Structural effect of glyme – Li + salt solvate ionic liquids on the conformation of poly ( ethylene oxide ) †

The conformation of 36 kDa polyethylene oxide (PEO) dissolved in three glyme–Li solvate ionic liquids (SILs) has been investigated by small angle neutron scattering (SANS) and rheology as a function of concentration and compared to a previously studied SIL. The solvent quality of a SIL for PEO can be tuned by changing the glyme length and anion type. Thermogravimetric analysis (TGA) reveals that PEO is dissolved in the SILs through Li–PEO coordinate bonds. All SILs (lithium triglyme bis(trifluoromethanesulfonyl)imide ([Li(G3)]TFSI), lithium tetraglyme bis(pentafluoroethanesulfonyl)imide ([Li(G4)]BETI), lithium tetraglyme perchlorate ([Li(G4)]ClO4) and the recently published [Li(G4)]TFSI) are found to be moderately good solvents for PEO but solvent quality decreases in the order [Li(G4)]TFSI B [Li(G4)]BETI 4 [Li(G4)]ClO4 4 [Li(G3)]TFSI due to decreased availability of Li + for PEO coordination. For the same glyme length, the solvent qualities of SILs with TFSI and BETI anions ([Li(G4)]TFSI and [Li(G4)]BETI) are very similar because they weakly coordinate with Li, which facilitates Li–PEO interactions. [Li(G4)]ClO4 presents a poorer solvent environment for PEO than [Li(G4)]BETI because ClO4 binds more strongly to Li and thereby hinders interactions with PEO. [Li(G3)]TFSI is the poorest PEO solvent of these SILs because G3 binds more strongly to Li than G4. Rheological and radius of gyration (Rg) data as a function of PEO concentration show that the PEO overlap concentrations, c* and c**, are similar in the three SILs.


Introduction
Ionic liquids (ILs) are salts with melting points below 100 1C.[3][4][5][6][7][8][9][10] Solvate ionic liquids (SILs) form when addition of a ligand molecule to a salt results in the formation of complex cations (or anions) and thereby reduces the melting point to less than 100 1C.2][13][14][15][16][17][18] Li + based SILs are attractive electrolytes for secondary lithium ion batteries due to their high lithium content. 19 Li + -glyme SILs, the glyme molecules bind to the Li + ion to produce large complex cations.SILs are defined as ''good'' or ''poor'' depending on the relative coordination strengths of the glyme and anion with Li + .In a good SIL, strong and long lived [Li(glyme)] + cations are formed, resulting in negligible free glyme.Conversely, in a poor SIL, Li + -anion interactions are stronger than Li + -glyme interactions, and there can be up to 90% uncoordinated free glyme in the SIL. 14Good SILs have properties similar to a conventional IL, whereas the properties of poor SILs are like those of concentrated salt solutions. 202][23] In general, good SILs are more likely to form when the ionic association strength in the salt is relatively weak.The length of glyme has a more complex effect, however, it appears that the SILs with longer glyme are more stable, as they are more slowly exchangeable. 21he structure and interactions of equimolar mixtures of glyme and Li + salt (bis(trifluoromethylsulfonyl)imide (TFSI), nitrate or trifluoroacetate salts) have recently been examined using molecular dynamic simulations. 24It was found that glyme-Li + interactions were dominant in the mixture of glyme (G3 or G4)-LiTFSI (good solvate ILs) but were much less in the other two salts.A subsequent experimental study of the bulk structure of a good SIL ([Li(G4)]TFSI) and poor SIL (lithium tetraglyme nitrate ([Li(G4)]NO 3 )) has been completed by our group using neutron diffraction and empirical potential structure refinement (EPSR) simulated fits. 18In [Li(G4)]TFSI, the coordination number between Li + -G4 is 2-3 times higher than that for Li + -anion, whereas the coordination number is lower in [Li(G4)]NO 3 .This produces Li + rich and depleted regions.
