Li-Nafion Membrane Plasticised with Ethylene Carbonate/Sulfolane

polymers

Article Li-Naﬁon Membrane Plasticised with Ethylene Carbonate/Sulfolane: Inﬂuence of Mixing Temperature on the Physicochemical Properties

Aigul S. Istomina 1, Tatyana V. Yaroslavtseva 1, Olga G. Reznitskikh 1, Ruslan R. Kayumov 2 , Lyubov V. Shmygleva 2, Evgeny A. Sanginov 2, Yury A. Dobrovolsky 2 and Olga V. Bushkova 1,*

1 The Institute of Solid-State Chemistry, Ural Branch of the Russian Academy of Sciences, 620990 Ekaterinburg, Russia; [email protected] (A.S.I.); [email protected] (T.V.Y.); [email protected] (O.G.R.) 2 The Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Russia; [email protected] (R.R.K.); [email protected] (L.V.S.); [email protected] (E.A.S.); [email protected] (Y.A.D.) * Correspondence: [email protected] or [email protected]; Tel.: +7-343-362-3036

Abstract: The use of dipolar aprotic solvents to swell lithiated Naﬁon ionomer membranes simultaneously serving as electrolyte and separator is of great interest for lithium battery applications. This

work attempts to gain an insight into the physicochemical nature of a Li-Naﬁon ionomer material whose phase-separated nanostructure has been enhanced with a binary plasticiser comprising non-

Citation: Istomina, A.S.; volatile high-boiling ethylene carbonate (EC) and sulfolane (SL). Gravimetric studies evaluating the ◦ Yaroslavtseva, T.V.; Reznitskikh, O.G.; influence both of mixing temperature (25 to 80 C) and plasticiser composition (EC/SL ratio) on the Kayumov, R.R.; Shmygleva, L.V.; solvent uptake of Li-Nafion revealed a hysteresis between heating and cooling modes. Differential Sanginov, E.A.; Dobrovolsky, Y.A.; scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) revealed that the saturation Bushkova, O.V. Li-Nafion Membrane of a Nafion membrane with such a plasticiser led to a re-organisation of its amorphous structure, Plasticised with Ethylene with crystalline regions remaining practically unchanged. Regardless of mixing temperature, the Carbonate/Sulfolane: Influence of preservation of crystallites upon swelling is critical due to ionomer crosslinking provided by crys- Mixing Temperature on the talline regions, which ensures membrane integrity even at very high solvent uptake (≈200% at a Physicochemical Properties. Polymers mixing temperature of 80 ◦C). The physicochemical properties of a swollen membrane have much 2021, 13, 1150. https://doi.org/ in common with those of a chemically crosslinked polymer gel. The conductivity of ≈10−4 S cm−1 10.3390/polym13071150 demonstrated by Li-Nafion membranes saturated with EC/SL at room temperature is promising for

Academic Editor: Srabanti Ghosh various practical applications.

Received: 27 February 2021 Keywords: lithiated Naﬁon; ethylene carbonate; sulfolane; swelling; X-ray diffraction; differential Accepted: 2 April 2021 scanning calorimetry Published: 3 April 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- Along with its analogues (such as Flemion (Asahi Glass), Fumapem (Dow Chemi- iations. cal, 3M, FuMA-Tech), Aquivion (Solvay), etc.), the perfluorinated sulfonic-acid (PFSA) ionomer Nafion® developed by DuPont in the 1960s is widely used in polymer electrolyte membrane fuel cells, sensors, vanadium flow batteries, ionistors, as well as various other electrochemical devices and electrochemical technologies [1–3]. The widespread applica- Copyright: © 2021 by the authors. tions of this ionomer material are due to its excellent transport properties, high thermal and Licensee MDPI, Basel, Switzerland. chemical stability, combined with stable mechanical behaviour. Ion exchange reactions can This article is an open access article be used to convert PFSA ionomers from their original acid form to a salt form using various distributed under the terms and cations without loss of strength or thermal and chemical stability [4–14]. These properties conditions of the Creative Commons allow PFSA salts to be used as polymer electrolytes with unipolar cation conductivity. Attribution (CC BY) license (https:// Single-ion cationic transfer eliminates the concentration gradient in an electrochemical cell creativecommons.org/licenses/by/ along with its problematic consequences [15,16]. At present, the lithium form of the mem- 4.0/).

Polymers 2021, 13, 1150. https://doi.org/10.3390/polym13071150 https://www.mdpi.com/journal/polymers Polymers 2021, 13, x FOR PEER REVIEW 2 of 15

Polymers 2021, 13, 1150 2 of 14 conductivity. Single-ion cationic transfer eliminates the concentration gradient in an electrochemical cell along with its problematic consequences [15,16]. At present, the lithium braneform of is ofthe particular membrane interest is of particular for simultaneously interest for application simultaneously as electrolyte application and as separator electrolyte in lithium-ionand separator [17 –in21 lithium] and lithium-sulfur-ion [17–21] and [22 lithium–24] batteries.-sulfur [22–24] batteries. NafionNafion isis aa randomrandom copolymercopolymer ofof tetrafluoroethylene, tetrafluoroethylene, comprising comprising aa semicrystalline semicrystalline polytetrafluoroethylenepolytetrafluoroethylene (PTFE) backbone backbone and and a per a perfluorovinylfluorovinyl ether ether-bearing-bearing terminal terminal sul- sulfonicfonic group group (Figure (Figure 1a).1a). The The outstanding outstanding properties properties of these of these materials materials are are due due to the to thedis- − dissimilarsimilar nature nature of of the the covalently covalently bounded bounded pendant ionicionic groupgroup (SO (SO3 3−) and perfluorinatedperfluorinated backbone,backbone, resultingresulting inin aa phase-separatedphase-separated nanostructurenanostructure [[1].1]. IonicIonic speciesspecies inin NafionNafion aggre-aggre- gategate toto formform hydrophilichydrophilic ionicionic domainsdomains (clusters)(clusters) distributeddistributed evenlyevenly withinwithin aa hydrophobichydrophobic perfluorinatedperfluorinated polymer polymer matrix matrix comprising comprising both both amorphous amorphous and and crystalline crystallin regions.e regions. Similar Sim- toilar the to initial the initial protonated protonated form, theform, lithiated the lithiated Nafion Nafion (Li-Nafion) (Li-Nafion) membrane membrane is poorly is conduc- poorly tiveconductive in its dry in state.its dry However, state. However, upon saturation upon saturation with dipolar with dipolar aprotic aprotic solvents, solvents, a polymer a pol- electrolyteymer electrolyte offering offering good transport good transport properties properties can be can obtained be obta [4–ined14,17 [4––2414,17]. The–24 introduc-]. The in- tiontroduction of solvent of solvent molecules molecules enhances enhances nanoscale nanoscale phase separation. phase separation. The morphology The morphology of swollen of Nafionswollen is Nafion typically is typically described described by the Gierke by the cluster-network Gierke cluster-network model [25 model] or equivalent [25] or equiv- ap- proachesalent approaches as summarised as summarised in a comprehensive in a comprehensive review ofreview Kusoglu of Kusoglu [1]. As emphasised[1]. As empha- in Ref.sised [1 in], “theRef. sorption[1], “the behavioursorption behaviour of PFSA ionomers of PFSA isionomers a very important is a very phenomenonimportant phenom- affect- ingenon their affecting phase-separated their phase morphology-separated morphology and, hence, and, their hence, overall their transport overall and transport structural and properties”. structural properties”.

