (60% CTC)/Zn using the following equations:
| (1) |
| (2) |
| (3) |
The Gibbs free energy for the cell was found to be 173.7 kJ mol-1. The enthalpy and entropy values were found to be 115.2 kJ mol-1 and 193.1 J K-1, respectively from the temperature dependence of the cell[3].
Nature of electrode reaction: The ac impedance and dielectric properties of these cells have been studied in the frequency range of 40 Hz to 100 kHz at the various time of cell discharge. Then these measured data were simulated in the frequency range of 10-3 to 1011 Hz using a Complex Non-linear Least Square (CNLS) analysis software developed by EG and G Parc, USA. It has been already reported that the metal halides formed due to the cell reactions in these cells, which formed an insulating layer between cathode and anode[8]. The formation of insulating layer can be assessed by the change in the impedance plots as a function of time. The semicircle corresponding to the electrode contribution of the cell increased with the time, indicating the formation of insulating layer between the cathode and anode. The impedance response of these cells revealed a progressive expansion of electrode contribution (Fig. 5). Similarly the capacitance of the cells also increases with the time, which further confirms the formation of insulating layers.
The capacitance of different cells at different frequencies was measured. Some of these data for 80% charge-transfer complex have been given in Table 3 as a representative of all these systems. It is evident from these data that the capacitance of 13.77, 3.97, 2.02 and 1.13 nF at 100 Hz, 1, 10 and 100 kHz, respectively, increases to 46.79, 10.53, 5.72 and 2.02 nF, respectively after 48 h for the composite of PVC having 80% of charge-transfer complex. This also confirms the formation of insulating layer between cathode materials and anode metals.
CONCLUSIONS
Solid-state electrochemical cells based on polymer composites of charge-transfer complex have been fabricated and characterized. These cells have high mechanical strengths, compare to that of pure charge transfer complex. Although the short-circuit current for these composite cathode was lower than the pure charge-transfer materials, the open-circuit voltage of these systems were comparable. Therefore, these polymer composites could be used as an alternative for preparing organic batteries.
ACKNOWLEDGMENT
The authors are thankful to CSIR, New Delhi for providing SRF to R.K. Gupta.