Solid polymer electrolytes based on lithium bis(trifluoromethanesulfonyl)imide/poly(vinylidene fluoride -co-hexafluoropropylene) for safer rechargeable lithium-ion batteries

https://doi.org/10.1016/j.susmat.2019.e00104Get rights and content

Highlights

  • Solid polymer electrolytes based on PVDF-HFP and LiTFSI are proposed.

  • Experimental and theoretical studies are performed as a function of LiTFSI content.

  • Ionic conductivities of 0.23 mS/cm are obtained for PVDF-HFP/80 wt% LiTFSI composites.

  • Theoretical simulations allow optimizing lithium concentration and percentage of free ions.

  • This solid electrolyte allows the development of environmentally friendlier batteries.

Abstract

The increasing use of electronic portable systems and the consequent energy demand, leads to the need to improve energy storage systems. According to that and due to safety issues, high-performance non-flammable electrolytes and solid polymer electrolytes (SPE) are needed.

SPE containing different amounts of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) into a poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, polymer matrix have been prepared by solvent casting. The addition of LiTFSI into PVDF-HFP allows to tailor thermal, mechanical and electrical properties of the composite.

In particular, the ionic conductivity of the composites increases with LiTFSI content, the best ionic conductivities of 0.0011 mS/cm at 25 °C and 0.23 mS/cm at 90 °C were obtained for the PVDF-HFP/LiTFSI composites with 80 wt% of LiTFSI.

This solid electrolyte allows the fabrication of Li metallic/SPE/C-LiFePO4 half-cells with a discharge capacity of 51.2 mAh/g at C/20. Further, theoretical simulations show that the discharge capacity value depends on the lithium concentration and percentage of free ions and is independent of the solid polymer electrolyte thickness. On the other hand, the voltage plateau depends on the SPE thickness. Thus, a solid electrolyte is presented for the next generation of safer solid-state batteries.

Introduction

The present highly technological and energy dependent society continuously demands efficient energy storage devices with higher energy density and safety for portable consumer devices and electric vehicles, lithium-ion batteries playing an increasing role [1,2]. Rechargeable lithium-ion batteries still show the highest energy density when compared to other battery systems, such as NiCd (nickel‑cadmium) and NiMH (nickel-metal hydride), and dominate the global market for energy storage systems [3].

To improve Li-ion battery energy density and safety, further developments are needed at the levels of its components: electrodes and separator/electrolyte [4,5]. A relevant and major issue that requires special attention is the safety of the batteries, with the separator/electrolyte playing an essential role [6].

There are several types of electrolytes but the most used are still the liquid electrolytes with different lithium salts types. Those electrolytes are generally based on organic alkyl carbonates that are volatile and flammable, and therefore, represent a problem with respect to battery safety [7]. Another problem of the liquid electrolytes is its reaction with lithium metal that results in the growth of Li dendrites which render internal short circuits that often lead to overheating and ignition, causing battery explosion [8].

In order to solve these safety problems, the use of non-flammable electrolytes without organic alkyl carbonates and solid polymer electrolytes (SPE) have been intensively studied, considered as suitable candidates [9] for solid-state rechargeable batteries [10].

SPEs mostly consist of dissolved lithium salts in a polymer matrix [11]. In addition to the lithium salts, different nanofillers, such as ceramic or metals, can be also added to improve the mechanical and electrochemical properties [12]. 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 cathodic environment, is poly(vinylidene fluoride) (PVDF) and its copolymers PVDF-co-trifluoroethylene (PVDF-TrFE) and PVDF-co-hexafluoropropene (PVDF-HFP) [13].

When compared to the others PVDF copolymers, PVDF-HFP is an excellent polymer matrix for SPEs due to its low degree of crystallinity, which allows improving ionic conductivity. It also shows excellent mechanical properties and high dielectric constant (~7 to 9), as well as highly polar functional groups (-C-F) [14].

In relation to lithium salts, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is widely used in SPE development, considering its excellent electrochemical properties, high chemical and thermal stability and because its large and bulky anions can be highly delocalized to facilitate the salt dissociation and solubility [15,16].

The literature reports some works on SPEs based on PVDF-HFP and LiTFSI based on different approaches with respect to the preparation conditions [17] as well as with respect to the addition of different fillers (1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) [18], silica [19], lithium aluminum titanium phosphate (LATP) [20], nickel-1,3,5-benzene tricarboxylate metal organic framework (Ni3-(BTC)2-MOF) [21], and 1-cyanomethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([CCNIm+][TFSI]) [22]. For these SPE, the ionic conductivity value is between 10−5 S/cm and 0.25 mS/cm, but electrical properties, thermal and mechanical stability, must still be improved.

Polymer gel electrolytes (PGEs) have been prepared by dissolving LiTFSI in 1-methyl-3-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (P13TFSI) ionic liquid mixing the electrolyte solution with PVDF-HFP copolymer. Further, small amounts of ethylene carbonate were added to the PGEs in order to improve the ionic conductivity and Li ion transport kinetics of the electrolytes [23].