Conventional electrolytes in secondary Li + batteries are limited by the use of volatile and flammable organic solvents for lithium salts. 25Polymer electrolytes (PEs) have been explored as replacements for organic solvents due to their negligible volatility and high thermal stability. 26,27Poly(ethylene oxide) (PEO) is one of the most popular PEs as it is stable to Li + metal electrodes.9][30] However, the conductivities of PEs is only about 10% of typical conventional electrolytes because PEs are usually gels or solids.Recently, researchers have added conventional ILs to PEs in order to improve ionic conductivity and transport properties with some promising results. 29,31Kido et al. recently studied the ionic conductivity of SIL-polymer mixtures, showing greatly enhanced ionic conductivity (by an order of magnitude) compared to conventional PEs. 32The very high Li + content of SILs suggests that their addition to PEs could improve charge storage and overall battery performance in addition to ionic conductivity.Therefore, understanding the mechanism of PEO dissolution in SILs, and how solvent quality varies with changes in the glyme length and anion type, is of both applied and fundamental importance.
The polymer radius of gyration (R g ), which is the average root-mean-squared distance of any point in the polymer coil from its centre of mass, is determined by the solvent quality which is categorised as poor, theta (y) or good. 33In a good solvent, polymer chains are expanded (swollen) because interactions between the solvent and the polymer are energetically favourable.In a poor solvent, polymer chains are collapsed as polymerpolymer interactions are preferred over polymer-solvent interactions.In a theta solvent, the excluded volume expansion is cancelled and the chains adopt their random flight conformations (ideal chains).R g is related to the Flory exponent (n) by R g = K(M w ) n , where K is a constant based on the polymer type and M w is the polymer molecular weight.Generally, for a good, theta and poor solvent, n = 0.6, 0.5 and 0.33, respectively.
In moderately good (i.e. between a theta and good solvent) or good solvents, polymer solutions can generally be divided into three regimes: dilute, semi-dilute and concentrated regimes. 33n the dilute regime, the polymer coils are fully swollen by solvent, and R g is almost independent of concentration.In the semi-dilute regime, there is overlap of neighbouring polymer coils, so R g decreases with increasing polymer concentration, and the intra-chain excluded volume effect is reduced.In the concentrated regime, R g is once again relatively constant as a function of polymer concentration.Here the high frequency of inter-chain polymer-polymer interactions means that the theta condition is reached, as the polymer essentially solvates itself.The crossover concentrations from dilute to semi-dilute, and semi-dilute to concentrated, are denoted c* and c**, respectively.
PEO is soluble in several imidazolium based aprotic ILs 34-40 due to hydrogen bonding with the imidazolium cations. 41or the protic ILs ethylammonium nitrate (EAN) and propylammonium nitrate (PAN), 42,43 PEO dissolves via hydrogen bonding between the ether oxygen and alkylammonium hydrogens.While PAN is a theta solvent for PEO, EAN is a moderately good solvent.
Recently, we reported the first study of PEO conformation dissolved in a SIL, [Li(G4)]TFSI, from the dilute through to the concentrated regime. 44[Li(G4)]TFSI was found to be a poorer solvent for PEO than water (a good solvent), but better than both EAN and PAN.37][38]41,45 In this work, we examine the relationships between the chemical structure of the SIL and the solvent environment it provides for PEO using small angle neutron scattering (SANS), thermogravimetric analysis (TGA), and rheology.For SANS, at high concentrations, a mixture of hydrogenous and deuterated PEO is used to satisfy zero average contrast (ZAC) conditions, which allows R g to be extracted despite significant overlap of neighbouring polymer coils. 33,44,46,47The present study will examine the effect of (1) shortening the glyme length from G4 to G3, (2) increasing the anion size from TFSI À to bis(pentafluoroethanesulfonyl)imide (BETI À ) and (3) employing a poor SIL ([Li(G4)]ClO 4 ) as the solvent on PEO conformation.
