Solid polymer electrolytes based on lithium bis(trifluoromethanesulfonyl)imide/poly(vinylidene fluoride -co-hexafluoropropylene) for safer rechargeable lithium-ion batteries
Graphical abstract
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
References (60)
- et al.
Recent advances in all-solid-state rechargeable lithium batteries
Nano Energy
(2017) Polymer electrolytes—the early days
Electrochim. Acta
(1998)- et al.
Polymer composites and blends for battery separators: state of the art, challenges and future trends
J. Power Sources
(2015) - et al.
New high-throughput methods of investigating polymer electrolytes
J. Power Sources
(2011) - et al.
Lithium ion conducting PVdF-HFP composite gel electrolytes based on N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide ionic liquid
J. Power Sources
(2010) - et al.
Preparation and performance of gel polymer electrolytes doped with ionic liquids and surface-modified inorganic fillers
Electrochim. Acta
(2014) - et al.
A durable and safe solid-state lithium battery with a hybrid electrolyte membrane
Nano Energy
(2018) - et al.
A promising TPU/PEO blend polymer electrolyte for all-solid-state lithium ion batteries
Electrochim. Acta
(2017) - et al.
Effects of pulse charging on the performances of lithium-ion batteries
Nano Energy
(2019) - et al.
Electroactive phases of poly(vinylidene fluoride): determination, processing and applications
Prog. Polym. Sci.
(2014)
Gel electrolyte membranes derived from co-continuous polymer blends
Polymer
Highly porous lithium-ion conducting solvent-free poly(vinylidene fluoride-co-hexafluoropropylene)/poly(ethyl methacrylate) based polymer blend electrolytes for Li battery applications
Electrochim. Acta
Microstructural variations of poly(vinylidene fluoride co-hexafluoropropylene) and their influence on the thermal, dielectric and piezoelectric properties
Polym. Test.
Preparation of porous, chemically cross-linked, PVdF-based gel polymer electrolytes for rechargeable lithium batteries
J. Power Sources
The a.C. Impedance of powdered and sintered solid ionic conductors
J. Electroanal. Chem. Interfacial Electrochem.
Physicochemical properties of poly(vinylidene fluoride-trifluoroethylene)/poly(ethylene oxide) blend membranes for lithium ion battery applications: influence of poly(ethylene oxide) molecular weight
Solid State Ionics
Composite electrolytes of pyrrolidone-derivatives-PEO enable to enhance performance of all solid state lithium-ion batteries
Electrochim. Acta
Solid polymer electrolytes containing poly(ethylene glycol) and renewable cardanol moieties for all-solid-state rechargeable lithium batteries
Polymer
Enhanced cycling performance for all-solid-state lithium ion battery with LiFePO4 composite cathode encapsulated by poly (ethylene glycol) (PEG) based polymer electrolyte
Solid State Ionics
A novel composite solid polymer electrolyte based on copolymer P(LA-co-TMC) for all-solid-state lithium ionic batteries
Solid State Ionics
Solid electrolyte based on waterborne polyurethane and poly(ethylene oxide) blend polymer for all-solid-state lithium ion batteries
Solid State Ionics
Physical and electrochemical properties of LiFePO4/C composite cathode prepared from various polymer-containing precursors
J. Power Sources
Ionic plastic crystal-polymeric ionic liquid solid-state electrolytes with high ionic conductivity for lithium ion batteries
Mater. Lett.
High temperature electrical energy storage: advances, challenges, and frontiers
Chem. Soc. Rev.
An outlook on lithium ion battery technology
ACS Cent. Sci.
The development and future of lithium ion batteries
J. Electrochem. Soc.
Higher, stronger, better… A review of 5 volt cathode materials for advanced lithium-ion batteries
Adv. Energy Mater.
A review of recent developments in membrane separators for rechargeable lithium-ion batteries
Energy Environ. Sci.
Ionic liquid-based membranes as electrolytes for advanced lithium polymer batteries
ChemSusChem
Advanced liquid electrolyte solutions
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Equal contribution.