Synthesis and characterization of P(VdF-HFP) based polymer electrolytes complexed with LiClO₄

Prasuryya Borah

Kaziranga University, Koraikhuwa, Jorhat 785006

Dr Sunil Kumar

Indian Institute of Technology Indore, Simrol, Indore, Madhya Pradesh, 453552

Abstract

In the present work, the structural and electrical properties of polymer electrolytes fabricated using PVDF-HFP and LiClO4 are discussed. The fabricated polymer electrolytes have been characterized by complex impedance spectroscopy, X-ray diffraction, FTIR, and SEM. The ionic conductivity measurements have been done in the temperature range from 230 K to 270 K. FTIR results show a strong interaction between polymer and salt. XRD results show no extra peaks in the complex containing PVDF-HFP and LiClO4 suggesting that the salt has completely dissolved in the polymer matrix. From SEM results it has been observed that the porosity of PVDF-HFP has increased significantly, which in turn can contain more liquid electrolytes resulting in better ionic conductivity.
Keywords: polymer electrolyte, ionic conductivity, solid polymer electrolyte

Abbreviations

INTRODUCTION

Recent increases in demand for oil, with the associated environment sustainable issues, are continuing to exert pressure on an already stretched and strained world energy infrastructure. Nowadays, significant attention is being paid to the development of batteries in order to reduce fossil fuel dependence and emission gases responsible for the greenhouse effect and therefore to reduce the environmental impact associated with the energies used for mobility. The growing demand for computers, mobile phones, tablets, and other portable devices leads to an increasing need for battery autonomy and performance. The main goal of the battery is to obtain specific level of battery performance for the different applications with low production cost. Consequently, detailed research is being done on the development of rechargeable batteries with high power and energy densities. Rechargeable batteries have largely replaced primary cells as they save resources and reduce pollution. Among the secondary batteries, lead-acid batteries and Nickle Metal Hydrides (NI-MH) batteries have stepped back from market since a new and strong system comes into our sight, Li-ion batteries. These batteries have higher energy densities compared to Ni-MH and lead-acid batteries. Li-ion batteries are composed of three different components: anode, cathode, and electrolyte. The Li-ions are stored in the cathode and anode. The electrolyte carries the positively charged Li-ion from the anode to the cathode during the discharging period and vice versa through the separator. The movement of the Li-ion creates free electron at the anode which creates a positive charge in the positive current collector. The electrical current then flows from the positive current collector through a device being powered to the negative current collector.

Fig 1 Working principle of Li-ion battery

In the all-solid-state Li-ion batteries, electrolyte plays a crucial rule. It functions as both the ionic conductor and the separator. During charging Li-ion de-intercalate from the lattice of the cathode and transfer to the anode. The conventional Li-ion batteries use liquid electrolytes, which have relatively low ionic resistance, but have some drawbacks such as safety issues, insufficient lifetime, high cost, limited electrochemical stability window, and low power density. One of the major problems associated with the liquid electrolytes is the growth of Li dendrites which often lead to overheating and ignition. However, all-solid-state Li batteries with non-flammable solid electrolytes can avoid some of the issues, in particular, the safety related ones. Relatively speaking, when compared to liquid-electrolyte Li batteries, all-solid-state ones are believed to be safer and to have longer cycle life, higher energy density, fewer requirements on packaging and state-of-charge monitoring circuits. With respect to this, there is a growing interest in all-solid-state batteries.

The uses of solid electrolytes overcome the need for containment of liquid electrolytes which simplifies the cell design, as well as improves durability and safety. There are two general types of solid electrolytes: inorganic ceramics and organic polymers. The main obvious reason for differentiating between these two polymers is their mechanical properties. The high elastic module of ceramics makes them perfect for rigid battery design whereas the low elastic module of polymers is more suitable for flexible battery design. Polymers are also generally easier to process than ceramics, which reduces the fabrication costs. On the other hand, ceramics are more suitable for high temperature or other aggressive environments. One of the most important properties of electrolyte is ionic conductivity, which is the main focus of the paper. The paper contains a compilation of recent conductivity data from the literature. The purpose of the compilation is to determine the ranges of conductivity for two particular electrolytes and provide a basis for comparison between the electrolytes.

