Polymer-Ion Interactions

Measuring the rate of increase of Tg as a function of lithium salt concentration provides

insights into the interaction of the polymer with lithium salts. The increase in Tg is attributed to

physical, ionic crosslinks that form among polymer chains and lithium ions. Generally, a larger slope of the Tg vs. [LiTFSI] plot indicates stronger, longer-lived, or more numerous polymer-Li+

interactions. We compared the rate of Tg increase as a function of lithium salt concentration among

PFPED10-DMC, PFEG3DMC, and PFEG4DMC electrolytes. Despite the identical end groups of

each material, the rate of Tg increase as a function of lithium salt concentration differed

significantly, as shown in Figure 4.9 and Table 4.4.

Table 4.4 Rate of increase in Tg as a function of salt concentration in PFPE-DMC electrolytes.

PFPE Slope, Tg vs. R (°C)

PFEG3DMC 3.3 ± 2.2

PFEG4DMC 70.1 ± 3.0

0 .0 0 .2 0 .4 0 .6 - 1 0 0 - 8 0 - 6 0 - 4 0 R Tg ( ° C ) P F P E_{D 1 0}- D M C P F E G₃D M C P F E G₄D M C

Figure 4.9 Glass transition temperature of DMC-terminated PFPEs as a function of LiTFSI concentration (R=[Li+_{]/[end group]).}

To better understand the differences in Tg increase among the PFPE-DMC materials, IR

spectroscopy was used to probe the interaction between Li+ and carbonyl oxygen atoms. Consistent with the small increase in Tg as a function of lithium salt concentration, the C=O stretching

frequency of PFEG3DMC exhibited no significant shift upon addition of lithium salt. Peak

broadening is apparent in the salt-saturated sample, indicating minimal interaction between the carbonyl oxygen atom and Li+/ion pairs. PFEG4DMC and PFPED10-DMC exhibited significant and

comparable shifts in stretching frequency as expected based on the materials’ similar salt-solvating ability. But despite the comparable strength of interaction between Li+ _{and the carbonyl oxygen}

atoms, PFEG4DMC’s Tg increase is higher than that of D10-DMC. We propose that this is caused

by higher end group concentrations in PFEG4DMC, creating closely spaced ionic crosslinks that

1 6 8 0 1 7 6 0 1 8 4 0 0 . 4 0 . 6 0 . 8 1 . 0 W a v e n u m b e r ( c m- 1) T r a n s m it ta n c e 1 6 8 0 1 7 6 0 1 8 4 0 0 . 4 0 . 6 0 . 8 1 . 0 W a v e n u m b e r ( c m- 1) 1 6 8 0 1 7 6 0 1 8 4 0 0 . 7 0 . 8 0 . 9 1 . 0 W a v e n u m b e r ( c m- 1) 2 2 c m- 1 _{2 4 c m}- 1

Figure 4.10 IR spectra of PFPE-DMC materials (red: PFEG3DMC, blue: PFEG4DMC, black: D10-DMC;

solid = neat, dashed = saturated with LiTFSI).

4.3.2.5 Ionic Conductivity

Ionic conductivity of the PFPE-DMC materials was then measured at 30°C across a range of lithium salt concentrations, as shown in Figure 4.11. At low salt concentrations (~5 wt.% LiTFSI), the ionic conductivity of the PFEGs was more than an order of magnitude higher than that of PFPED10-DMC. However, PFPED10-DMC exhibits a monotonic increase in ionic

conductivity as a function of salt concentration, while the fluorinated glycols exhibit maxima in ionic conductivity. As a result, the maximum achievable ionic conductivity in PFEG4DMC is only

~2.5x higher than that of PFPED10-DMC. In the dilute regime of polymer electrolytes, conductivity

increases upon addition of lithium salt due to elevations in the number of available charge carriers.18 _{At higher salt concentrations, addition of lithium salt causes conductivity to decrease}

because elevations in Tg and viscosity more than offset any increase in the number of charge

0 1 0 2 0 3 0 4 0 1 0- 8 1 0- 7 1 0- 6 1 0- 5 1 0- 4 w t . % L iT F S I  ( S c m- 1) P F E G₃D M C P F E G₄D M C D 1 0 - D M C

Figure 4.11 Ionic conductivity of PFPE-DMC electrolytes at 30°C as a function of salt concentration.

