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Conductivity and Morphology Correlations of Ionic-Liquid/ Lithium-salt/Block Copolymer Nanostructured Hybrid Electrolytes Ezzeldin Metwalli, Max V. Kaeppel, Simon Jakob Schaper, Armin Kriele, Ralph Gilles, Konstantinos N. Raftopoulos, and Peter Muller-Buschbaum ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00173 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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ACS Applied Energy Materials

Conductivity and Morphology Correlations of IonicLiquid/Lithium-salt/Block Copolymer Nanostructured Hybrid Electrolytes Ezzeldin Metwalli,*,† Maximilian V. Kaeppel,† Simon J. Schaper,† Armin Kriele,‡ Ralph Gilles,§ Konstantinos N. Raftopoulos,†,ǁ Peter Müller-Buschbaum*,† †

Technische Universität München, Physik-Department, Lehrstuhl für Funktionelle Materialien,

James-Franck-Str. 1, 85748 Garching, Germany ‡

Helmholtz-Zentrum Geesthacht, German Engineering Materials Science Center (GEMS), Max-

Planck-Straße 1, 21502 Geesthacht, Germany §

Technische Universität München, Heinz Maier Leibnitz Zentrum MLZ, Lichtenbergstr 1, 85747

Garching, Germany ǁ

Cracow University of Technology, Department of Chemistry and Technology of Polymers,

Warszawska 24, 31-155 Kraków, Poland

KEYWORDS battery, hybrid electrolyte, block copolymer, self-assembly, SAXS

ACS Paragon Plus Environment

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ABSTRACT

The most daunting challenge in solid-state polymer electrolyte membranes (PEMs) is to achieve high ionic-conductivity close to that of the liquid electrolytes, while maintaining enhanced thermal and mechanical performances. The ionic conductivity in relation to the morphology of PEMs composed of diblock copolymer (polystyrene-block-polyethylene oxide; PS-b-PEO), lithium

salt

(lithium

trifluoromethanesulfonate;

LiTf),

and

ionic

liquid

(1-ethyl-3-

methylimidazolium trifluoromethanesulfonate; EMIMTf) is investigated. The optimized functional nanostructured PEMs are achieved with room-temperature ionic conductivities higher than 1 mS cm-1 benchmark. The morphology of these microphase-separated electrolytes is composed of a major soft high ionic-conductive PEO/LiTf/IL matrix with minor glassy highmodulus PS nanodomains. The ionic liquid upload in hybrid electrolytes inhibits the PEO crystallization, reduces the PEO glass transition temperature, promotes an extended PEO chain conformation, and enhances the solubilization of the non-dissociated lithium salt at the PS-PEO domain interfaces. These intrinsic properties caused by the ionic liquid loading serve to achieve stable and robust nanostructured electrolyte membranes and can explain the achieved benchmark conductivity.

INTRODUCTION Lithium-ion batteries (LIBs) have been rapidly developed for portable consumer electronics in recent years. The success of LIBs relies on their unique characteristics including a high energy density, a lightweight, a long life time, and a rapid charge/discharge.1 Despite, the success of LIBs several accidents of fire and explosion have raised major concerns with respect to their


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general safety. Such safety issues of LIBs are a major handicap of their further optimization and mainly arise from the use of flammable liquid organic electrolytes. Therefore, solid-state electrolytes2–4 in particular the polymer electrolyte membranes (PEMs) have recently begun to achieve increasing attention.5–8 The PEMs enhance not only safety but also open the pathways for fabrication of flexible and ultrathin LIBs. Despite such obvious advantages, the implementation of PEMs in LIBs has not been commercially exploited due to their inherent low ionic conductivity. Extensive research efforts have been reported recently for PEMs, especially regarding the use of block copolymer electrolytes. For instance, polyethylene oxide (PEO) based block copolymers such as polystyrene-block-polyethylene oxide (PS-b-PEO),9–20 polyethyleneblock-polyethylene oxide dendrons (PE-b-PEOd),21 and polystyrene-block-polyisoprene-blockpolyethylene oxide (PS-b-PI-b-PEO)22,23 were recently studied as PEMs for LIB applications. Also cross-linked PEMs based on PEO for LIB or Li-S batteries attract more and more interest in research due to their high ionic conductivity upon high mechanical stability.8,24,25 Generally, block copolymers (BCs) have two or more chemically different polymer blocks linked via covalent bonds. The BCs undergo a microphase separation and self-assembly processes and thereby create various nanoscale morphologies, such as spheres, cylinders, gyroids, or lamella.26– 32

The lithium-ion doping in PEO-based BCs has been mainly investigated as a potential solid-

state membrane because the conductive PEO block has the ability to solvate lithium ions. The PEO is combined with glassy blocks such as polystyrene (PS) to achieve a high modulus and thereby the necessary mechanical stability of the PEMs.10–12,14,15,17 In the bulk as well as in films, the doping with lithium ions showed a significant effect on both, the structure and the conductivity of PEO-based BCs.15,20 Ionic conductive PS-b-PEO BCs were reported for a certain lithium ion concentration threshold ([Li]/[EO] = 0.1, mole ratio) with an enhanced mechanical


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stability.5,15,17 At much higher salt concentrations, aggregates of lithium salt and a limited conductivity performance were observed.20 As compared with the conductivity of liquid electrolytes being in the order of 10-2 S cm-1, the conductivities of PEMs based on BCs are still relatively low (< 10-4 S cm-1), which hampers application of the BCs in LIBs. Many good reasons for such low conductivity values were discussed in previous investigations.9–11,15,33,34 In particular, the conductivity of BCs can be estimated using the equation σBC = σcφcf where σc and

