Recent progress in NMR spectroscopy of polymer electrolytes for lithium batteries

Current Opinion in Colloid & Interface Science 18 (2013) 228–244

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Recent progress in NMR spectroscopy of polymer electrolytes for lithium batteries Sabina Abbrent a, Steve Greenbaum b,⁎ a b

University of South Bohemia, Czech Republic Hunter College of the City University of New York, NY, NY, USA

a r t i c l e

i n f o

Article history: Received 27 March 2013 Accepted 28 March 2013 Available online 6 April 2013 Keywords: Polymer electrolytes Lithium batteries NMR

a b s t r a c t Recent progress on the use of nuclear magnetic resonance (NMR) spectroscopy to investigate structure and dynamics of polymer electrolytes for advanced lithium batteries is reviewed in this article. The survey includes a list of both standard and relatively novel techniques, with many examples of their applications drawn from the literature. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Lithium-metal polymer (LMP) batteries are considered one of the most promising candidates to exploit the full energy of the Li +/Li electrochemical couple for electric vehicle and other applications. The ultimate goal in the development of a polymer electrolyte for LMP batteries is to allow high performance operation with a high specific energy density. More recently the prospect of dispensing almost entirely with the bulky cathode (up to 70% of the total battery mass) leading to a corresponding increase in gravimetric energy density has led to the concept and pursuit of the lithium-air battery, which also requires a robust solid ionic conductor [1]. To reach these targets, it is very important to take into account the ionic transport, electrochemical and interfacial properties of the polymer electrolyte. To date, polyethylene oxide (PEO)-based polymer electrolytes have been regarded as one of the most suitable electrolytes for lithium batteries. They are also attractive for lithium ion batteries because of their dual functions as electrolyte and separator. Despite over 30 years of worldwide research on polymer electrolytes (PEs) since their initial discovery [2,3], the requirement of sufficiently high cationic conductivity for lithium battery applications, still remains somewhat elusive. Furthermore, there is still considerable scientific controversy about the very nature of the ion transport mechanism and the precise factors governing cation–anion interactions in polymer–salt complexes based on PEO, the most widely studied host polymer. Other efforts have focused on the introduction of nanosized ceramic particles into the polymer matrix. Many reports have indicated improvements in the ionic conductivity, due to the enhanced segmental motion of amorphous regions, and interfacial stability of ⁎ Corresponding author. Tel.: +1 212 772 4973; fax: +1 212 772 5390. E-mail address: [email protected] (S. Greenbaum). 1359-0294/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cocis.2013.03.008

PEO-based polymer electrolytes by the incorporation of nanosized ceramic fillers [4]. Furthermore, when nanoscopic inorganic particles are added to form composites, a whole host of new technical and fundamental scientific issues not limited to mechanical/rheological properties come into play. Among these are the details concerning particle surface/polymer and surface/ion interactions. In the interim, while PEO complexes still garner much scientific attention, alternative strategies for preparing flexible polymeric ion conductors have been conceived, executed, and even developed commercially. Principal among these are gel electrolytes in which a lithium salt liquid carbonate solution is immobilized in a polymeric matrix, typically PVdF [5]. More recently, the exponentially growing list of ionic liquids (IL) and their applications has led to their incorporation into gel electrolytes, offering enhanced electrochemical and thermal stability [6]. From the “early days” (late 1970s) of PEs, nuclear magnetic resonance (NMR) spectroscopy has been successfully and productively applied to investigate ion polymer coordination and dynamics [7]. The relevant nuclei include 7Li and 19F, for probing the environments and mobility of the ions, and 1H and 13C for the polymer. Recently, with renewed interest in sodium batteries, 23Na is returning to the scene as well. One of the hallmarks of any ionic conductor is the observation of motional line narrowing of the nuclear resonance corresponding to the mobile ion (i.e. 7Li) and this diagnostic commonly reported during the initial studies of PEs remains in use today [7]. In fact, this approach was the first to establish unambiguously that ionic conduction is strongly coupled to polymer segmental motion occurring above the glass transition temperature (Tg) in the amorphous phase of the PE [8]. As discussed previously, this finding has driven the PE community to focus on the development of amorphous materials with low Tg. This remains a popular approach, though as mentioned, there is much recent emphasis on new combinations of materials, including fillers, solvents, and ILs, and even ordered crystalline phases as discussed later.

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NMR is also a powerful method to probe ion and polymer dynamical process over a span of some ten orders of magnitude (in time or frequency) from spin-lattice relaxation to pulsed field gradient diffusion. After NMR excitation, the return to thermal equilibrium magnetization along the applied static field direction is characterized by spin-lattice or longitudinal relaxation time T1. It is the process of energy transfer from the excited nucleus to the surroundings or the lattice, usually meaning the neighboring molecules, that returns the nuclear spin system to thermal equilibrium. Since the first comprehensive treatise of relaxation published over sixty years ago [9], measurements of T1 as a function of temperature have provided a useful molecular level probe of dynamics because the most effective relaxation mechanisms, whether mediated by nuclear dipole–dipole, quadrupole, or electron spins (in samples containing paramagnetic centers), must have significant spectral density components close to the NMR frequency. Thus T1 is very sensitive to motional processes with a time-scale of ~10 −10 s. Regarding motional processes more directly related to long-range ionic transport, pulsed field gradient NMR has been used to measure molecular and ionic self-diffusion coefficients since the mid-1960s and has become a relatively common tool to characterize PEs [7]. The main purpose of this review is to highlight recent examples in the literature of the use of NMR to characterize both structure and dynamics in both the solvent-free and gel types of polymer electrolytes, with most of these examples having appeared in another review published by the present authors [7]. 2. NMR spectroscopy used for structural characterization in polymer electrolyte design In polymer electrolyte field of research 1D 1H high resolution NMR is often used in combination with other characterization and material analysis methods. Structural characterization of compounds used in the preparation of heterogeneous polymer electrolytes gives insight into the properties of the “finished product”. In the study by Baskaran et al. [10] the plasticized polymer electrolytes composed of the PVAc:DMF:LiClO4 were studied. The effect of the plasticizer N,N-dimethylformamide (DMF) concentration on the conductivity was discussed on the basis of conductance and impedance spectroscopy analysis at various temperatures. 1H NMR measurements have been carried out to confirm the existence of DMF in the polymer electrolyte matrix. Structural characteristics of these electrolytes have been studied by XRD and SEM analyses. Glass transition temperature (Tg) has been obtained from the DSC analysis and thermal stability of these gel electrolytes has been discussed using TG/DTA. In another work [11] a group of star-hyper-branched polymer with PS arms and hyper-branched polyglycidol blocks at the ends of arms (sPS-b-HPG) was synthesized and the suitability of sPS-b-HPG as polymer electrolyte was studied. sPS-b-HPG was proposed to combine the mechanical stability of PS with the ionic conductivity of polyglycidol. The ionic conductivity of sPS-b-HPG/LiClO4 electrolyte was measured. Structure verification was conducted by 1H NMR (300 MHz) and 13C NMR (75 MHz) with tetramethylsilane (TMS) used as internal standard. High molecular weight polyelectrolytes, amorphous at ambient temperature and with a highly delocalized negative charge to minimize ion pairing were considered by Meziane et al. [12]. Ion pairing is known to severely diminish ion transport. Thus, a monomer anion, potassium 4-styrenesulfonyl(trifluoromethylsulfonyl)imide (STFSIK CH2 = CHC6H4SO2N(K)SO2CF3) was synthesized. The corresponding polymer, PSTFSI (a), was subsequently obtained by free radical polymerization. Characterization was done using NMR (13C, 1H, 19F), FTIR and thermal analysis (DSC and TGA). Formation of STFSIK monomer was proven by 19F NMR with a singlet at 21.715 ppm, to be compared with the signal of the starting compound trifluoromethyl sulfonamide CF3SO2NH2, which is at 20.030 ppm. 13C NMR was also helpful, with appearance of a quadruplet at 126.36–122.06–117.77–113.47 ppm, corresponding to the CF3 group, and displacement of the other carbon