Molecular weights were determined using size exclusion chromatography (SEC) on a Shimadzu CBM-20A liquid chromatography system with an Agilent Polargel-M guard column and three Phenomenex Phenogel 5 mm columns (10e3A/10e4A/10e5A) at 50 1C.The eluent was dimethylacetamide at a flow rate of 1.0 mL min À1 , injections were 100 mL, and detection was with a Shimadzu RID-10A differential refractive index detector.Samples were filtered through a 0.45 mm pore polytetrafluoroethylene (PTFE) membrane prior to injection.The SEC was calibrated with poly(methyl methacrylate) standards (PDI o 1.1), which was adjusted using an 11.4 kDa PEO standard that was predetermined using a Bruker MALDI-TOF mass spectrometer.
Small angle neutron scattering (SANS) experiments were conducted using the QUOKKA beamline 48 at ANSTO (Australia) using 1 mm path-length Hellma Cells.The incident neutron wavelength was 5.0 Å.Two sample-to-detector distances of 2 m and 12 m with count time 30 min and 120 min, respectively, were used to provide a q range of 0.006 to 0.34 Å À1 .These count times are B16 times longer than would normally be necessary for deuterated solvents on a high flux instrument such as QUOKKA, but are necessary here because of the high incoherent scattering due to the high hydrogen content of our solvents.After subtraction of the solvent scattering, the uncertainties in scattered intensities are less than 1.5%.Raw 2D SANS data were reduced to 1D data in IGOR Pro with reduction procedures provided by NIST modified for use on QUOKKA. 49In order to produce the PEO-SIL data on an absolute scale, the empty beam and an empty 1 mm Hellma cell were run before measurement and subtracted from the PEO-SIL SANS data.The pure SILs were run under the same conditions as PEO-SIL solutions and the solvent backgrounds were subtracted from the PEO-SIL solutions during the data reduction process.All experiments were carried out at 25 1C except for the PEO-[Li(G4)]BETI solutions which were analysed at 30 1C to ensure the samples were liquid.
Shear rate dependent viscosities of PEO-SIL solutions were measured on a TA Instruments AR-G2 rheometer using the cone and plate arrangement with a cone of 40 mm diameter and angle of 1159 0 36 00 by increasing the shear rate from 1 to 1000 s À1 .All measurements were maintained at 25 1C except for PEO-[Li(G4)]BETI solutions which were measured at 30 1C.A thermogravimetric analysis (TGA) of 5 wt% PEO in each SIL was performed on a TG/TDA analyser (PerkinElmer) from room temperature to 600 1C at a heating rate of 10 1C min À1 under nitrogen gas flow (20 mL min À1 ).

SANS data analysis
The method for analysing the reduced SANS data was described in detail in the previous work. 44Generally, the coherent neutron cross section per unit volume, I(q), for solutions of identical deuterated and hydrogenous polymers is given by: 33 where P(q) is the single-chain (intramolecular) form factor, which contains information on R g , and S(q) is the total scattering structure factor, which describes both intra-and intermolecular correlations between monomer units, and is related to both R g and the correlation length (x).q is the scattering vector expressed as 4pl À1 sin y, where l is the neutron wavelength and 2y is the scattering angle; n and N are the number density and degree of polymerization of the polymer molecules; b H , b D , and b s are the neutron scattering lengths of the hydrogenous monomer units, deuterated monomer units and the solvent, respectively; v p /v s is the ratio of specific volumes for polymer and solvent; and j is the volume fraction of deuterated polymer chains.