Nafion EC SL

(CF2 CF2)mCF CF2 n O O O O CF2 CF O CF2 CF2 S O M O S O O CF3 O (a) (b) (c) FigureFigure 1.1.Structural Structural formulasformulas ofof Li-NaﬁonLi-Nafion (where(where MM++ isis an an exchangeable exchangeable counterion) counterion) ( (aa),), ethylene ethylene carbonatecarbonate ((bb),), andand sulfolane sulfolane ( c(c).).

However,However, lithium-conductinglithium-conducting membranemembrane requirementsrequirements forfor lithiumlithium batteriesbatteries areare notnot limitedlimited toto highhigh conductivity,conductivity, mechanical mechanical strength, strength, and and chemical chemical stability. stability. For For the the practical practical usageusage ofof newnew polymerpolymer electrolytes,electrolytes, especiallyespecially inin high-powerhigh-power lithium-ionlithium-ion batteriesbatteries (LIBs),(LIBs), equallyequally importantimportant factorsfactors areare aa widewide rangerange ofof operatingoperating temperaturestemperatures (to(to atat leastleast includeinclude ambientambient temperatures),temperatures), resistanceresistance toto locallocal overheating,overheating, absenceabsence ofof gassinggassing and and electro- electro- chemicalchemical stability.stability. TheseThese factorsfactors allowallow thethe safetysafety ofof anan LIBLIB toto bebe significantlysignificantly increased,increased, justifyingjustifying the the replacement replacement of of liquid liquid electrolyte electrolyte onto onto a polyelectrolyte a polyelectrolyte membrane. membrane. However, How- theever, development the development of a suitable of a suitable plasticiser plasticiser remains rema anins urgent an urgent problem, problem, preventing preventing the full the potentialfull potential of Li-Nafion of Li-Nafion membranes membranes from from being being realised. realised. InIn recentrecent workwork [[26],26], wewe showed showed thatthat suitablesuitable plasticisersplasticisers forfor lithiatedlithiated NafionNafion cancan be be obtainedobtained among among binary binary mixtures mixtures of solventsof solvents combining combining high high boiling boiling points points and flashpoints and flash- withpoints low with vapour low pressuresvapour pressures and relatively and relatively high melting high pointsmelting (above points room (above temperature). room tem- Inperature). particular, In binaryparticular, mixtures binary of mixtures ethylene of carbonate ethylene (EC) carbonate and sulfolane (EC) and (SL) sulfolane were used (SL) towere plasticise used to a plasticise lithiated Nafiona lithiated 115 Nafion membrane. 115 membrane. In the best In sample,the best single-ionsample, single lithium-ion −6 −4 −1 conductivitylithium conductivity of 6.3 × of10 6.3 ×to 10 6.9−6 to× 6.910 × 10S−4 cm S cm−was1 was achieved achieved within within a a temperaturetemperature ◦ −9 −1 ◦ rangerange ofof− −4040 toto +80+80 °C,C, withwith a a negligible negligible electronic electronic conductivity conductivity (≈ (10≈10−9 SS cmcm−1 at 2525 °C)C) + contributioncontribution andand anan electrochemical stability window window of of 0 0 to to 6 6 V V vs vs Li/L Li/Lii+. For. For a single a single--ion + σ ≥ −5 −1 ionLi+ conductor, Li conductor, σ ≥ 10−5 S cm10 −1 isS acceptable cm is acceptable for practical for use practical in electrochemical use in electrochemical cells [15,16]. cellsThus, [15 a ,Li16-].Nafion Thus, membrane a Li-Nafion swollen membrane with swollen the EC/SL with mixtu the EC/SLre seems mixture suitable seems for practical suitable forapplications practical applications in LIBs. in LIBs. In the present study, we evaluated the influence of mixing temperature (40 to 80 ◦C) In the present study, we evaluated the influence of mixing temperature (40 to 80 °C) on the swelling degree of Li-Nafion in binary plasticiser EC/SL and revealed a hysteresis on the swelling degree of Li-Nafion in binary plasticiser EC/SL and revealed a hysteresis between heating and cooling modes. Following its saturation with a plasticiser, while the between heating and cooling modes. Following its saturation with a plasticiser, while the crystalline regions remained nearly unchanged, a re-organisation of the membrane’s amor- crystalline regions remained nearly unchanged, a re-organisation of the membrane’s phous structure was observed. These results provide an insight into the physicochemical nature of Li-Nafion membranes swollen with aprotic solvents.

Polymers 2021, 13, x FOR PEER REVIEW 3 of 15

amorphous structure was observed. These results provide an insight into the physicochemical nature of Li-Nafion membranes swollen with aprotic solvents.

Polymers 2021, 13, 1150 3 of 14 2. Materials and Methods 2.1. Membrane Preparation In this work, a commercial Nafion® 115 (125 µm) (Du Pont de Nemours, Wilmington, 2. Materials and Methods DE, USA) extrusion membrane was used. The Li+ form of the membrane was prepared as described2.1. Membrane in our Preparation previous work [9] and shown in Figure 2 over the course of four consec- ® utive In stages: this work, purification, a commercial neutralisation Nafion 115 by LiOH,(125 µm) thorough (Du Pont washing de Nemours, and drying. Wilmington, The + samplesDE, USA) were extrusion first dried membrane in air at 60 was °C used.for 1 h Theand Lithenform in a desiccator of the membrane at room was temperature prepared overas described P2O5 for seven in our days. previous Residual work water [9] content and shown was monitored in Figure2 byover infrared the course Fourier of spec- four troscopyconsecutive (Bruker stages: Vertex purification, 70v, Bruker neutralisation Corporation, by Karlsruhe, LiOH, thorough Germany washing) and simultaneous and drying. ◦ thermalThe samples analysis were (STA first 449 dried F3 Jupite in airr, NETZSCH, at 60 C for Selb, 1 h Germany and then). in a desiccator at room temperatureAll further over manipulations P2O5 for seven of days. the Li Residual-Nafion water membrane content and was plasticisers monitored bywere infrared per- formedFourier in spectroscopy an MBRAUN (Bruker UNILAB Vertex glove 70v,box Bruker (MBraun Corporation, Inertgas- Karlsruhe,Systeme, München Germany), Ger- and simultaneous thermal analysis (STA 449 F3 Jupiter, NETZSCH, Selb, Germany). many) in an argon atmosphere (the content of H2O and O2 < 1 ppm).

FigureFigure 2. 2. SchemeScheme of of the the Nafion Naﬁon membrane membrane lithiation lithiation process process..