φc are the ionic conductivity and the volume fraction of the conductive microdomains, and f is the morphology factor.35 This factor f includes the conductive microdomain shape, the orientation, and the connectivity. In theory the morphology factor f can reach unity in case of an isotropic bicontinuous morphology. However, it is only 2/3, 1/3, and even 0 for a lamella, a cylinder, and a spherical morphology of the conductive microdomains, respectively.35 Additional effects such as grain boundaries and defects as well as the presence of low mobility salt at the conductive/nonconductive boundaries are not considered in the above equation and result in an even lower conductivity.35,36 Another subset of BC-based electrolytes are those mixed with ionic liquids (ILs).6,37–44 ILs are neoteric liquid materials which have low-melting point and are typically composed of bulky ions.45 ILs also offer other advantages such as a low vapor pressure, a high conductivity, and a good electrochemical stability.46 The high affinity of ILs to the PEO block of PEO-based BCs creates hybrid electrolytes with conductive and nonconductive microdomains. The IL/BC mixed systems also offer the antagonistic properties of mechanical stability and relatively good conductivity which are of interest for LIBs. Several investigations have studied the morphology, ionic conductivity, glass transition temperature, and tensile strength of binary IL/BC mixed systems as function of the IL content for battery applications.6,37–44 For instance, ILs


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incorporation in BCs, such as PS-b-PEO, polystyrene-block-polymethyl methacrylate (PS-bPMMA), polypropylene oxide-block-polyethylene oxide-block-polypropylene oxide (PPO-bPEO-b-PPO) generally decreases the glass transition temperature as well increases the segmental mobility of the polar block, thereby enhancing the conductivity of the electrolytes.6,41–43 In a recent study8, PEMs have been prepared via a polymerization process of styrene, macromolecular PEO, lithium salt (LiTFSI), and ionic liquid to achieve nanostructured PEMs. In this latter investigation8, a promising high ionic-conductivity has been achieved. Considering all the advantages of ILs and knowing that imidazolium-based ILs are capable of both, dissolving lithium salts,39,47 and promoting the PEO chain mobility,6 the aim of the current study is to investigate a mixture of tri-component comprised of lithium salt (LiTf), IL and diblock copolymer (DBC) to achieve high ionic-conductive hybrid nanostructured electrolytes. The selected lithium salt and IL have the same anionic moiety. The aim of the use of a tricomponent system is to enhance both, the lithium salt ionization and the overall conductivity of the BC electrolytes, to exceed the values of the corresponding binary systems with an acceptable mechanical and thermal performance. Moreover, since ILs have the capacity to plasticize ioncontaining polymers, it would improve the ion-containing PEO chain mobility and hence, enhances possible salt dissociation/solvation at the conductive/nonconductive polymer interfaces. Based on the above possible mentioned advantageous, we use the PS-b-PEO DBC, a fixed lithium salt concentration, and different IL contents to systematically examine both, crystallinity and morphology of these hybrid electrolytes as well as their corresponding conductivity correlations. Small angle and wide angle X-ray scattering (SAXS/WAXS) are used to probe both, crystallinity and structure of the hybrid electrolytes, respectively. The conductivity


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of the hybrid BC electrolytes is investigated with impedance spectroscopy. The thermal properties of the hybrid system are evaluated with differential scanning calorimetry (DSC).

RESULTS AND DISCUSSION Inhibition of PEO Crystallinity. For PS-b-PEO DBC the major PEO block forms a soft conductive PEO matrix in which non-conductive glassy, hard PS nanodomains are embedded. Conductivity of these DBC electrolytes (BCEs) is assumed to be present only in the low glass transition temperature (Tg) PEO domains, because the PEO block have a Tg value (~ -60 °C) far below room temperature. Thus, at room temperature the PEO chain mobility creates a dynamic and soft environment that facilitates the lithium ion transport. However, PEO is also a semicrystalline polymer, and has a strong crystallization tendency that may ruin the possible pathways for the mobile lithium ions. The possible PEO chain folding during the crystallization process would favor the formation of a non-dissociated lithium salt near the PS-PEO interface, resulting in a low ionic conductivity. Our investigations show that the PEO crystallization can only be avoided under certain conditions, using an appropriate amount of lithium salt and possibly adding other additives. Surprisingly, the PEO crystallization of BC electrolytes is generally underestimated in many previous investigations. In the present study, the crystallinity of binary LiTf/DBC and ternary IL/LiTf/DBC hybrid electrolytes is investigated with in-situ temperature dependent WAXS measurements.


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Figure 1. a,b) Temperature dependent 1D WAXS profiles and c,d) temperature dependent 1D SAXS profiles of lithium salt doped DBCs at two different [Li]/[EO] ratios of a,c) 0.1 and b,d) 0.2. In the WAXS data the solid lines mark the positions of characteristic PEO Bragg peaks. In the SAXS data (symbols) the fits using c) a lamellar and d) a hexagonal cylinder model as described in the text are shown with solid lines. The corresponding temperatures are indicated. The main Bragg reflections of the PEO crystal (vertical solid lines) and P(EO)3:LiCF3SO3 complex crystal (vertical red dashed lines) are indicated.

