Article pubs.acs.org/JPCC
Effect of Lithium Salt in Nanostructured Silica−Polyethylene Glycol Solid Electrolytes for Li-Ion Battery Applications John Fredy Vélez, Mario Aparicio, and Jadra Mosa* Instituto de Cerámica y Vidrio (CSIC), 28049 Madrid, Spain ABSTRACT: Organic−inorganic hybrid solid electrolytes based on silica−polyethylene glycol PEG ( 2 0 0 , 4 0 0 ) with bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) or lithium trifluoromethanesulfonate (LiOTf) were synthesized by a sol−gel process. The thermal and structural properties of the materials thus obtained were systematically investigated by a variety of techniques including SEM, TGA-DTA, DSC, FTIR, Raman, and multinuclear (1H, 13C, 7Li) solid-state NMR. Arrehenius behavior was observed for solid hybrid electrolyte membranes, implying that segmental motions of the organic network were involved in ionic conductivity. The Li-ion transfer number was determined and correlated with their ionic conductivities, and the interfacial stability against lithium was evaluated. Maximum ionic conductivities for the solid hybrid electrolyte membrane SiO2−PEG400 with LiTFSI and a [Li+]/[O] = 0.10 (class II hybrid) of 3.9 × 10−4 and 4.5 × 10−3 S/cm−1 were obtained at room temperature and 60 °C, respectively. The good value of electrochemical stability window (∼6 V) and stable interfacial impedances against lithium metal make these hybrid electrolytes a promising candidate for all-solidstate lithium battery applications.
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INTRODUCTION The development of lithium-ion rechargeable batteries with high energy density is decisive to meet the demand for power sources in portable devices, electric vehicles, and storage of electricity in order to address energy and environmental concerns.1,2 Liquid electrolytes has been used in Li-ion batteries due to it elevate ionic conductivities; however, they have several disadvantages such as the difficult hermetic sealing, limited temperature range operation, corrosion of electrodes, and lack of mechanical stability.3 All-solid-state lithium ion batteries using solid electrolytes have been studied as the safest and the most reliable concept, due to several advantages, for example, nonvolatility, low flammability, a high chemical and electrochemical stability, and structurally flexible design.4 However, a major limitation of these electrolytes to be used in lithium-ion batteries is their relatively low ionic conductivity as compared to liquid electrolytes. To date, poly(ethylene oxide) (PEO) based electrolytes are extensively studied because of their effective ion solvating properties and ionic conductivities higher than 10−4 S cm−1 above 70 °C.5 However, the ionic conductivity of PEO-based electrolytes decreases drastically at room temperature because it has a highly crystalline structure that restricts ionic mobility, which cannot be applied in electrochemical devices.6 As a consequence, various approaches have been developed, in order to enhance the dimensional stability of the polymers: addition of nanosized particles to form composite polymer electrolytes,6,7 synthesis of block copolymers,8,9 use of cross-linking electrolytes,10,11 development of gel polymer electrolytes,6,12 pore-filling polymer electrolytes,13 incorporation of plasticizer agents,14 branched/ star-branched/comb structures,15,16 and incorporation of ionic liquids,17−19 in order to reduce crystallinity while retaining the © 2016 American Chemical Society
solvating properties. Although these alternatives help to enhance the room temperature ionic conductivity, some are lack of structural integrity and high conductivity that require commercial applications. A more promising approach able to substitute liquid electrolytes is to synthesized organic− inorganic hybrid electrolytes through control of inorganic and organic polymerization.20−25 The combination of organic and inorganic components in the hybrid materials provides advantageous properties such as a stable three-dimensional framework, which offers a good mechanical stability such as enough flexibility to manipulate the membranes and enhanced thermal stability. The hybrid network can be readily tailored as a function of organic and inorganic ratio, synthesis conditions (pH, polymer molecular weights, Li concentration, etc.) and mobility of structural network and active species, to obtain the desired structural and electrical properties.25 Also, adequate