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chemical shifts was compared to sodium styrene sulfonate carbon chemical shifts. NMR spectroscopy was employed in a fundamental way to determine the size and oxidation state of oligomers used for the preparation of polymer electrolytes [13]. The aim of the investigation by del Valle et al. was, by controlling polyaniline (PANI) and poly(o-anisidine) electro-synthesis, to obtain fragments of known size characterized by NMR and UV–vis techniques. 1H NMR samples were prepared in deuterated DMSO and the obtained spectra showed characteristic signals of aniline dimers. Integral analysis revealed that the ratio between signals in the aromatic region and protons attached to the nitrogen atom was about 5:1, indicating that most of the obtained molecules were trimers. The integral area of the aromatic region was not an exact value which further suggested the presence of small traces of oligomers. The approach adapted by Liao et al. [14] in order to increase the alleged poor electrochemical and mechanical performance of the GPE (gel polymer electrolytes) with monopolymer as matrix was to use copolymers from monomers with different functions. A new copolymer was developed, namely poly(methyl methacrylate–acrylonitrile–vinyl acetate) (P(MMA–AN–VAc)), to obtain a GPE with good comprehensive performance by employing the individual advantages of MMA, AN and vinyl acetate (VAc). AN provides the copolymer with good processability, electrochemical and thermal stability. MMA provides good electrolyte uptake and reduce the brittleness of the copolymer due to the use of AN. VAc further provides strong adhesion to the anode or cathode materials and excellent mechanical stability. Compared with P(MMA– AN)-based GPE, P(MMA–AN–VAc)-based GPE has higher ionic conductivity, better mechanical strength, electrochemical and thermal stability. Structure verification of the new polymer was also done using 1H NMR technique. Similar approach was used in the study by Vargun et al. [15], where PAN, PEMA homopolymers, and P(AN-EMA) copolymers from 92/8 to 84/16 M ratios were synthesized by emulsion polymerization technique. This technique enabled them to obtain high molecular weight polymers with high polymerization rates in an environmentally friendly medium. The structural and thermal analyses were performed for all homopolymers and copolymers. The 1H NMR spectra of the purified copolymers were recorded in d6-DMSO. The chemical structure and the copolymer composition were thus confirmed (Fig. 1). The chemical structures of PAN–PEMA copolymers were also clarified with 13C NMR (Fig. 2) and the assignment of the observed peaks of PAN and PEMA carbons is illustrated in the same figure. The characteristic \C_N group carbons corresponding to AN and the \C_O carbons relating to EMA were observed at 120 (b) and 174 (a) ppm, respectively. The other carbons concerned with the \CH3(g), \CH2(e), \CH(f), and \OCH2(c) groups can be identified clearly at around 22, 33, 28, and 61 ppm, respectively. This approach has also been applied to electrolytes using solvent-free polymers. Nishihara et al. [16] report the synthesis of a new class of organoborate polymers, poly(lithium tetraarylpentaborate) via LiOMe-promoted condensation of bifunctional arylboronic acids tethered by oligo(ethylene oxide). Here the anionic charges are immobilized on the polymer framework through covalent bonding thus freeing the lithium cations and creating so called single ion conductive polymers. The structure of the polymers was confirmed by 1H and 11 B NMR spectra. Similarly, in their work Çelik et al. [17], synthesized poly(4vinylbenzeneboronic acid), PVBBA via free-radical polymerization of 4-vinylbenzeneboronic acid (4-VBBA) and continued by modification with polyethylene glycol monomethylether (PEGME) with different molecular weights to produce boron containing comb-branched copolymers. The attachment of PEGs to the polymer was investigated by FT-IR, 1H NMR and 1B NMR. In the work of Aydın et al. [18] PVA was modified with PEGME using BH3/THF with different molecular weights of PVA and PEGME to

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Fig. 1. 1H NMR spectra of P(AN-co-EMA) copolymers from 92/8 to 84/16 mol%.

produce boron containing comb-branched polymers. The attachment of PEGs to the polymer was investigated by FT-IR, 1H NMR and 11B NMR. 1 H NMR spectrum of the synthesized boron containing comb-branched polymer is shown in Fig. 3a. The chemical shift at 1–2 ppm [2] was attributed to the hydrogens of the CH2 groups of PVA backbone. The methoxy groups of PEGME were observed near 3 ppm (d). The peaks between 3.20 and 3.70 ppm (c, 1) belong to ethylene oxide protons of PEG and backbone of PVA chains. The ethylene oxide protons which are linked to the boronate center appeared at 4.20 and 4.60 ppm (b, a). Fig. 3b shows the 11B NMR spectrum of PVA2PEGME550 ester copolymer. In a previous work, it was reported that four coordinated boron sites (ring C–BO3) usually appear close to −15 ppm and three coordinated boron sites (ring C–BO2) appear at higher chemical shifts. The shift differs due to major structural features, like the coordination of the boron sites. 11BNMR spectrum demonstrated that the resonance covering the chemical shift at −15 ppm is four coordinated and from −5 to 5 ppm is due to three coordinated boron site. The results demonstrated that the grafting occurs predominantly via three coordination. In order to enhance the targeted properties Zhang et al. [19] synthesized a class of low viscosity compounds from disiloxane starting materials, with each compound containing only one or two oligo(ethylene glycol) chains. NMR has also been employed productively to characterize gel polymer electrolytes (GPEs) containing ionic liquids (ILs). A novel

Fig. 2.

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C NMR spectra of P(AN-co-EMA) copolymers from 92/8 to 84/16 mol%.

kind of GPE based on comb-like copolymers of poly(ethylene glycol) monomethylether (mPEG) grafted carboxylated butadieneacrylonitrile rubber (XNBR) was prepared by introducing ionic liquids and LiClO4 into polymer framework. FTIR spectra confirmed the grafting of mPEG to XNBR as side chains, and the content of grafted mPEG was calculated from the integral area of related peaks in 1H NMR spectra [20]. The influence of the nature of the CnH2n + 1 alkyl side group on the synthesis and the electrochemical properties, e.g., ionic conductivity and electrochemical stability, of PYR1ATFSI ionic liquids was investigated [21]. For this purpose, PYR1ATFSI ionic liquids containing alkyl groups of different lengths (from 1 to 10 carbons) and structures (linear or ramified) were synthesized. The PYR1ATFSI ionic liquids were characterized in terms of NMR and DSC measurements, ionic conductivity, viscosity and electrochemical stability. Polymer–ionic liquid nano-composite polymer electrolytes have been prepared by incorporating IL in PVdF-HFP with different average molecular weights and carbon nanotube concentrations [22]. Room temperature (20 °C) 1H, 13C and 19F NMR spectra were obtained. The main result was the synthesis of a new IL, 1-methyl-3-hexylimidazolium imide and its structure was confirmed by the 1H NMR analysis.

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Fig. 4. 250 MHz 1H NMR spectra of poly(diallyldimethylammonium) chloride and poly(diallyldimethylammonium)TFSI.

Fig. 3. (a) 1H NMR spectra of PVA2PEGME550 ester copolymer; (b) 11B NMR spectra of PVA2PEGME550 ester copolymer.

Pont et al. report the synthesis and characterization of new pyrrolidinium-based polymeric ionic liquids and its application as mechanically and electrochemically stable polymer electrolytes for lithium batteries [23]. The optimization of the thermal and electrochemical properties of transparent and highly conductive polymeric films based on pyrrolidinium polymer, single molecule ionic liquid and lithium salt blends is presented. The synthesized polymers were characterized by NMR and FTIR spectroscopy. Fig. 4 presents the 1H NMR spectra of poly(diallyldimethyl ammonium) chloride and poly(diallyldimethyl ammonium)TFSI in deuterated water and acetone, respectively. In their work Chen and Fang [24], synthesized hyperbranched polymer with terminal IL groups (HBP-IL) in which PF6− ion can migrate, whereas imidazolium cation is immobilized. The chemical structure, thermal behavior, and ionic conductive property of HBP-IL were investigated by 1H NMR, FTIR, differential scanning calorimetry (DSC), thermogravimetric analyzer (TG), and complex impedance analysis, respectively. The influence of some selected ILs (ionic liquids) on the formation of model complexes showing coordination of lithium ions by bidentate N-donor ligands was investigated [25]. The ILs were 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) imide ([emim][NTf2]), 1-ethyl-3-methylimidazolium ethylsulfate ([emim] [EtSO4]), and 1-ethyl-3-methylimidazolium perchlorate ([emim][ClO4]).