2.2. BinaryAll further Plasticiser manipulations Preparation of the Li-Nafion membrane and plasticisers were performed in an MBRAUN UNILAB glovebox (MBraun Inertgas-Systeme, München, Germany) in an A binary EC/SL plasticiser was prepared from anhydrous ethylene carbonate (99%) argon atmosphere (the content of H O and O < 1 ppm). and sulfolane (99%), both from Sigma2 Aldrich 2(St. Louis, MO, USA)). The mass fraction of sulfolane2.2. Binary (ω(SL)) Plasticiser in the Preparation EC/SL mixtures varied from 0 to 1 with an increment of 0.05–0.10. Since the melting points of ethylene carbonate and sulfolane are above room temperature A binary EC/SL plasticiser was prepared from anhydrous ethylene carbonate (99%) (≈36 and ≈29 °C, respectively), they were heated until wholly melted (up to 40 °C for EC and sulfolane (99%), both from Sigma Aldrich (St. Louis, MO, USA). The mass fraction of and up to 30 °C for SL). Then, the mixtures were stirred for 30 min using a magnetic stirrer sulfolane (ω(SL)) in the EC/SL mixtures varied from 0 to 1 with an increment of 0.05–0.10. and kept at room temperature for at least 24 h before use. Since the melting points of ethylene carbonate and sulfolane are above room temperature (≈36 and ≈29 ◦C, respectively), they were heated until wholly melted (up to 40 ◦C for EC 2.3. Membrane Swelling and up to 30 ◦C for SL). Then, the mixtures were stirred for 30 min using a magnetic stirrer andThe kept swelling at room of temperature the Li-Nafion for atmembrane least 24 h was before inve use.stigated gravimetrically; each ex- periment was repeated in triplicate. A dry membrane sample was placed in an excess vol- ume2.3. of Membrane solvent (EC, Swelling SL, or EC/SL) over activated 3Å molecular sieves. To achieve an equi- libriumThe degree swelling of swelling, of the Li-Nafion the samples membrane were held was under investigated isothermal gravimetrically; conditions for 24 each h (fromexperiment 40 to 80 was °C). repeated The initial in triplicate. and final Asample dry membrane weights were sample measured. was placed To remove in an excess the volume of solvent (EC, SL, or EC/SL) over activated 3Å molecular sieves. To achieve an equilibrium degree of swelling, the samples were held under isothermal conditions for 24 h (from 40 to 80 ◦C). The initial and final sample weights were measured. To remove the

surface solvent, the samples were blotted dry before measurements. The weight uptake W (%) was calculated as: W = (msw − mdry)/mdry × 100%, (1) Polymers 2021, 13, 1150 4 of 14

where mdry and msw are weights of membrane samples before and after swelling, respectively. The uncertainty of W measurement did not exceed 1–1.5%. The molar uptake of solvent λ (equal to the number of solvents molecules per lithium cation) was calculated from W values using the formula:

λ = (W × EW)/(Mrsolv × 100%), (2)

where EW is the equivalent weight of the dried Li-Naﬁon samples (equal to 1106) and Mrsolv is the average molar mass of the used binary solvent.

2.4. Thermal Analysis The thermal behaviour of the samples was investigated using a DSC 214 Polyma differential scanning calorimeter (NETZSCH, Selb, Germany). A test sample was cut out of the membrane in the form of a disk, placed into a special Concavus Al pan, and hermetically sealed. The measurements were performed under dry nitrogen ﬂow at 40 mL/min within a temperature range from −150 to 300 ◦C. The following temperature programme was used: heating to 50 ◦C and isothermal exposure (15 min) for additional homogenisation of the sample; cooling to −150 ◦C at 20 ◦C/min and isothermal exposure for 3 min; heating to 300 ◦C at 40 ◦C/min.

2.5. Wide-Angle X-ray Diffraction (WAXD) Wide-angle X-ray diffraction data were collected using a Stoe STADI-P diffractometer ◦ (Stoe, Darmstadt, Germany) at following conditions: Cu Kα radiation; 2θ = 5–90 ; rotating sample; symmetrical transmission mode. In order to prevent water contamination, the membrane sample under study was placed between two lavsan films. A baseline correction was applied to the data prior to further analysis. The measured profile was decomposed into the fundamental profiles by a least-squares fit, with Lorentz functions used to approximate each fundamental profile. The position, height, width, and area of obtained peaks were evaluated. The degree of crystallinity Xc (%) was determined for “dry” membrane samples from XRD data according to the following formula:

Xc = Ac/(Aa + Ac) × 100%, (3)

where Ac is the integrated area of the Bragg peak (100); Aa is the integrated area of the amorphous halo superimposed on the crystalline peak.

2.6. AC Conductivity Measurements Through-plane lithium conductivity was measured at a frequency range 1 Hz to 100 kHz using a Z-350M analyser (Elins LLC, Chernogolovka, Russia) with a potential amplitude of 10 mV. The membrane samples (10 mm in diameter) were placed between two blocking Ni electrodes within a hermetically sealed two-probe test cell and equilibrated ◦ for 24 h at 25 C before measurement. The electrolyte resistance Re was determined by intercepting the high-frequency part of the impedance curve with the real axis. The speciﬁc conductivity σ of the samples was obtained as follows:

σ = d/(Re × A), (4)

where d and Re are the thickness and resistance of the sample, respectively; while A is the area of electrical contact. Conductivity was determined by impedance spectroscopy with an error about 10%. Polymers 2021, 13, x FOR PEER REVIEW 5 of 15 Polymers 2021, 13, 1150 5 of 14

3. Results and Discussion 3.3.1. Results Characterisation and Discussion of the Plasticiser 3.1. Characterisation of the Plasticiser An important point in selecting EC and SL as components of the binary plasticiser was Anthe importantobvious similarity point in of selecting their molecular EC and structure SL as components (Figure 1b,c). of the Both binary of these plasticiser solvents wasreadily the obviousdissolve similarity lithium salts of their due molecular to the intermediate structure (Figure (SL) and1b,c). high Both (EC) of these dielectric solvents con- readilystants (60 dissolve and 90, lithium respectively salts due [27]) to the, high intermediate dipole moment (SL) and (4.93 high for (EC) EC and dielectric 4.81 for constants SL [28]), (60 and 90, respectively [27]), high dipole moment (4.93 for EC and 4.81 for SL [28]), and the and the presence of electron donor groups capable of coordinating the lithium cation (Fig- presence of electron donor groups capable of coordinating the lithium cation (Figure1b,c) . ure 1b,c). This explains the effective solvation of Li+ in lithiated Nafion upon swelling with This explains the effective solvation of Li+ in lithiated Nafion upon swelling with EC, SL, EC, SL, or an EC/SL mixture. The combination of high boiling points with low vapour or an EC/SL mixture. The combination of high boiling points with low vapour pressure at pressure at T < 100 °C (0.003 and 0.009 kPa, respectively [29,30]) and high flash points T < 100 ◦C (0.003 and 0.009 kPa, respectively [29,30]) and high flash points (145.5 ± 4 and (145.5 ± 4 and 165 ± 4 °C [30]) offered by both EC and SL solvents is an important factor in 165 ± 4 ◦C[30]) offered by both EC and SL solvents is an important factor in improving improving the safety of LIBs. the safety of LIBs. While EC and SL solvents have individual melting points above room temperature While EC and SL solvents have individual melting points above room temperature (36.4 and 28.5 °C, respectively [28]), the mixture formed therefrom offers a significant de- (36.4 and 28.5 ◦C, respectively [28]), the mixture formed therefrom offers a significant decrease in the melting point of the plasticiser. Indeed, as established in our recent work crease in the melting point of the plasticiser. Indeed, as established in our recent work [31], [31], the EC–SL system belongs to a simple eutectic type with a eutectic temperature of— the EC–SL system belongs to a simple eutectic type with a eutectic temperature of—16 ◦C and16 °С a eutecticand a eutectic point at point 70 wt at % 70 SL. wt Moreover, % SL. Moreover, binary EC/SLbinary mixturesEC/SL mixtures in a moderate in a moderate range ofrange compositions of compositions 50 to 7550 wtto 75 % wt SL % are SL prone are prone to transition to transition into into the the metastable metastable state state of aof supercooleda supercooled liquid liquid without without crystallisation crystallisation of components of components (i.e., becoming (i.e., becoming glassy) glassy) [31], which [31], makeswhich themmakes nearly them ideal nearly plasticisers. ideal plasticisers. As shown As in shown our previous in our previous work [26 ],work it is advisable[26], it is ad- to saturatevisable to membranes saturate membranes with ES/SL with binary ES/ plasticiserSL binary plasticiser at temperatures at temperatures≥40 ◦C when ≥40 both°C when EC andboth SL EC components and SL components are in a liquid are in state. a liquid state.