4.3.2.6 Transference Number

In polymer electrolytes, solvent chains are entangled and immobilized. The prevalent conduction mechanism for Li+ is ion hopping from solvation site to solvation site along the polymer chain during segmental rearrangements.20 In small molecule electrolytes, on the other hand, solvent molecules diffuse freely. Vehicular motion occurs: Li+ diffuses with the solvent molecules in its coordination sphere. Lithium ion transference numbers range from 0.2 to 0.4 in small molecule electrolytes because Li+ moves at slower speeds with its solvation shell than the relatively “free” anions.16 _{Shi and Vincent previously reported that in PEO electrolytes below the}

critical molecular weight for entanglements (3200 g/mol), vehicular motion becomes a major cation transport mechanism.4 The PFEG materials are likely below the critical molecular weight for entanglement, indicating that vehicular transport may be a major mechanism for ion transport in the PFEG electrolytes. In theory, this would lower the t+ value of fluorinated glycols relative to their higher molecular weight analogs, mitigating some of the benefits of perfluoroether-based electrolytes.

The transference numbers of PFEG3DMC and PFEG4DMC were measured using the

potentiostatic polarization method at 30°C. At ~3.5wt.% LiTFSI, t+(PFEG3DMC) = 0.97 ± 0.02, and

t+(PFEG4DMC) = 0.98 ± 0.02. These values are in good agreement with previously reported values for

the transference number of PFPE electrolytes (t+

(D10-DMC)=0.91 at 9.2wt.% LiTFSI, 30°C).7 In spite

of the likelihood that vehicular transport occurs in these systems, Li+ remains significantly more mobile than the anion. We propose that the perfluorinated PFPE backbone interacts favorably with the fluorinated TFSI- anion, creating a solvation shell that slows the motion of the anion in an analogous mechanism to the slowed mobility of Li+ in its solvation shell in commercial electrolytes.16

Chapter 3 introduced a hypothesized mechanism for ion transport in PFPE electrolytes, in which Li+ hops among ion aggregates that are prevalent in these systems.15 This mechanism presents another possible explanation for the high t+ values observed in the fluorinated glycols, as Li+ hopping among ion aggregates is expected to be faster than anion diffusion. It should be noted, however, that efficient Li+ _{transport was proposed to only “switch on” at high salt concentrations}

where ion aggregates are spaced sufficiently close together. To test whether this mechanism applies to the perfluorinated glycols, the dependence of t+ in PFEG4DMC electrolytes on LiTFSI

salt concentration was measured at 30°C.

As shown in Figure 4.12, t+decreased from 0.98 to 0.83 with increasing salt concentration. Contrary to the proposed mechanism for ion transport in other PFPE systems, Li+ _{transport actually}

becomes less efficient at higher salt concentrations. This has been observed in propylene carbonate-based electrolytes21,22 and is tentatively attributed to ion aggregation yielding the positive triplet, slowing the motion of Li+. A conflicting report showed that t+ increases in PEO electrolytes as a function of salt concentration, which was attributed to formation of the negative

triplet, slowing the motion of the anion.23 While ion aggregation is the likely cause of the reduction in t+ at high salt concentrations, further work is needed to clarify the transport mechanisms contributing to this phenomenon.

0 1 0 2 0 3 0 4 0 0 .7 0 .8 0 .9 1 .0 [ L iT F S I] ( w t . % ) t+

Figure 4.12 Transference number of PFEG4DMC as a function of LiTFSI concentration.