When the polymer chains are fully deuterated (j = 1), the first term of the right-hand side of eqn (1) is zero.This allows the characteristic dimensions to be determined from S(q).Fits to the data using the polymer excluded volume effect (eqn (2) and ( 3)) 50 yield apparent R g and the Flory exponent (n) as a function of polymer concentration.This approach is generally useful in the relatively low polymer concentration regime, but is also used in the concentrated region, see below, where this model reduces to the Debye equation for a Gaussian polymer by setting n = 0.5 (see eqn ( 6)). where In eqn ( 2) and ( 3), I 0 is the scaling factor and g(x,U) is the incomplete gamma function. 50he correlation length or ''blob size'' (x), is independent of the molecular weight but decreases with increasing polymer concentration. 51x can be determined from S(q) using the Ornstein-Zernike formula: 52 Zimm plot analysis allows further insight on the PEO-SIL solutions using the equation: 53

Kc
IðqÞ where K = (Dr/r PEO ) 2 N A À1 is the contrast factor, Dr is the difference in neutron scattering length density between d-PEO and the solvent, r PEO is the apparent density of PEO (1.2 g cm À3 ), N A is the Avogadro constant, c is the polymer concentration, M w is the weight-average molecular weight, and A 2 is the second virial coefficient, which indicates the polymer-polymer interactions in a solvent.In general, A 2 4 0 for a good solvent due to repulsions between polymers, A 2 = 0 for the y condition and A 2 o 0 for a poor solvent due to attractions between polymers.At higher polymer concentrations, zero average contrast (ZAC) between the polymer and solvent is achieved by mixing hydrogenous and deuterated PEO such that j = 0.16 for [Li(G3)]TFSI, 0.2 for [Li(G4)]BETI and 0.06 for [Li(G4)]ClO 4 (see ESI, † Table S1 for the neutron scattering length densities).This means that jb D + (1 À j)b H = b s (v p /v s ), so that the second term of the righthand side of eqn ( 1) is zero, and R g is determined from P(q) only.The unperturbed chain dimension in theta condition or melt can be determined by the Debye equation: 33  where y = q 2 R g 2 and B is the background.It should be noted that R g determined by the ZAC method is the radius of gyration of an individual chain (real R g ) whereas the R g from non-ZAC method (bulk contrast) (eqn ( 2) and ( 3)) is an ''apparent'' R g , but not the R g of an individual chain.

Results
The  3)) are also shown in Fig. 1 (solid curves) and the fitting parameters for each system (apparent radius of gyration, R g , and Flory exponent, n) are listed in Tables 1-3, respectively, together with correlation lengths, x, derived from fits to the Ornstein-Zernike formula (eqn ( 4)).Fits are shown in the ESI † in Fig. S2.
The apparent R g of PEO in [Li(G3)]TFSI is 65 Å at 1.5 mg mL À1 , and decreases with increasing PEO concentration.The same PEO is slightly more swollen in [Li(G4)]BETI (R g = 72 Å) and [Li(G4)]ClO 4 (R g = 71 Å), but these also shrink with increasing concentration (c) across the range examined.Correlation lengths consistently follow the same trend as apparent R g for all PEO concentrations in these SILs, which is also seen in the aqueous systems. 54As the polymer concentration increases, more inter-chain interactions occur and adjacent polymer chains are closer together, thus reducing x.
These results are also consistent with our previous study of [Li(G4)]TFSI; 44 in the most dilute solutions and at comparable concentrations, apparent R g of 69 Å, x = 45 Å and n = 0.54 are identical to those found here for [Li(G4)]BETI.This is not surprising given the chemical similarity between the anions.
Zimm plots (Kc/I(q) vs. (q 2 + c), eqn ( 5)) for the low concentration SANS data are shown in Fig. 2. Extrapolations to c = 0 and q 2 = 0 in Fig. 2 are used to determine the fully swollen (infinite dilution) R g , 55  Zimm plot analysis also yields second virial coefficients (A 2 ) between 0.4 and 0.7 Â 10 À3 cm 3 mol g À2 for the three SILs.This is consistent with 0.7 Â 10 À3 cm 3 mol g À2 obtained in [Li(G4)]TFSI (M w = 38 kDa PEO), 44 but much smaller than that found in water (A 2 = 2.2 Â 10 À3 cm 3 mol g À2 , M w = 38 kDa PEO) 56 and the aprotic IL [BMIm]BF 4 (A 2 = 2.0 Â 10 À3 cm 3 mol g À2 , M w = 27.3 kDa PEO), 39 which are good solvents for PEO.However, the solvent quality of these SILs is better than that of the protic IL EAN, which has a much smaller A 2 value of 1.6 Â 10 À6 cm 3 mol g À2 (M w = 38 kDa PEO), and is closer to a theta solvent for PEO. 43Therefore, our analysis reveals that SILs are moderately good solvents for PEO.