The latter was used for the very first time for studies in coordination chemistry. Depending on their solubility in the applied IL, the bidentate N-donor ligands 2,20-bipyridine (bipy) and 1,10-phenanthroline(phen) were used for the complex-formation reactions of Li+. To determine the coordination number in solution, the chemical shift of the 7Li NMR signal (abundance: 92.6%) was studied as a function of the added ligand concentration in reference to an external standard. All operations were performed under nitrogen atmosphere. In a typical series of measurements, a solution of bipy or phen (1.0Min the IL) was mixed in different volume ratios with a 0.05 M solution of the appropriate lithium salt. The 7Li NMR spectra were recorded at a frequency of 155 MHz and ambient temperature. The number of bidentate ligand molecules coordinated to a Li + ion in solution was determined by applying 7Li NMR measurements. In a typical experiment the concentration of the appropriate lithium salt was kept constant, while the concentration of the bidentate ligand was varied over a wide range. The resulting chemical shift of the 7Li signal was then plotted against the mole ratio of [ligand]:[Li +]. When such a plot shows a remarkable discontinuity in the chemical shift, the appropriate [ligand]:[Li +] ratio can be taken as the coordination number relative to the bidentate ligand. Although such experiments cannot reveal unequivocal information to which extent the remaining coordination site is indeed occupied by an anion of the employed IL, one can at least estimate whether the first coordination sphere provides enough space for the coordination of a solvent anion. On the basis that a maximum of three phen or bipy molecules can occupy six coordination sites found previously, the number of remaining vacant coordination sites can be estimated. As illustrated by the NMR spectra in Fig. 5, the successive addition of bipy leads to a significant downfield shift of the 7Li signal up to a molar ratio of about [bipy]:[Li +] = 4:1, suggesting a strong interaction between Li + and bipy. At higher concentration levels of bipy, the 7Li signal is only slightly further shifted and the first coordination sphere of the Li + center is almost saturated. In Fig. 6 the chemical shift was plotted as a function of the molar ratio [bipy]:[Li +], and the straight lines (blue color) clearly indicate a break point in the chemical shift at a molar ratio of [bipy]: [Li +] = 2:1. This suggests that under these conditions two bipy molecules occupy the first coordination sphere of the Li + ion. As a result of the curvature observed in the data, the measured chemical shift at this ratio is not identical to the value expected from the crossing point of the straight lines. The design of new lithium salts for polymer electrolytes can also benefit from NMR characterization methods. Structure verification for a new class of dilithium (DL) salts prepared by a newly developed

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signals near 8–9 ppm. Further analysis was necessary here, and was conducted with IR and GPC. The main idea of the study by Niedzicki et al. [28] was to design salt structure that would not have disadvantages of big bulky anions causing high viscosity when dissolved in organic solvents, therefore a decrease in conductivity. Also, ions of these new salts should not form agglomerates after dissolution, due to ion pairs' and triplet's negative effects on conductivity of an electrolyte, mechanism of lithium cation insertion into the electrodes (in both charging and discharging processes) and transference number of a lithium cation. Following imidazole derivatives have been synthesized and characterized by 13C and 19F NMR: lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (working name LiTDI), lithium 4,5-dicyano-2-(pentafluoroethyl)imidazole (LiPDI) and lithium 4,5-dicyano-2-(n-heptafluoropropyl)imidazole (LiHDI). 3. High resolution NMR characterization in model compounds

Fig. 5. 7Li NMR spectra recorded as a function of the molar ratio [bipy]:[Li+] in [emim] [NTf2] at 25 °C; Li[NTf2] was used as a source of Li+ ions.

synthetic strategy from relatively inexpensive starting materials was conducted by 1H NMR measurements [26]. The interesting feature of this class of salts is their striking similarity to LiTFSI, in the aspect that both are imides and the negative charge on the nitrogen atom is delocalized by two sulfone groups, making lithium ion highly mobile. This salt was further compared with other already existing lithium salts using other methods. The aim of the work of Chetri et al. [27] was to synthesize and study the conductivity of some salts of poly(2vinylpyridine) (P-2VP) with acids of iodine. This system unlike most of the well-known polymer electrolytes based on PEO is complexed with alkali metal salts and needs no external salt to impart ionic character. Proton NMR data indicated that the two signals observed at 1.75 and 2.6 ppm are due to \CH2\ and \CH\ protons, respectively. The signals ranging from 6.7 to 7.4 are due to the protons of pyridine molecule of P-2VP. The addition of HI, HIO3, or HIO4 molecules was confirmed by the appearance of the

Fig. 6. Chemical shift of the 7Li signal as a function of the molar ratio [N, N-ligand]:[Li+] at 25 °C.

In order to better understand the correlations and interactions between individual species in a polymer electrolyte, a liquid model system can sometimes be used, where the advantage of high resolution characteristic of the liquid state can be exploited. Such is the case when using NMR spectroscopy, low molecular weight analogs of PEO (glyme, diglyme, triglyme) systems were used for the study of ionic pair formation constants [29]. Results from 7Li, 11B and 19F NMR measurements are compared and the role of the nuclei studied is discussed. The study shows that even in the solvents of very similar coordination (in terms of the donor and acceptor number values) and similar dielectric properties, the ionic pair formation constant depends on the solvent used. Its role is discussed in terms of effects related to ion agglomerate formation, non-covalent interactions between ions and liquid matrix as well as the number of interacting centers present in the molecules of the solvent. The discrepancies between the results obtained using various nuclei NMR as well as discrepancies between ion-pair formation constant calculated for different experimental regimes are also analyzed. 4. Solid state NMR characterization The spectrum of a given sample and its variation on changing structure, temperature or composition can be followed. PVdF-HFP based gel electrolytes, swollen in a weight ratio 30:70 by a 0.41 mol kg −1 solution of LiTFSI in PYRA12O1TFSI were investigated [30]. Two types of silica fillers were dispersed in the gels, a commercial nano-size SiO2 (HiSilTM T700) and a hierarchical one (SBA-15), in order to discuss the role of the filler microstructure on the conductivity and on the electrochemical properties of the polymer electrolytes. Each membrane was thoroughly characterized from the physico-chemical and electrochemical points of view. 13 C { 1H} CP-MAS NMR spectra were obtained at room temperature at 100.63 MHz for the 13C nucleus. Fig. 7 shows the 13C { 1H} CP-MAS NMR spectra of the electrolyte membrane, and of the electrolyte membrane with 5 wt.% of SBA-15. The two peaks at ∼ 43.0 and ∼ 120 ppm represent the resonances of the \CH2\ and \CF2\ groups, respectively. The peaks of the HFP carbons are expected to fall at ∼ 92 (\CFH–), ∼ 123 (\CF2\) and ∼ 127 ppm (\CF3). The two high-frequency features are masked by the very large peak due to the CF2 groups of the vinylidene chains, whereas the resonance at ∼ 92 ppm is only barely observable above the baseline. Despite of the large quantity of ionic liquid absorbed in the membrane, the narrow feature characteristics of the liquid state are not observed in both the spectra, which confirm that the samples are gel electrolytes, where the polymer strands are swollen by the liquid phase. By comparing the two spectra of Fig. 7, we can conclude that the addition of the filler does not cause relevant changes to the NMR

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Fig. 8.

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C CPMAS NMR spectra of PED-2 blend polymer membrane.