Ceramic Electrolyte

The uneven deposition of the highly reactive metal anodes, as well as chemical instability of electrolyte, reduced the performance of the battery. One straight forward strategy for addressing these problems involves utilizing ceramic electrolytes because they comfortably meet the modulus requirement for preventing surface roughness. Their fundamental structure is mechanically strong enough to reduce the deposition of lithium and they are also effective in reducing the formation of lithium dendrite. Ceramic electrolytes own many attractive physical and electrochemical properties including high elastic modulus, high thermal stability, low flammability, wide electrochemical stability windows, and favoured lithium ion transference number. One of the most significant disadvantages of ceramic is their fluctuation of ionic conductivity (10^-8 to 10^-4 Scm^-1, at room temperature) for being applied in high-energy battery. Ionic conduction in ceramic compounds occurs by movement of ionic point defects, the creation and/or movement of which requires energy, so the conductivity of these compounds increases with increasing temperature. However, ionic conduction in some compounds is reasonably high even at relatively low temperatures, so there are several types of lithium-ion conducting inorganic ceramics that have been investigated for use in lithium-ion batteries. Among the numerous advantages of ceramic electrolytes, their shortcoming cannot be neglected: (a) some of solid inorganic electrolyte can not be used in directly contact with lithium metal as they get reduced causing significant electronic conductivity and resultant serious safety hazards; (b) several ceramic solid electrolytes are not stable while contacting with moisture and CO₂; (c) the interfacial resistance increases significantly between metal electrode and electrolyte which limits the overall power density of the all-solid state battery.

Polymer Electrolyte

Polymer electrolytes are ion-conducting membranes with moderate ionic conductivity (≤ 10^-4 Scm^-1) at room temperature. It is usually accepted that fast ion conduct in SPE was predominately due to the presence of the amorphous phase in the polymeric host. Consequently, it was thought that the lesser the degree of crystallinity, the higher would be the ionic conduction in SPE. For the purpose of all reliable solid-state electrochemical device application, the polymer electrolyte should have inherently possessed the following properties:

⦁ Ionic conductivity σ ~10⁻⁴ S cm⁻¹ at room temperature. This enables us to achieve a performance level close to that of the liquid electrolyte-based devices.

⦁ Ionic transference number ion ∼ 1. This is not only desirable but the polymer electrolyte should preferably be a single-ion (namely, cation) conducting system.

⦁ In order to avoid the undesired chemical reactions, wider temperature range due to operation of battery SPE should have high chemical, thermal and electrochemical stabilities

⦁ Due to the aware of scaling up and large-scale manufacturing of the devices, SPE should have high mechanically strength.

⦁ Finally, the electrolyte should be compatible with the variety of electrodes materials.

In contrast to the ceramic solid electrolyte, SPE can impressively lighten the disadvantages of ceramic electrolytes. They exhibit better flexibility to overcome the poor interfacial resistance and are more cost-effective compared with ceramic electrolytes, which is beneficial from the manufacturing point of view. Due to the moderate mechanical strength (106 to 108 pa) of SPE, it is shown to be effective to suppress the growth of lithium dendrite formation as ceramics. These conveniences provide SPE a bright future to be applied in all-solid-state batteries. Challenges for SPE being used in high-energy batteries mainly arise from poor ionic conductivity due to the intrinsic nature of SPE. For example, possessing only around 10-6 Scm^-1, limiting its practical applications.

SPE mostly consist of dissolve Li-salt in a polymer matrix. In addition to the Li-salt different nano fillers such as ceramic or metals, can be also used to improve their mechanical, electrochemical properties of SPE.

Statement of the Problem

Although the organic/solid polymer electrolytes have numerous advantages, challenges for SPE being used in high-energy batteries occur due to poor ionic conductivity. Therefore, one of the attempts to increase the ionic conductivity of SPE is the addition of Li-salts such as LiClO₄. The polymers in salt system possess good ionic conductivity along with the good mechanical properties.

Objectives of the Research

a) Synthesis of PVDF-HFP and PVDF-HFP/LiClO4 by solution cast method

b) XRD analysis

c) FTIR analysis

d) SEM analysis

e) Impedance measurement

LITERATURE REVIEW

Information about PVDF-HFP

A polymer matrix that stands out due to its exceptional properties and characteristics, including high polarity, excellent thermal and mechanical properties, being chemically inert and stable in the cathodic environment, is poly (vinylidene fluoride) (PVDF) and its copolymers PVDF-co-trifluoroethylene (PVDF-TrFE) and PVDF-co-hexafluoropropene (PVDF-HFP). It has also been observed that the electrolyte properties could be improved substantially when PVDF was co-polymerized with hexafluoropropylene (HPF). PVDF-HPF co-polymer exhibited greater solubility towards organic solvents as well as having lower crystallinity with a reduced glass transition temperature than pure PVDF polymer.