At high PEO concentrations, R g cannot be determined through the excluded volume model because of inter-chain interactions, 47 so the zero average contrast (ZAC) method is used.SANS results for PEO in [Li(G3)]TFSI, [Li(G4)]BETI and [Li(G4)]ClO 4 at PEO concentrations from 50 to 150 mg mL À1 with corresponding fits to the Debye model (eqn ( 6)) are shown in Fig. 3, with best-fit parameters listed in Table 4.
At high PEO concentrations, R g = 65 AE 2 Å is independent of the SIL and PEO concentration within experimental uncertainty.The R g value (B65 AE 2 Å) for the PEO is close to the measured chain size of 70 Å for the same M w in the melt, 57 indicating that these solutions in the concentrated regime have already achieved the limiting R g value for that of the melt.Viscosities of PEO-SIL solutions at various PEO concentrations were measured as a function of shear rate from 1 to 1000 s À1 (see ESI, † Fig. S3).The viscosities of pure [Li(G3)]TFSI, [Li(G4)]BETI and [Li(G4)]ClO 4 are consistent with previous reports. 15For the SIL-PEO systems, viscosity increases notably with increasing PEO concentration, and slight shear thinning was observed at shear rates above 200 s À1 .This is typical for PEO in various ILs such as EAN and PAN, 58 and also in water, 59 but the microscopic explanation of shear thinning is still a matter of debate. 60he c* and c** concentrations, which define the transitions from the dilute to semi-dilute and semi-dilute to concentrated regimes, can be determined from slope changes in plots of apparent R g , x, or viscosity as a function of polymer concentration. 44In Fig. 4, apparent R g and x for PEO decrease slightly at low PEO concentrations, then follows an apparent R g B c À0.25 dependence at higher PEO concentrations.Note that this is an apparent R g , and is not expected to follow R g B c À0.125 , 61 as stated in the experimental section.The concentration at which the slope changes is c*, and gives values of 16, 12 and 14 mg mL À1 for [Li(G3)]TFSI, [Li(G4)]BETI and [Li(G4)]ClO 4 respectively.These experimental values can be compared to c* values calculated from R g at infinite dilution using c* = (M w /N A )/(4p R g 3 /3), and give corresponding values of 31, 24 and 29 mg mL À1 respectively.The experimental values are about a factor of two smaller than the calculated values, but within the difference of about one experimental data point.R g at higher PEO concentrations is determined from ZAC data, which enabled c** to be measured at 50, 43 and 45 mg mL À1 for [Li(G3)]TFSI, [Li(G4)]BETI and [Li(G4)]ClO 4 respectively.Viscosity as a function of polymer concentration is also shown in Fig. 4 for the three PEO-SIL systems.In the dilute regime, viscosity increases weakly with polymer concentration as chains are mostly isolated.In the semi-dilute regime, the viscosity increases more steeply due to increasing entanglement of polymer chains.In the concentrated regime, the polymerpolymer interactions are dominant, which results in a much sharper increase in the viscosity with increasing concentration.The crossovers c* (from dilute to semi-dilute) and c** (from semi-dilute to concentrated), determined from the viscosity trend agree very well with those determined from R g , as shown in Fig. 4.
For the same polymer, better solvent quality leads to a smaller c* and c** as the polymer chains are more extended.The crossover concentrations therefore show that the solvent quality for PEO increases in the order of The c* for the same PEO polymer in EAN was reported as between 24 and 60 mg mL À1 , 43 suggesting that EAN is a poorer solvent than the SILs.
