Elsevier

Electrochimica Acta

Volume 431, 1 November 2022, 141186
Electrochimica Acta

Poly(vinylidene fluoride-co-hexafluoropropylene) based tri-composites with zeolite and ionic liquid for electromechanical actuator and lithium-ion battery applications

https://doi.org/10.1016/j.electacta.2022.141186Get rights and content

Highlights

  • Novel tri-composites for advanced application were prepared by solvent casting.

  • Tri-composites are based on MFI and [BMIM][SCN] IL fillers dispersed in a PVDF-HFP polymer matrix.

  • Mechanical and electric properties can be tuned depending on filler type and content.

  • The tri-composites show a high ionic conductivity of 3.9 × 10−5 S.cm−1 for the composite with 30 wt.% IL content.

  • The tri-composites are suitable for lithium-ion battery and bending actuator applications.

Abstract

In order to improve the multifunctionality of polymer composites for advanced applications a novel tri-composite material is presented based on the inclusion of MFI zeolite and the ionic liquid (IL) 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]) in an electroactive poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer matrix. Tri-composites were produced by solvent casting with a relative overall filler content up to 40 wt.%. A compact film morphology is obtained, and the polymer degradation temperature and degree of crystallinity are independent on the filler content and type. On the other hand, the polymer crystalline phase depends on the presence of the IL but is independent of its content. Regarding the mechanical properties, it is observed that the IL has a plasticizing effect with a significant contribution to the reduction of the Young's modulus from 151 MPa for the pristine polymer to 92 MPa for the 30 wt.%/10 wt.% IL/MFI content sample. A high ionic conductivity value at room temperature of 3.9 × 10−5 S.cm−1 was obtained for the tri-composite with the highest IL content (30 wt.%/10 wt.% IL/MFI). The tri-composite system was studied as solid polymer electrolyte (SPE) for battery and bending actuator applications, demonstrating the multifunctionality of the material. The sample with the best cycling performance was observed for the highest MFI content (10 wt.%/30 wt.% IL/MFI) with a discharge capacity value of 110 mAh.g−1 and maintaining almost 80% of its initial capacity after 30 cycles at the C/10 rate. Regarding to the bending performance, the best bending actuation was detected for highest IL content (30 wt.%/10 wt.% IL/MFI), related to the ionic conductivity of the sample. Thus, it is demonstrated that the new tri-composite system, due to its tunable characteristics as a function of the relative IL and zeolite contents, is suitable for a next generation of multifunctional polymer composite materials for advanced applications.

Introduction

In recent years, the development of smart devices has grown substantially driven by the technological evolution in areas such as sensors/actuators, biomedicine and energy generation and storage, among others. These smart devices are increasingly required in the context of the Industry 4.0 and Internet of Things (IoT) paradigms [1]. IoT is based on interconnected physical objects, relying on sensors/actuators, power systems and data communication and management [2], [3], [4].

Smart devices based on smart materials have the ability to respond to a given stimulus in a predictable and reproducible way [5]. Smart materials can react to one or more stimuli providing a response associated with different varying conditions such as temperature, pressure, light, pH, electric, magnetic or mechanical action [6,7]. Furthermore, depending on the specific stimulus, these materials can lead to different responses in terms of thermal, colour, shape or electrical variations, among others [8].

Polymer-based composites are widely used for smart materials development and are based on the combination of a polymer matrix with one or more fillers, allowing to tailor micro and macroscopic, mechanical and functional properties, as well as processability [9].

One of the most interesting polymer matrices for composite development are poly(vinylidene fluoride) (PVDF) and (vinylidene fluoride) (VDF) copolymers such poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) that are known for their excellent chemical resistance, mechanical and thermal characteristics and outstanding electroactive characteristics, including piezo-, pyro- and ferroelectricity [10], [11], [12].

Regarding fillers, ceramic, magnetic and conductive fillers have been extensively used over the years [13,14]. Currently, important efforts are being devoted to the development of composites with micro/mesoporous fillers such as zeolites and metal-organic frameworks (MOFs)) [15,16], considering their intrinsic properties and tailorability. Further, ionic liquids (ILs) are also being explored to develop hybrid polymer structures also based on their wide range of functional properties together with their compatibility with polymer structures [17].

Mesoporous fillers are characterized by high porosity and surface area, tuneable structure, and ion exchange capacity, among others [18]. Regarding ILs, they are interesting due to their chemical and electrochemical stability, non-flammability, negligible vapour pressure and high ionic conductivity [19,20], with characteristics such as density, vapour pressures, viscosity, or electrical conductivity depending on the nature of cation and anion [19,21,22].

Polymer composites based either on mesoporous materials [23] or in ILs [17] have emerged as promising materials for sensors/actuators, energy storage, environmental applications, among others, due to their unique and easily tuneable properties [17]. In the polymer composites field, a three-component approach is being explored where the selection of the fillers and their synergy allows to improve functionality and/or different characteristics of the polymer [24].