properties can be achieved varying the connectivity between the organic and inorganic components. Therefore, the hybrid solid electrolytes with adequate properties are promising candidates to overcome the shortcomings associated with conventional PEO-based electrolytes. Two types of hybrids have been classified according to the interactions between organic and inorganic components: class I, in which weak interactions such as van der Waals forces, hydrogen bonding, or electrostatic interactions are established between both components, and class II, in which both components are covalently bonded (strong bonds). Class I organic−inorganic hybrids based on polymers (e.g., PEG, PEO, PPG, or EO chain monomers) and Received: July 18, 2016 Revised: September 12, 2016 Published: September 16, 2016 22852
DOI: 10.1021/acs.jpcc.6b07181 J. Phys. Chem. C 2016, 120, 22852−22864
Article
The Journal of Physical Chemistry C
mixing half amount of solvent with TEOS. After homogenization, PEG was added, and the solution was stirred at room temperature for 30 min. Then, an acidic water solution was added and vigorously stirred for 2 h in order to complete the hydrolysis−condensation reactions. The amount of water corresponds to a molar composition of H2O/TEOS = 2. Second solution consists of a simple mixture of half calculated amount of ethanol with the lithium salt (LiTFSI or LiOTf) in three molar compositions [Li]/[O] = 0.07, 0.10, and 0.14 and, then, stirred at room temperature for 2 h. Afterward, solutions A and B were mixed and stirred for 1 h resulting in a clear and homogeneous solution. A similar procedure was followed to synthesize a sol without lithium in order to analyze the role of lithium in the hybrid network. All the processes were carried out at room temperature in argon atmosphere inside a glovebox. The sol was used to prepare self-supported membranes in polytetrafluoroethylene (6 cm diameter) and silicone (4 cm diameter) molds, dried first for 2 days at room temperature. Thin-films were also performed using a dip-coating machine inside a glovebox at room temperature with a scan rate of 16 cm min−1 using soda-lime glass slides as substrates (2.5 × 7 cm2). The hybrid materials were first dried at room temperature for 30 min and then heat treated at 120 °C for 12 h using a heating rate of 1 °C/min, in an HOBERSAL Model JB-1 oven inside the glovebox to dry and consolidate. Characterization. The viscosity of different sols was measured at 25 °C by a Vibro Viscometer AX-SV-34 (A&D Ltd. Co.). Coating thickness was obtained by profilometry (Talystep, Taylor-Hobson U.K.). Coating homogeneity was examined with a HITACHI S-4700 field emission scanning electron microscope (FESEM). Thermal stability of the membranes cured at 120 °C was studied by thermogravimetric analysis (TGA−DTA) in argon up to 600 °C with a heating rate of 10 °C/min using a Netzsch STA 409. DSC scans were performed using a DSC 220U SEIKO calorimeter, by initial cooling (at 5 °C min−1) from room temperature to −50 °C, the temperature was maintained at −50 °C over a period of 10 min, then raised (also at 5 °C min −1) to 200 °C, and finally cooled (at 5 °C min −1) from 200 to −50 °C. This cycle was repeated two times because the second run was used to obtain information related to a recrystallized process after melting. The crystal structure of the samples was characterized by grazing incidence X-ray diffraction (step, 0.040°; angle, 0.5°) using a Siemens D-5000. FTIR spectra for hybrid sols and membranes were recorded on a PerkinElmer Spectrum 100 spectrometer in the 650−1650 cm−1 range with a resolution of 2 cm−1 using the attenuated total reflectance (ATR) accessory and transmittance mode, respectively. Raman spectra of hybrid electrolytes were recorded in the range 200−1600 cm−1 using a confocal raman microscope (model alpha300 WITec GmbH, Germany). Laser with 532 nm wavelength excitation was used for this purpose. Several spectra have been performed to obtain an average spectrum. Solid-state NMR experiments were obtained at 302 K on a Bruker (AV 400 WB) NMR spectrometer. The measurements were performed on 50 mg samples of milled SiO2−PEGn selfsupported membranes contained in a sealed Pyrex 4 mm (outlet diameter) NMR tube. The Larmor frequencies for 1H and 7Li were 400.1 and 155.5 MHz, respectively. 1H and 7Li NMR spectra were acquired under conditions of magic angle spinning (MAS) with a spinning speed of 10 kHz. 13C-CP−