Figure 1 shows the temperature dependent 1D WAXS profiles of lithium-doped PS-b-PEO binary (LiTf/DBC) electrolytes at two different [Li]/[EO] ratios of 0.1 and 0.2. At [Li]/[EO] =


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0.1 (Figure 1a) two main (120) and (032) Bragg reflections at q = 13.6 and 16.7 nm-1 are observed at low temperatures and assigned to PEO crystallites.48,49 The intensities of both Bragg peaks vanishes at temperatures (> 65 °C) near the melting point of the PEO crystals (Tm,PEO = 61 °C). The used salt LiTf does not totally prohibit the PEO crystallization at a [Li]/[EO] ratio of 0.1 (Figure 1a) at temperatures below the PEO melting temperature (< Tm,PEO). In contrast, in our previous investigation5 the crystallization of PEO was totally suppressed at a [Li]/[EO] ratio of 0.1 using another lithium salt (Li[N(CF3SO2)2], referred to as LiTFSI). This means that the current selected salt LiTf (LiCF3SO3) is less soluble in the PEO block as compared with the LiTFSI salt, and significantly exists in a non-dissociated form near the PSPEO interface. Considering Li+ and its corresponding anion interactions, the larger is the anion with many more electron-drawing elements (F, N, etc.), the lower is the lattice energy. Calculations by Savoie et al. reveal that LiTFSI has a lower lattice energy (~ 650 kJ mol-1)50 than LiTf (725 kJ mol-1)51 as well as a lower solvation free energy in PEO.50 Thus, the lithium salt of the weakly coordinating TFSI anion is expected to be better soluble in PEO as compared with the Tf anion. The behavior of the LiTf salt can be quantitatively attributed to the Gutmann donor number (DN). This number (DN) indicates the electron donating properties of the anion and hence its ability to interact with cations such as Li+. The higher is DN, the higher is the anion basicity, resulting in a strong interaction with Lewis acids such as Li+. Most anions (PF6, AsF6, BF4, TFSI) have only relative small DN values (2.5 - 6.0). In contrast, a high DN value (16.9) is present for the Tf anion in the employed LiTf salt, which is an exceptional property.52 Thus, the same concentration of various types of lithium salts may influence the crystallinity behavior of the PEO block in PEO-b-PS DBCs in a different manner. For instance, in contrast to LiTFSI, at a [Li]/[EO] ratio of 0.1, the LiTf salt used in the present work does not totally suppress the PEO


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crystallization. However, at a high salt upload of 0.2, a nearly complete suppression of PEO crystallization is revealed by the weak intensity of the PEO (120) and (032) Bragg reflections (Figure 1b).

Figure 2. Room temperature 1D WAXS profiles of bare, LiTf/DBC ([Li]/[EO] = 0.1) and IL/LiTf/DBC hybrid electrolytes with different [IL]/[EO] ratios, viz., 0.04, 0.1, 0.2, and 0.3. The main Bragg reflections of the PEO crystal (vertical solid lines) and P(EO)3:LiCF3SO3 complex crystal (vertical dashed lines) are indicated.

The 1D WAXS profiles for the ternary hybrid electrolytes composed of DBC, lithium salt at a fixed [Li]/[EO] ratio of 0.1 and various IL contents are plotted in Figure 2. In comparison to the salt-free DBC (bottom curve in Figure 2) the 1D WAXS profiles of the LiTf containing BCE


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show two additional high intensity peaks at low q values. In the binary LiTf/DBC system, these two new peaks at q = 7.26 and 8.68 nm-1 and many small additional peaks seem to be unchanged during the whole thermal process up to 125 °C (see Figure 1a,b). The latter peaks are assumed to occur due to formation of a LiTf-PEO complex crystalline phase.53 Most of these observed peaks were previously reported and were assigned to the P(EO)3:LiCF3SO3 crystalline phase.53 For the latter crystalline phase, the PEO chains form a helical column chelating the lithium ions forming a complex with the PEO chain along the columns and no ionic crosslinking among these extended helical columns was approved.53 Lightfoot et al.54 suggested that the lithium cation in this LiTf-PEO complex-crystal is coordinated by five oxygen atoms: Three atoms from the ethylene oxide units and two oxygen atoms from two different Tf anions. Thus, the anion bridges two lithium ions, thereby leading to an anion chain running parallel to the polymer chain. Zardalidis et al.55 confirmed that this complex builds up in the stoichiometric ratio of P(EO)3:LiCF3SO3 independent of the [Li]/[EO] ratio. Our results (Figure 1) further prove that these crystals are thermally stable up to high temperatures (125 °C). Thus, the studied LiTf/DBC binary system is assumed to have a local structure which comprises crystalline PEO, crystalline P(EO)3:LiCF3SO3, and a disordered LiTf/PEO phase. Rhodes and Frech53 further proved that the local structure of the disordered PEO/LiTf phase resembles that of the crystalline P(EO)3:LiCF3SO3 phase using Raman spectroscopy. Such complex-crystalline phase formed in the LiTf/PEO system is unique compared with many other Li-salt types (PF6, AsF6, BF4, TFSI). With such knowledge on the crystallinity of the LiTf/DBC binary system, the [Li]/[EO] ratio of 0.1 is chosen for further investigation on the IL/LiTf/DBC ternary system, to study the effect of IL contents on the overall crystallinity behavior. For the IL/LiTf/DBC ternary system, the Bragg peaks of crystalline PEO reduce in intensity with