The ionic conduction of SPE is based on diffusion of lithium-ion through their free volume, so it is important that lithium salts be dispersed in polymers at the molecular level. As we all know, LiClO₄ has small ionic radius and low dissociation energy, and it can be dissolved in most of the organic solvents. So, LiClO₄ salt is selected as a lithium salt for this experiment.

METHODOLOGY

The PVDF-HFP and PVDF-HFP/LiClO₄ polymer electrolytes were prepared by solution casting method. The solution casting method is one of the traditional methods for casting polymer electrolytes film as well as gel. In this method appropriate amount of polymer and complex salt are dissolved separately in common solvent, then mixed together and stirred magnetically for sufficient time until salt complexation in the polymer host. The obtained viscous mother liquor is then poured into a Petri dish for the film formation through slow evaporation of the solvent followed by vacuum drying.

Experimental Procedure

Sample preparation

PVDF-HFP/LiClO₄ composites were prepared by dissolving the appropriate amount of LiClO₄ at an 85/15 polymer/salt ratio. First, 0.68 gm of PVDF-HFP (Aldrich reagent Mw = 400000) was dissolved in 9 ml of mixed solvent of 1 mthyle-2 pyrodiene and then 0.12 gm of LiClO₄ were dispersed in the above PVDF/HFP solution, and the mixed LiClO₄ and PVDF-HFP solution was magnetically stirred for a couple of time to until a homogeneous and transparent solution was obtained. Then, the resultant viscous and homogeneous solution was spread on a Petri dish and placed in oven at 60°C for 48 hrs to obtain a solid membrane. The uniform membrane was punched into circles with diameter of 16 mm.

Ionic conductivity measurements

The ionic conductivity of the sample was measured by sandwiching the sample between two stainless steel blocking electrodes. The measurements were performed using an electrochemical impedance analyzer between 1 Hz to 1 MHz at room temperature.
The comparison of ionic conductivity between PVDF-HFP and PVDF-HFP/ LiClO4 was performed by utilizing the same procedure for ionic conductivity.

RESULTS AND DISCUSSION

X-ray Diffraction Analysis

Fig 2 XRD analysis of PVDF-HFP/LiClO₄

XRD is a powerful tool to determine the crystallinity and structural changes in a polymer electrolyte system. Figure 2 shows the X-ray diffraction pattern of pure PVdF-HFP, PVdF-HFP/LiClO4. The peaks found 2Ɵ = 18.6°, 20.06° and 43° reveal the partial crystallization of PVdF units present in the complexes, giving an overall semi-crystalline nature of PVdF-HFP, and the presence of broad hump in the complexes confirms the partial amorphous nature of electrolytes. Furthermore, no peaks are found for LiClO4 which reveals the complete dissolution of lithium salt in the polymer complex.

Fourier-Transform Infrared Spectroscopy Analysis

Fig 3 FTIR spectra of PVDF-HFP

Fig 4 FTIR spectra of PVDF-HFP + LiClO₄

FTIR has been used as a powerful technique to characterise the molecular and structural changes in the polymer electrolyte systems. Figure 5 shows the FTIR spectra of pure PVDF-HFP, PVDF-HFP + LiClO₄. The vibrational bands at 490 cm⁻¹ are assigned to the wagging vibrations of the CF₂ group. The peaks at 1071, 759, and 607 cm⁻¹ belong to the vibration of the crystalline phase of (PVDF-HFP), whereas the frequencies of 883 and 841 cm⁻¹ are assigned to the vibration of amorphous phase of (PVDF-HFP). The CH2 ring and CH groups absorb in the regions of 3100–2990 cm^-1. The peaks appearing at 1192 and 2923 cm⁻¹ are assigned to the asymmetrical stretching vibrations of CF₂ and CH₂ groups, respectively. The deformation vibration of CH₂ group, which appears at the frequency 1402 cm⁻¹ will move to a higher frequency/position with the weakening of interaction between H atoms of CH₂ groups and F atoms of CF₂ groups.