3.2.3.2. InﬂuenceInfluenceof ofPlasticiser Plasticiser Composition Composition on on Solvent Solvent Uptake Uptake and and Conductivity Conductivity TheThe membranes membranes were were saturated saturated with with EC/SL EC/SL binarybinary mixturesmixtures of of varying varying composition composition atat 4040◦ °C.C. TheThe solventsolvent uptakeuptake waswas deﬁneddefined byby the the gravimetric gravimetric technique technique asas described described in in SectionSection 2.3 2.3.. FigureFigure3 a3a illustrates illustrates the the dependence dependence of of the the weight weight uptake uptakeW W(calculated (calculated by by EquationEquation (1)) (1) vs.) vs.ω ω(SL).(SL). It canIt can be be seen seen that that the highestthe highestW value W value relates relates to the to pure the EC, pure while EC, thewhile solvent the solvent uptake uptake decreases decreases as ω(SL) as increasesω(SL) increases from 132.5% from 132.5% (pure EC) (pure down EC) todown 88.1% to (pure88.1% SL). (pure SL).

FigureFigure 3.3. WeightWeight uptake uptake (W (W) )and and molar molar uptake uptake (λ) ((λa)() anda) and conductivity conductivity of swollen of swollen Li-Nafion Li-Naﬁon mem- membranebrane at 25 at °C 25 (b◦C() asb a) asfunction a function of ω(SL) of ω (SL)(mixing (mixing temperature temperature of 40 of °C) 40 ◦C).

OnOn the the assumption assumption that that the the mixed mixed solvent solvent composition composition absorbed absorbed by by Li-Naﬁon Li-Nafion is is identicalidentical toto thethe initialinitial mixturemixture usedused (no(no selective selective sorption), sorption), thethe totaltotal number number ofof binary binary plasticiserplasticiser moleculesmolecules perper LiLi cation cation inin thethe swollenswollen Li-NaﬁonLi-Nafion membranemembrane (molar(molar uptakeuptake λλ)) waswas calculatedcalculated usingusing EquationEquation (2).(2). AsAs cancan bebe seenseen fromfrom FigureFigure3 a,3a, the theW Wand andλ λvalues values decreasedecrease rapidly rapidly as asω ω(SL)(SL) increases, increases, while while the theλ values λ values differ differ twice twice for pure for pure solvents solvents EC and EC SLand (λ SL= 16.6 (λ = and 16.6 8.1, and respectively). 8.1, respectively). Here, Here, it is important it is important to note to that note the thatλ values the λ arevalues much are

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higher than the usual solvation number of free lithium cation equal to 4. The solvation of anions in dipolar aprotic solvents is known to be much weaker than cations. From the above, it follows that the sorption of solvent occurs not only due to the solvation of lithium cations by the highly polar EC and/or SL molecules but also due to some other changes in the membrane. Figure3b shows the conductivity isotherm at room temperature for Li-Nafion membranes saturated with EC/SL mixtures of varying compositions at a mixing temperature of 40 ◦C. For samples saturated with the EC/SL mixtures enriched with ethylene carbonate (ω(SL) < 0.2), it was not possible to measure the conductivity: on cooling from 40 to 25 ◦C, an excess amount of EC released from the membrane (syneresis) was solidified on the membrane surface. This phase separation agrees with the phase diagram of the EC–SL system [31]: liquidus temperatures for EC/SL mixtures with ω(SL) < 0.2 are above 25 ◦C. Thus, the non-conductive layer of solid plasticiser forming between the membrane and electrodes makes impedance measurements impossible. The best transport properties of Li-Nafion provided EC/SL mixtures enriched with ethylene carbonate. The membrane conductivity rapidly decreases with ω(SL) in the range of 0.2 to 0.7 (eutectic composition), but then it remains practically unchanged up to pure sulfolane (Figure3b). Although the decrease in conductivity may be caused by an increase in viscosity with SL content in the mixture [32], this can also be explained in terms of changes in ionic equilibria with the EC/SL mixture composition (since EC and SL differ in solvating ability [26]) or a decrease in the solvent uptake (Figure3a) influencing the conduction channels within the membrane [1].

3.3. Influence of Mixing Temperature on Solvent Uptake In order to study the influence of the mixing temperature on the solvent uptake of Li-Nafion, pure EC and SL were selected along with binary EC/SL mixtures of two compositions: ω(SL) = 0.2 and 0.7. The choice of these binary compositions was due to the EC-enriched mixture with ω(SL) = 0.2 providing the highest conductivity values of Li-Nafion (Figure3b) and being homogeneous at room temperature, which prevents phase separation upon cooling, while the mixture with ω(SL) = 0.7 corresponds to the eutectic point of the EC–SL system [31]. Measurements of the W–T dependencies were carried out by sequential heating (40 to 80 ◦C with 10 ◦C steps) and cooling (80 to 40 ◦C with 20 ◦C steps). Figure4 displays the results obtained. It can be seen that the mixing temperature has a powerful effect on the solvent uptake of Li-Nafion regardless of the plasticiser composition: the higher the mixing temperature, the greater the solvent uptake. However, this characteristic was observed only in the heating cycle (Figure4); during the cooling cycle, the decrease in solvent uptake was barely noticeable. As a result, the W value obtained for the highest temperature was virtually maintained for all other annealing temperatures at the cooling mode for all plasticiser compositions studied (Figure4). Upon cooling from 80 ◦C, the W value for this temperature was almost preserved after thermal treatment at 60 and 40 ◦C. If cooling was carried out from 60 ◦C, again, a similar W value was retained after thermal treatment at 40 ◦C (Figure4a). At the end of measurements, long-term (60 days) isothermal treatment was carried out at 40 ◦C to check that the equilibrium state of the swollen samples during the cooling cycle was achieved. However, this procedure did not influence the obtained W values. This observation suggests either that the samples are close to the equilibrium state or that syneresis requires a very long time. The observed hysteresis in solvent uptake between the heating and cooling curves supports the hypothesis that an irreversible rearrangement of the supermolecular structure occurs under the influence of a plasticiser and elevated temperatures. Here, it should be noted that all membrane samples retained their integrity upon swelling despite significant mechanical properties changes. Polymers 2021, 13, 1150 7 of 14

Figure 4. Dependence of the weight uptake (W) and molar uptake (λ) of a solvent of the Li-Naﬁon membrane vs. mixing temperature at heating and cooling mode for the following solvents: ethylene carbonate (EC) (a); sulfolane (SL) (b); EC/SL with ω(SL) = 0.2 (c); EC/SL with ω(SL) = 0.7 (d).