Fig. 7. 13C{1H} CP-MAS NMR spectra of the polymer electrolyte (blue) and the polymer electrolyte with 5 wt.% of SBA-15 (red).

signature, so suggesting that the interactions among the filler and the polymer gel electrolyte are weak. With the aim of developing a highly conductive polymer electrolyte, a triblock copolymer poly(propylene glycol)-block-poly(ethylene glycol)block-poly(propylene glycol) bis(2-aminopropyl ether) (PPG-PEG-PPG diamine, H2N-PPG-PEGPPG-NH2, denoted as ED2000) is blended with PVdF-HFP copolymer to synthesize the blended polymer electrolytes [31]. The presence of soft segment in ED2000 (ester carbonyls or ether oxygens) makes it more flexible to trap sufficient amount of electrolyte. A series of blend polymer electrolytes has been synthesized by varying the weight ratio of PVdF-HFP and ED2000. The structural and electrochemical properties of the electrolytes thus obtained were systematically investigated by a variety of techniques including differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), 13C and 29Si solid-state NMR, AC impedance, linear sweep voltammetry (LSV) and charge–discharge measurements. The FTIR and NMR results provided the information about the interaction among the constituents in the blend polymer membrane. 29Si and 13C magic angle spinning (MAS) NMR spectra were acquired at a spinning speed of 5 kHz. Solid-state 13C CPMAS NMR experiment was performed to obtain the backbone structure of the blend polymer membrane. Fig. 8 shows the 13C CPMAS NMR spectrum of the blend polymer membrane acquired at a short contact time of 1.0 ms. With the help of the spinning technique individual peaks of the polymer electrolyte are partly resolved. The condensation degree of the silica network architecture inside the materials can be directly characterized by using 29Si MAS NMR. As shown in Fig. 9, different silicated species are resolved and can be assigned. According to the authors, a dominant signal at −70 ppm, corresponds to T 3 (RSi(OSi)3, where R refers to an alkyl group and a small peak around −61 ppm corresponds to T 2 (RSi(OSi)2(OH)) sites. The observation of T groups indicated the presence of organosilane in the material. This organosilane (i.e., MPEOP) was stable under synthesis conditions since the Si–C cleavage was not observed due to the formation of Q (Si(OSi)4) groups. A series of P(VdF-HFP)–(PC + DEC)–LiClO4–SAG (silica aerogel) composite gel polymer electrolytes was prepared [32]. 7Li solid-state NMR spectroscopy has been carried out to investigate the polymer– salt–filler interactions in the CGPEs. Solid-state 7Li magic angle spinning (MAS) NMR spectra were recorded with a spinning speed of 2 kHz. 7Li

MAS NMR spectra of CGPEs with varying plasticizer–filler and salt–filler ratios are depicted in Figs. 10 and 11, respectively. The observed results can be explained by considering the chemical shift values, as they are the most sensitive structural changes in the sample being investigated. Due to the presence of different coordination sites in the copolymer, plasticizer and SAG, various complexes can be formed by the interaction of these coordination sites with the Li+ ions. It is revealed that with increase in the amount of SAG, appearance of shoulder peaks and linewidth broadening occurred. From this analysis, more than one resonance caused by the presence of various local environments of Li + ions becomes clearly observed. The interaction with SAG particles causes these resonance peaks to shift and varies in intensity. The results indicate that the lithium ion interacts with the silica aerogel particles as well as the plasticizer and P(VdF-HFP). Such interactions can induce structural modification of the polymer chain and provide a favorable conduction path for faster migration of Li-ions on the surface of the loaded SAG particles, leading to higher ionic conductivity.

Fig. 9. 29Si MAS NMR spectra of PED-2 membrane. The dashed lines represent the components used for the spectral deconvolution.

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In another study, poly(AN-co-GMA-IDA) was prepared by copolymerizing acrylonitrile (AN) and 2-methylacrylic acid 3-(biscarboxymethylamino-2-hydroxy-propyl ester) (GMA-IDA) [34]. The polymers were mixed with the plasticizer ethylene carbonate (EC) and lithium perchlorate (LiClO4) to form gel polymer electrolytes (GPE). Solid-state 7Li NMR spectrometry was used to elucidate the interaction between the plasticizer EC and GMA-IDA. 7Li MAS NMR spectra with high power 1H decoupling were recorded. Fig. 13 shows the deconvolution analysis of the resulting spectra where three different coordination sites can be distinguished. This result was also confirmed by FT-IR results.

5. Dynamics: linewidth

Fig. 10. Least square peak fits of 7Li NMR spectra of P(VdFHFP)–(PC + DEC)–LiClO4–SAG CGPE with varying plasticizer–filler ratios (wt.%): (a) 20:70:10:0, (b) 20:69:10:1, (c) 20:66:10:4 and (d) 20:64:10:6. Spinning speed 2 kHz.

It is instructive to observe the effect of MAS on spectra, and a typical example of static and spun sample (see Fig. 12) was reported by Marinin et al. [33]. The study involves gel polymer electrolyte material based on polyester diacrylate (PEDA) and lithium perchlorate in EC added in different amounts, and the MAS technique was used as the primary tool to confirm chemical compositions and structures of the resulting electrolytes.

Fig. 11. Least square peak fits of 7Li NMR spectra of P(VdFHFP)–(PC + DEC)–LiClO4–SAG CGPE with varying salt–filler ratios (wt.%): (a) 20:70:10:0, (b) 20:66:13:1, (c) 20:66:10:4 and (d) 20:66:8:6. Spinning speed 2 kHz.

Since the early days of NMR, linewidth measurements, in particular, the full width at half-maximum (FWHM), have been used to describe the extent of interaction and relative motion in materials [35]. In the case of polymer electrolytes the FWHM usually decreases with decreasing interaction of ion with the polymer host. Motional line narrowing with increasing temperature indicates an averaging of both 1H–7Li and 7Li–7Li hetero- and homonuclear dipole–dipole interactions caused by an increase in ion hopping motion and segmental motion [36]. 1 H and 7Li NMR were used [37] to detect the surroundings of the electrolyte and the status of the lithium ions dissociated in the electrolyte. The structure of electrolyte EMC (ethyl methyl carbonate) was shown to be better than electrolytes DMC (dimethyl carbonate) and DEC (diethyl carbonate) at dissociating the lithium carbonate into pure ions (Fig. 14), and it should in addition make an excellent environment for ionic migration and diffusion. Room temperature 7Li NMR studies were carried out [38] to compare the behavior of SPE (solid polymer electrolytes) and NCSPE (silica-based SPE). It was observed that addition of nanoparticles to the SPEs leads to a sharp decrease in linewidth which can be attributed to the decrease of lithium ion interaction with the polymer and hence increase in lithium ion mobility resulting in enhancement of ionic conductivity of the samples. The linewidth of various SPEs and NCSPEs given in Fig. 15 shows the NMR spectra of (PEG)20 LiBr: y(SiO2). Two types of gels containing the same IL (i) polymer-in-IL gel electrolytes containing 2,3-dimethyl-1-hexyl imidazolium bis(trifluoromethanesulfonyl)Imide (DMHxImTFSI) and polyethylene oxide (PEO) and (ii) polymer gel electrolytes containing DMHxImTFSI, PEO and PC have been prepared [39]. As the IL used (DMHxImTFSI) contains NMR sensitive nuclei (1H and 19F) in the cations (DMHxIm+) and anions (TFSI–) of the IL respectively, so 1H and 19F NMR spectra have been recorded to study the nature of mobile species. The 1H and 19F NMR spectra of the gel in the 80–350 K temperature range are given in Fig. 16. Both show line narrowing with an increase in temperature. Linewidth (FWHM, full width at half maximum) has been calculated from Fig. 16 for the 7H and 19F NMR peaks at different temperatures and the variation of 1H and 19F NMR linewidths as a function of temperature has been plotted in Fig. 17. Line narrowing observed in 1H and 19F NMR with increasing temperature suggests that both cations and anions are mobile in these gel electrolytes. PEO has been found to act mainly as a stiffener in gels without PC whereas in polymer gels it plays an active role and also results in a small increase in ionic conductivity. Similar study was conducted by Sharma et al. [40], where the effect of the addition of propylene carbonate (PC) to the nanocomposite polymer electrolytes containing polyethylene oxide (PEO), hexafluorophosphoric acid (HPF6) and fumed silica was studied. The linewidth (full width at half maximum, FWHM) has been determined at different temperatures from 1H NMR data. Two line narrowing areas were found, where the first takes place at the glass transition temperature of the electrolytes,

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Fig. 12. The 1H MAS NMR spectrum (horizontal scale in ppm) of the polymer electrolyte containing 54.8% of EC by mass. The sample spinning rate is 5 kHz. Inset: 1H NMR spectrum recorded with no sample spinning.