Adding propylene carbonate (PC) contents up to 30 wt% into the SPE acts as plasticizer and improves ionic conductivity. Thus, SPEs based on PVDF-HFP and LiTFSI and PC with solid-like mechanical stability presents a high conductivity of 1 × 10−5 S/cm at the composition 0.55/0.15/0.30 wt% PVdF-HFP/LiTFSI/PC [17]. Another SPE based on different materials are (PVDF-HFP with Li7La3Zr2O12 [24], (PEO)-LiClO4-

Li1.3Al0.3Ti1.7(PO4)3 (LATP)) [25] and PEO-MIL-53(Al)-LiTFSI [26] which have been prepared for different types of solid-state batteries, including sodium-ion [27], Al batteries based on NASICON materials [28] and lithium‑vanadium batteries [26].

In SPE ions may be present either as free ions, contact ion pairs or diffusion ion pairs, the ionic conductivity value being essential for obtaining high capacity in the battery [29]. In solid state batteries it is essential to improve the ionic conductivity of the SPEs, at room and/or battery operation temperature, and consequently battery performance [30]. This can be only achieved through proper understating of the interactions between ions and polymeric matrix and the contribution of ions to the electrical response.

Based on the literature and maintaining a fixed PC content, the goal of this work is to prepare and optimize SPE based on PVDF-HFP polymer and high amounts of LiTFSI salt. Samples were prepared by solvent casting with different LiTFSI contents up to 80 wt%. The influence of LiTFSI content in the microstructure, ion-polymer interaction, ionic conductivity of the SPE, and charge-discharge performance of the SPE in the cathodic C-LiFePO4 half-cells were evaluated.

Also, the charge-discharge behavior and cycling performance of the SPE are correlated with the theoretical 1D model simulation of Li-ion Solid State Batteries (SSB) in order to properly understand and optimize its performance.

The theoretical simulations were performed as a function of lithium concentration, percentage of free ions and solid polymer electrolyte thickness.

Section snippets

Materials

Poly(vinylidene fluoride-co-hexafluropropylene), PVDF-HFP, Solef 21,216 (Mw = 600.000 Da, VDF/HFP mole ratio equal to 88/12) and lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) were acquired from Solvay and Solvionic, respectively. Propylene carbonate (PC, anhydrous 99.0%) and N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich and Merck, respectively.

Solid polymer electrolyte preparation

PVDF-HFP/LiTFSI composites were prepared after the protocol presented in [31] by dissolving the appropriate amounts of LiTFSI at

Theoretical simulation model

The theoretical simulations were performed in order to predict the performance of the solid-state lithium ion batteries when subjected to variations of the electrochemical and geometric variables of the separator component, such as, concentration of lithium, free lithium ions concentration and separator thickness. Further, battery performance was evaluated at different discharge rates to account for variations in the energy efficiencies [33].

The simulations were carried out by implementing the

Composites morphology

In order to evaluate the effect of LiTFSI salts on the morphology of the PVDF-HFP composites, SEM images are presented in Fig. 2a and b first for the PVDF-HFP films without LiTFSI salts in both surface (a) and cross-section (b) views, revealing a compact structure without pores, which is attributed to the melting and recrystallization process during sample preparation [31,37].

When the LiTFSI salts are introduced into the PVDF-HFP matrix, a porous microstructure is observed regardless of the

Conclusions

Solid polymer electrolytes (SPE) based on poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-HFP, copolymer and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) have been prepared by solvent casting and the effect of LiTFSI content on ionic conductivity and other physical properties evaluated.

PVDF-HFP/LiTFSI composites show a porous morphology. Their thermal and mechanical properties correlate with the LiTFSI content. Incorporation of LiTFSI into PVDF-HFP decreases the melting enthalpy

Nomenclature

    cLi+

    concentration of lithium-ions, mol/m3

    c0,Li+

    initial concentration of lithium-ions, mol/m3

    cLi

    solid lithium concentration, mol/m3

    cLi,max

    maximal lithium activity in the positive electrode, mol/m3

    cLi,min

    minimum lithium concentration in the positive electrode, mol/m3

    cn

    concentration of negative ions, mol/m3

    Di

    diffusion coefficient for species i (i = Li+, n), m2/s

    DLi

    diffusion coefficient of solid lithium through the electrolyte at positive electrode, m2/s

    Eeq,i

    equilibrium potential in the electrode

Acknowledgments

The authors thank the FCT (Fundação para a Ciência e Tecnologia) for financial support under the framework of Strategic Funding grants UID/FIS/04650/2013, UID/EEA/04436/2013 and UID/QUI/0686/2016; and project no. PTDC/FIS-MAC/28157/2017. The authors also thank the FCT for financial support under grant SFRH/BPD/112547/2015 (C.M.C.). Financial support from the Basque Government Industry Department under the ELKARTEK and HAZITEK programs is also acknowledged. JMMD and JLGR acknowledge funding by

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