The effect of temperature on equilibrium water content is well known for the protonated form of Nafion. Nevertheless, for H+ Nafion, the increase in W with temperature is much less pronounced (see, for example, data for Nafion 115 in Ref. [33]; for Nafion 212 and Nafion XL, Ref. [34]): within the same temperature range of 40 to 80 ◦C, the water uptake increased by less than 10% and W did not exceed 40%. Such a dramatic difference in behaviour upon swelling suggests significant variations in the molecular mechanisms of solvent absorption between the protonated Nafion–water and lithiated Nafion–EC/SL systems. To the best of our knowledge, the present work is the first to report the strong influence of temperature on the aprotic solvent uptake for lithiated Nafion and observe the heating-cooling hysteresis.

3.4. Thermal Analysis and Wide-Angle X-ray Diffraction of “Dry” Membranes Despite having a large number of bulky side chains (Figure1a), Nafion membranes are known to have significant levels of crystallinity [1,25,35]. This crystallinity is one of the most critical properties of Nafion, providing both its structural integrity and mechanical stability even at high levels of swelling [1]. As shown in Starkweather’s paper [35], the three-dimensional hexagonal structure of Nafion is similar to that of PTFE at elevated temperatures. Compared with PTFE, the X-ray diffraction peaks of Nafion are broadened and shifted toward smaller angles, indicating that crystals are smaller and less perfect [35,36]. It has been proposed that the lamellar morphology of crystallites is randomly distributed over the amorphous Nafion matrix [35,37,38]. For extruded Nafion membranes, the relative degree of crystallinity of about 7–17% is lower than that of PTFE (55–75%) [1]; the difference can be attributed to the effect of side chains and ionic groups hindering crystallisation [1,25,35]. In the present work, wide-angle X-ray diffraction was carried out as described in Section 2.5 for “dry” samples of Nafion 115 membrane in its H+ (as received) and Li+ forms in order to reveal the influence of the lithiation procedure (Section 2.1) on the degree of crystallinity and structural characteristics. Diffraction patterns of the samples in the 2θ 1

Polymers 2021, 13, x FOR PEER REVIEW 8 of 15

the difference can be attributed to the effect of side chains and ionic groups hindering crystallisation [1,25,35]. Polymers 2021, 13, 1150 In the present work, wide-angle X-ray diffraction was carried out as described in8 of Sec- 14 tion 2.5 for “dry” samples of Nafion 115 membrane in its H+ (as received) and Li+ forms in order to reveal the influence of the lithiation procedure (Section 2.1) on the degree of crys- rangetallinity from and 7 tostructural 22◦ (where characteristics. the most intensive Diffraction peak patterns is located) of togetherthe samples with in decomposed the 2θ range profilesfrom 7 to are 22° presented (where the in Figuremost int5;ensive their characteristics peak is located) are together shown with in Table decomposed1. For both pro- samples,files are presented the measured in Figure curves 5; their were characteristics decomposed are into shown a broad in Table peak at1. For 2θ ≈ both15◦ samples,with a fullthe widthmeasured at half-maximum curves were decomposed (fwhm) of into≈4◦ a(amorphous broad peak at halo), 2θ ≈ and15° with a relatively a full width sharp at peakhalf-maximum at 2θ ≈ 17 (fwhm)◦ with fwhmof ≈4° of(amorphous≈2◦ (indexed halo), as and the a (100) relatively reflection sharp of peak the hexagonalat 2θ ≈ 17° structurewith fwhm [35 ]).of Taking≈2° (indexed into account as the (100) the significant reflection error of the in hexagonal the decomposition structure of [35]). broadened Taking profile,into account the WAXD the significant results for error H+ Nafion in the decomposition are in reasonable of agreementbroadened withprofile, the the literature WAXD dataresults [1,25 for,35 H–+41 Nafion]. For are lithiated in reasonable Nafion, agreement crystallographic with the data literature are extremely data [1,25,35 scarce–41 (only]. For Ref.lithiated [40] was Nafion, found). crystallographic data are extremely scarce (only Ref. [40] was found).

FigureFigure 5. 5.Wide-angle Wide-angle diffraction diffraction pattern pattern of of “dry” “dry” Naﬁon Nafion 115 115 membrane membrane before before (a) ( anda) and after after (b) ( lithia-b) tion.lithiation.

TableTable 1. 1.X-ray X-ray characteristics characteristics of of “dry” “dry” Naﬁon Nafion 115 115 membrane membrane before before and and after after lithiation. lithiation.

AmorphousAmorphous Galo Galo (100) SampleSample CrystallineCrystalline Degree,Degree, % % 2θ2θ,, Deg Deg FwhmFwhm 22θ,θ, DegDeg Fwhm Fwhm Nafion 115 (H+) 15.00 ± 0.10 4.29 17.03 ± 0.04 2.25 29 Nafion 115 (H+) 15.00 ± 0.10 4.29 17.03 ± 0.04 2.25 29 Nafion 115 (Li+) 15.41 ± 0.06 5.14 17.01 ± 0.03 2.27 20 Nafion 115 (Li+) 15.41 ± 0.06 5.14 17.01 ± 0.03 2.27 20 As shown in Table 1, the conversion of protonated Nafion into the lithiated form barelyAs changed shown in the Table most1, the intensive conversion crystalline of protonated reflection Nafion (100). into However, the lithiated the maximum form barely of changedthe amorphous the most halo intensive shifted crystallinenoticeably reflectiontoward larger (100). angles, However, while the the maximumdegree of crystal- of the amorphouslinity (calculated halo shifted by Equation noticeably (3)) significantly toward larger decreased angles, while as a result the degree of lithiation. of crystallinity (calculatedThe morphological by Equation (3)) features significantly of the Nafion decreased membrane, as a result which of lithiation. contains crystalline in- clusionsThe morphologicaland ionic clusters features along with of the amorphous Nafion membrane, regions, clearly which influence contains thermal crystalline tran- inclusionssitions. It is and known ionic that clusters crystalline along regions with amorphous of polymers regions, at temperatures clearly influence well below thermal their transitions.melting point It is behave known similarly that crystalline to chemical regions crosslinking of polymers [42], at while temperatures electrostatic well interac- below theirtions meltingwithin ionic point clusters behave act similarly as physical to chemical crosslinking crosslinking [1,43]. Limiting [42], while the segmental electrostatic dy- interactionsnamics of Nafion within ionicmacromolecules, clusters act asall physical these factors crosslinking are clearly [1,43 ].manifested Limiting the in segmentalits thermal dynamicsbehaviour. of As Nafion mentioned macromolecules, in the review all these of Kusoglu factors [1], are clearlyNafion manifestedmembranes in exhibit its thermal three behaviour.thermal transitions As mentioned labelled in theα, β review and γ, of which Kusoglu correspond [1], Nafion to distinct membranes relaxation exhibit mecha- three thermalnisms. However, transitions at labelled present,α, evenβ and forγ, the which protonated correspond form to of distinct Nafion, relaxation there is mechanisms.no consensus However,about their at assignment. present, even The for γ the relaxation protonated around form −1 of20 Nafion, to −90 (nearly there is independent no consensus of about water γ − − theirabsorption assignment. and cation The type)relaxation is usually around attributed120 to to short90-r (nearlyange molecular independent motions of water of the absorption and cation type) is usually attributed to short-range molecular motions of the –CF2– backbone [1,40]; nevertheless, Corti et al. [44] reasonably reassigned this transition –CF2– backbone [1,40]; nevertheless, Corti et al. [44] reasonably reassigned this transition as the true glass transition temperature of the polymer. The β relaxation between −40 and +20 ◦C and the α relaxation between 90 and 120 ◦C (both sensitive to cation type and water content) were attributed to glass transition temperatures of the polar regions (ionic clusters) and non-polar matrix of the Nafion membrane. However, there is no consensus in the assignment of α and β relaxation to the former or latter process (see, for example, Polymers 2021, 13, x FOR PEER REVIEW 9 of 15