whereas the second corresponds to the melting temperature in these PEO-based polymer electrolytes. Linewidth analysis was also conducted by Kaur et al. [41] where polymer gel electrolytes were studied containing different concentrations of PMMA, a trisubstituted imidazolium ionic liquid with imide anion (DMHxImTFSI) and propylene carbonate (PC). The temperature at which 1H and 19F NMR line narrowing takes place in different electrolytes based on the type of IL used was correlated with the conductivity results for these electrolytes. It was concluded that it is possible to design polymer gel electrolytes containing suitable ionic liquid, solvent, and polymer so that it possesses higher value of ionic conductivity above a certain temperature, which can also be controlled depending upon the applications, for which such gel electrolytes are to be used. NMR studies conducted by Kumar et al. [42] confirm lithium ion contribution to the overall ionic conduction in polymer electrolyte PEO-LiTf system with added IL 1-ethyl 3-methyl imidazolium trifluoromethanesulfonate (EMICF3SO3 or EMITf). A line narrowing in the spectra of (PEO)25 LiTf film (without ionic liquid) is distinctly observed as the temperature increases from room temperature to 100 °C. The effect of addition of ionic liquid on the Li mobility can be ascertained by comparison of spectra without and with ionic liquid. A substantial line decreasing of the linewidth can be observed

due to ~ 30 wt.% addition of ionic liquid in PEO-complex, even at room temperature. This indicates not only the increase in Li mobility in the PEO-complex due to ionic liquid addition, but the plasticization effect of ionic liquid EMITf is also observed with the enhancement in the flexibility of the polymer backbone. Such improvement in flexibility has also been observed in terms of glass transition temperature (Tg) lowering found by DSC measurements. Echelmeyer et al. [43] presented a first study on a ternary composite in which an ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4), together with a lithium salt, lithium tetrafluoromethane sulfonate (LiTf), is confined within a SiO2 glass. The network structure of the silica network and the dynamic state of the various anions and cations confined within the silica network were analyzed by solid-state NMR techniques. The mobility of the Li cations and triflate and tetrafluoroborate anions were found to increase in synchrony with the [BMIM]BF4/LiTf ratio. For the sample with the highest [BMIM]BF4/LiTf ratio, an almost liquidlike dynamic behavior was observed already at temperatures below ambient, entailing a very high ionic conductivity of 0.5 × 10−2 S cm−1 at 298 K. High Li mobility down to subambient temperatures constitutes a prerequisite for a good Li ion conductor for use in battery applications

Fig. 13. 7Li MAS NMR spectra of AG3/EC 50:50 wt.% at 213 K.

Fig. 14. Comparison of FWHM between different kinds of linear carbonate electrolytes.

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Fig. 15. NMR spectra of (PEG)20 LiBr: y(SiO2) showing the variation in linewidth with addition of nanoparticles.

as confirmed by temperature-dependent 19F MAS, 1H, and 7Li NMR on one of the samples. The corresponding spectra (cf. Fig. 18) show mobility of the cations ([BMIM] +, Li +) and anions (triflate, BF4−) down to a temperature of 240 K. Below this temperature, the distinct line broadening in the 1H and 7Li NMR spectra and the appearance of considerable spinning sideband intensity in the 19F MAS NMR spectra indicate a freezing of the motion of the various ionic species present in the system. The line narrowing can be further used to calculate the correlation time, activation energy and finally conductivity of the measured species, as was the case in the study by Borgohain et al. [44]. The influence of added nanoparticles of hydrotalcite [Mg0.67Al0.33(OH)2] [(CO3)0.17·mH2O] as the filler in the SPE PEG:LiClO4 to form the NCPE was investigated by several experimental techniques. To find out whether an increase in mobility of the charge carrier has taken place with increasing doping level, 7Li NMR experiments were carried out on the polymer electrolyte and the NCPE with doping level of 3.6 wt.%. From the obtained half-widths of the obtained spectra, the correlation time can be calculated.

Fig. 16. 1 1H (a) and 19F (b) NMR spectra at different temperatures for ion gel, DMHx-ImTFSI + 2 wt.% PEO. Temperature range is 80–350 K. 2 1H (a) and 19F (b) NMR spectra at different temperatures for polymer gel electrolytes (PC + 0.5 M DMHxImTFSI + 0.5 wt.% PEO). Temperature range is 80–350 K.

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Fig. 17. 1 Variation of 1H ( ), 19F (▼) NMR linewidth (FWHM) and ionic conductivity (□) with temperature for ion gel DMHxImTFSI + 2 wt.% PEO. 2 Variation of 1H ( ), 19F (▼) NMR linewidth (FWHM) and ionic conductivity (□) with temperature for polymer gel electrolytes PC + 0.5 M DMHxImTFSI + 0.5 wt.% PEO.

The ionic conductivity σNMR is calculated using the Nernst–Einstein equation, 2 2

σ NMR ¼

Nq d 6τ c kT

where N is the number density of Li+ ions, d is the average ionic jump distance, and q is the ionic charge. Considering an average Li–Li distance to be nearly 4 Å, from the calculated N value of ∼3.3 × 10 26 m −3 we can calculate the ionic conductivity. There is a marginal decrease in the activation energy for the doped SPE and therefore a slight increase in the mobility of Li+. Ionic mobility was also investigated by similar approach for electrolyte materials that were irradiated or quenched in order to decrease the crystalline region [45]. The 1H NMR spectra for low salt composition (x > 150, where x is the PEG/Li ratio) and for pure PEG consist of two components, a strong broad signal and a relatively narrow and weaker signal on top of it. For the higher salt concentration (x b 150), the broad component merges with the baseline. It is generally known that the broad component is

due to the crystalline regions and the narrow component corresponds to the amorphous regions. A Gaussian curve fits to the broad signal and a Lorentzian to the narrow signal. Fig. 19 shows the variation of 7H NMR Lorentzian component linewidth as a function of salt concentration. As the salt concentration increases, the linewidth decreases sharply in a similar way to the decrease in Tg as observed in DSC measurements and reaches a minimum at around x = 46 and then increases. Further increase in linewidth with salt concentration though x decreases could be due to the decrease in polymer chain flexibility as indicated by an increase in Tg at higher salt concentration. 1H NMR signals for few selected compositions of (PEG)x:NH4NO3 have been recorded as a function of temperature in the range 200–330 K. From the linewidth data, correlation times, τcs, have been calculated. The observed σ(NMR) was much higher than that measured from the complex impedance plot. Such discrepancies have been observed earlier and could be caused by the sensitivity of NMR to local dynamics in contrast with the conductivity measurement, which measures only the long-range transport. Another reason for this difference could be that while conductivity measurement responds to the motion of only

Fig. 18. 19F MAS (a), static 1H (b), and static 7Li spin echo NMR spectra (c) for ionogel-glass ig_3 for the indicated temperatures. νMAS (10 kHz; π/2-pulse lengths) 4.25 μs (19F), 10 μs (1H), and 3.5 μs (7Li); repetition time: 5–20 s (19F), 30–60 s (1H), and 15 s (7Li). Exponential line broadening of 10 Hz (19F), 1 Hz (1H), and 5 Hz (7Li) was used to smooth the data.

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Fig. 19. Room temperature 1H NMR linewidths vs salt concentration for (PEG)x: NH4NO3 system.

charged (i.e. dissociated) species, NMR can sense the motion of undissociated molecules as well. Such differences in the conductivities determined by the two techniques have been discussed earlier [46].

6. Dynamics: relaxation NMR relaxation times and lineshape measurements provide information on motions modulating nuclear spin interactions and sample dynamics over multiple timescales on the Hz to MHz frequency scale. The possibility of measuring spin-lattice relaxation times (T1) for several nuclei and also for individual peaks in a spectrum will give information about the different species in a sample individually, i.e. the bulk polymer, plasticizer, lithium ions, the anions or other species separately. The temperature dependence of T1 values along with the presence of minima in T1 as a function of temperature enables correlation times and activation energies to be obtained for the motion of each species, such as the segmental motion of the polymer backbone or the hopping motion of lithium cations. A typical example including short theory description is given in the paper by Roach et al. [36] and the references therein. In their work 1H high resolution NMR was initially used to confirm the structure of the polymer and determine the number-average molecular weights for the prepared samples consisting of ionomers made from polyethylene glycol (PEG) with Mn = 600 and a varying ratio of dimethyl 5-sulfo-isophthalate lithium salt to neutral dimethyl isophthalate producing a range (6%, 11%, 17%, 100%) of ion contents. These materials are amorphous liquids with Tg far below room temperature, which facilitates thermal equilibration of the system. 1H and 7Li correlation times along with activation energies provide a detailed perspective as to whether lithium ion hopping motion is correlated to the segmental motion of the polymer backbone, which is important for understanding the fundamental cation transport mechanism(s) within these ionomer systems. 1 H and 7Li NMR spin-lattice relaxation measurements were obtained by a standard inversion-recovery sequence and were aptly described by single exponential fits. The position of this relaxation rate maximum indicates the temperature at which the motional correlation time is comparable to the reciprocal of the Larmor frequency of the nuclei of interest. The relative mobilities of nuclei can be evaluated by comparing the temperature position of this maximum for samples with different compositions such that a higher temperature maximum implies that the nuclei are less mobile [9].