silicon alkoxides result in materials with properties as high room temperature conductivity, amorphous at room temperature, excellent mechanical, and thermal stability, optical transparency, ease to molding into a desired shape and a wide electrochemical voltage window (0−4 V).17,25,26 Sol−gel process offers the possibility to introduce organic chains into an inorganic gel network, allowing the mixture of the components on a nanoscale. This may be accomplished by hydrolysis and condensation reactions of organically modified alkoxides or monomers with alkoxides.27−30 Kao et al.31 reported higher room-temperature ionic conductivities (up to 10−4 S/cm) for class I hybrids than for class II, but unfortunately present low chemical stability. The interface between the inorganic and polymeric components plays an important role in the enhancement of the solid hybrid electrolyte properties.2,32 In class II materials, the design of a specific hybrid organic− inorganic network at molecular level can provide the potential for higher conductivity and tLi+. These improved properties will help to increase cell performance and operate cells at lower temperatures and higher currents, thus increasing the delivered power.33 There are three important parameters affecting the hybrid structure: the type of the class hybrid material, the length of PEG chains, and the type of Li salts. In this article, we report on results considering the three approaches. Lithium bis(trifluoromethane)sulfonimide (LiTFSI) has been used widely due to the high dissociating ability, leading to enhanced ionic mobility.22 This salt also presents a useful plasticizing characteristic by being free-volume; this is a significant advantage in polymer hosts that have an inherent tendency to crystallize, such as oxyethylene-based electrolytes.22,23 However, lithium trifluoromethanesulfonate (LiCF3SO3) presents high thermal and electronic stabilities and does not tend to form ion pairs, being weakly coordinated.33 This work reports on the preparation and properties of nanosized hybrid organic−inorganic electrolytes as selfsupported materials and thin-films for Li-ion batteries. Silica (SiO2)−polyethylene glycol (PEGn) hybrid materials have been prepared using a sol−gel approach. Chain length of the organic precursor PEG (MW: 200, 400), lithium salt (LiTFSI, LiOTf), and [O]/[Li] ratio (0.07, 0.10, 0.14) have been evaluated for the TEOS/PEGn = 1 (molar ratio), in order to establish structure−conductivity relationships.
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MATERIALS AND METHODS Materials. The hybrid electrolytes were prepared by sol−gel method using tetraethoxysilane (TEOS 99% from ABCR) as inorganic precursor, polyethylene glycol (PEG200 average MW = 200 g/mol from Sigma-Aldrich and PEG400 average MW = 400 g/mol from Panreac) as organic precursor, and bis(trifluoromethane)sulfonimide lithium salt LiN(CF 3SO2)2 (LiTFSI 99.95% from Sigma-Aldrich) and lithium trifluoromethanesulfonate CF3SO3Li (LiOTf 99.995% from SigmaAldrich) as Li salts for different [Li+]/[O] molar ratios (where oxygen is only from the ether group). Lithium salts (LiTFSI and LiOTf) were introduced into a vial inside a glovebox, and then dried at 120 °C in a glovebox (Argon atmosphere) for 3 days. Absolute ethanol (EtOH, 99.5% from Panreac) was used as solvent and acidulated water (HCl 0.1 N) as catalyst of the sol−gel reaction. Synthesis Procedure. The sol (molar composition TEOS/ PEG = 1 and TEOS + PEG/EtOH = 0.1) was prepared by mixing two solutions (A and B). First solution was prepared by 22853
DOI: 10.1021/acs.jpcc.6b07181 J. Phys. Chem. C 2016, 120, 22852−22864
The Journal of Physical Chemistry C
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RESULTS AND DISCUSSION Sols and Hybrid Organic−Inorganic Electrolytes. Homogeneous and transparent sols with no phase separation or precipitates were obtained for all compositions. As-prepared sols present similar viscosity values at room temperature (between 3.04 and 3.63 mPa s) independently of Li content. However, LiOTf compositions present the lowest values, probably due to slower rates of hydrolysis and condensation reactions.33 Sols maintain a constant viscosity during several weeks (in Ar atmosphere, RT), revealing an excellent stability and the possibility to use them for a long time. pHs around 2 were obtained for all compositions independently of aging time. Acidic conditions promote linear polycondensations and prevent steric hindrance of the