as the true glass transition temperature of the polymer. The β relaxation between −40 and +20 °C and the α relaxation between 90 and 120 °C (both sensitive to cation type and water content) were attributed to glass transition temperatures of the polar regions (ionic clusters) and non-polar matrix of the Nafion membrane. However, there is no consensus in the assignment of α and β relaxation to the former or latter process (see, for example, Refs. [1,41,42–48]). Moreover, the transition temperatures do not always fall within the indi- cated limits; they are also influenced by residual water (see, for example, work of Almeida and Kawano [43]). The melting point of H+ Nafion determined by DSC was about 275 °C [25]. It is significant that the melting in Nafion occurs over a very broad temperature range below 275 °C [25]. Therefore, thermal transitions in H+ Nafion above 200 °C are usually associated with crystalline melting [43,46–48]. The replacement of mobile protons by alkali metal cations increases both α and β relaxation temperatures by over 100 °C, while the γ relaxation temperature is around 100 °C [1]. It should be noted that for some ionic forms of Nafion, the β relaxation occurs as a shoulder on the low-temperature side of the α peak [44]. However, very little is known about thermal transitions in Li-Nafion; differential scanning calorimetry (DSC) measurements observed relaxations at 81 °C [49] and 212 ± 18 °C [1]. In the present work, DSC measurements were carried out to analyse the thermal behaviour of lithiated Nafion compared with its protonated form. Figure 6 provides the DSC Polymers 2021, 13, 1150 9 of 14 curves for the “dry” samples of Nafion 115 in H+ and Li+ forms. The region near –100 °C at the γ transition was not analysed due to instrumental effects. As can be seen from Fig- ure 6, the DSC curve of H+ Nafion demonstrates thermal effects typical for a protonated Refs.membrane, [1,41–48 including]). Moreover, the well the-defined transition endothermi temperaturesc effect doof crystalline not always melting fall within at 274 the °C, indicatedwhich is entirely limits; they consistent are also with influenced Gierke’s by results residual [25] waterand our (see, WAXD for example, data (see work Section of Almeida3.2). and Kawano [43]). The melting point of H+ Nafion determined by DSC was about 275 ◦C[For25 Li].- ItNafion, is significant the DSC that curve the melting shows in a Nafiontransition occurs near over 77 a°C, very which broad is temperatureclose to that rangereported below by 275the ◦groupC[25 ].of Therefore, Changchun thermal Institute transitions of Applied in HChemistry+ Nafion above[49], along 200 ◦ withC are a usuallydiffused associated double effect with at crystalline 198 and melting218 °C ( [Figure43,46– 486).]. However, The replacement the available of mobile data protons did not ◦ byallow alkali us metalto identify cations the increasesdouble peak both atα 198and andβ relaxation218 °C reliably. temperatures The latter by effect’s over 100temper-C, ◦ whileature theis closeγ relaxation to the value temperature reported is in around the literature 100 C[ for1]. the It should α relaxation be noted in lithiated that for somePFSA ionic[1]. However, forms of Nafion, it seems the moreβ relaxation likely that occurs the effect as a shoulderat 218 °C on is thea weak low-temperature endothermic sidepeak ofrelated the α peakto crystallite [44]. However, melting, very while little the iseffect known at 198 about °C may thermal be attributed transitions to in α Li-Nafion;relaxation. ◦ differentialAlthough a scanning double effect calorimetry also observed (DSC) measurements by Ozerin et al. observed [46] for relaxations the protonated at 81 formC[49 of] ◦ andPFSA 212 at± 22718 andC[ 1239]. °C was attributed by the authors to crystalline melting, the reasons for itsIn splitting the present were work, not discussed DSC measurements in the cited were work. carried DSC curves out to for analyse Na, K, the Rb, thermal and Cs behaviourNafion salts of presented lithiated Nafion by Almeida compared and Kawano with its [43] protonated exhibit a form. very similar Figure 6diffused provides double the DSC curves for the “dry” samples of Nafion 115 in H+ and Li+ forms. The region near effect,◦ with the higher value attributed by the authors to crystallite melting. Considering –100the literatureC at the γ data,transition we tend was to not attribute analysed the due transition to instrumental at 218 °C effects. to crystal As canmelting. be seen A fromlikely Figure6, the DSC curve of H + Nafion demonstrates thermal effects typical for a protonated explanation for the remarkable decrease in the temperature from 274 °C (protonated form)◦ membrane,to 218 °C (Li including-Nafion) the(Figure well-defined 6) is a reduction endothermic in the effect average of crystalline size of crystallites melting at resulting 274 C, whichfrom the is entirely lithiation consistent procedure. with Gierke’s results [25] and our WAXD data (see Section 3.2).

FigureFigure 6. 6.Differential Differential scanning scanning calorimetry calorimetry (DSC) (DSC) curves curves for for the the “dry” “dry” samples samples of of Naﬁon Nafion 115 115 in in H H++ andand Li Li++ forms.forms.

For Li-Nafion, the DSC curve shows a transition near 77 ◦C, which is close to that reported by the group of Changchun Institute of Applied Chemistry [49], along with a diffused double effect at 198 and 218 ◦C (Figure6). However, the available data did not allow us to identify the double peak at 198 and 218 ◦C reliably. The latter effect’s temperature is close to the value reported in the literature for the α relaxation in lithiated PFSA [1]. However, it seems more likely that the effect at 218 ◦C is a weak endothermic peak related to crystallite melting, while the effect at 198 ◦C may be attributed to α relaxation. Although a double effect also observed by Ozerin et al. [46] for the protonated form of PFSA at 227 and 239 ◦C was attributed by the authors to crystalline melting, the reasons for its splitting were not discussed in the cited work. DSC curves for Na, K, Rb, and Cs Nafion salts presented by Almeida and Kawano [43] exhibit a very similar diffused double effect, with the higher value attributed by the authors to crystallite melting. Considering the literature data, we tend to attribute the transition at 218 ◦C to crystal melting. A likely explanation for the remarkable decrease in the temperature from 274 ◦C (protonated form) to 218 ◦C (Li-Nafion) (Figure6) is a reduction in the average size of crystallites resulting from the lithiation procedure.

