Enhanced ionic conductivity in poly(vinylidene fluoride) electrospun separator membranes blended with different ionic liquids for lithium ion batteries

Highlights

• Electrospun PVDF membranes with ionic liquids (IL) were produced for battery separators.

• Different types of cations and IL contents were evaluated.

• The morphology, β-phase and thermal properties of the polymer are affected by the ionic liquids.

• The correlation between ionic conductivity and battery performance is observed.

• The best membrane is the 15% of [Emim][TFSI]/PVDF one, being suitable for lithium-ion battery applications.

Abstract

Electrospun poly(vinylidene fluoride) (PVDF) fiber membranes doped with different ionic liquids (ILs) and sharing the same anion were produced and their potential as separator membranes for battery applications was evaluated. Different types of ILs containing the same anion, bis(trifluoromethylsulfonyl)imide [TFSI]⁻, were used with IL concentrations ranging between 0 and 15 wt% The morphology, microstructure, thermal and electrical properties (ionic conductivity and electrochemical window) of the membranes were evaluated. The presence of ILs in the PVDF polymer matrix influences the fiber diameter and the content of the polar β phase within the polymer, as well as the degree of crystallinity. The thermal stability of the membranes decreases with the incorporation of IL. Impedance spectroscopy tests show a maximum ionic conductivity of 2.8 mS.cm⁻¹ for 15% of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][TFSI]) at room temperature. The electrochemical stability of the samples ranges from 0.0 to 6.0 V. When evaluated as battery separator membranes in C-LiFePO₄ half-cells, a maximum discharge capacity of 119 mAh.g⁻¹ at C-rate was obtained for the PVDF membrane with 15% [Emim][TFSI], with a coulombic efficiency close to 100%. The results demonstrate that the produced electrospun membranes are suitable for applications as separators for lithium ion batteries (LIBs).

Introduction

Since the industrial revolution, a continuous growth in world energy demand has been observed. In an economy dependent on the consumption of fossil fuels, such as oil and coal, the growing demand for energy also increases the pressure on the environment, with a continuous extraction of non-renewable natural resources, impacts on the climate and problems associated with greenhouse gases emissions [1]. Hence, a transition to a sustainable development, based on cleaner and renewable energy resources to decarbonize the economic system is needed. However, as renewable energies do not guarantee a permanent supply of power into the grids, complementary systems allowing the storage of energy in hours of high production, to be used later, whenever needed, are sought [2].

Lithium ion batteries (LIBs) are widely used to power mobile devices, such as smartphones, laptops or electric vehicles, mainly due to their high energy density and long cycle life [3]. A typical LIB is composed by two electrodes (anode, negative electrode and cathode, positive electrode) and a separator/electrolyte [4].

The separator/electrolyte is therefore a key component in LIBs. The separator is placed between the electrodes, preventing their physical contact, avoiding short circuits and allowing the flow of ions between them. Its ideal properties are high ionic conductivity and chemical stability, high wettability and excellent mechanical strength [5]. Some of the most used materials in separators are poly(ethylene oxide) (PEO) [6], [7], [8], [9], poly(acrylonitrile) (PAN) [8], [10], [11] and poly(vinylidene fluoride) (PVDF) and its copolymers [10], [12], [13], [14].

PVDF is an electroactive polymer suitable for application in energy storage devices, due to its non-toxicity, mechanical stability, high dielectric constant and dipolar moment, and the ability to be processed in a wide variety of forms and shapes [15]. However, this material does not meet all the desired properties of battery separators. Consequently, the addition of electrolytes is essential to facilitate the ion flow, increasing the ionic conductivity. Another valid approach is the addition of fillers into the polymeric matrix to enhance the ionic conductivity without the requirement to introduce liquid components that hinder both the safety and the cycle life of the LIB. Some well-studied fillers are silicon dioxide (SiO₂) [16], aluminum oxide (Al₂O₃) [17] and ionic liquids (ILs) [18], [19], [20].

ILs are defined as salts with melting temperatures below 100 °C. Their high ionic conductivity and non-volatility make them excellent candidates as fillers in LIBs, as organic solvents or electrolytes [21]. Several experimental techniques can be used to produce membranes with ILs, including solvent casting [22], doctor blade [23], hot pressing [24] and electrospinning [25].

Electrospinning involves the application of a high voltage to a polymer solution to form nonwoven fibers. It is a simple, low-cost, efficient and highly reproducible technique, successfully employed in a wide range of applications [26], [27], with special relevance in separators for battery application in the form of monolayer, multilayer, composite, surface modified, and gel polymer electrolytes [28].

Typically, electrospun composite membranes are based on different polymer matrices, being polyimide (PI) [29], poly(acrylonitrile) (PAN) [30] and PVDF [31] the most used. For these polymer matrices, the most studied fillers are SiO₂ [32], titanium dioxide (TiO₂) [33], Al₂O₃ [34], lithium lanthanum titania (LLTO) [35] and lithium aluminum titanium phosphate (LATP) [36]. Electrospinning has been used to obtain fiber membranes based on poly(ethylene)/SiO₂ composite membranes with suitable electrochemical properties [37], [38], [39]. SiO₂ nanoparticles were also dispersed in a poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) matrix. The addition of ILs into these membranes allows the formation of a gel polymer electrolyte with improved safety and cycling stability for lithium sulfur batteries [40], and LIBs [41]. ILs are still not well studied as fillers. Just one study has been reported, in which the use of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Bmim][TFSI]) allowed high electrochemical stability, wettability and battery performance [42]. ILs are also utilized as electrolytes, where electrospun PAN membranes using ILs as electrolyte presented higher porosity and wettability than commercial Celgard® separators, as well as suitable mechanical properties. They also showed high cycling stability and discharge capacity at all tested rates [43]. The addition of 1-propyl-1- methyl piperidinium bis(trifluoromethanesulfonyl)imide ([Pmpip][TFSI]) into an electrospun PVDF-HFP gel polymer electrolyte matrix proved to suppress the lithium dendrite formation in LIBs and showed high electrolyte uptake and lithium transference number [44].

Despite these interesting works, there is still a lack of systematic knowledge that allows the proper selection of ILs in order to improve the membrane performance for battery applications. Thus, the objective of this work is to understand the effects of different ILs sharing the same anion, on separator membrane conductivity and battery performance, in order to optimize separator membranes for LIB applications. This study reports on the production of electrospun separator membranes based on PVDF and ILs sharing the same anion, [TFSI]⁻, but different cations (trihexyl(tetradecyl)phosphonium [P66614]⁺, 1-methyl-1-propylpiperidinium [Pmpip]⁺, 1-ethyl-3-methylimidazolium [Emim]⁺, 1-ethyl-3-dimethylimidazolium [Edmim]⁺) and different amounts of [Emim][TFSI]. The effect of the ILs on the PVDF membrane microstructure, thermal stability and ionic conductivity were evaluated. In addition, the battery performance incorporating these membranes is presented.