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increasing the IL content up to a [IL]/[EO] ratio of 0.2 and completely vanish at 0.3 (Figure 2). Moreover, the main characteristic PEO Bragg peaks are getting broader and slightly shift to the lower q values. Thus, the PEO crystallites get smaller (Figure S1 in Supporting Information) and slightly increase in the d-spacing. This interesting observation proves the progressive amorphization role of the used IL on the PEO crystals. The IL incorporation seems to implement a stress induced strain in the PEO crystals, as revealed from a slight increase of the characteristic d-spacing of the PEO crystals. Thus, a favorable molecular incorporation of the IL in the PEO chain is assumed. In contrast, the characteristic diffraction peaks of P(EO)3:LiCF3SO3 persist upon IL upload up to a [IL]/[EO] ratio of 0.1 and then diminish at 0.2 without any noticeable changes in their q positions (Figure 2). Thus, the P(EO)3:LiCF3SO3 crystals are assumed to be more stable than the PEO crystals concerning IL addition, indicating that the IL addition is more effective to suppress the PEO crystals than possible disruption of the P(EO)3:LiCF3SO3 complexcrystals. At a [IL]/[EO] ratio of 0.3, all characteristic Bragg peaks of both types of PEO containing crystals are absent. Instead the characteristic scattering of an amorphous phase is seen. All observed changes regarding the crystallinity are important to understand the overall microscopic structures of the complex binary and ternary systems. Phase Transformation of the Hybrid Electrolytes. The morphologies of the LiTf/DBC binary and IL/LiTf/DBC ternary samples are investigated with SAXS measurements. The temperaturedependent 1D SAXS data and the corresponding fits of the lithium salt doped PS-b-PEO DBC are shown in Figure 1c and 1d for two different [Li]/[EO] ratios of 0.1 and 0.2. For the selected DBC with a PEO volume fraction of 0.71, one would have expected hexagonally packed PS cylinders in a PEO matrix, however, the SAXS data are best fitted with a lamellar morphology in case of [Li]/[EO] = 0.1 (Figure 1c). The used fitting parameters to describe the lamellar


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morphology are the inter-domain spacing D, which gives an insight into the periodicity of the structure, the domain shape and size (form factor). The lamellar morphology of the LiTf/DBC binary is caused by the strong crystallization tendency of PEO and is consistent with our previous report on PEO-based BC system using LiTFSI as a salt at [Li]/[EO] < 0.1.5 With increasing temperature, the SAXS profiles show that the first intensity maximum stays at the same q value. For LiTf/DBC binary system with [Li]/[EO] ratio of 0.2, the 1D SAXS data are alternatively fitted with a hexagonally packed cylinder (HPC) morphology (Figure 1d). This morphology stays constant over the entire probed temperature range. With increasing the lithium salt content ([Li]/[EO] ratio) from 0.1 to 0.2 an induced change in the morphology from a lamellar to a cylinder structure is concomitant with the formation or absence of PEO crystals, as evident from the WAXS data (Figure 1b). In other words, the formation of crystallized PEO domains drives the system to form a lamellar morphology. Thus, the PEO crystallization is an important factor, inducing a change in the microphase separation structure. Depending on the lithium salt content the observed morphologies can deviate from simplified calculated ones.18,56


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Figure 3. Room temperature 1D SAXS data of bare DBC, LiTf/DBC ([Li]/[EO] = 0.1) and IL/LiTf/DBC hybrid electrolytes with different [IL]/[EO] ratios from bottom to top, viz., 0.02, 0.04, 0.1, 0.2, and 0.3. The bottom three data are fitted using a lamella model, while the top four curves are fitted using a hexagonally packed cylinder model.

For the ternary IL/LiTf/DBC hybrid samples, the 1D SAXS data are shown at a fixed [Li]/[EO] ratio of 0.1 for different IL contents in Figure 3. The 1D SAXS data are stacked on top of each other in order to better emphasize possible changes of the microscopic structure induced by microphase separation. In Figure 3, the first two bottom SAXS curves represent both, the bare and the LiTf/DBC ([Li]/[EO] = 0.1) binary samples. Next, from bottom to top the SAXS curves of ternary IL/LiTf/DBC samples are shown with an increasing IL content ([IL]/[EO] ratios = 0.02, 0.04, 0.1, 0.2, and 0.3). In Figure 3, the three bottom SAXS curves are fitted with a


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lamellar morphology, while SAXS curves of IL/LiTf/DBC ternary samples are fitted with a hexagonally packed cylinder morphology at IL concentrations ([IL]/[EO] ratio) ≥ 0.04. From a comparison of the main characteristic peaks of both, the bare DBC and LiTf/DBC samples (the two bottom curves in Figure 3), an initial strong shift towards a small q value is observed. Such shift shows that the inter-domain periodic distance D is strongly increased upon salt incorporation in the DBC. In literature, the thermodynamic properties and the morphological modifications of PS-b-PEO DBCs upon adding lithium salts were extensively investigated.11,18,57 So far, a linear as well as a nonlinear increase of the domain spacing of the self-assembled PEO based DBCs were reported for lithium salt doping. Generally, an expansion of the domain spacing D of the microphase separation structure is observed and related to a selective incorporation of the lithium salt in the PEO block. Three possible reasons are suggested for such increase: i) a salt volume effect, ii) a chemical incompatibility enhancement upon selective salt incorporation and iii) an extended chain conformation effect. Epps and coworkers9 showed a linear increase of the domain spacing with salt doping and mainly attributed it to the high effective interaction parameter (χeff) of the salt-doped DBC. The parameter χeff is a measure of the enhanced incompatibility between polymer blocks upon adding lithium salt. Recently, Gunkel et al.57 reported such a linear increase of χeff with salt concentration, however, at about 10 orders of magnitude less than that expected for the estimated domain spacing expansion in the ordered phase, implying that the incompatibility parameter (χeff) is not the major reason for the increase in domain spacing. Instead, an extended chain conformation upon lithium salt incorporation was assumed. Moreover, Teran et al.18 indicated a nonlinear dependence of χeff on the salt concentration for a PS-b-PEO DBC.


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Figure 4. a) Domain spacing D of the bare DBC and LiTf/DBC with [Li]/[EO] ratio of 0.1 measured at different temperatures as obtained from the fitting of the SAXS data. b) D as a function of IL concentration. The arrow indicates the anomalous behavior upon adding the lithium salt to the bare DBC. The lines are guides to the eyes and indicate the linear behavior of increasing of D spacing of the IL/LiTf/DBC hybrid electrolytes upon increasing the IL content.