In fact, tri-composites based on ILs and zeolites have been reported in the literature for proton exchange membrane fuel cells (PEMFC) [25,26], catalysis [26], and solid polymer electrolytes (SPEs) in lithium-ion batteries (LIBs) [27]. Further, electric double layer capacitor (EDLC) applications have been developed based on 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid and TiO2 nanoparticles [28], gel electrolyte membranes have been developed relying on composites based on ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [BMIM][TFSI] with and without the Li-salt [29] and imidazole-grafted PVDF-HFP polymeric ionic liquids have been developed for quasi-solid-state dye-sensitized solar cells [30], among others. Composites for PEMFC applications based on poly(ether ether ketone) (SPEEK)/zeolite/ionic-liquid have been prepared with different amounts of ILs, showing strong potential for this application as they maintain proton conduction at high temperatures > 80 °C under anhydrous conditions [31]. Also for SPE applications, three-component composites based on PVDF-HFP, the IL 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]) and clinoptilolite zeolite (CPT), have been produced based on different processing strategies. The developed SPEs show high room temperature ionic conductivity (0.19 mS.cm−1) and a discharge capacity value of 160.3 mAh.g−1 at a C/15-rate [27].

Thus, taking into account the advantages of using two fillers, the goal of this work is to develop a new three-component composite material based on PVDF-HFP as polymer, MFI zeolite and [BMIM][SCN] IL and explore its applicability in two areas with different working priciples: on the one hand, in the area of electromechanical actuators and, on the other hand, in the area of SPE for electrochemical LIBs, towards room temperature solid-state batteries. Composites containing MFI and [BMIM][SCN] with different relative filler contents up to a maximum of 40wt.% were prepared by solvent casting technique, the maximum filler content was chosen to maintain the composite mechanical stability. This specific zeolite was selected due to its high thermal stability, porous structure consisting of a 10-ring channel system, and receptivity to doping with foreign atoms [32]. The IL was selected because of its chemical structure and high ionic conductivity at 25 °C (8.98 mS.cm−1) [27]. The tricomposites were studied as a function of filler content and evaluated in terms of morphology, polymer phase, thermal, mechanical and ionic conductivity properties. In addition, their applicability as bending actuators and in SPEs was demonstrated.

Section snippets

Materials

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, Kynarflex PVDF-HFP 2801–00107, HFP content of 12%) was purchased from Arkema. 1-butyl-3-methylimidazolium thiocyanate ([BMIM][SCN]) was supplied by Iolitec. N,N-dimethylformamide (99%) (DMF) and N-methyl-2-pyrrolidone (99%), (NMP) were obtained from Merck. For cathode preparation, LiFePO4 (LFP) and Super P conductive carbon black were purchased from Phostech Lithium and Timcal Graphite & Carbon, respectively. For the MFI zeolite

MFI characteristic features

Fig. 3 shows the experimental and calculated powder XRD pattern of the as-synthesized MFI-type zeolite (silicalite-1). The structure consists of a combination of two interconnected channel systems. The MFI framework forms sinusoidal channels with 10 member rings (MR) (5.1 × 5.5 Å) along the a-axis, which are connected to straight channels with 10 MR (5.3 × 5.6 Å) running along the b-axis [36,37]. The silicalite-1 was selected due to its high thermal stability (above 1000 °C) [38], hydrophobic

Conclusions

A tri-composite materials platform suitable for solid polymer electrolytes, SPE, in battery applications and bending actuators has been developed. These tri-composites were produced by solvent casting based on PVDF-HFP as polymer matrix, MFI zeolite and IL [BMIM][SCN] as fillers with varying relative filler contents up to overall 40 wt.%. The composite morphology is compact and independent of the filler type and content, which also slightly affect thermal properties and degree of crystallinity.

CRediT authorship contribution statement

João C. Barbosa: Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. Rafael S. Pinto: Investigation, Validation, Methodology, Writing – original draft. Daniela M. Correia: Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. Arkaitz Fidalgo-Marijuan: Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Writing – review & editing. Renato Gonçalves:

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank FCT (Fundação para a Ciência e a Tecnologia) for financial support under the framework of Strategic Funding grants UIDB/04650/2020, UID/FIS/04650/2021, UID/EEA/04436/2021 and UID/QUI/0686/2021; and support from FEDER funds through the COMPETE 2020 Programme (projects PTDC/FIS-MAC/28157/2017 and POCI-01-0145-FEDER-007688) and MIT-EXPL/TDI/0033/2021, POCI-01-0247-FEDER-046985. Grants SFRH/BD/140842/2018 (J.C.B.), 2021.07361.BD (R.S.P.), and SFRH/BPD/121526/2016 (D.M.C) and

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