7 Li spin-lattice relaxation processes are governed primarily by two main mechanisms: (I) quadrupolar relaxation and (II) dipolar relaxation. Quadrupolar relaxation is mediated by interactions of the nuclear electric quadrupole moment of the 7Li nuclei with the fluctuating electric field gradient produced by the local charge distribution at the site of the nucleus, while the dipolar relaxation is caused by fluctuations of lithium homo- and hetero-nuclear dipole–dipole interactions. 1 H spin-lattice relaxation is mainly governed by dipolar relaxation due to random fluctuations in homo- and hetero-nuclear dipole–dipole interactions. Solid state 7Li MAS NMR was used to show how a chosen kind of lithium salt affects the mobility of the poly(3-(2-cyanoethoxymethyl)3-ethyloxetane) P(CYAMEO) matrix with the kind of lithium salt used [47]. Figs. 20 and 21 show the solid state 7Li NMR spectra of the P(CYAMEO)10(LiX), X = N(CF3SO2)2, CF3SO2, BF4 or ClO4, at various temperatures (from − 30 °C to 50 °C). The results suggest that the lithium ions in the matrix exist in two different chemical environments of P(CYAMEO) electrolyte with LiBF4 at higher temperature (>20 °C). Fig. 22 shows the temperature dependence of T1 ( 7L). An approximately common slope on the (presumably) low temperature side of the T1 minima was observed for all the P(CYAMEO)-based electrolyte films and suggest that a diffusion mechanism of the Li ions in the electrolyte systems is similar. Analysis of the relaxation data based on Bloembergen–Purcell–Pound (BPP) model [9] was performed for the electrolytes [48] resulting in estimated K, τ0, and Ea. K depends on the particular spin interaction responsible for the relaxation. τ0 is the average dwell or “rattling” time for a lithium ion in a potential well before hopping to a next site. Ea is the activation energy. The K values for all electrolyte films are 1.75–2.54 × 10 9 s −2. This suggests that spin interaction responsible for the relaxation is very similar to each electrolyte film. The results also suggest that the mobility of

Fig. 20. Solid state 7LiNMRspectra of P(CYAMEO)10(LiN(CF3SO2)2) electrolyte films under various temperature conditions.

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Fig. 21. Solid state 7Li NMR spectra of P(CYAMEO)10(LiBF4) electrolyte films under various temperature conditions.

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finding to an increase in the segmental mobility of the polymer chains, due to an increase amount of the amorphous polymer phase. As the mobility of the chain segments within amorphous phase is higher than that in the crystalline phase, additional experimental evidence was thus provided for the enhancement of ionic conductivity associated with the increased amorphous phase of the polymer electrolyte on the addition of filler. At higher filler concentrations, physical constrictions restrict chain mobility resulting in a decrease in T2 and overall mobility of the system. Poly(ethylene glycol)-monomethacrylate (PEM) mixed with self-assembled copolymer stearylmethacrylate (SMA), known to align spontaneously in polar media, was proposed and synthesized by a simple photo-induced radical polymerization technique [50]. High-resolution solid-state 13C NMR spectra were obtained by the combined use of cross-polarization (CP) and magic angle spinning (MAS) with 1H high-power dipolar decoupling (CPMAS). The 1H spin-lattice relaxation-time in the rotating frame (T1ρ) was indirectly measured from well-resolved 13C signals enhanced by CP of 500 μs applied after the 1H π pulse and spin-locking of 1H nucleus. The T1ρ decay curves for the copolymer, P(PEM/SMA) and for each homopolymer for comparison were plotted as the logarithm of the relative intensity, M/M0, obtained from the peak area originated from EO and ethylene units in 13C NMR spectrum, against the relaxation-time, τ, for each polymer system. As the relaxation-time of the EO unit and the ethylene unit in the copolymer becomes close to each other, the so-called spin diffusion phenomena should occur in the copolymer system. That is, the NMR results strongly suggest that each polymer chain of PEM/SMA will be several nanometers apart. Thus, the present P(PEM/SMA) system would not form a phase-separation structure in macro-scale, but each polymer chain is partly intertwined in nanometer scale. 7. Dynamics: diffusion

lithium ions in the P(CYAMEO) matrix is affected with the kind of lithium salts. Larger τ0 value for LiBF4 indicates slower mobility of lithium ions in the matrix, while the τ0 value of P(CYAMEO) 10(LiN(CF3SO2)2) being the smallest suggests higher mobility than in the other P(CYAMEO)-based electrolytes. The influence of added weight percentage of alumina filler Al2O3 on (PEO)9Mg(ClO4)2 polymer electrolyte system was investigated by Dissanayake and Ekanayake [49]. 1H NMR measurements of spin–spin relaxation times T2 were performed on a series of electrolytes, where the probing nuclei were protons ( 1H) bound to the polymer chain. The 1H NMR spin–spin relaxation time T2 was shown to increase up to 15 wt.% of Al2O3 and then decrease again. The authors attributed this

Fig. 22. 7Li nuclear spin-lattice relaxation times (T1) for P(CYAMEO)-based electrolyte films, P(CYAMEO)10(LiX), plotted as a function of inverse temperature, open circle: X = ClO4, open square: X = BF4 (higher magnetic field peak), closed square: X = BF4(lower magnetic field), open triangle: X = CF3SO3, and closed diamond: X = N(CF3SO2)2.

As an alternative or complementary technique to impedance spectroscopy conductivity measurements an NMR technique can be applied using pulsed field gradient (PFG). Contrary to classic conductivity results, the NMR technique enables different species to be measured separately, thus for a typical PE system, the mobility of cation, anion and polymer chain or plasticizer can be estimated. Furthermore, diffusion measurements for individual peaks in a spectrum can be conducted separately. Background theory and thorough description of PFG NMR measurements and their application on liquids and particularly ionic liquids are described by Söderman et al. [51], while polymer electrolytes are the main topic of a concise work by Walderhaug et al. [52]. The pulsed-field-gradient spin-echo NMR measurements were performed on 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide ([emim][FSI]) and 1-ethyl-3-methylimidazolium [bis[(trifluoromethyl) sulfonyl]imide] ([emim][TFSI]) over a wide temperature range from 233 to 400 K and compared to molecular dynamics (MD) simulations performed on [emim][FSI], [emim][TFSI], [N-methyl-Npropylpyrrolidinium][FSI] ([pyr13][FSI]), and [pyr13][TFSI] utilizing a many-body polarizable force field. An excellent agreement between the ion self-diffusion coefficients from MD simulations and pfg-NMR experiments has been observed for [emim][FSI] and [emim][TFSI] ILs [53]. The influence of lithium salt and its concentration on the mechanism of ionic conductivity and mobility of the segmental motion was investigated by Bandyopadhyay et al. [54] on the system of polyester polyol and isocyanate-based polyurethanes doped with lithium trifluoromethane sulfon imide (LiTFSI) and lithium perchlorate (LiClO4). Charge carrier densities estimated from the Nernst–Einstein relation, using measured NMR diffusivity values and ionic conductivities were found to range from approximately 8% to 29% of total Li densities for LiTFSI, indicating that a significant fraction of Li is involved in room-temperature ionic conduction in this material. For