3.5. Thermal Analysis and Wide-Angle X-ray Diffraction of Swollen Li-Naﬁon Samples It is known that the morphology of an ionomer re-organises as the dry membrane absorbs a solvent; these structural changes following swelling can be observed by DSC and WAXD measurements. Therefore, the inﬂuence of a solvent on the thermal behaviour Polymers 2021, 13, x FOR PEER REVIEW 10 of 15

3.5. Thermal Analysis and Wide-Angle X-Ray Diffraction of Swollen Li-Nafion Samples It is known that the morphology of an ionomer re-organises as the dry membrane absorbs a solvent; these structural changes following swelling can be observed by DSC Polymers 2021, 13, 1150 and WAXD measurements. Therefore, the influence of a solvent on the thermal behaviour10 of 14 of lithiated Nafion was studied step-by-step during a gradual increase in solvent uptake. Ethylene carbonate was used as a solvent in these measurements. A weighted portion of EC was added to a piece of membrane to obtain a sample with a specified solvent uptake; after that,of lithiated samples Nafion were stored was studied for 24 h step-by-step at the mixing during temperature a gradual of increase40 °C for in homogeni- solvent uptake. sation.Ethylene An Li-Nafion carbonate sample was usedsaturated as a solvent with EC in theseat 40 measurements.°C was prepared A weighted as described portion in of EC Sectionwas 2.3. added to a piece of membrane to obtain a sample with a specified solvent uptake; after that, samples were stored for 24 h at the mixing temperature of 40 ◦C for homogenisation. Figure 7 demonstrates DSC curves for Li-Nafion◦ samples having different EC contents. AnIt can Li-Nafion be seen samplethat the saturated introduction with of EC a atrelatively 40 C was small prepared quantity as described of EC (W in= Section19%) 2.3. Figure7 demonstrates DSC curves for Li-Nafion samples having different EC contents. led to an abrupt decrease in the glass transition temperature from 77 °C of a dry membrane It can be seen that the introduction of a relatively small quantity of EC (W = 19%) led to an to −2 °C without phase separation. With an increase in W to 32%, the glass transition tem- abrupt decrease in the glass transition temperature from 77 ◦C of a dry membrane to −2 ◦C perature shifted even more to the negative region to reach –39 °C. Here, it is likely that without phase separation. With an increase in W to 32%, the glass transition temperature saturation with EC (W = 132%) shifts the glass transition temperature of the membrane to shifted even more to the negative region to reach −39 ◦C. Here, it is likely that saturation below –80 °C (not observed in this work), while the DSC curve shows only the endother- with EC (W = 132%) shifts the glass transition temperature of the membrane to below mic effect of the first-order phase transition at 36.5 °C associated with the melting of eth- −80 ◦C (not observed in this work), while the DSC curve shows only the endothermic ylene carbonate released from the membrane upon cooling to 25 °C (Figure 7). effect of the first-order phase transition at 36.5 ◦C associated with the melting of ethylene carbonate released from the membrane upon cooling to 25 ◦C (Figure7).

Figure 7. DSC curves for Li-Nafion with different contents of EC (40 ◦C/min). Figure 7. DSC curves for Li-Nafion with different contents of EC (40 °C/min). The obtained results indicate that even pure EC, which is solid at room temperature, Therepresents obtained a results very effective indicate plasticiser that even forpure Li-Nafion. EC, which The is solid destruction at room of temperature, ionic crosslinking representsdue toa very the solvationeffective plasticiser of lithium for cations Li-Nafion. by EC The and destruction charge screening of ionic releases crosslinking segmental due tomobility the solvation of perfluorinated of lithium Nafioncations backbones.by EC and The charge structural screening rearrangements releases segmental in the swollen mobility of perfluorinated Nafion backbones. The structural rearrangements in the swol- membrane result from the decrease in Tg to very low values. len membraneThe result WAXD from results the fullydecrease support in Tg the to assumptionvery low values. of structural changes. For the study, Thewe WAXD selected results samples fully saturated support withthe assumption a mixed solvent of structural EC/SL cha withnges.ω(SL) For the = 0.2 study, at various we selectedmixing samples temperatures: saturated 25, with 40, 60,a mixed and 80 solvent◦C. (Mixtures EC/SL with with ω(SL)ω(SL) = =0.2 0.2 at werevarious used for mixingmembrane temperatures: swelling 25, 40, instead 60, and of80 pure°C. (Mixtures EC due with to being ω(SL) entirely = 0.2 were homogeneous used for mem- at 25 ◦C brane accordingswelling instead to the EC/SLof pure phaseEC due diagram to being [ 31entirely]). Figure homoge8 showsneous diffractograms at 25 °C according of swollen to the membranesEC/SL phase having diagram a diverse [31]). Figure thermal 8 shows history diffractograms in comparison of with swollen a dry membranes membrane. The havingappearance a diverse thermal of a new history shoulder in comparison on the side ofwith large a dry angles membrane. at the most The intense appearance peak (100) of can a newbe shoulder clearly seen.on the side of large angles at the most intense peak (100) can be clearly seen. Figure9 shows an example of the decomposition into components of the measured profile in the region 2θ = 7–33◦. As can be seen from the figure, the saturation with the plasticiser causes the appearance of a new broad peak (new amorphous halo) having a maximum at 2θ = 19.1–19.4◦, which was absent in the dry membrane WAXD profile (right shoulder in Figure8). The intensity of this peak noticeably increases with mixing temperature (and, accordingly, with solvent uptake; see Figure4). Simultaneously, the intensity of the amorphous halo, which is identical to that appearing in the diffraction pattern of a “dry” membrane (Figure5), decreases significantly. The WAXD technique Polymers 2021, 13, 1150 11 of 14

used in the present study (transmission mode, see Section 2.5) allows the changes in the intensity of all three components of the complicated diffraction peak in the (100) region to be directly compared. Figure 10 presents the component intensities as a function of the mixing temperature and data for the “dry” membrane for comparison. The diagram demonstrates that the saturation with the plasticiser somewhat reduces the intensity of the crystal reflection (100) compared to the solvent-free membrane; an increase in the mixing temperature in the range of 25–80 ◦C also leads to a slight decrease in its intensity. However, the influence of the plasticiser on the state of the amorphous regions is much more significant: the proportion of the halo observed in the “dry” membrane sharply decreases (Figure 10). A new, very intense halo with a maximum at 19.1–19.4◦, which Polymers 2021, 13, x FOR PEER REVIEW 11 of 15 appeared due to the plasticiser sorption and noticeably increases with mixing temperature up to 60 ◦C does not change significantly at 80 ◦C.

Polymers 2021, 13, x FOR PEER REVIEWFigureFigure 8. 8.Wide-angle Wide-angle X-ray X-ray diffraction diffraction patterns patterns for for “dry” “dry” Li-Naﬁon Li-Nafion and and membrane membrane samples samples swelled12 of 15

withswelled EC/SL with mixture EC/SL withmixtureω(SL) with = 0.2ω(SL) at different= 0.2 at different mixing temperatures mixing temperatures speciﬁed specified in the ﬁgure. in the figure.

Figure 9 shows an example of the decomposition into components of the measured profile in the region 2θ = 7–33°. As can be seen from the figure, the saturation with the plasticiser causes the appearance of a new broad peak (new amorphous halo) having a maximum at 2θ = 19.1–19.4°, which was absent in the dry membrane WAXD profile (right shoulder in Figure 8). The intensity of this peak noticeably increases with mixing temperature (and, accordingly, with solvent uptake; see Figure 4). Simultaneously, the intensity of the amorphous halo, which is identical to that appearing in the diffraction pattern of a “dry” membrane (Figure 5), decreases significantly. The WAXD technique used in the present study (transmission mode, see Section 2.5) allows the changes in the intensity of all three components of the complicated diffraction peak in the (100) region to be directly compared. Figure 10 presents the component intensities as a function of the mixing temperature and data for the “dry” membrane for comparison. The diagram demonstrates that the saturation with the plasticiser somewhat reduces the intensity of the crystal reflection (100) compared to the solvent-free membrane; an increase in the mixing temperature in the range of 25–80 °C also leads to a slight decrease in its intensity. However, the influence of the plasticiser on the state of the amorphous regions is much more significant: Figure 9. Decomposition of X X-ray-ray diffraction profile proﬁle near near (100) (100) Bregg Bregg peak peak for for Li Li-Naﬁon-Nafion saturated saturated the proportion of the halo observed in the “dry” membrane sharply decreases (Figure 10). withwith EC/SLEC/SL mixture mixture wit withh ω(SL)ω(SL) = = 0.2 0.2 at at the the mixing mixing temperature temperature of of 25 25 °C◦C.. A new, very intense halo with a maximum at 19.1–19.4°, which appeared due to the plasticiser sorption and noticeably increases with mixing temperature up to 60 °C does not change significantly at 80 °C.