Thus, in general, the overall increase (either linear or nonlinear) of the domain spacing D is assumed to originate from the integrated effect of the salt volume contribution, the increase of


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the incompatibility parameter and a possible extended chain conformation upon salt incorporation. In the present study, an increase of the domain spacing from 47 nm to 58 nm is observed (Figure 4) upon lithium salt loading ([Li]/[EO] = 0.1). Figure 4a shows the temperature dependent inter-domain spacing D for both, salt-free and salt-containing DBC. This change of the domain spacing of the DBC upon salt doping seems only partially due to salt volume effect. The increase of D due to the salt volume effect can be estimated as follows: d/d* =1+xsalt ρBCE/ρsalt , where xsalt is the mass ratio of the salt known from the sample preparation, ρ is the density, d* and d are the domain spacing of the bare DBC and of the LiTf/DBC samples, respectively. Thus, the anomalous effect of lithium ion incorporation within PEO based DBCs on the domain spacing is consistent with those reported in previous studies.10,11,15,18,58 As a new question may now arise, “what is the role of the IL incorporation on the domain spacing in the IL/LiTf/DBC ternary system?”. Interestingly, the incorporation of small amounts of IL results in a pronounced drop of the domain spacing D approaching the value of the bare DBC (Figure 4b). Such decrease can have two possible reasons: On the one hand, a preferential solvation of the Li cations in the IL/LiTf/DBC sample promoted by the IL molecules. On the other hand, a strong affinity of the IL to the PEO block can significantly reduce the Li+ interaction with the PEO chains. In addition to the reported potential solubility of lithium salt in ionic liquids at room temperature,39,47,59 and the strong affinity of the IL to the PEO block,6 IL incorporation is considered to be the main reason for weakening the strong lithium salt-PEO interactions. Such effect will also enhance the lithium ion mobility along or between the PEO chains. At [IL]/[EO] ≥ 0.04, the morphology changes to a hexagonal packed cylinder structure, which remains for all investigated IL concentrations up to [IL]/[EO] = 0.3 (Figure 3). Four peaks related to the hexagonally packed cylinder morphology can be clearly observed in the SAXS data


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of the IL/LiTf/DBC hybrid samples. The domain spacing D increases with IL content, which underlines that the IL is mainly incorporated in the PEO matrix since the amophorization of the PEO crystals leads to a pronounced increase of the D values. The radius of the PS nano-cylinders remains unchanged at about 13.2 ± 0.3 nm (as obtained from the fittings of the SAXS data) when IL is added into the IL/LiTf/DBC samples indicating the fact that the IL is only present in the PEO domains of the nanostructured hybrid electrolytes.

Figure 5. a) DSC curves of the third heating cycle (10 °C min-1) for all materials under investigation, from bottom to top shown are the bare DBC, the LiTf/DBC sample at a [Li]/[EO] ratio of 0.1 and the IL/LiTf/DBC samples for different IL concentrations as indicated. b-c) The


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“zoom-in” into regions of the DSC curves showing in particular the Tm,PEO and Tg region. d) Crystallinity χc of PEO in the hybrid electrolytes and e) Tg as a function of IL content expressed in [IL]/[EO] molar ratios.

Depression of the glass transition temperatures. The DSC curves of the investigated hybrid BCEs recorded during the third heating cycle are plotted in Figure 5. Different “zoom in” regions are shown to emphasize on the glass transition temperatures (Tg) and on the melting temperatures of the PEO domains (Tm, PEO). The Tm, PEO values are determined from the melting peak maxima (Figure 5b). The Tm, PEO decreases upon LiTf incorporation into the bare DBC. A further decrease of Tm, PEO with ionic liquid addition is found up to a [IL]/[EO] ratio of 0.15. At a concentration of [IL]/[EO] = 0.2, the melting peak vanishes−no more crystalline PEO is found. A complete amorphous phase of PEO is observed at a lower concentration of IL ([IL]/[EO] = 0.2) compared with the WAXS measurements from which at [IL]/[EO] = 0.3, due to different thermal treatment protocols employed in both measurements. As seen from Figure 5, the melting enthaply ∆H has its highest value for the bare DBC sample, while upon lithium salt addition at [Li]/[EO] ratio of 0.1, the enthalpy significantly decreases due to the formation of PEO-LiTf complex-crystals, as revealed from the WAXS measurements. In case of IL addition, the enthalpy ∆H systematically decreases, meaning that less PEO crystals exists in the hybrid electrolytes. The percentage of crystalline phase of PEO (χc) is calculated from the relation χc = ∆Hm/∆H°m, where ∆Hm is the melting enthalpy of the sample and ∆H°m is the melting enthalpy of 100% crystalline PEO (namely 213.7 J g-1).60 The dependence of χc on the IL concentration is shown in Figure 5d. Thus, the overall amorphous phase increases with the increase of the IL concentration. We speculate that the interaction of the IL with the ether oxygen of PEO inhibits the PEO