TGA curves for 5 wt% PEO in pure glyme (G3 or G4) and the three SILs are shown in Fig. 5.The data for PEO in G3 and G4 are similar to that for the pure glymes 15 until the decomposition temperatures of 167 1C for G3 and 206 1C for G4 are reached. 15bove these temperatures, approximately 5 wt% of the total mass (PEO polymer) is retained until the PEO begins to decompose at B340 1C.This confirms no strong interactions between PEO and the pure glymes, consistent with only weak dispersion interactions between ethylene oxide groups.TGA profiles for the PEO-SILs solutions are similar to those of pure SILs 15 over the entire temperature range.All PEO-SIL solutions exhibit thermal stability up to B200 1C, as shown in   lost at B420 1C.The remaining mass is residual carbon.For the PEO in the poor SIL [Li(G4)]ClO 4 , weight loss is more rapid above 200 1C due to the markedly greater amount of free glyme. 15At 341 1C, 30 wt% of the original mass remains, which is equal to the sum of PEO and LiClO 4 , i.e. all of the glyme has been removed.At higher temperatures, PEO begins to decompose, and LiClO 4 decomposition commences at 390 1C. 62The 12 wt% mass fraction present at temperatures greater than 470 1C and its white appearance is consistent with decomposition of LiClO 4 into LiCl. 62

Discussion
The conformation of a polymer in solution is dictated by the solvent quality, which is governed by the balance of polymerpolymer and polymer-solvent interactions.Fits to the SANS data (R g coil size and R g at infinite dilution, Flory exponent n) revealed that the three SILs are moderately good solvents, with solvent quality increasing in the order [Li [Li(G3)]TFSI is a poorer solvent for PEO than either [Li(G4)]TFSI or [Li(G4)]BETI.In pure Li + -glyme SILs, the glyme molecules compete with the anion for Li + . 14,15It has previously been shown that the Li + -O distance of B2.2 Å in [Li(G4)]TFSI is reduced to B2.0 Å in [Li(G3)]TFSI, 16 indicating tighter Li + -glyme binding.Tighter binding leads to higher ionicity (cation-anion separation) 16,63 and higher viscosity. 15,63For PEO to be solvated by the SIL it must compete effectively for Li + coordination sites with both the glyme and anion.Molecular Dynamics simulations have shown that the number of anions not attached to any Li + doubles from 6% in [Li(G3)]TFSI to 12.2% in [Li(G4)]TFSI, 24 meaning that anions are more strongly associated with Li + in [Li(G3)]TFSI.Tighter binding of G3 to Li + than G4 means that Li + is less available to bind PEO, resulting in poorer solvency.Employing the same glyme length enables the effect of the anion on PEO solvation to be assessed.The solvent quality of PEO dissolved in its monomeric liquids, ethylene glycol and monoglyme has been studied using rheology and photon correlation spectroscopy (PCS). 64The polymer-solvent and solvent-solvent interactions were greatly influenced by the number of H-bond donors available in the solvent, with increased H-bond capacity leading to better solvency.This is consistent with our observations.Given that lithium acts a hydrogen bond donor, as the Li + concentration per unit volume increases (from [Li(G4)]ClO 4 to [Li(G4)]TFSI and [Li(G4)]BETI) the solvent quality increases.
Strong coordination of ClO 4 À with Li + means that [Li(G4)]ClO 4 is a poor SIL, with around 20% free glyme. 14This means PEO in [Li(G4)]ClO 4 can be solvated by both free glyme as well as complex cation and anion.Given that the TGA data confirms that interaction between G4 and PEO are weak, this will also contribute to [Li(G4)]ClO 4 being a poorer solvent.Coordination between the Li and glyme leads to a greater thermal stability for the three SILs up to B200 1C. 14,15For PEO-[Li(G4)]ClO 4 , the more severe mass loss is due to the weaker coordination between glyme and Li + . 15At 341 1C, 30 wt% of the mass is retained which is equal to the sum of PEO and LiClO 4 .The great similarity in the thermal behaviour between the PEO-SIL solutions and their respective SILs suggests that PEO does not completely replace the glyme from Li + , 44 irrespective of the change of the structure on the SIL.