Figure 10. Amorphous and crystalline peak areas for Li-Nafion samples before and after swelling with an EC/SL mixture with ω(SL) = 0.2 at different mixing temperatures (specified in the diagram).

Based on changes in Tg and diffraction profiles of swollen membranes, it can be as- sumed that the penetration of the solvent into the amorphous regions of the polymer results in a new amorphous structure that includes not only macromolecules but also low-

molecular-weight solvent. Unlike water, molecules of some polar organic solvents can in- teract with PFSA chains and penetrate amorphous regions to weaken interchain interac- tions [4,45,50]. Moreover, it is known that saturation with water does not cause any noticeable changes in the Nafion diffraction profile [46]. The excellent preservation of a membrane’s crystalline regions upon saturation with a plasticiser regardless of mixing temperature is a highly significant factor. This fact is due to two reasons. Firstly, the high melting point of the crystallites (≈218 °C) is significantly above the studied temperature range (25–80 °C). Secondly, the higher chain packing density of the crystalline structure makes it much more resistant to dissolution [45]. Here, the integrity of the membrane is ensured by crosslinking due to crystalline regions even when very high degrees of swelling are achieved (>200 at 80 °C for EC); this also explains the apparent similarity of their characteristics with those of chemically crosslinked gels.

4. Conclusions Mixed solvent EC/SL is a suitable plasticiser for Li-Nafion, since it provides high ionic conductivity (10−4 Sm cm−1 at room temperature) due to high solvent uptake. The

Polymers 2021, 13, x FOR PEER REVIEW 12 of 15

Polymers 2021, 13, 1150 Figure 9. Decomposition of X-ray diffraction profile near (100) Bregg peak for Li-Nafion saturated12 of 14 with EC/SL mixture with ω(SL) = 0.2 at the mixing temperature of 25 °C.

FigureFigure 10. Amorphous 10. Amorphous and andcrystalline crystalline peak peak areas areas for Li for-Nafion Li-Nafion samples samples before before and after and afterswelling swelling withwith an EC/SL an EC/SL mixture mixture with with ω(SL)ω (SL)= 0.2 = at 0.2 different at different mixing mixing temperatures temperatures (specified (specified in the in thedia- diagram). gram). Based on changes in Tg and diffraction profiles of swollen membranes, it can be assumedBased on that changes the penetration in Tg and diffraction of the solvent profiles into of the swollen amorphous membranes, regions it of can the be polymer as- sumedresults that in the a penetration new amorphous of the structure solvent into that the includes amorphous not only regions macromolecules of the polymer but re- also sultslow-molecular-weight in a new amorphous structure solvent. Unlikethat includes water, not molecules only macromolecules of some polar organicbut also solventslow- moleculacan interactr-weight with solvent. PFSA Unlike chains water, and penetrate molecules amorphous of some polar regions organic to weakensolvents interchaincan in- teractinteractions with PFSA [4 ,chains45,50]. and Moreover, penetrate it is amorphous known that regions saturation to weaken with water interchain does not interac- cause any tionsnoticeable [4,45,50]. changes Moreover, inthe it is Nafion known diffraction that saturation profile with [46]. water does not cause any noticeable changesThe excellent in the preservation Nafion diffraction of a membrane’s profile [46]. crystalline regions upon saturation with a plasticiserThe excellent regardless preservation of mixing of a temperaturemembrane’s iscrystalline a highly significantregions upon factor. saturation This fact with is due ◦ a plasticiserto two reasons. regardless Firstly, of mixing the high temperature melting point is a highly of the crystallitessignificant (factor.≈218 ThisC) is fact significantly is due ◦ to twoabove reasons. the studied Firstly, temperature the high melting range (25–80 point ofC). the Secondly, crystallites the higher(≈218 °C) chain is significantly packing density aboveof the crystallinestudied temperature structure range makes (2 it5– much80 °C). more Secondly, resistant the tohigher dissolution chain packing [45]. Here, den- the sityintegrity of the crystalline of the membrane structure is makes ensured it much by crosslinking more resistant due toto crystallinedissolution regions [45]. Here, even the when ◦ integrveryity highof the degrees membrane of swelling is ensured are by achieved crosslinking (>200 due at80 to crystallineC for EC); regions this also even explains when the veryapparent high degrees similarity of swelling of their are characteristics achieved (>200 with at those 80 ° ofC for chemically EC); this crosslinked also explains gels. the apparent similarity of their characteristics with those of chemically crosslinked gels. 4. Conclusions 4. ConclusionsMixed solvent EC/SL is a suitable plasticiser for Li-Nafion, since it provides high ionic conductivity (10−4 Sm cm−1 at room temperature) due to high solvent uptake. The Mixed solvent EC/SL is a suitable plasticiser for Li-Nafion, since it provides high degree of swelling of the membrane depends on the composition of the binary plasticiser ionic conductivity (10−4 Sm cm−1 at room temperature) due to high solvent uptake. The and the mixing temperature; moreover, hysteresis occurs between the heating and cooling mode curves. The sorption of even small amounts of a plasticiser shifts the glass transition temperature of the ionomer from 77 ◦C to negative temperatures; at saturation with EC/SL, ◦ it decreases to below −80 C. While the introduction of the plasticiser leads to the critical reorganisation of the amorphous structure of the ionomer, it does not significantly influence the crystalline regions, which remain nearly unchanged in the studied mixing temperature range of 25–80 ◦C. Due to their high melting point (about 218 ◦C), crystallites in Li-Nafion act as chemical crosslinking elements, maintaining the integrity of the membrane even at a very high solvent uptake (>200%) and making its characteristics similar to those of a chemically crosslinked gel. The obtained results provide a more in-depth insight into the physicochemical nature of the ionomer. Thus, Li-Nafion plasticised with EC/SL may be promising for use as a polymer electrolyte in developing a new generation of LIBs with enhanced safety.

Author Contributions: Conceptualisation, E.A.S., Y.A.D. and O.V.B.; methodology, O.V.B.; software, T.V.Y. and O.G.R.; validation, T.V.Y., O.G.R. and L.V.S.; investigation, A.S.I., T.V.Y., O.G.R., L.V.S. and R.R.K.; data curation, L.V.S.; writing—original draft preparation, A.S.I. and O.G.R.; writing—review and editing, O.V.B.; visualisation, A.S.I., T.V.Y., O.G.R.; supervision, Y.A.D.; project administration, O.V.B.; funding acquisition, O.V.B. All authors have read and agreed to the published version of the manuscript. Polymers 2021, 13, 1150 13 of 14

Funding: This research was funded by RUSSIAN SCIENCE FOUNDATION, grant number 18-19- 00014. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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