Poly(Vinylidene Fluoride-co-Hexafluoropropylene) (PVdF-co-HFP)-Based Gel Polymer Electrolyte for Lithium-Ion Batteries

Akhila Das, Neethu T. M. Balakrishnan,

Jishnu N. S., ]arin D. Joyner: Jou-Hyeon Ahn, Jabeen Fatima M. J., and Prasanth Raghavan

Introduction

Lithium-ion batteries (LIBs) have played a significant role in next-generation electrochemical energy storage devices, being used for electric/hybrid vehicles, power stations, portable electronics, etc. [I]. LIBs attracted considerable attention because of their superior properties, such as high energy density, compatibility, low selfdischarge, light weight, etc. Electrolytes play a vital role in the efficient working of LIBs [2-5]. Conventional electrolytes impede the development of next-generation batteries, because these electrolytes pose a high risk due to their potential for leakage, low working temperature, low electrochemical window, high resistance at solid electrode-electrolyte interfaces, etc. An efficient and safer electrolyte would be desirable to get the most out of this promising battery technology, which is capable of revolutionizing portable devices. Polymer electrolytes are suitable electrolytes for LIBs due to their advantages with respect to being safe, lightweight, more flexible, with wide electrochemical stability windows, etc. Different polymers used as electrolytes include polyvinylidene difluoride (PVdF) [6], polyethylene oxide (PEO) [7], polyvinyl acetate (PVAc) [8] and polyvinylidene fluoride-co-hexafluoropropylene (PVdF-co- HFP) etc. [9, 10]. PEO is considered to be a dry, solid polymer electrolyte working at ambient temperature, which suffers from low ionic conductivity and poor cycling performance. Polyacrylonitrile (PAN)-based electrolytes possess high ionic conductivity but are expensive [11]. PVAc shows low mechanical stability whereas polyvinyl chloride (PVC) is the least-soluble of the polymers. PVdF-based electrolytes are anodically stable due to the presence of a strong electron-withdrawing group of fluorine atoms, and possess high dielectric constants (8.4 6) having greater dissolution of salts. The co-polymer PVdF-co-HFP shows greater ionic conductivity, compared with PVdF, because of the amorphous nature of HFP, leading to the ease of migration of Li⁺, along with a high dielectric constant (8.4 5). Because of these attractive properties of PVdF-co-HFP, the potential value of this polymer has been explored in different fields, such as sensors, batteries, supercapacitors, scaffold engineering and textiles.

Crystal Phases of PVdF-co-HFP

One of the important co-polymers of PVdF is PVdF-co-HFP. The most appropriate ratio of PVdF: HFP, for use as a polymer electrolyte in a battery, is 88:12, which is highly amorphous. The more amorphous the nature, the greater the movement of ions and the higher the ionic conductance. The membrane can easily trap the lithium salts and support the mechanical efficiency of free-standing films. It exhibits both the crystalline phase as well as the amorphous phase. The crystalline phase exhibited by PVdF enhances the mechanical stability of the polymer matrix, which can also act as a separator, and the amorphous nature of HFP improves the ionic conduction of the electrolytes in batteries. The efficiency of this co-polymer matrix depends on the type and amount of doping salts, solvents, etc. PVdF-co-HFP exists in different phases, such as Phase I, Phase II, Phase III, etc. Phase I is also known as the a phase, which is the most common phase under normal conditions, and these exist in the TGTG' (trans-gauche-trans-gauche') nonpolar conformation. When the polymer is

FIGURE 6.1 Schematic illustration of different crystal phases (a phase and p phase) of polyvinylidene difluoride (PVdF). Adapted and reproduced with permission from Ref. [12]. Copyright © 2007 Elsevier.

strained and stretched, a polar conformation, called phase II (or (3 phase) is formed in a zig-zag fashion [12] (Figure 6.1). Phase III has TTTGTTTG' conformation, which forms when the polymer is stressed moderately. The last phase, IV, is also known as the 6 phase, which exists only under specific conditions and parameters [13, 14]. The addition of plasticizers, such as ethylene carbonate (EC) and propylene carbonate (PC), etc., can transform the crystalline phase from phase I to phase III [15, 16].

Preparation of PVdF-co-HFP-Based Polymer Electrolytes

Polymer electrolytes are electrolytic materials which have a wide range of applications in electrochemical energy storage materials. These materials are synthesized by the dissolution of lithium salts in high-molecular-weight polymers. There are different techniques for the synthesis of polymer electrolytes, including (a) solvent casting [17], (b) phase inversion [18, 19], (c) plasticizer extraction and (d) electrospinning [20]. Plasticizer extraction is a preparation method in which plasticizers are added to improve the mechanical and thermal properties of the polymer matrix.

These matrices provide nanoscale pore sizes, having lower porosity (~50%). This technique is a complex process and is expensive, compared with other techniques. Later, advanced synthetic techniques, such as phase-inversion and electrospinning processes, were also introduced, which resulted in greater porosity and conductivity. Detailed explanation of solution- casting, phase-inversion and electrospinning techniques, and their effects on the electrochemical properties of PVdF-co-HFP are discussed in the subsequent sections.