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crystallization and an increased amount of amorphous PEO is formed, supporting the results obtained from the WAXS measurements. It is worth noticing that for all IL containing samples a small exothermic peak is seen at ~ 18 °C. This is a well-known phenomenon called cold crystallization.61 During the cooling step a PEO crystallization takes place only partially because the kinetics of the crystallization process is slower than the cooling rate. If the temperature is too low, close or below the Tg, no more PEO crystals are formed because their segmental mobility is also too slow or even totally arrested. Therefore, the crystallization continues when the chains regain mobility during heating. In the present study this is the case right before the PEO melting peak occurs. As an alternatively explanation, the existence of a second melting temperature to the melting of the salt-rich P(EO)3:LiCF3SO3 crystalline phase was reported in literature.62,63 As seen in Figure 5e, the Tg values are not observed for the bare DBC and binary lithium-containing LiTf/DBC sample. The absence of a Tg detection in the IL free samples reflects the weak-segregation behavior of these samples. Tg values are not always detectable even in case of better defined copolymer compositions.33 However, for the ternary electrolyte samples, the Tg related step in the DSC curves is well pronounced (Figure 5c). Systematic addition of IL (EMIM-Tf) decreases the Tg, indicating a possible faster chain dynamics. The presence of IL seems to release the constraints imposed by the PEO-LiTf complexes. From the DSC measurements, the IL acts as a plasticizer, increasing the amorphous PEO phase content and making the interactions between the PEO chains less pronounced in the hybrid electrolytes. These properties suggest a possible enhanced lithium ion mobility. Conductivity of the hybrid electrolytes. The ionic conductivity is the most important parameter for polymer electrolytes and can be obtained by fitting the complex impedance plots employing


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an appropriate equivalent circuit. The as-prepared and annealed hybrid electrolytes are stable under the mechanical force of the employed spring even at relatively high temperatures. The possible failure of the electrolytes under the spring force (0.5 N mm-2) at different temperatures would result in a short circuit within the impedance cell, a sketch of the measurement cell is provided in the Supporting Information (Figure S2). For comparison, the bare PEO is tested at the same experimental conditions and shows comparable stability only near room temperatures but fails near the PEO melting temperature (50 °C). The present investigation on electrolytes is aiming on getting a thermally and mechanically stable ionic conductive membrane for thin film batteries. In thin film batteries, the electrolyte should mainly be applied to the electrode via solution-based processes. The solution-based techniques will offer a better contact and fewer defects at the electrode-electrolyte interface as compared with free-standing membranes. Our initial trials have indicated that a free-standing film applied to smooth gold-coated copper metal electrodes have higher resistances as compared with direct solution-cast films. Moreover, the employed thermal post-treatment step further improved the conductivity of a membrane sandwiched between two electrodes via nano-scale structuring the electrolyte, removing the possible residual solvent content and forming defectfree electrode/electrolyte contacts. Following the thermal treatment step at 120 °C for 24 hours, the samples are cooled back to room temperature prior to the impedance measurements. For the selected polymer electrolytes, the variation of the ionic conductivity with reciprocal temperature (Arrhenius plot) is shown in Figure 6a. For the LiTf/DBC and the IL/LiTf/DBC (with [IL]/[EO] ≤ 0.08) electrolytes, the plots exhibit a jump in the conductivity around 60 °C (near the melting point of PEO), above which the PEO block is in the amorphous phase. This behavior is


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correlated to the results obtained from the DSC which show that the main crystalline-amorphous transition occurs at Tm,PEO.

Figure 6. a) Ionic conductivity as a function of reciprocal temperature for IL/LiTf/DBC hybrid


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electrolytes for three selected IL content at [IL]/[EO] ratios of 0.08, 0.15 and 0.3; b) conductivity versus IL content in the hybrid electrolytes at three selected temperatures of 35 °C, 45 °C, and 75 °C. The solid line in are a) VTF fits and b) a guide to the eyes, indicating the conductivity enhancements with IL content. Error bars are smaller than the symbols.

The conductivity data have been fitted using Vogel-Tamman-Fulcher (VTF) equation (1). The VTF equation is effectively used to describe various dynamical processes in glassy and polymeric systems:64 = .

(1)

( )

Where σ0 is a pre-exponential factor, Ea is the activation energy, KB is the Boltzmann constant, T0 denotes temperatures where the conductivity approaches zero and T is the absolute temperature. The lines in Figure 6a show the results of the non-linear least square fitting of the conductivity data using VTF equation. The activation energy parameter obtained from the fits are 0.45, 0.36, 0.28 (±0.03) eV for DBC/LiTf/IL with [IL]/[EO] ratios of 0.08, 0.15, and 0.3, respectively. The successful fit using VTF model indicates that ionic mobility is intimately connected with the segmental motion of the polymer chains in the matrix.34 In other words, the VTF fittings indicate a strong coupling between the ions and the polymer chain segmental motions of the DBC/LiTf/IL hybrid electrolytes. The overall conductivity behavior of the IL/LiTf/BCE hybrid electrolytes shows a progressive improvement at temperatures up to 95 °C. In general, three detrimental factors for the ionic conductivity of polymer electrolytes are:10,14,15,65 1) The solvation ability of the PEO polymer chains, forcing the lithium salt to be mainly in a dissociated form. 2) The flexibility of the polymer chains, promoting a fast Li+ ion mobility along and between the polymer chains, and 3)