Upon incorporation of LiClO₄ in polymer host, the peak at 758 cm⁻¹ shifts to 748 cm⁻¹ attributed to the wagging band of CH₂Cl. Frequencies 840–560 cm⁻¹ are assigned to C–Cl stretching vibrations. The characteristic absorption vibrations of LiClO₄ 1150–1080 cm^-1, 941 cm⁻¹ are assigned to symmetrical vibration of ionic pairs between Li+ and ClO₄, 624 cm⁻¹ is stretching vibration of ClO₄ and the sharp peaks 3597 and 1637 cm^-1 are stretching and bending vibrations of OH bonds of the absorbed water. The absorption peaks at 2983 and 3025 cm⁻¹, which are assigned to the symmetrical and non-symmetrical stretching vibration of CH₂ groups, appear after addition of LiClO₄ to polymer film, because of the interaction between lithium ions and F atoms.

Scanning Electron Microscope Analysis

Fig 5 SEM micrograph of the SPE films containing PVDF-HFP

Fig 6 SEM micrograph of the SPE films containing PVDF-HFP+ 15% of LiClO₄.

The surface morphology of the PE films with PVDF-HFP and PVDF-HFP+LiClO₄ salt content of 15 wt.% are shown in the Fig. 3. The image shows uniformly distributed spherical structures in the PE film with PVDF-HFP. It shows normal porous surface with small pore size. The morphology changes to greater pore size when 15 wt.% of LiClO₄ is added into the polymer electrolyte.

Complex Impedance Spectroscopy

Fig 7 Room temperature impedance measurement of PVDF-HFP (in black) PVDF-HFP/LiClO₄ (in red).

Fig 8 Ionic conductivity of PVDF-HFP/LiClO4

The electrical properties of the SPE films have been studied using the complex impedance spectroscopy (CIS)technique. It is well known that Li ions migrate in two ways: (a) move along the molecular chains of polymer, and (b) move in the amorphous phase of polymer electrolyte. The former is slow transport whereas the latter is fast. Figure 7 illustrates that the room temperature impedance measurement for SPE with and without salt. The typical complex impedance plot of SPE at room temperature shows a high frequency depressed semicircle portion which corresponds to the resistor Rb (bulk resistor) and a tail at low frequency. The Rb gives information about the ionic conductivity and tail region occurs due to the accumulation of charges (double-layer formation) at the electrolyte–electrode (blocking electrode) interface. The SPE-15 shows better ionic conductivity compared to SPE. Further, the temperature dependent complex impedance plot for other SPE-15 has been shown within the temparature between 230 K to 270 K. This shows improvement in conductivity with increase in temperature since the resistance of the polymer started to decrease.

CONCLUSION

In the study, a series of PVdF-co-HFP based electrolytes with different LiClO₄ loadings was prepared using solution casting method. XRD, FTIR, SEM and complex impedence spectroscopy analysis of PVdF-co-HFP/LiClO₄ were performed to characterize their structural, thermal and dielectric properties, respectively. XRD patterns indicated that crystalline region of PVdF-co-HFP was turned into amorphous fraction and the diffraction peaks were broadened with LiClO₄ loading in the complex composition. The dielectric constant, loss, and conductivity values were obtained in a range from 0 mHz to 4 MHz frequencies. The effects of added LiClO₄ amount on PVdF-co-HFP for the conductivity were investigated, so the conductivities of PVdF-co-HFP/LiClO₄ increased significantly with increasing frequency for all samples independent on LiClO₄ content. The electrolyte showed the maximum conductivity of 10^-2 S/cm at room temperature. Thus, the optimum conductivity can be observed with a low LiClO₄ ratio.

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ACKNOWLEDGEMENTS

I owe a great thanks to Prof. Sunil Kumar, department of Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore for imparting his valuable guidance and giving me such a wonderful opportunity to learn in his lab. He has been very kind and graceful throughout the project.

My deep sense of gratitude to all the labmates working in this lab, who made this summer project enjoyable and memorable by being constant source of support and fun as well, especially to Ms. Tanvi Pareek, Ms. Sushmita Dwivedi, Mr. Manish Badole, Mr. Shadab Ali Ahmed for being a constant support of guidance and inspiration from the rest day to the very last day of my stay. Without their painstaking efforts, this project would not be able to complete.