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LiClO4 the carrier fraction is smaller, implying that Li + is more tightly bound to its anion site. The cause for the decrease in capacity of solid-state lithium batteries after cycling was investigated by Kaneko et al. [55]. Due to a relatively low ionic conductivity (approx. 10−7 to 10−5 S cm−1) at room temperature for the PEO-based SPEs tris(methoxy poly(-ethylene glycol)) borate ester (B-PEG) and tris(methoxy poly(ethylene glycol)) aluminate ester (Al-PEG) were introduced as new plasticizers, and the ionic conductivity was shown to successfully increase. It was presumed that the Lewis acidity of group 3 elements would attract the lithium salt anions to the plasticizer molecules, enhancing the dissociation of lithium salts and the transport numbers of Li ions, thus improving the ionic conductivity. Diffusion coefficients of the Li and F nuclei, DLi and DF were obtained by pulsed field gradient stimulated-echo sequence (PGStE)-NMR spectroscopy while the overall electrolyte conductivity was measured by AC impedance both before and after charge–discharge cycles using SPE samples. The ionic conductivity showed an obvious drop (about 56% with respect to initial conductivity). On the other hand, both of the diffusion coefficients, DLi and DF, and the transport number of lithium, tLi, remained constant before and after cycling. These unchanged values suggested that the ionic conduction mechanism inside the SPE bulk was maintained before and after cycling. Therefore, the authors argue that the increase in bulk resistance stemmed from a decrease of Li salt concentration. As a possible cause they mention Li salt being consumed by a side-reaction that occurs at the SPE cathode interface. Accordingly, it was concluded that the selection of Li salt would be one of the crucial factors in designing long-lifetime LPBs. Extensive investigation of suitable salts for battery application was conducted [56] based on the concept of “Hückel Anions”, i.e. the delocalization of 6 “π” electrons on an aromatic 5-membered ring. A wealth of compounds with ring nitrogen and/or CN in the periphery have been modeled and show very weak Li +–anion interactions, especially with increased CN substitution. The simple representative, 4,5-dicyano-1,2,3-triazole (DCTA) was shown to possess favorable conductivities in PEO electrolytes. To further increase the resistance to oxidation and lessen the ion-pair formation, replacement of central N by C–CF3 or C2F5 to 4,5-dicyano-2-trifluoromethyl-imidazole and 4,5-dicyano-2-pentafuoroethyl-imidazole (TDI and PDI respectively) has, also in PEO electrolytes, showed excellent conductivities and favorable phase diagram. In this latest work published by this group these salts were compared with the non-fluorinated LiDCTA and LiPF6 with several experimental techniques. Diffusion coefficients of 1H, 19F and 7Li nuclei were obtained by pfg-NMR and compared to conductivities measured by AC impedance spectroscopy. Transport numbers of cations were calculated according to equation T+ = Dcation/(Dcation + Danion), that assumes the validity of the Nernst–Einstein equation. Translational mobility of the low-molecular-weight solvent in the gel polymer electrolytes was studied by 1H PFG NMR spectroscopy [33]. Three different phases with the molecular self-diffusion coefficients differing by an order of magnitude were revealed. They correspond to the EC molecules bound to the polymer (phase I), EC molecules solvating lithium ions (phase II), and cyclic dimer of 2-hydroxyethyl acrylate molecules (phase III). The dependences of the self-diffusion coefficients on the EC content and on the diffusion time (in the range from 20 to 600 ms) were obtained and the exchange time of the EC molecules between phases I and II was estimated. PFG-NMR was used by Joost et al. [57] to measure the self-diffusion coefficients D of 1H, 1Li, and 19F for ionic liquid consisting of N-methylN-butylpyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI) and the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt used together with poly(ethylene oxide) (PEO). The ternary polymer electrolyte membranes were prepared by a solvent-free technique in which

the components are mixed together and hot pressed. The highest diffusion coefficients were recorded for the Pyr14 + cation, and the TFSI − anion exhibits slightly slower diffusion. Nevertheless, on increasing temperatures, the increase of Pyr14 + diffusion coefficient was found steeper than that of TFSI − anions. Li + diffusion coefficient, which is always one order of magnitude lower than those of the other ions, progressively increases with increasing IL molar fractions. The differences between results from NMR diffusion and conductivity measurements were also discussed. Comparison between two kinds of polysiloxanes with lithium triflate, LiSO3CF3 (LiTf),as the source of ions, one almost liquid H3C(R2SiO)nCH3 with the average value of n around 35 (named PSO) and the other cross-linkable H3C-([R(CH3)SiO]n x-[R0(CH3)SiO]x)-CH3 with R = \CH2CH2CH2O(CH2CH2O)4CH3 and R0 = \CH2CH2CH2Si(OCH3)3 (PSO-X) was conducted concerning the relative contributions of cation and anion transports to the effective total conductivity, the extent of correlation between cation and anion transports as well as between ion transport and segmental motion of the polymers and their corresponding temperature dependences [58]. Differences between shortrange motion estimated from T1 relaxation data and long-range motion derived from pgf-NMR technique for 19F, 1H, and 7Li nuclei were established. The results suggest that the long range mobility of the polymer chains, which is reduced by cross-linking, has a direct effect on the local dynamics, significantly reducing the local mobility of cations, anions and side chains. Besides the direct influence of the cross-links, the added salt has a major influence on the local dynamics of the polymer segments and the cations and anions as well. An interesting dependence on the amount of salt is also evident, where for PSO there is mainly not only a decrease of the relaxation rates with an increase of the salt content, but only a moderate shift of the relaxation maxima, the 7Li relaxation maximum of PSO-X shows a significant high-temperature shift upon increasing the salt content from 5 to 20 wt.% An analogous shift is observed for the 19F relaxation rates, whereas the trend is somewhat less pronounced for the 1H nuclei. This implies a stronger effect of salt on the local dynamics, if the polymer is crosslinked. The diffusion coefficients of fluorine and lithium are very similar, with fluorine diffusion always being somewhat faster than lithium diffusion. The protons are the slowest component in either system, as expected for long polymeric chains. Analogous trends can be observed for the corresponding activation energies of diffusion and rationalized in the same manner. In the cross-linked system, all diffusion coefficients are strongly reduced as compared to the non-cross-linked system. With increasing salt content there is a small increase of the diffusion coefficients measured for all the nuclear species, i.e. 1H, 7Li and 19F in the non-cross-linked systems. The authors attribute this observation to a plasticizing effect — the dissolved salt component leads to an increased separation of the polymer chains from each other and therefore to a larger free volume and an enhanced mobility. In the cross-linked system, the chain diffusion is hardly affected by the salt content. Plasticization does not seem to play a role here, due to the polymeric network structure formed by crosslinking. Room temperature ionic liquids (ILs) based on 1-ethyl-3-methylimidazolium (EMIm, EMI) combined with two amide-type anions, bis(trifluoromethanesulfonyl) amide [N(SO2CF3)2, TFSA, TFSI, NTf2, or Tf2N] and bis(fluorosulfonyl)amide [N(SO2F)2, FSA, or FSI] were investigated by 1H, 19F, and 7Li NMR spectroscopy. The spin-lattice relaxation times (T1) were measured by the corresponding correlation times and compared to the bulk viscosity and diffusion coefficients [59]. Not many T1 data sets for ILs are available. Since the NMR frequency is usually 50–500 MHz, the correlation times derived from the T1 minimum are in order of 10−8–10−11 s and their experimental activation energies are about 10–25 kJ/mol in the temperature range of 253–353 K.