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the accessibility of the diffusive paths of the ions in the electrolyte, by avoiding possible deadend pathways. It seems that these three conditions can be adequately fulfilled by IL addition to the hybrid electrolytes. The ability of the PEO chains to solvate the lithium salt is assumed to be present only far from the PS-PEO interfaces, as the chains would have a limited mobility near the interface to fold around the ions. In contrast, the tendency of the PEO to fold and crystallize is also favored to take place far from the interface and near the middle of the PEO domains which is illustrated in a sketch in the Supporting Information (Figure S3). A preferential middle localization of lithium salt in an amorphous PEO domain was experimentally observed using energy-filtered transmission electron microscopy.16 Thus, in the presence of PEO crystals, the lithium salt tends to reside near the PS-PEO interface, mainly in a non-dissociated form, leading to low conductivity values. At temperatures above Tm,PEO, the PEO crystals melt and the salt can subsequently migrate to the middle of the domains, enhancing its dissociation (hence, the conductivity). The conductivity jump near Tm,PEO is mainly due to better lithium salt dissociation via migration from the PS-PEO interface to the middle of the PEO domains. At even higher temperatures, the conductivity is further improved due to improved mobility of the PEO domains. For the samples containing IL (with [IL]/[EO] ratio > 0.08), the absence of a step-like jump in the conductivity near Tm,PEO is due to a low crystallinity of the samples as evident from the DSC measurements. Generally, crystallinity χc calculated from both WAXS and DSC data are matching within only 10% error due to different thermal history of both measurements. The strong tendency of IL incorporation to inhibit PEO crystallization may explain both, the improved lithium salt solvation in the PEO domains and the absence of a characteristic jump in the conductivity near the Tm,PEO. The Li+ mobility in the PEO chains may also be improved by


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disruption of the PEO ether linkage to the Li+ ions. This disruption process is assumed to improve with enhancing the spontaneous PEO chain conformation changes. The observed depression in the Tg,PEO upon IL addition, as revealed from the DSC data, would result in an enhanced PEO chain mobility which in turn will favor the disruption of the PEO ether linage with Li+ ions, hence causing an enhanced conductivity performance. Moreover, the increase of the inter-domain distance (D) with IL addition, as revealed from the SAXS measurements, indicates a preferential incorporation of IL inside the PEO chains, promoting an extended PEO chain conformation. As well, the lamella-cylinder phase transformation upon IL upload would offer an improvement of the PEO percolation in the samples. The appearance of PS cylinders in a PEO matrix of the DBC with larger PEO block is confirmed by the extended volume of the PEO domain due to the preferential incorporation of lithium salt and IL into the PEO domain as well as the suppression of PEO crystals with increasing IL content, proven by WAXS (figure 2) and DSC (figure 5) data, which results in an increasing domain spacing D (figure 4b) due to volume increase as well as an elongated PEO chain conformation. This would likely offer open and easy pathways for Li+ along the PEO domains. Combining, crystallization inhibition, improved PEO chain mobility, enhanced salt dissociation, morphological transition and extended PEO chain conformation upon IL addition, a progressive improvement of the conductivity performance is observed. In the present study, the good ionic conductive hybrid electrolytes are obtained while additionally maintaining a relatively mechanically stable electrolyte (under a spring force of 0.5 N mm-2) in combination with a good thermal stability (up to 95 °C). Up to an IL content of [IL]/[EO] < 0.15 in the hybrid electrolytes, the samples seem solid and dry, while at higher IL contents the DSC measurements indicate that the electrolyte is gel-like in nature. The high room temperature ionic conductivity (> 1 mS cm-1) in combination with good thermal


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and mechanical performance of the hybrid electrolytes render these systems to be promising candidates for further battery applications.

CONCLUSIONS The present study demonstrates the potential of a block copolymer based electrolyte as a solidstate membrane in LIB applications. Hybrid electrolyte membranes comprised of the PS-b-PEO DBC, lithium trifluoromethane sulfonate salt (LiTf) and 1-ethyl-3-methylimidazolium trifluoromethane sulfonate (EMIM-Tf) ionic liquid (IL) are investigated as a function of the IL content. The ternary IL/LiTf/DBC hybrid electrolytes are prepared via a simple solution-casting method and include two types of cations in the electrolyte, namely [EMIM]+ and [Li]+ but only one type of anion [Tf]-. Nanostructured membranes composed of a matrix of IL/LiTf/PEO enable high ion-conducting pathways, while cylindrical PS nanodomains implement relatively good mechanical and thermal stability. Only a simple thermal post-treatment step is required. With such approach, a benchmark ion conductive hybrid electrolyte with high conductivity of > 1 mS cm-1, which is stable under a spring force of 0.5 N mm-2 at temperatures up to 95 °C is demonstrated. To understand this behavior, the crystallinity, morphology and thermal properties of the hybrid electrolytes are investigated with SAXS/WAXS and DSC techniques. A large fraction of crystalline PEO and crystalline P(EO)3:LiCF3SO3 are observed in the IL/LiTf/DBC hybrid electrolyte at a low IL content. The strong crystallization behavior of the PEO chains may drive the incorporated salt in these hybrid systems to reside at the PS-PEO interfaces, prohibiting the salt dissociation. Upon IL addition, the hybrid electrolytes exhibit a gradual increase of the amorphous PEO phase. The WAXS data reveal that P(EO)3:LiCF3SO3 complex-crystals are


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relatively stable as compared with the PEO crystals upon IL addition. The amorphous nature of PEO favors the lithium salt dissociation, preferably by salt migrating away from the PS-PEO interfaces. The Tg decreases with the increase of IL concentration suggesting an increase of the polymer chain flexibility, hence the lithium ion mobility. The observed increase of the domain spacing with IL upload is related a favorable interaction with the PEO chain forming a disordered extended chain conformation. The expanded conformation of the PEO chain eliminates possible conductivity-suppressing interfaces and enhances the random walk of the lithium cations along the chains. Combining all these effects we succeeded in the fabrication of long-range ordered nanostructured materials offering a promising thermally stable solid-state conductive electrolyte with conductivities values beyond the 1 mS cm-1 benchmark. Owing to its outstanding properties for reliable and safe devices, the current electrolyte membranes pave the way to design and implement all-solid-state thin film LIBs.