poly(ethylene oxide) glycoxy groups and thus is considered as a necessary condition to synthesize SiO2−PEGn hybrids.35 Self-supported hybrid membranes without lithium content after 120 °C of thermal treatment are homogeneous, transparent, and crack-free with thickness between 350 and 600 μm. Incorporation of lithium salt (LiTFSI or LiOTf) does not produce significant changes in the self-supported membranes keeping flexibility. Shorter PEG chain (PEG200) compositions present better mechanical stability in terms of manageability and flexibility. Heat treated thin-films present high optical quality and are transparent without precipitates. No changes were observed when Li content or length chains increase, meaning that organic and inorganic components and Li salts were combined at molecular level. Thin-films with thickness 450−900 nm at the withdrawal rate used were obtained. Figure 1 shows SEM characterization of thin-film solid electrolyte compositions T/P200 = 1 (a) and T/P400 = 1 (b)
MAS (cross-polarization magic angle spinning) NMR spectra were obtained at 100.62 MHz for the 13C nucleus. The ionic conductivity of the solid electrolytes was measured by electrochemical AC impedance spectroscopy (EIS) using a potenciostat/galvanostat (BioLogic VMP3 Versatile Multichannel) with a four-probe device, Pt wire electrode (0.33 mm diameter). In this method, the two inner probes served as voltage sensors, and the two outer Pt wires served as current injectors. The coating was sandwiched between two Teflon blocks. The amplitude of the AC signal was therefore 50 mV over frequency range 106 to 0.01 Hz (60 points/decade). All measurements were performed from 20 to 110 °C in an inert closed system, and the system was thermally equilibrated at each selected temperature at last for 1 h. Before the impedance measurements, the sample was kept at 110 °C in inert atmosphere (Ar) for 12 h, then the temperature was decreased to RT while measuring conductivity. The ionic conductivity σ of the samples is calculated with the distance between Pt wire electrodes (0.92 cm) (L), the thickness of the coating (t), and the width of the substrate (W), and R, the electrolyte resistance, was obtained from the intersection of the semicircle with the real impedance axis (fitted using a software program) in the Nyquist plot, using the eq 1:32
σ = L /RWt
(1)
The electrochemical stability windows (ESW) was determined by cyclic voltammetry (CV) using stainless steel (SS) as a working electrode and 0.38 mm-thick lithium metal foil (99.9% from Sigma-Aldrich) as counter and reference electrodes with the structure Li/electrolyte/SS cell at scan rate of 5 mV s−1 from 0 to 7 V vs Li/Li+. The lithium transference number (tLi+) was determined by using a combination method of dc polarization and ac impedance measurements, which was proposed by Evans et al.34 using the following equation: t Li + =
Is(ΔV − I0R 0) I0(ΔV − IsR s)
Article
(2)
The initial current (I0) was calculated by
I0 =
ΔV R 0′ + R 0
Figure 1. FE-SEM photographs of a fracture surface of hybrid thin-film electrolytes for SiO2−PEG compositions (a) T/P200 = 1 and (b) T/ P400 = 1, using LiTFSI [Li+]/[O] = 0.1.
(3)
where ΔV is the applied potential across the cell, Is is the steady-state dc current, R0 and Rs are the initial and steady-state resistances of the passivating layers, and R′0 is the electrolyte bulk resistance. All the resistance values were obtained by a fitting procedure of impedance spectra. The current and resistance were measured using a potentiostat/galvanostat (BioLogic VMP3 Versatile Multichannel). The sample was sandwiched between two 0.38 mm-thick lithium foils as nonblocking electrodes to constitute a symmetric test cell with the structure of Li/electrolyte/Li. The electrochemical impedance (amplitude voltage of 10 mV) was first measured before a dc bias of 300 mV was applied to the cell. The current response of the cell was monitored over time until a steady state was reached. Interfacial resistance of lithium interface was performed by monitoring the time dependence of the impedance of symmetrical Li/solid electrolyte/Li cells. The same Swagelok cell (argon atmosphere at room temperature and 60 °C) for both electrochemical stability windows and Li+ transfer numbers was used.
using LiTFSI [Li+]/[O] = 0.1. The image of the fracture surface for T/P200 = 1 thin-film shows a very homogeneous material formed by particles of 10−30 nm without phase separation, and low size porosity (