We argued in our previous work that the dissolution of PEO in [Li(G4)]TFSI was via the coordination between Li + and PEO. 44ypical coordination numbers for LiX-PEO complexes are 4 to 6. 65,66 Recent molecular dynamics simulations suggest that the average coordination number (Li + /O) in both [Li(G3)]TFSI and [Li(G4)]TFSI is 5, with 1 and 0.5 oxygens contributed from the anion, respectively. 13Therefore, in SILs such as [Li(G3)]TFSI and [Li(G4)]TFSI, it is more likely that PEO is solvated by displacing the anion from the coordination sphere of lithium ions, and we expect that solvation in [Li(G4)]BETI will be similar.The recent work on thermal, ionic and electrochemical properties of PEO-[Li(G4)]TFSI by Kido et al. showed that some Li-PEO complex could also form, 32 which may effect the PEO structure.

Conclusions
PEO is dissolved in [Li(G3)]TFSI, [Li(G4)]BETI and [Li(G4)]ClO 4 via the coordination between PEO ether oxygens and Li + , although interactions between PEO and free glyme may also play a role in [Li(G4)]ClO 4 .All three solvate ionic liquids are moderately good solvents for PEO as revealed by the Flory exponent values, Zimm analysis, and viscosity behaviour.
Solvent quality increases in the order of [Li(G3)]TFSI o [Li(G4)]ClO 4 o [Li(G4)]BETI B [Li(G4)]TFSI.The R g values and the crossover concentrations determined from trends in R g and viscosity as a function of PEO concentration show that PEO conformation in the SILs is similar, and much less affected by changing ion structures compared to that in common ionic liquids. 67This is likely a consequence of the mechanism of solvation being coordination bonds with Li + ions in SILs rather than hydrogen bonds in conventional ILs.
PEO must compete with anion and glyme for Li + coordination sites to be solvated.If there is a stronger coordination between Li + and the anion (ClO 4 À vs. TFSI À or BETI À ) or Li + and glyme (e.g.G3 vs. G4), then the solvent quality is poorer for PEO.
For the same glyme length, weakly coordinating anions (good SILs) are better solvents than strongly coordinating anions (poor SILs) because the lithium ion is more available for binding PEO.
The poor SIL contains a substantial proportion of free glyme which could also contribute to poorer solvation because glyme-PEO interactions are weak.

Fig. 5 .
Fig. 5.For PEO in the good SILs, [Li(G3)]TFSI and [Li(G4)]BETI, there is a gradual decrease in weight due to glyme decomposition above 200 1C and PEO above 350 1C until almost all mass is

Fig. 5
Fig.5TGA curves of 5 wt% PEO in pure glymes and SILs under N 2 flow.Note: the 5 wt% PEO in G4 data (dashed line) was reproduced from our previous work44 for comparison.

Table 1
Fitted apparent radius of gyration, R g , correlation length, x and Flory exponent, n for d-PEO-[Li(G3)]TFSI solutions

Table 2
Fitted apparent radius of gyration, R g , correlation length, x and Flory exponent, n for d-PEO-[Li(G4)]BETI solutions

Table 4
Fitted radius of gyration, R g , for PEO-SIL solutions under ZAC conditions The solvent quality for PEO for G4 based SILs is [Li(G4)]TFSI B [Li(G4)]BETI 4 [Li(G4)]ClO 4 .Henderson et al. reported the association strength for LiX salts in glymes is BETI À B TFSI À o ClO 4 À . 21This means ClO 4 À coordinates much more strongly to Li + than the other two anions.While BETI À is slightly larger than TFSI À due to its two additional CF 2 groups, it has similar coordination strength.