EXPERIMENTAL PROCEDURES Preparation of Hybrid BCEs. The diblock copolymer PS-b-PEO was obtained from Polymer Standard Service (Mainz, Germany) with a total molecular weight of 47 kg mol-1, a PEO volume fraction (fPEO) of 0.71 and with a polydispersity index (PDI) of 1.07. The lithium salt lithium trifluoromethanesulfonate (LiTf) was purchased from Sigma-Aldrich with a molecular weight of 156.01 g mol-1 and a purity of 99.99%. The ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIM-Tf) was also purchased from Sigma-Aldrich. The ionic liquid (EMIM-Tf) has a molecular weight of 260.23 g mol-1 and is referred to as IL throughout the text. It has a high boiling point around 350 °C and a low melting point of -13 °C. Both EMIM and Tf


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refer to the cation (1-ethyl-3-methylimidazolium) and the anion (triflate) of the IL. Thus, the anion is the same anion (Tf) as in the lithium salt. As a consequence, there are two types of cations in the electrolyte, namely [EMIM]+ and [Li]+, but only one type of anion, namely [Tf]-. All solvents and other reagents were anhydrous grade. Without further purification, all solid materials and solvent bottles were opened and used inside a glove box providing argon atmosphere. The PS-b-PEO diblock copolymer (DBC) was dissolved in THF at a concentration of 50 mg mL-1 and mixed with the desired proportions of lithium salt and ionic liquid to obtain various [Li]/[EO] and [IL]/[EO] molar ratios. The solution-cast hybrid films have final thicknesses in the range of 40-50 µm. The samples were sandwiched between either 2 mica windows or 2 gold-coated copper plates for both, the SAXS/WAXS and impedance spectroscopy measurements, respectively. Small/Wide Angle X-ray Scattering (SAXS/WAXS). The crystallinity and structure of the prepared LiTf/DBC binary and IL/LiTf/DBC ternary hybrid samples were probed using small angle and wide angle X-ray scattering (SAXS/WAXS) with a system which was previously described in details.5 The X-ray source was operated at 50 kV/0.6 mA with a Cu anode (Kα, λ = 0.1542 nm). A 2-dimensional Pilatus 300K (Dectris) detector was used. The sample-todetector distances were 1051 and 101 mm for SAXS and WAXS, respectively. The hybrid films were initially deposited on one mica window, allowed to dry in the glove box overnight, then a second mica window was placed on top of the dry films. The whole stack as assembled with a stainless steel metal holder. The samples were thermally annealed at 125 °C for 24 h in a vacuum oven. In-situ temperature dependent SAXS/WAXS measurements were performed using an electrical thermal stage Linkam HFSX350 suitable for both, heating and cooling processes at an accuracy close to 0.2 °C. Starting from room temperature, the samples were heated at a fixed rate


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of 10 °C min-1. The SAXS and WAXS scattering data were collected for 30 min and 10 min, respectively. The 2D SAXS/WAXS data were radially averaged and presented in logarithmic scattering intensity versus the magnitude of the scattering vector, q = 2π*sin(2θ)/λ, where 2θ is the scattering angle. All SAXS data were fitted using the “Scatter” software.66 AC Impedance Spectroscopy. The hybrid BC-based electrolytes were assembled by sandwiching the samples between two gold-coated copper disk electrodes (1 cm in diameter) and placed in a home-made cell. The thicknesses of the prepared hybrid films were kept fixed. The samples were initially deposited on one electrode using the same protocol as for the X-ray scattering measurements. Then a top electrode was placed on the electrolyte and pushed against it at a constant force using a spring, ensuring a fixed force on all tested hybrid electrolytes. The spring constant was measured with an electric force sensor. A spring constant of about 0.48 N mm-2 was used. Impedance spectroscopy was used to determine the ionic conductivity of the material using a potentiostat (VMP3, Bio-Logic). The impedance of the cell was measured over a frequency range from 10 Hz to 1 MHz with applied AC voltage amplitude of 10 mV. Ionic conductivity was collected from the complex impedance data (Z* = Z′ - iZ″) at different temperatures. The spectra (Nyquist plots) were modelled with an equivalent electric circuit and fitted using the EC-lab software package (Bio-Logic Instrument, V10.20). The thickness of the films was measured by a Bruker Dektak XT stylus profiler after lifting off part of the deposited film from the electrode with the help of a plastic knife. The conductivity was calculated as

σ = t/(R*a) where t is the sample thickness, a is the electrode metal surface area, and R is the bulk resistance, as determined from of the Nyquist plot fittings. Differential Scanning Calorimetry (DSC). Thermal transitions were measured using a Perkin Elmer differential scanning calorimetry (DSC) instrument DSC8500a. Empty aluminum pans


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were filled with the solution and dried in the glove box. The aluminum pans were crimped close, and nitrogen gas was served as a purge gas for the DSC measurements. The glass transition temperature (Tg) was determined from the third heating cycle using the midpoint method. The first two heating cycles were performed by heating the samples to 125 °C using a rate of 10 °C min-1 followed by an equilibration for 5 min. All heating cycles were started at -85 °C and equilibrated for 5 min prior to each heating step.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: PEO crystal size, impedance cell sketch, sketch of the PS-b-PEO DBC, table with reported conductivity values. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-Mail: [email protected]. Present Addresses ǁ

Cracow University of Technology, Faculty of Chemical Engineering and Technology,

Department of Chemistry and Technology of Polymers, Warszawska 24, 31-155, Kraków, Poland


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ACKNOWLEDGMENT This work was supported by funding the Excellence Cluster “Nanosystems Initiative Munich” (NIM), the International Research Training Group 2022 Alberta/Technical University of Munich International Graduate School for Environmentally Responsible Functional Hybrid Materials (ATUMS) and the Center for NanoScience (CeNS).

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ToC-FIGURE


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Conductivity and Morphology Correlations of Ionic ... - ACS Publications

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