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Obtained values were analyzed by Stokes–Einstein, Nernst–Einstein, and Stokes–Einstein–Debye equations for the neat and binary ILs to clarify the physico-chemical properties and mobility of individual ions. Gelation is a simple method that allows a good compromise between the retention of the IL and its fluidity inside the polymeric network [60]. This strategy is said to be quite different from the traditional solid polymer electrolytes that result either from doping of a given polymer matrix with an IL or by the introduction of polymerizable groups on IL structures. The resulting ion gels are simpler than solid polymer electrolytes and have been shown to exhibit improved conductivities. In this work these ion gels are based on 1-butyl-3-methyl imidazolium dicyanamide (BMIMDCA). The dicyanamide (DCA) compounds are liquid at room temperature and characterized by their low viscosity, water miscibility and high thermo (over 373 K) and electrochemical stability (over 3.5 V). Classical Vogel–Tammann–Fulcher (VTF) behavior was shown to be consistent for both results from NMR diffusion and conductivity measurements. Salt-in-polymer electrolytes based on graft copolymers with oligoether side chains and added LiCF3SO3 (LiTf) are investigated concerning the transport and dynamics of the ionic species with respect to applications as Li + ion conductors [61]. Polymer architectures are based on polysiloxane or polyphosphazene backbones with one or two side chains per monomer, respectively. NMR methods provide information about molecular dynamics on different length scales: The mechanisms governing local dynamics and long range mass transport are studied on the basis of temperature dependent spin-lattice relaxation rates and pulse field gradient diffusion measurements for 7Li, 19F and 1H, respectively. The correlation times characterizing local ion dynamics reflect the complexation of the cations by the oligoether side chains of the polymer. 7Li and 19F diffusion coefficients and their activation energies are rather similar, suggesting the formation of ion pairs and clusters with similar activation barriers for cation and/or anion long-range transport. Activation energies of local reorientations are generally significantly smaller than activation energies of long range diffusion. Polyvinyl butyral (PVB)-type polymers were synthesized by acetalization of the hydroxyl (OH) groups of polyvinyl alcohol (PVA) with n-butyl aldehyde and, consequently, it contains OH end-groups. By controlling the degree of acetalization the relative content of the OH and BA groups can be systematically changed. The OH group is a strong Lewis base and attracts lithium, affecting the cation mobility. Research on the dependence of OH concentration on dynamic properties and its influence on the interactions of the ionic species in the PVB gel electrolytes doped with 1 M LiTFSI/EC/DMC solutions was conducted by Y. Saito et al. [62] as a fundamental study of gel electrolytes. Diffusion coefficients, DLi, DF, and DH, for samples with different OH end-cap concentrations were measured at 25 °C and the differences or similarities between the obtained values were attributed to free ions or ion-pairs respectively. The authors conclude that when designing polymer gel electrolytes, increases of the dissociation degree of the salt and cation mobility would be the dominant evaluation index for the practical performance of lithium batteries. Salt dissociation is defined by the degree of solvation of the liquid polar-solvent species and, in some cases, the attraction of the polar site of the polymer on the lithium ion. However, from the aspect of cation mobility enhancement, it is desirable to reduce the cation size by removing the solvating species, although an appropriate interactive effect of the polar sites of the polymer could promote the salt dissociation without a change in the cation size. Polymeric gel electrolyte network was formed from polyesterdiacrylate (PEDA) based on oligohydroxyethylacrylate with the molecular mass of 2500–3000 with the addition of ethylene carbonate (EC) [63]. In addition to the high ionic conductivity 10–3 S/

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cm, these materials exhibit good mechanical properties and were thermally stable in the temperature range from 20 to 80 °С. At room temperature, the 7Li NMR spectra for polymer samples with different contents (0–54.8 wt.%) of EC represent a broad asymmetric line, associated with the superposition of two lines with different chemical shifts and widths. The “narrow” line traditionally corresponds to the mobile state of solvent-coordinated 7Li, whereas the “wide” component can be assigned to lithium ions coordinated by the polymer matrix. Correlation time of motions modulating the process of magnetic relaxation of 7Li nucleus was calculated from longitudinal (spin-lattice) relaxation. Temperature dependence of the relaxation has a characteristic minimum in the range of temperatures corresponding to the following condition: ωτс ≈ 0.62 (ω in the NMR frequency, τс is the correlation time). In the physical sense, this condition means the time of an elementary translational jump for a distance comparable with the size of a solvated cation. Based on the results it is argued that for small solvent contents, the lithium ions move together with their solvate sheaths, whereas for high EC concentrations, the correlation time is also determined by the quick exchange with solvent molecules between the first and the next solvate sheaths of lithium cations. Lithium ion transfer on macroscale distances then occurs as the result of small-scale translational jumps over the distances compared with the sizes of solvated cationic complexes. It should be expected that the translational motion of lithium cations is regulated by molecular motions of the solvate environment. At least two components could be distinguished for 1H diffusion measurements which were assigned to protons belonging to PEDA and polymer electrolyte molecules.

8. Oriented and ordered systems Despite over 30 years of research and development into non-crystalline solvent-free polyether based solid electrolytes, there remains a conductivity plateau of about 10 −5 S/cm (at ambient temperature), still over an order of magnitude too low for applications. Studies by Bruce et al. [64,65] showed that crystalline polymer electrolytes, previously thought to be insulators, exhibit marked ionic conductivity, even exceeding that of their amorphous counterparts, Gao et al. [66] demonstrated that helical jump motions (both single step and multiple steps) of polymer segments occur at ambient temperatures in well-developed complex crystals. Their work was conducted through studies on polyethylene oxide (PEO)/LiClO4 complexes using state-of-the-art solid-state 13C NMR spectroscopy. Because of the strong coordination between the polymer segments and the Li+ ions, the authors believe that the jump motions induce the transportation of the coordinated Li+ ions along the crystallographic c axis, providing a novel mechanism of ionic conductivity of the complex crystals. In addition, this work also shows that solid-state high-resolution 13C NMR spectroscopy can be a powerful and general tool for elucidating phase structures, dynamics, and subsequently the conduction mechanism of crystalline polymer electrolytes. Fig. 23 shows the 13C CP/MAS spectra of neat PEO, PEO6:LiClO4, and PEO3:LiClO4 at room temperature. Because of the short contact time (100 μs) employed, the crystalline signals in these spectra are greatly emphasized and selectively observed. For neat PEO, the spectrum shows a very broad crystalline peak. The loss of resolution of the crystalline signal is due to the inefficient heteronuclear dipolar decoupling caused by the high mobility of the crystalline polymer chains at room temperature. Up to ten well-resolved peaks ranging from d = 70.2 to 64.5 ppm for PEO6:LiClO4 and five resolved peaks ranging from δ = 71.2 to 68.8 ppm for PEO3:LiClO4 are observed. These multiple signals are all due to CH2 carbon atoms of the PEO segments confined in the crystalline structures, whereas the differences

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Fig. 23. 13C CP/MAS spectra of (a) neat PEO, (b) PEO6:LiClO4, and (c) PEO3:LiClO4. These spectra were acquired at room temperature, and the contact time was 100 ms. The schematic pictures of two complex crystal structures are shown in the right column.

in the spectral features of the two complex samples can be attributed to the different crystal structures. Prior work by Golodnitsky and coworkers and our group has shown that crystalline-phase conductivity can be significant if the helical chain structures through which the Li + ions can hop are oriented, whether that orientation is achieved through mechanical stretching [67] or the presence of a strong and inhomogeneous magnetic field during solvent casting [68].

9. In situ NMR imaging This newly developed NMR imaging technique [69,70] allows an intuitive image of chemical and morphological relationships, and reveals the degradation mechanism of lithium polymer batteries. So far, two areas remain unclear regarding the cyclic durability of LPBs: [1] the factors that trigger anion decomposition and [2] appropriate methods for suppressing the decomposition of Li salt in LPBs. These two questions were attempted through investigation of LPBs by electrochemical measurements, in situ 19F NMR imaging, and real-time monitoring of the thickness of LPBs before and after degradation. Fig. 24 shows the schematic figure of the cell inside the NMR equipment and the definition of the x-, y-, and z-axes for the SPE, where xz- and xy-sections represent vertical and horizontal cross sections with respect to the cathode and Li metal sheets, respectively. The corresponding 19F NMR images for SPEs are shown in Fig. 25. The xz-section reveals the segregation of fluorine components in the vicinity of the cathode sheets (Fig. 25(b1)). This segregation implies the electrolysis of anion components at the LiFePO4 | SPE interface, since fluorine components were included only in the TFSI anions. For a fresh SPE, the xy-section in the vicinity of the cathode sheets had a uniformed distribution of fluorine components (Fig. 25(a2)). On the other hand, fluorine components (maybe the by-products of electrolysis) were unevenly distributed after degradation. This indicates that the electrolysis of anion components did not occur uniformly at the interface of the cathode sheet and SPE film, but rather proceeded at specific points on the cathode sheet.

10. Concluding remarks Since the early days of polymer electrolytes dating back to the late 1970s, NMR has made contributions to understanding ion and polymer mobility processes through basic measurements such as linewidth and relaxation. With the development of more sophisticated methods, including multidimensional NMR and in situ imaging, our understanding of the relationship between ionic conduction and polymer electrolyte structure has evolved, even as the definition of “polymer electrolyte” has broadened from solvent-free polyether salt complexes to heterogeneous mixtures of polymer ionic liquids and similarly intricate ionically conducting media. Herein lies a

Fig. 24. Schematic diagram of cell for in situ NMR imaging, and definition of x-, y- and z-axes of inserted LPB.

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Fig. 25. Fluorine distribution in SPE measured by 19F NMR (a1, a2) before and (b1, b2) after electrochemical cycling. Blue and red (darker and brighter) areas correspond to low and high concentrations, respectively.

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